Forces holding the particles of a condensed phasetogether (cohesive forces) become anisotropic inthe phase boundary region, and their normalcomponent becomes smaller compared to theparallel component. To simplify the descriptionand facilitate theoretical treatment, the propertiesof the boundary region of a condensed phase areprojected onto a two-dimensional surface (Gibbs‘‘dividing surface’’). The tensile stress resulting
from the anisotropic forces in the boundaryregion is then termed the interfacial tension. Thisquantity corresponds to the reversible workrequired to bring particles from the volume phaseto the interface during enlargement of the former,and thus corresponds to the increase in freeenthalpy of the system per unit surface area:
dG
dF
� �P;T
¼ g
DOI: 10.1002/14356007.a25_747
where G is the free enthalpy, F the surface area,and g the interfacial tension.
The unit of interfacial tension is accordinglyN � m/m2, i.e., N/m, and is normally expressed asmN/m. The interfacial tension in most casesdecreases with increasing temperature and withincreasing pressure.
Interfaces exist between all contacting butimmiscible phases; in current usage the terminterface is understood to mean the contactregionof twocondensedphases,whereas the termsurface refers to the boundary region between acondensed phase and a gas. Surface tensions ofliquids and interfacial tensions between two liq-uid phases can easily be measured. The capillaryrise of a liquid in a capillary tube can be used tocalculate the surface tensionof a liquid if thewallsof the capillary tube are completely wetted by theliquid (capillary attraction).
The value of g can be obtained directly bymeasurement of the force required to enlarge thesurface or interface between two liquids over aspecific length. A small wettable measuring plate(Wilhelmy plate) or a ring slowly withdrawnfrom the liquid (Lecomte du No€uy tensiometer)is used for this purpose. In the droplet-volumemethod the volume of a droplet released from acapillary tube, which is directly proportional tothe interfacial tension, is measured. The surfacetension of a liquid can also be determined bymeasuring the maximum pressure in air bubblesforced through a capillary tube into a liquid, thispressure being directly proportional to the sur-face tension. Extremely low interfacial tensionsin the range from 10�1 to 10�5 mN/m can bemeasured with a spinning drop tensiometer, inwhich a droplet of the lighter phase is suspendedin the heavier liquid contained in a rotatingcapillary [12], [13]. Also, capillary wave spec-troscopy is used to measure the interfacial ten-sion between two liquids [14].
In the equilibrium state only very small energyinteractions occur between the particles at thesurface of a condensed phase and those of thegaseous phase. The surface tension can thereforebe regarded simply as an expression of the inter-actions between the particles in the surface ofthe condensed phase. However, at the interfacebetween two condensed phases interactionsbetween the particles of the two phases occurthrough the interface, resulting in a reductionof the interfacial tension. When examining the
interactions between the particles of condensedphases a distinction has to be made betweenLondon dispersion forces and polar forces.Accordingly, the surface tension comprises com-ponents that may be attributed to pure dispersionforces (gd) and to polar interaction (gp):g ¼ gdþgp
For example, water has a surface tension of72.8 mN/m, composed of 21.8 mN/m for gd and51.0 mN/m for gp.
In contrast, alkanes and also polyethylenehave no surface tension attributable to polarforces, and in this case g ¼ gd. If one of thephases at an interface is completely nonpolar,then the interaction through the interface is re-stricted to dispersion forces and the interfacialtension between two condensed phases is givenby Fowkes equation:
If a droplet of a liquid is brought into contact withthe surface of a first liquid – the two liquidsbeing immiscible – two phenomena can occur,depending on the magnitude of g12. If g12 issmaller than (g1 � g2), the second liquid spreadsover the surface of the first liquid in the form of athin (in the limiting case monomolecular) film.The liquid that has spread exerts a surface pres-sure p in the opposite direction to the surfacetension g1 of the underlying phase, resulting in areduced surface tension g 01 of this phase:
g1�p ¼ g01
The surface tension g 01 can be measured by theaforementioned methods, and the surface pres-sure p by a Langmuir surface balance, whichmeasures the force acting on a barrier that re-stricts the spreading film. If the particles of thespread liquid are sufficiently far apart, the liquidfilm behaves as a two-dimensional gas describedby the equation of state:
gF ¼ RT
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where F is the available surface, R the gasconstant, and T the temperature. Such a surfacefilm is elastic. If the film is compressed, a regioncharacterized by a sharp increase in surfacepressure is reached since the forces of repulsionbetween the particles then come into play. Thisregion corresponds to the state of a condensedliquid. On compressing the surface film furtherthe latter may finally break up due to furrowformation, layer stacking, etc., whereupon thesurface pressure decreases.
If the interfacial tension g12 between a firstliquid having a surface tension g1 and a secondliquid having a surface tension g2 applied as adroplet to the surface of the first liquid is greaterthan (g1 � g2), the applied droplet remains in theform of a lens in the surface of the underlyingliquid (Fig. 1). The forces resulting from the twosurface tensions g1 and g2, as well as from theinterfacial tension g12, act at the point of contactof the two liquid phases with the gas phase. If theresultant anglesa, b, and f are measured and twoof the tensions are known, the third can becalculated (Neumann triangle). If the underlyingphase 1 is a solid and its surface is not deformed,then g1 and g2 act in opposite directions along astraight line, and the contact or wetting angle fbetween the applied liquid and the surface of thesolid is formed at the point of contact of the threephases (Fig. 2). The contact angle f is given byYoung’s equation:
cosf ¼ g1�g12g2
The contact angle can be measured by directobservation of a droplet on a horizontallymounted plate of the solid. If the liquid is ad-vancing the contact angle generally becomeslarger than in the stationary state on account ofthe restricted elasticity of the liquid and theroughness of the solid surface, and if the liquid
is receding the contact angle becomes smaller(Fig. 3).
The determination of the contact angle isimportant for evaluating the wetting of solidsand for indirect determination of the surfacetension g1 of a solid. The nonpolar and polarcomponents of the surface tension of the solidmust be determined separately by measuring thecontact angles with nonpolar and polar liquidswhich are then added to obtain the total surfacetension. The surface tension g1 of the solidemployed in Young’s equation accordingly cor-responds only to the force resulting from theeffective interactions (g12) between liquid andsolid on the one hand, and the surface tension ofthe liquid (g2) on the other. This surface tensiong1 therefore generally does not correspond to thetrue surface tension of the solid, but only to a partthereof.
The reversible work per unit area that isexchanged with the surroundings in the forma-tion of a liquid – solid interface is the adhesionor wetting tension g j, which expresses the adhe-sion forces between liquid and solid surface:
g j ¼ �g2cosf ¼ g12�g1
If the solid is wetted by the liquid the adhesiontension is negative (f < 90�), whereas if it is not
Figure 1. Lens formed by a nonspreading liquid 2 on anotherliquid 1 (schematic)
Figure 2. Formation of a contact (wetting) anglef between aliquid 2 and a solid surface 1 (schematic)
Figure 3. Contact anglef of a liquid on a solid surface in theadvancing state (a) and receding state (r) (diagrammatic)
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wetted the tension is positive (f > 90�). Forexample, the adhesion tension between waterand glass is negative, i.e., energy is released tothe surroundings on formation of the interface,and the contact angle is < 90�. Thus water wetsglass and rises in a glass capillary. In contrast, theadhesion tension between water and polyethyl-ene is positive, i.e., work must be expended toproduce a water – polyethylene interface, andthe contact angle is > 90�. Hence water doesnot wet polyethylene completely and is displacedfrom a polyethylene capillary.
The wetting of a solid surface by a liquid isanalogous to the spreading of a substance on aliquid surface.
The afore-described phenomena are observedfor phases of a pure substance and for multicom-ponent phases. In the latter case, as a result ofanisotropic interactions in the boundary regionof a condensed phase, accumulation or deple-tion of components in this region may occur(positive or negative adsorption at the inter-face). Substances that accumulate from a liquidphase at the interface (or surface) are referred toas interface-active, usually as surface-active or,more loosely, capillary-active; substances thataccumulate in the volume phase, rather than atthe interface, are accordingly termed inactive.
Interfacial activity leads to a decrease of theinterfacial tension compared to that of the puresubstance, whereas interfacial inactivity in-creases the interfacial tension. Most water-solu-ble organic substances lower the interfacial andsurface tensions ofwater, whereas inorganic saltsincrease them. For example, 20 wt% of butyricacid reduces the surface tension of water from 73to 27 mN/m, whereas the addition of 20 wt% ofwater to pure butyric acid does not reduce thesurface tension of butyric acid of 27 mN/m. Thusbutyric acid is surface-active in water, but wateris not surface-active in butyric acid. A 10%sodium chloride solution has a surface tensionthat is 3 mN/m higher than that of pure water; thestrongly hydrated sodium chloride ions in thevolume phase are surface-inactive.
Strong accumulation of substances at the in-terface is termed adsorption. The adsorption ofsubstances from a liquid phase at its interface isdescribed by the Gibbs equation:
� dgd lnc
¼ GRTx
where g denotes the surface tension, c the con-centration of a substance in the volume phase inmol/cm3, g the excess concentration of this sub-stance in the interface over the concentration inthe volume phase in mol/cm2, R the gas constant,T the temperature, and x is a correction factorbetween 1 and 2 that depends on the degree ofdissociation of the adsorbed substance. For non-dissociated substances x ¼ 1, and for completelydissociated substances x ¼ 2.
If g is negative, i.e., if the concentration of adissolved substance in the interface is lower thanin the volume phase, the interfacial and surfacetensions increase, whereas if g is positive, whichmeans a higher concentration of the dissolvedsubstance in the interface than in the volumephase, the interfacial and surface tensions of thecondensed phase decrease with increasing con-centration of the dissolved substance in the vol-ume phase. The greater the excess concentrationg of a substance at the interface, the more strong-ly surface-active it is. Surfactants, also referred toas tensides, are active substances whose mole-cules or ions have the property that when acharacteristic concentration in aqueous solutionis exceeded, they associate by reversible aggre-gation to form larger particles, known asmicelles, which impart colloidal behavior to thesolution.
The reason for the large adsorption at theinterface lies in the chemical structure of theadsorbed substance. Surfactants are amphiphilicsubstances: their molecules contain endophilicand exophilic groups. Endophilic groups arethose that interact strongly with the condensedphase, whereas exophilic groups are those thatinteract more stronglywith one another thanwiththe volume phase, and are therefore expelledfrom the latter. With amphiphilic substances theexophilic group is forced out of the volume phaseand the endophilic group is drawn in, an accu-mulation in the form of an oriented adsorptionoccurrs at the interface. In aqueous systems theendophilic groups are termed hydrophilic, andthe exophilic groups are termed hydrophobic.
In addition to the interfacial phenomena,which can be described with the aid of thermo-dynamics, electrical potentials are generallyproduced between condensed phases in ionizablesystems. By dissociation, adsorption, desorption,and exchange of ions in the interface, anelectrical double layer is formed consisting of
434 Surfactants Vol. 35
two components: a relatively rigid adsorptionlayer and a diffuse component that extends toa greater or lesser extent into the volume phase,depending on the ionic strength. The electricalpotential of the rigid adsorption layer (Sternpotential) can be determined approximately byelectrophoresis or flow potential measurements(zeta potential [15]).
2. Overview of Surfactants
The highly diagrammatic tail – head model iswidely used in the graphical representation ofsimple surfactants; the tail symbolizes the hydro-phobic group, and the head the hydrophilicgroup.
The hydrophobic groups are mainly alkyl oralkylaryl hydrocarbon groups, but fluoroalkyl,silaalkyl, thiaalkyl, oxaalkyl groups, etc., are alsopossible. Some important hydrophobic andhydrophilic groups are listed below:
Since surfactants mainly act in aqueous systems,it is expedient to classify surfactants according tothe chemical structure of their hydrophilicgroups. The hydrophilic groups may be ionic ornonionic, and their chemical structure can varywidely.
The following classification of surfactants hasproved convenient (arranged in order of indus-trial importance).
Anionic Surfactants, anionics, anion-activecompounds, are amphiphilic compounds inwhich the hydrophobic residues carry anionicgroups with small counterions such as sodium,potassium, or ammonium ions which only slight-ly influence the surface-active properties of thesubstance. Examples include soaps, alkylbenze-nesulfonates, alkyl sulfates, and alkylphosphates.
Nonionic Surfactants, niosurfactants, areamphiphilic compounds that are unable to disso-ciate into ions in aqueous solution, for example,alkyl and alkylphenyl polyethylene glycol ethers,fatty acid alkylolamides, sucrose fatty acid es-ters, alkyl polyglucosides, or trialkylamineoxides.
Cationic Surfactants, cationics, cation-active compounds, are amphiphilic compoundsin which the hydrophobic residues exist ascations with counterions such as chloride, sulfate,or acetate that only slightly influence the activeproperties of the compound. Examples includetetraalkyl ammonium chloride or N-alkylpyridi-nium chloride.
Amphoteric Surfactants, amphosurfactants,(i.e., ampholytes and betaines) have zwitterionichydrophilic groups. Examples include aminocar-boxylic acids, betaines, and sulfobetaines:
The electrical charge state of the amphosurfac-tants, which are often sparingly soluble at theisoelectric point, depends on the pH of thesolution.
The subdivision of surfactants according tothis classification is sometimes unambiguous.
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Nonionic surfactants may assume a cationiccharacter in acid solution due to protonation;amine oxides are an example of this:
Anion – Cation Surfactants, in which an-ions and cations are amphiphilic and which areobtained by mixing anion and cation surfactants,are a special case. They are often sparinglysoluble in water and in their properties resemblenonionic surfactants rather than ionic surfactants.
The structures of surfactants are not restrictedto the simple tail – head model. Surfactants maycontain several hydrophilic or hydrophobicgroups. Examples include the sodium salts ofthe dialkyl sulfosuccinates:
with a hydrophilic group flanked by two hydro-phobic groups, or the disodium salt of 1,14-disulfatotetradecane with two hydrophilicgroups at both ends of a long hydrophobic resi-due:
Surfactants of the latter type are also termed bolasurfactants.
Industrial surfactants [8] are not pure definedsubstances, but mixtures of a large number ofisomeric, homologous, and chemically similarsubstances. Lowmolecularmass products accountfor thebulkof the industrialproducts.Amphiphilichigher molecular mass compounds also havesurface-active properties and are produced fornumerous special applications. Examples includesulfated or ethoxylated condensation products ofalkylphenolsandformaldehyde,ethyleneoxide –proylene oxide block copolymers, modified poly-siloxanes, polyvinyl-N-alkylpyridinium salts, orcopolymersofvinylpyridineandmethacrylicacid,the last-mentioned being an example of polymericamphosurfactants. The ‘‘polysoaps’’ combinepolymericcations (e.g., thecationofpolyvinylpyr-idine) with fatty acid anions.
In the theoretical treatment of surfactants thedemarcation with respect to other amphiphiles
that form higher ordered aggregates such asvesicles or membranes but not micelles is notalways sharp [16], [17]. Substances such as polarlipids or water-soluble proteins play an extreme-ly important role in the physiological processesof living organisms [18].
The vast majority of surfactants have hydro-phobic groups derived from hydrocarbons. Onlya few groups of surfactants bear hydrophobicresidues containing heteroatoms. Hydrocarbon-derived surfactants are discussed here accordingto the nature of their hydrophilic groups (anionic,nonionic, cationic, amphoteric); the remainingsurfactants are treated according to the structuralfeatures of their hydrophobic groups (polypro-pylene glycol derivatives, silicone-based surfac-tants, fluorosurfactants).
The so- called biosurfactants produced bymicroorganisms, are treated in [19]; for highpolymer surfactants, see [20].
3. Properties of Aqueous SurfactantSolutions [21–23]
In highly dilute aqueous solutions surfactantsexist in monodisperse form and are concentratedat the interfaces by hydrophilic – hydrophobicoriented adsorption. The surface concentration gof a surfactant can be calculated from the Gibbsequation by measuring the surface tension asa function of the volume concentration (seeChap. 1). With a rise in volume concentrationthe surface concentration increases until thesurface is completely covered by the surfactantmolecules or ions. Further surfactant particlescannot be accommodated at the surface andGibbs’ equation is no longer valid. The arearequirement of a surfactant molecule or ion inthe adsorption layer can be calculated from thesurface concentration at maximum occupancy.Such calculations show that the surfactantmolecules are adsorbed with their longitudinalaxis perpendicular to the surface. The projectionof the largest surface-parallel cross section of thesurfactant particle onto the surface gives its arearequirement.
An entropy effect is extremely important forthe interfacial activity. The hydrophobic groupsof the surfactant molecules or ions are hydro-phobically solvated in aqueous solution. Thusthe water molecules are highly ordered in the
436 Surfactants Vol. 35
immediate vicinity of the hydrophobic species,which is associated with a decrease in entropy. Ifthe hydrophobic group is displaced from theaqueous phase, the state of disorder of the waterand thus the entropy of the system increases. Thiseffect is also termed the hydrophobic effect. If,under total occupancy of the surface, the volumeconcentration of the surfactant in equilibriumwith the surface concentration prevailing in thiscase is exceeded, the surfactant molecules orions, which up to this concentration are mainlypresent in the volume phase in monodisperseform, congregate in an entropy-governedmannerto form larger aggregates (micelles) in whichthey are oriented with the hydrophilic groupspointing towards the aqueous volume phase andthe hydrophobic groups towards the interior ofthe micelles.
The aggregation of surfactant molecules orions into micelles takes place in a strictly limitedconcentration range characteristic of each sur-factant; if the surfactant concentration rises fur-ther, the number of micelles per unit volumeincreases, but not, however, the number ofmono-disperse – dissolved surfactant molecules orions (more correctly, their activity). Sincemicelle formation occurs at the volume concen-tration of the surfactant at which the surface islargely, if not completely, covered so that thesurface tension becomes independent of anyincrease in volume concentration, measurementof the surface tension as a function of the con-centration represents a simple method of deter-mining the so- called critical micelle concentra-tion (CMC) at which micellization or micelleformation begins (Fig. 4).
The increase in the number of particles in asurfactant solution with concentration suddenlydrops at the critical micelle concentration due tomicelle formation, and consequently the concen-tration dependence of all colligative propertiessuch as vapor pressure and osmotic pressure, andin the case of ionic surfactants also the equivalentconductance, changes at this concentration.
Micelles are dynamic structures that are inequilibrium with the surrounding monodispersesurfactant solution; their average aggregationnumber fluctuates in purely aqueous solutionsat the critical micelle concentration around amean value characteristic of each surfactant. Thisvalue is between 100 and 1000 in simple nonionicsurfactants, and is generally below 100 in ionic
surfactants; with the latter, the electrostaticrepulsion acting between the ionic head groupsopposes further aggregation.
Above the CMC surfactant solutions are col-loidal solutions; the surfactants form ‘‘associa-tion colloids’’ and behave like sols, and at higherconcentrations behave like gels. This behavior isconnected with the shape of the micelles thatform under particular conditions. A distinction ismade between spherical, rod-shaped, and disk-shaped micelles [24].
The shape of the micelle is determined pri-marily by the ratio of the area requirement of thehead group of the surfactant under the prevailingconditions to the volume of the hydrophobic tailgroup. If this ratio is large, for example in the caseof ionic n-alkyl surfactants, spherical micellesare formed. For small ratios, such as in the caseof nonionic alkyl polyglycol ethers, rod-shapedmicelles are formed. However, simple ionicsurfactants too can aggregate to form rod-shapedmicelles if the electrostatic repulsion of the headgroups is screened by adding an electrolyte andtheir spatial requirement on the surface of themicelle is reduced.
Surfactants with voluminous or severalhydrophobic groups, as well as fluorosurfactants,form disk-shaped micelles. Some micelles asconstituents of lyotropic mesophases are illus-trated schematically in Figure 5.
Spherical micelles can transform at highersurfactant concentrations into rod-shaped ordisk-shaped micelles. Such a transition may be
Figure 4. Surface tension g as a function of concentration c,and critical micelle concentration (CMC)a) Nonionic surfactant (dodecylheptaglycol ether) in water;b) Ionic surfactant (dodecyl sulfate) in water; c) Dodecylsulfate in 0.1 N sodium chloride solution
Vol. 35 Surfactants 437
indicated by a second critical micelle concentra-tion, at which the colligative properties of thesolution once more change abruptly.
At higher surfactant concentrations spatiallyextensive, ordered structures, known as liquidcrystal mesophases or lyotropic liquid crystals,are formed. In a number of nonionic surfactantsof the poly(ethylene glycol) ether type suchliquid crystal phases can be attributed to theformation of specific hydrates [25].
Provided they do not transform into rod-shaped or disk-shaped micelles, sphericalmicelles condense into cubic phases, rod-shapedmicelles into hexagonal phases, and disk-shapedmicelles into lamellar phases, whose basic struc-ture resembles the bilayer lipid membranes(BLM) of living organisms. The bilayer struc-tures present in lamellar phases transform underhigh shear forces (e.g., by ultrasound) intovesicles – namely spherical structures having an
Figure 5. Structure of lyotropic mesophases formed from micellar aggregates [24] (reproduced from [6], with permission)A) Cubic phase; B) Hexagonal phase; C) Lamellar phase; D) Inverse hexagonal phase; E) Nematic rod phase; F) Nematicdisc phase
438 Surfactants Vol. 35
outer bilayer membrane (Fig. 6) [26–28]. Thisstructure was first observed in natural fat parti-cles (liposomes), formed from lipids.
The ordered and oriented, monolayer andmultilayer adsorption of surfactants or amphi-philes leads to the formation of Langmuir –Blodgett films [29], which are important in sur-face treatment and coating technology.
Solutions of spherical micelles and cubicphases formed therefrom are isotropic, whereassolutions of rod-shaped and disk-shapedmicellesand their liquid crystal phases are anisotropic.Due to this anisotropy the flow of surfactant solsand gels deviates from Newtonian behavior. A
knowledge of this divergence from Newtonianflow is important in the preparation and formu-lation of surfactants, and in the design of pipe-lines and conveying and mixing equipment. Adistinction is made between pseudoplasticity(shear thinning), namely decreasing in viscositywith increasing shear rate; and dilatancy (shearthickening), namely increasing viscosity withincreasing shear.
When the mechanical stress is released pseu-doplastic and dilatant liquids return to their orig-inal state.
Thixotropy is a decrease in viscosity as a resultof sustained mechanical stress, and rheopexy an
Figure 6. From bilayer lipid membranes (BLM) to vesiclesA) Amphiphilic molecule; B) BLM (section); C) Myelin structure; D) Partially broken myelin structure; E) Cross sectionthrough a vesicle (from [26], with permission)
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increase in viscosity as a result of sustainedmechanical stress. The terms thixotropy andrheopexy imply that the liquids return to theiroriginal state, even after a time delay, when themechanical stress is released; in practice, how-ever, both expressions are used even if this is notthe case.
Gelscanalsoexhibitplasticbehavior,actingaselastic solids below a certain critical shear stress(the yield stress or yield point) and transforminginto the liquidNewtonian or non-Newtonian stateonly when the yield stress is exceeded.
Pseudoplasticity, thixotropy, and plasticityare often observed in surfactant solutions, where-as dilatancy and rheopexy occur less often. Theflow behavior of surfactant solutions dependslargely on the temperature, concentration, andon the presence of other additives, especiallyelectrolytes. Transitions from one flow behaviorto another are often produced by slight changes inthe conditions, which must be taken into accountwhen diluting ormixing surfactant solutions. The‘‘thickening’’ of surfactant solutions, e.g., ofalkyl polyethylene glycol ether sulfate solutionswith sodium chloride is utilized in the formula-tion of such solutions. Conversely, the addition ofa number of compounds such as urea or short-chain alkylbenzene sulfonates leads to the disso-ciation of the gel structure and thus decreasedviscosity. Such substances are termed hydrotro-pics, and the effect, hydrotropy.
In addition to being viscous, many surfactantsolutions at relatively high concentrations,exhibit limited elastic deformation, which ismainifested inter alia in vibrational phenomena.The flow behavior of such liquids is termedviscoelastic [30].
The flow behavior of surfactant solutions ismeasured in rotational viscometers (! Rheome-try). The solution is added to the gap between twoconcentric cylinders, one of which is at rest,while the other rotates. A shear rate gradient isgenerated due to the relative movement of thecylinders. The torque necessary for this move-ment is measured, from which the shear stressand viscosity of the solution are calculated. Thecone-and-plate system is based on the sameprinciple. A cone having an obtuse angle rotatesover a plate and the liquid is subjected to shearforces in the gap between cone and plate.
Surfactant sols and gels often exhibit segre-gation phenomena. Coacervation is the separa-
tion of an initially uniform phase into two phases,one of which contains more of the colloidalcomponent, and the other less. A further propertyof sols and gels is aging, known as deswelling orsyneresis, i.e., the coagulation of gel particles andtheir dehydration, with the formation of a com-pacted gelatinous mass and a low- colloid liquidphase. To prevent such segregations, which canlead to difficulties in the storage and metering ofsurfactant solutions, mixing equipment must of-ten be provided in storage vessels.
The fact that dissolved surfactants areadsorbed at interfaces and form micelles confersa number of properties on their solutions, whichare described below.
Wetting. The formation of a liquid – solidinterface inplace of a gas – solid interface istermed wetting. Young’s equation describes therelevant relationships (see 1). A prerequisite,however, is that the surface-active substancesare adsorbed at the interface so that their hydro-philic groups are directed towards the aqueousphase. If these hydrophilic groups are adsorbedon the solid surface and the hydrophobic groupsproject into the aqueous phase (inverse adsorp-tion), the solid surface becomes hydrophobic, aneffect that is exploited in water-repellent textiles,fabric softeners, and ore flotation, for example.
Rewetting. If a liquid, in particular a surfac-tant solution, displaces a liquid droplet or a solidparticle insoluble in the latter from the surface ofa solid, with emulsification or suspension of thedisplaced substrate, this is termed rewetting.
Solubilization is the apparent dissolution ofsubstances that are insoluble or slightly soluble inwater, in a surfactant solution by incorporationinto the micelles. On the other hand, water can besolubilized in hydrophobic solvents by addingsurfactants. Thereby, inverse micelles areformed, with the hydrophobic groups alignedoutward and the hydrophilic groups inward (seeFig. 5D). Solubilization, like the decrease insurface tension and interfacial tension, is re-garded as a characteristic property of surfactantsin aqueous solutions.
Emulsion Stabilization (see also! Emul-sions) is the stabilization of disperse systems oftwo liquids that are insoluble or only slightly
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soluble in one another. This stabilization is basedon covering the interfaces with surfactants thatcounteract coalescence of the droplets of thedisperse phase. Such emulsions are metastablesystems, and are also termed macroemulsions todistinguish them from the thermodynamicallystable microemulsions [31–33].
Suspension Stabilization is the preventionof coagulation and delay of sedimentation ofsolids finely dispersed in a liquid. The surfactantsare solid – liquid adsorbed at the phase interfaceand prevent the aggregation and coagulation ofthe dispersed solid particles by means of stericscreening and, in the case of ionic surfactants,also by electrostatic repulsion. Furthermore, un-der certain conditions surfactants are also able todeflocculate, without mechanical action, suspen-sions that have coagulated.
Protective colloid action is the suspensionstabilization of hydrophobic colloids, especiallyagainst the coagulating action of inorganic salts.
Lime Soap Dispersion. A number of sur-factants having relatively large or several hydro-philic groups can disperse sparingly soluble cal-cium and magnesium salts of fatty acids (limesoaps) in water. Examples of such surfactantsinclude:
C9H19 ��C6H4 ��ðOCH2CH2Þ10 ��OH
C17H35CONHðCH2Þ3NþðCH3Þ2ðCH2Þ3SO�3
C10H21 ��C6H4 ��SO2NHCH2CH2SO3Na
Foams (see also ! Foams and Foam Con-trol). Surfactants accumulate at the interface of agas bubble in aqueous solution, with their hydro-phobic residues directed to the interior of thebubble. When the ascending bubble burststhrough the surface of the liquid phase the filmof surfactants present there becomes attached tothe surface of the bubble. A lamella consisting oftwo films of surface-active substance is obtained,with hydrophilic groups pointing into the interiorof the lamella and hydrophobic groups pointingaway from the lamella. Liquid entrained from thesolution is found between the films. The combi-nation of numerous such bubbles constitutes afoam. The properties of foams are connectedwiththe mechanical properties of the lamellae and the
dynamic processes in and on the lamellae (vis-cosities of the surface films and interlamellarliquid, elasticity of the surface films, diffusionand spreading rate of the surface-active sub-stances, electrical repulsion, Van der Waals at-traction between the two films of the lamella, andgas permeability, leading to drying and breakingof the lamella).
The properties of surfactant solutions are theresult of adsorption and aggregation of the sur-factant molecules and ions; the attainment of theequilibrium state is determined by the rate ofdiffusion, orientation, and organization of thesurfactant molecules and ions. Many propertiesof surfactant solutions are therefore time-depen-dent. The attainment of the equilibrium value forthe surface tension of a surfactant solution is anexample. Another example is the restorationforce that spontaneously occurs in the expansionof a surface of a surfactant solution, as the resultof a depletion in surfactant per unit area of thesurface, resulting in an increase in surface ten-sion. This increased surface tension is restored tothe original value by diffusion of surfactant fromthe volume phase to the surface, or, in the case ofspatially limited regions of increased surfacetension, also by expansion of the surfactant filmof the neighboring regions. The Marangonieffect, i.e., the transportation of a liquid layeradhering to the surfactant film to the site of highersurface tension, is observed in the latter case.
For the sake of convenience, the properties ofthe boundary region between two phases areregarded as properties of a two-dimensional sur-face (see Chap. 1). Although this model repre-sentation satisfactorily explains many phenome-na, it does not provide a detailed understanding ofthe conditions in boundary regions between twophases. For example, lamellae, membranes, ormultimolecular surface films should no longer beregarded as two-dimensional; rather, they formthree-dimensional structures between two phases(i.e., a third phase, known as the interphase).
4. Relationship between Structureand Properties of Surfactants
The nature of the hydrophilic group of a surfac-tant exerts a decisive influence on the behavior ofthe surfactant in aqueous solution. The solubilityof ionic surfactants depends on the electrolytic
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dissociation and solvation of the hydrophilicgroup, whereas the solubility of nonionic surfac-tants depends essentially on the hydration of thepolarized but undissociated hydrophilic group.The solubility of ionic surfactants generally in-creases with increasing temperature, whereasthat of nonionic surfactants decreases becausethe extent of hydration via hydrogen bond for-mation decreases with increasing temperature(see Figs. 7 and 8). Aqueous anionic-surfactantgels (Fig. 7) dissolve in a monodisperse fashionat low concentration, and in a micellar manner athigher concentrations. The solution temperaturerises only slightly with increasing concentration,and likewise the critical micelle concentrationincreases only slightly with temperature. Thetriple point monodisperse solution – gel –micellar solution is known as the Krafft point
(or temperature), and is characteristic for eachionic surfactant. It can be determined sufficientlyaccurately for most purposes as the point atwhich a surfactant solution above the criticalmicelle concentration becomes clear on heating.This is because above the critical micelle con-centration the solution temperature (clear point)depends only slightly on the concentration.
The phase diagram of a nonionic alkyl poly-ethylene glycol ether is illustrated schematicallyin Figure 8. The critical micelle concentrationdecreases with increasing temperature. At ele-vated temperature micellar solutions separateinto two phases; thus, initially clear solutionsbecome turbid on heating. The temperature atwhich a solution of a nonionic surfactantbecomes turbid is termed the turbidity or cloudpoint, and is a characteristic parameter of anonionic surfactant.
Ionic surfactants aggregate into micelles atmolar concentrations that are a factor of tenhigher than in the case of nonionic surfactants.The reason for this is the electrostatic repulsionof the surfactant ions, which counteracts orien-tated association. In the presence of electrolytesthe electrostatic repulsion of the surfactant ions iscounteracted to some extent due to the increasedconcentration of the counterions; electrolytesthus greatly reduce the CMC of ionic surfactantsand increase the aggregation number. The CMCof nonionic surfactants is only slightly influencedby electrolytes (see Fig. 4). Critical micelle con-centrations of some well-defined chemical com-pounds are listed in Table 1, while those of someindustrial products are listed in Table 2. Thetables illustrate the influence of the chemicalstructure of the surfactants on the CMC and thesurface tension:
1. With linear alkyl groups in the hydrophobicpart, the amphiphilic substances require 7 – 9carbon atoms or more in the hydrocarbonchain to exhibit surface activity and micellesformation. Only with the very highly surface-active fluorosurfactants, whose fluorocarbonresidues are extremely hydrophobic, is inter-facial activity observed for shorter chains,starting with perfluorobutanecarboxylic acidand perfluorobutanesulfonic acid.
2. In a series of homologues the CMC decreaseswith increasing size of the hydrophobic resi-due. For linear alkyl hydrophobic groups,
Figure 8. Phase diagram of a nonionic surfactant of theethoxylate type in aqueous solution, as a function of concen-tration c and temperature T
Figure 7. Phase diagram of an anionic surfactant of thesulfonate type in aqueous solution, as a function of concen-tration c and temperature T
442 Surfactants Vol. 35
Table 1. CMCs of some surfactants of defined structure, and their surface tensions at the CMC
CMC
Surface tension,
Surfactant Temperature, �C mmol/L g/L mN/m
Na 1-decyl sulfate 50 34.0 8.3 40
Na 1-dodecyl sulfate 50 8.1 2.2 38
Na 1-tetradecyl sulfate 50 2.0 0.60 37
Na 1-hexadecyl sulfate 50 0.66 0.22 38
Na 1-octadecyl sulfate 50 0.23 0.76 38
Na 2-hexadecyl sulfate 20 0.48 0.17 37
Na 4-hexadecyl sulfate 20 0.62 0.21 36
Na 6-hexadecyl sulfate 20 1.20 0.41 35
Na 8-hexadecyl sulfate 20 2.20 0.76 30
Na p-1-tetradecylbenzenesulfonate 75 0.67 0.25 38
Na p-2-tetradecylbenzenesulfonate 75 0.72 0.27 37
Na p-3-tetradecylbenzenesulfonate 75 0.77 0.29 36
Na p-5-tetradecylbenzenesulfonate 75 0.93 0.35 34
Na p-7-tetradecylbenzenesulfonate 75 1.73 0.65 34
Na 2-dodecene-1-sulfonate 30 13.0 3.51 36
Na 2-tetradecene-1-sulfonate 30 2.7 0.77 32
Na 2-hexadecene-1-sulfonate 30 0.61 0.81 33
Na 2-octadecene-1-sulfonate 50 0.18 0.06 31
Na 3-hydroxydodecane-1-sulfonate 30 24.8 7.15 42
Na 3-hydroxytetradecane-1-sulfonate 30 6.33 1.91 41
Na 3-hydroxyhexadecane-1-sulfonate 30 1.45 0.46 39
Na 3-hydroxyoctadecane-1-sulfonate 50 0.38 0.13 37
1-Dodecyl pentaglycol 20 0.06 0.024 31
1-Dodecyl heptaglycol 20 0.09 0.045 34
1-Dodecyl nonaglycol 20 0.12 0.070 37
1-Dodecyl dodecaglycol 20 0.17 0.121 41
Perfluoropentanecarboxylic acid 20 0.09 0.028 17
Perfluoroheptanecarboxylic acid 20 0.01 0.04 17
Perfluorononanecarboxylic acid 20 0.001 0.005 20
Table 2. CMCs of some industrial surfactants at 20 �C and their surface tensions at this concentration
Surface tension
Surfactant CMC, g/L mN/m
Potassium oleate 0.35 25.5
Alkylbenzenesulfonatea 0.50 35.0
Alkanesulfonateb 0.44 34.5
Olefinsulfonatec 0.60 32.5
Fatty alcohol ether sulfated 0.17 36.5
Fatty alcohol ethoxylatee (6 EO)f 0.008 30.5
Fatty alcohol ethoxylate (9 EO) 0.015 30.5
Fatty alcohol ethoxylate (12 EO) 0.020 31.5
Nonylphenol ethoxylateg (9 EO) 0.054 34.0
Nonylphenol ethoxylate (12 EO) 0.084 35.5
Nonylphenol ethoxylate (14 EO) 0.096 35.5
Distearyldimethylammonium chloride 2.5 35.5
Cetylpyridinium chloride 0.6 39.5
aLinear C10 – C13 alkylbenzenesulfonate having a mean alkyl chain length of C12, as sodium salt.bLinear C13 – C17 alkanesulfonate having a mean alkane chain length of C15, as sodium salt.cFrom linear C14/C16 a-olefin, as sodium salt.dFrom linear C12 – C14 fatty alcohol with 3 mol EO/mol, as sodium salt.eFrom linear C12 – C14 fatty alcohol.fMol EO/mol.gFrom alkylphenol with branched nonyl residue.
Vol. 35 Surfactants 443
Stauff’s rule applies:
logðCMCÞ ¼ A�B n
where A and B are constants and n is thenumber of carbon atoms in the alkyl residue.Traube’s rule states that in a homologousseries the interfacial activity increases withincreasing size of the hydrophobic group andthat the surfactant concentration at which aspecific interfacial activity is achieved de-creases with increasing size of the hydropho-bic group. For simple linear hydrocarbonchains in the hydrophobic part
logCn ¼ �n logAþB
where Cn is the concentration of the homo-logue, n is the number of carbon atoms in thehydrocarbon chain, and A and B are constants.For aqueous solutions of sodium salts of fattyacids, for example, the concentration requiredto lower the surface tension by a specificamount is three times lower when the alkylchain is extended by one methylene group.
3. In a series of isomers the CMC and interfacialactivity increase with increasing internal po-sition of the hydrophilic group.
4. Branching of the hydrophobic hydrocarbonhas the same effect on the CMC and interfa-cial activity as a shortening of a linear hydro-carbon residue. A central rather than terminalsubstitution of the alkyl chain by a phenylgroup bearing the hydrophilic group has thesame effect (effective chain length – seeFig. 9).
5. For nonionic surfactants of the polyethyleneglycol ether type the water solubility andCMC increase with increasing length of thepolyethylene glycol chain.
Micelle formation, interfacial activity, andabsolute values of the interfacial tension aredecisively influenced by the size and structureof the hydrophobic groups, which determine thecohesive forces between the surfactant mole-cules and ions as well as their spatial requirementat the interface. Cohesive forces that increasewith increasing chain length exist between sur-factants having unbranched alkyl residues, andare stronger than the forces between branchedalkyl chains. Surfactants with linear alkyl groupsaccordingly associate at lower concentrationsthan those with branched chains, while surfac-tants with long chains associate at lower concen-trations than those with short chains.
Fluoroalkyl groups are substantially morehydrophobic than the corresponding aliphaticalkyl groups. Fluorosurfactants therefore formmicelles at much lower concentrations than theiraliphatic analogues.
Surfactants with straight- chain aliphaticgroups can form a tightly packed film at theinterface, with parallel linear chains; their spatialrequirement is small, and the strength (tension) ofthe film increases with increasing chain length. Incontrast to branched- chain surfactants, such sur-factants only reduce the surface tension slightly,though they do so already at low volume con-centrations. Surfactants with branched- chain hy-drophobic groups, which reach the CMC only atrelatively high concentrations, lead to lowersurface tensions because their surface films havea relatively low cohesion but relatively highspatial requirement. Surfactants with straight-chain aliphatic groups have been referred to as‘‘efficient’’ surfactants since they are alreadyactive at low concentrations, and surfactants withbranched hydrophobic residues as ‘‘effective’’surfactants, since they produce a marked de-crease in surface tension, albeit at higher con-centrations [34] (Fig. 10).
The low cohesion between the perfluoroalkylgroups in perfluoroalkyl surfactants results invery low interfacial tensions in solution. Surfac-tants with methylpolysiloxanes as hydrophobicgroup lie between surfactants with alkyl groups,and perfluoroalkyl surfactants as regards interfa-cial tension, due to the moderate cohesive forcesbetween the dimethylsiloxane groups.
On account of the marked decrease in theinterfacial and surface tension, solutions ofeffective surfactants generally exhibit goodFigure 9. Effective chain length of alkylbenzenesulfonates
444 Surfactants Vol. 35
wetting and foaming abilities but have a lowdetergent power due to the high CMC; the oppo-site is true for efficient surfactants. This rule ofthumb applies provided that themolecularmassesof the surfactants are not so large that the adjust-ment of the equilibrium state is adversely affectedby the reduced diffusion rate. Minima of theinterfacial and surface tensions as a function ofthe length of the hydrophobic or hydrophilic(ethoxylate) residue given in the relevant litera-ture refer to ‘‘dynamically’’ measured values, i.e.,values that have been obtained without waitingfor equilibrium to be established, which can takeseveral hours in the case of slowly diffusingsurfactants. For application technology, however,it is largely the dynamic interfacial tensions thatare decisive, since in processes of wetting, wash-ing, emulsification, etc., the instantaneous effectsare of most importance.
Surfactants of varying structures form mixedfilms at interfaces. At low concentration surface-active substances with the higher surface activitypreferentially accumulate in the interface. Withincreasing surfactant concentration the morestrongly surface-active species may be expelledfrom the interface and absorbed by micelles inthe volume phase, forming mixed micelles. As aresult the interfacial tension, which drops withincreasing surfactant concentration until theCMC is reached, may subsequently increase,reaching a constant end value (Fig. 11).
Such concentration-dependent interfacial ten-sion behavior is often observed for industrialsurfactants, which consist of mixtures of variouschemical species.At fairly high concentrations ofthe second component permanent mixed filmsmayform,whosepropertiesdeviatefromthefilms
of the individual components because of theirdifferent structure and, thus, stability. For exam-ple, lauryl alcohol in lauryl sulfate or lauric acidethanolamide in alkylbenzene sulfonate stabi-lizes the foam on aqueous solutions of the corre-sponding surfactants because the nonionic sub-stances are incorporated in thefilmandoriented ina hydrophilic – hydrophobic manner. Thus, thedistance between the electrically charged hydro-philic groups of the surfactant is increased andtheir electrostatic repulsion is reduced.
Similar to mixed films at interfaces mixedmicelles can form in solution, whose structurecan be greatly altered by the mixing proportionsup to the formation of ‘‘swollen’’ micelles, whichare regarded as constituents of microemulsions.
5. Industrial Development
The industrial development of the wide range ofsurfactants now available received its initial andmost important stimulus from the burgeoningtextile industry of the mid-1800s. The variousprocesses involved in the finishing and proces-sing of raw cotton and wool into finished textilesrequired, besides the soap and natural saponins(! Saponins) known from antiquity, specialsurface-active substances, dispersants, softeners,and sizing agents, with resistance to water hard-ness being one of the initial requirements. Thefirst steps in this directionwere taken byRUNGE in1834 and by MERCER in 1846 with sulfated oliveoil, followed in 1875 by turkey red oils, whichwere obtained by sulfating castor oil. Theseprocesses underwent many improvements, andare still of some importance.
Figure 10. Surface tension g of an efficient and an effectivesurfactant as a function of concentration c
Figure 11. Surface tension g of a contaminated surfactant asa function of concentration c
Vol. 35 Surfactants 445
Until 1930 industrial surfactant chemistrywasalmost exclusively an adjunct of the textileindustry, and only in the 1930s did developmentsoccur that opened up newmarkets for synthetic orsemi-synthetic surfactants, including householddetergents. Whereas oils and fats were formerly,with a few exceptions, the only raw materials forsurfactants including soaps, during this periodraw materials based on coal and petroleum alsobecame increasingly important, although thenatural materials based on oils and fats havenever completely lost their importance.
In 1963 P€uSCHEL identified five periods in thedevelopment of surface-active substances [35]:
1. Up to 1925: Turkey-red oils2. From 1925 to 1929: highly sulfonated oils3. From 1929 to 1935: invention of the principal
types of synthetic surfactants4. From 1935 to 1945: coal and petroleum were
exploited as raw material base5. From 1945 to 1960: new, special types of
surfactants were developed
This subdivision of surfactant developmentcan be concluded by a sixth period beginningaround 1960, characterized by increasing envi-ronmental awareness, which has considerablerepercussions on research and technology insurfactant chemistry. Nowadays, particularattention is being paid to ‘‘mild’’ surfactants,i.e., those with a low irritation potential for theskin and mucous membranes.
Two developments in the 1930s in Germanyhave permanently affected the development ofsurfactant chemistry.
SCHRAUTH et al. at the Deutsche Hydrierwerkein Rodleben accomplished the catalytic highpressure hydrogenation of fatty acids to fattyalcohols. BERTSCH et al. at H. T. B€oHME in Chem-nitz, who had already developed sulfated fattyacid esters (Avirol) and fatty acid amides (Hu-mectol), obtained primary alkyl sulfates bysulfating fatty alcohols, which already appearedon the market in 1932 as a constituent of thespecialty detergent FEWA, the first soap-freedetergent.
In 1930, SCH€oLLER and WITTWER at I.G. Far-benindustrie in Ludwigshafen invented ethoxy-lation, i.e., the reaction of compounds having adissociating hydrogen atom with ethylene oxide.This reaction leads to derivatives of poly(ethyl-
ene glycols) or also oligo(ethylene glycols),known as ethoxylates. This was the birth of anew class of surfactants, the nonionic surfactants.I.G. Farbenindustrie introduced in rapid succes-sion the following nonionic surfactants to themarket:
Fatty acid ethoxylates Soromin SG
Fatty alcohol ethoxylates Peregal O
Alkylphenol ethoxylates Igepal C
Parallel to these developments, I.G. Farbenin-dustrie offered a range of new oleic and fatty acidderivatives:
O-acylisethionate Igepon A
N-acyl-N-methyltaurine Igepon T
N-acylsarcosine Medialan
N-acylaminoethyl sulfate Igepon C
The particularly dermatologically friendlycondensation product formed from oleic acid andoligopeptides, Lamepon A, was developed atChemische Fabrik Gr€unau.
The first cationic surfactants, N-acylami-noethyltrialkyl ammonium salts, Sapamines,were produced in 1927 by Ciba. In 1934 DOMAGK
discovered the disinfectant action of cationicsurfactants, the so- called invert soaps; I.G. Far-benindustrie developed alkyldimethylbenzylammonium salts under the name Zephirol.Betaines (Du Pont) and ampholytes (Th. Gold-schmidt) came later, in 1937 and 1948.
In the mid-1940s Atlas Powder introduced, anew class of nonionic surfactants (Spans andTweens) to themarket with the fatty acid sorbitanesters and their ethoxylates.
The first surfactant to be based on a coalproduct was diisopropylnaphthalenesulfonate,which was synthesized in 1917 by GUNTHER atBASF. This compound and other short- chaindialkylnaphthalenesulfonates are still used aswetting agents under the generic name Nekal.Alkylbenzenesulfonates with longer chain alkylgroups were produced in 1933 by I.G. Farbenin-dustrie and, independently, in 1936 by NationalAniline in the United States. The predominantposition in terms of quantity that alkylbenzene-sulfonates nowadays occupy was achieved, how-ever, only after 1945. Up to then, besides soaps,alkyl sulfates and alkanesulfonates played animportant role in detergents and cleansing
446 Surfactants Vol. 35
agents. Alkyl sulfates are obtained by sulfatingprimary (natural) alcohols or by addition ofsulfuric acid to long- chain olefins. In the lattercase secondary alkyl sulfates are obtained, whichwere marketed as Teepol (Shell, 1934). Second-ary alkanesulfonates were produced on an indus-trial scale from 1941 onward at I.G. Farbenin-dustrie in Leuna by sulfochlorination followedby saponification; they are still important nowa-days as emulsifiers under the name Mersolat.
Until the expansion of refinery capacities andthe conversion of chemicals production to rawmaterials derived from petroleum, the mainlylinear hydrocarbons obtained from Fischer –Tropsch synthesis plants were used in Germanyas feedstocks for the industrial production ofsurfactants. C10 – C14 alkenes were used foralkylating benzene to produce alkylbenzenesul-fonates, while C13 – C18 alkanes were used forthe production of alkanesulfonates. The highermolecular mass hydrocarbons (Fischer –Tropsch slack wax) were oxidized to fatty acids,which were used among other things as syntheticsoaps.
With the expansion of petroleum refining lowmolecular mass olefins became available as start-ing materials. Ethylene was particularly valuableas a startingmaterial for ethylene oxide, the basicbuilding block of nonionic surfactants, and pro-pylene as the starting material for hydrophobicgroups. Propylene trimer was used for producingisononylphenol, the starting material for nonion-ic surfactants, while propylene tetramerwas usedfor producing branched- chain dodecylbenzene,the starting material for a new type of alkylben-zene sulfonates (tetrapropylbenzenesulfonate),which was primarily developed in the UnitedStates and was also widely used in Europe.
In the 1950s the alkylbenzenesulfonates, non-ylphenol ethoxylates, and fatty alcohol ethoxy-lates were produced in such amounts that a verylarge proportion of surfactant demand could bemet. Nevertheless, the development of specialsurfactants for particular purposes continuedafter 1950. During this period novel surfactantswere synthesized, in particular by incorporatingnew hydrophobic groups into the surfactant mo-lecules. Examples include the Pluronic andTetronic types produced byWyandotte, in whichpolypropylene glycol or polyoxypropylatedethylenediamine are used as hydrophobic build-ing blocks, and also the fluorosurfactants intro-
duced from1950 onward by 3 Mcontaining (per)fluoroalkyl groups, and the silicone surfactantsproduced by TH. GOLDSCHMIDT, containing alarge variety of polydialkylsiloxanes as hydro-phobic groups.
Although for a long time surfactant chemistryobtained a basic stimulus from the textile indus-try, nowadays the development of industrialproducts is largely determined by the require-ments of the detergent, cleansing agent, andcosmetics industries since the bulk of surfactantsis nowadays used in these sectors.
From 1960 onward the demand for biodegrad-ability of surfactants in detergents and cleansingagents led to major changes. The most importantproduct, tetrapropylenebenzenesulfonate, wasreplaced with great technical effort and expendi-ture by linear alkylbenzenesulfonate; in additionto nonylphenol ethoxylate, fatty alcohol ethox-ylates based not only on natural rawmaterials butalso on synthetic products appeared. A furtherecological factor – the demand for a reductionor elimination of phosphate in detergents –together with the demand for a more rapidbiodegradability, stimulated new technicaldevelopments: alkanesulfonates, alkenesulfo-nates, and a-sulfo fatty acid esters were consid-ered as potential industrial products. Alkanesul-fonates, produced by sulfoxidation of paraffins ina process developed in the 1940s by Hoechst,have in the meantime secured a firm market inEurope. The alkenesulfonates have achievedsome importance in the United States and Japan,whereas thea-sulfo fatty acid esters have still notfound any major use. With the expansion of oilpalm plantations (e.g., in Malaysia and thePhilippines), fatty alcohol sulfates based onthese renewable raw materials are becomingincreasingly important.
With the demand for particularly mild surfac-tants, besides the fatty alcohol ether sulfates andsulfosuccinic acid esters of fatty alcohol oligo-glycol esters, a number of surfactants that werealready developed in the 1930s have attractedrenewed interest, including fatty acylisethionatesand fatty acyltaurates, amino acid derivatives,ampholytes and betaines, carboxymethylated fat-ty alcohol ethoxylates, and so- called sugar sur-factants. In parallel to the demand for mildproducts, there has been a more emotionallymotivated wish for ‘‘natural’’ products, rawmaterials and surfactants based on renewable,
Vol. 35 Surfactants 447
preferably plant raw materials. There is thus apotential for substituting products from petro-chemical raw materials by those from renewableraw materials, which will be reflected in theextent to which corresponding amounts of re-newable raw materials, primarily coconut, palm,and palm kernel oils, are available at comparableprices for industrial purposes [36].
6. Anionic Surfactants
6.1. Carboxylates
Soaps, the salts of fatty acids, are still importantamong surfactants (! Soaps). Soaps areobtained mainly by saponification of natural fats.The oxidation of paraffins, olefins, aldehydes, oralcohols (Stass reaction) is no longer of impor-tance for the production of fatty acids. The saltsof carboxylic acids occurring in petroleum(naphthenic acids), which are also surfactants,are treated in ! Metallic Soaps.
6.1.1. Carboxymethylated Ethoxylates
The sensitivity of soaps to water hardness is adisadvantage for some applications (e.g., textilewashing). In contrast, the so-called super soaps,the sodium salts of carboxymethylated ethoxy-lates exhibit an extreme hardness resistance com-bined with good water solubility.
The carboxymethylated ethoxylates or poly-ether carboxylates, as they have also been named,
are known to be dermatologically friendly. Sincethey also have outstanding dispersion and emul-sification properties, they are suitable for a widerange of uses, for example as surfactants indetergents and cosmetics [37], [38], and as in-dustrial emulsifiers [39].
The carboxymethylated ethoxylates are pre-pared by reacting an alcohol ethoxylate or alkyl-phenol ethoxylate with chloroacetic acid andcaustic soda:
The sodium chloride byproduct is best removedby acidifying the reaction mixture, whereby thecarboxylate separates as water-insoluble poly-ethercarboxylic acid and the sodium chlorideremains in the aqueous phase. The polyetheracids are clear, mobile liquids that can be trans-ported in stainless steel or plastic containers,they are the most suitable form for handling thisgroup of products. By neutralizing them withcaustic soda colorless, highly viscous pastes areobtained that yield mobile solutions only atconcentrations below 25 wt%. Typical data forsome carboxymethylated products are listed inTable 3.
Manufacturers include Auschem, Chem-Y,and H€uls in Europe; Rhone-Poulenc Surfac-tant, Croda, Sandoz, Finetex, and Witco inthe United States; and Nikko Chemicals inJapan.
Table 3. Data for some commercial carboxymethylated fatty alcohol ethoxylates, R�[OCH2CH2]n�OCH2COOH and �OCH2COONa, based
Degree of carboxymethylation, % ca. 95 ca. 95 ca. 95 ca. 95
Acid number, mg KOH/g ca. 66 0 ca. 115 0
pH (1%) 2.5 – 3 6 – 7 2.5 – 3 6 – 7
Setting point, �C 8
Clear point, �C ca. 18 max. 10 ca. 11 max. 10
Density, 20 �C, g/mL 1.01 1.03 1.05 1.03
Brookfield viscosity, 20 �C, mPa � s ca. 200 ca. 30 ca. 300 ca. 100
*CAS numbers are [33939-64-9] and [50546-32-2].
448 Surfactants Vol. 35
6.1.2. Amino Acid Derivatives
Condensation products of fatty (oil) acids andamino acids are also less sensitive to hardnessand hydrolysis than soaps. Sarcosides, for exam-ple, are obtained by reacting a fatty acid chloridewith sodium sarcosinate according to the Schot-ten-Baumann reaction:
Sarcosides are goodwetting agents and lime soapdispersants.
Industrially more important than the conden-sation products of amino acids are those witholigopeptides, which are obtained by partial hy-drolysis of collagen, an animal skeletal proteinthat has no antigenic properties. The oligopep-tides have molecular masses of 200 – 600. Thecondensation products are prepared similarly tothe sarcosides, by condensation of a fatty acidderivative (chloride or anhydride) at the terminalamino group of the oligopeptide [40], [41].
The products can be freed from byproducts byacidifying the reaction mixture; the pure conden-sation products separate as acids, which can beredissolved with a base.
As salts, fatty acid/protein condensation pro-ducts are highly water soluble. They have a highlime soap dispersion power and foaming ability.Fatty acid/protein condensation products arepreferentially used as highly skin-friendly sur-factants in cosmetic preparations, bodycareagents, shampoos, and children’s bathing pro-ducts. They are stored and transported as 30 –40% solutions in steel or plastic vessels. Ifpreserved against bacterial growth, they can bestored for prolonged periods at 20 �C and above.Typical data of two commercial products arelisted in Table 4.
Producers. Sarcosides are produced byCroda, Hoechst, and ICI in Europe; Clough,Grace, and Stepan in the United States; NikkoChemicals in Japan; and Zohar in Israel.
Fatty acid/protein condensation products areproduced by Croda and Henkel in Europe; Aji-nomoto, Croda, Henkel, and Inolex in the UnitedStates; and Ajinomoto and Nikko Chemicals inJapan.
6.2. Sulfonates
Sulfonates are the salts of sulfonic acids, inwhicha hydroxysulfonyl group is bonded via the sulfuratom directly to a carbon atom of the hydropho-bic residue. This bond is thermally and chemi-cally very stable. The strong affinity of thesulfonate group for water results in good watersolubility, which decreases with increasingmolecular mass. The alkaline-earth salts ofsulfonic acids are less soluble in water than thealkali metal salts. The sensitivity of sulfonates towater hardness increases with increasing molec-ular mass. Being salts of strong acids, the alkaliand alkaline-earth sulfonates give a neutral reac-tion in aqueous solution.
Sulfonates are nowadays the most importantgroup among the synthetic surfactants. Besidesthe sulfonates discussed hereinafter, lignin sul-fonates (! Lignin, Chap. 5.) and petroleumsulfonates are industrially important. Being out-standing dispersants, lignin sulfonates are used to
Table 4. Typical data for the potassium salt of a dipeptide/coconut oil
acid condensate (A) and triethanolamine salt of a tetrapeptide/coconut
oil acid condensation product (B)
Property (A) (B)
Molecular mass, g/mol 500 – 600 790 – 890
Dry content, wt% 31.5 – 32.5 39.5 – 40.5
Active substance, wt% 29.0 – 30.0 37.5 – 39.0
pH of a 10% solution 6.2 – 7.0 6.5 – 7.0
Viscosity at 20 �C, mPa � s 1500 – 500 > 400
Density at 20 �C, g/mL 1.05 – 1.07 1.06 – 1.09
Gardner color index max. 8 max. 10
Protein content, wt% 10 – 13 14 – 16
Ash, wt% max. 8 max. 4
Vol. 35 Surfactants 449
improve the viscosity of concrete mixtures anddrilling muds, while petroleum sulfonates areused mainly as oil-soluble surfactants for pro-ducing water-in-oil emulsions. Both types ofsulfonates could become important as surfactantsin tertiary crude oil production.
6.2.1. Alkylbenzenesulfonates [42], [43]
p-Alkylbenzenesulfonates having (on average)8 – 20 carbon atoms in the alkyl chain have awide range of uses. p-Alkylbenzenesulfonateswith less than 6 carbon atoms in the alkyl groupare not surface-active.
The alkylbenzenesulfonates are mainly used inthe form of sodium salts, and occasionally also aspotassium and ammonium salts and as salts withaliphatic amines. The octyl- to decylbenzenesul-fonates have a good wetting action, but areunsuitable as emulsifiers and cleansing agentsdue to the shortness of the hydrophobic group.The most universally usable alkylbenzenesulfo-nates are those with an average of ca. 12 carbonatoms in the alkyl chain, comprising homologueswith 10 – 14 carbon atoms (dodecylbenzenesul-fonate). Because of their outstanding suitabilityas surfactants in detergents and cleansing agents(! Laundry Detergents) and their low produc-tion costs compared to other surfactants, thedodecylbenzenesulfonates are nowadays themost important group of synthetic surfactantsin terms of quantity. They are a constituentof powder and liquid heavy-duty householdand industrial detergents. They are used lessin fine detergents, shampoos, and cosmeticpreparations.
Alkylbenzenesulfonates with � 15 carbonatoms in the alkyl chain are sparingly soluble inwater, but are readily soluble in organic media.Solutions in mineral oils play a role as drillingand cutting oils in the metal-processing industry,and as spinning, dust-binding, and batch oils inthe textile industry.
Alkylbenzenesulfonates are produced byalkylation of benzene (Friedel – Crafts reac-tion), sulfonation of the alkylbenzene obtained,and neutralization. In the dodecylbenzenesulfo-
nates a distinction is made between those derivedfrom highly branched tetrapropylene (TPS ¼tetrapropylenebenzenesulfonate) and those withlinear alkyl groups (LAS ¼ linear alkylbenzene-sulfonate). Tetrapropylenebenzenesulfonate isless soluble in water than linear dodecylbenze-nesulfonate (ca. 10% and 20% at 20 �C), is lesstoxic to fish, but is a stronger skin irritant than thelatter. Sodium tetrapropylenebenzenesulfonateis less hygroscopic and less plastic than linearsodium dodecylbenzenesulfonate, and accord-ingly can be processed more easily into powderformulations. Tetrapropylenebenzenesulfonateis, however, becoming increasingly lessimportant because of its unsatisfactorybiodegradability.
In the first stage of the alkylbenzenesulfonatesynthesis, benzene is alkylated with the corre-sponding alkyl chlorides or olefins. Linear alkylchlorides are obtained by chlorination of alkanes,whereby conversion is limited to at most 30% tominimize the proportions of polychlorinatedhydrocarbons. Tetrapropylene, which can beobtained by oligomerization of propylene, isalready present as an olefin. Linear olefins arenowadays obtained by oligomerization of ethyl-ene or by catalytic dehydrogenation of linearparaffin cuts.
The oligomerization of ethylene using alumi-num alkyls (Alfen process) or by transition metalcomplexes (Shell Higher Olefin Process, SHOP)affords mixtures of homologous, even-num-bered, unbranched a-olefins with a broadSchulz – Flory molecular mass distribution[44], [45] (see also ! Hydrocarbons, Section2.1.2.2.). Such olefins are less suitable for pro-ducing commercial alkylbenzenemixtures, sincenarrow olefin cuts are required for this purpose.One possible way of narrowing the homologuedistribution is to isomerize a-olefins to olefinswith double bonds, as carried out in the SHOPprocess, followed by comproportionation orcometathesis of higher and lower olefins.Althoughmixtures of olefinswith internal doublebonds and a broad molecular mass distributionare again formed, the advantage of this process isthat the desired olefin cut (e.g., C10 – C13) can bedistilled from the mixture, and the unrequiredfractions can be returned to the cometathesisreaction, so that ultimately the mixture ofa-olefins with a broad distribution that isobtained in the oligomerization is converted to
450 Surfactants Vol. 35
a mixture of linear olefins of narrow distributionhaving internal double bonds.
The process most widely employed nowadaysfor producing olefins for industrial alkylbenzenesynthesis is the dehydrogenation of n-alkanes. Inconjunction with the back-integrated process forobtaining linear starting alkane from correspond-ing petroleum fractions (kerosene) by means ofmolecular sieves and the forward-integrated ben-zene alkylation catalyzed by hydrogen fluoride, itis nowadays carried out worldwide in numerousplants and is known as the UOP process (Univer-sal Oil Products) [46], [47].
Nowadays, exclusively aluminum chlorideand hydrogen fluoride are used as alkylationcatalysts in alkylbenzene synthesis, whereby hy-drogen fluoride can only be used for alkylationwith olefins but not with alkyl chlorides.
A fixed-bed catalyst was described for the firsttime in 1990, which has advantages compared tohydrogen fluoride- catalyzed alkylation and is tobe marketed by UOP under the name ‘‘Detal’’[47], [48].
Alkylation always gives mixtures of isomeric(and homologous) alkylbenzenes. The isomerdistribution of the alkylbenzenes obtained withthe two catalysts is different. Both processes areFriedel – Crafts reactions, in which only second-ary or tertiary alkyl residues occur on the benzenering. Regardless ofwhether primary or secondarylinear alkyl chlorides or linear olefins havingterminal or internal double bonds are used as
starting materials, alkylbenzenes are alwaysobtained in which the phenyl groups is bondedinternally along the alkane, but not in the termi-nal position.
The isomer distribution depends on the cata-lyst used. With aluminum chloride and linearalkyl chlorides or olefins roughly the same iso-mer distribution is obtained, in which 2-pheny-lalkanes predominate and 3-, 4-, and 5-pheny-lalkanes, etc., are present in lesser amounts. Ifbenzene is alkylated in the presence of hydrogenfluoride, then 2-phenylalkanes are present in thelowest concentration in the resultant isomer mix-ture, and the proportion of the more internalphenylalkanes increases the closer the positionof the phenyl group to the middle of the alkanechain. These relations are illustrated in Table 5for industrial alkylbenzenes. Various isomeriza-tion reactions are responsible for these isomerdistributions. Lewis acids such as aluminumchloride isomerize the phenylalkanes alreadyformed, whereas protic acids such as hydrogenfluoride isomerize the unreacted olefins but notthe phenylalkanes [49]. If branched olefins suchas tetrapropylene are used as starting material,the phenyl residue is always bonded to a tertiarycarbon atom, e.g.:
Table 5. Composition of industrial alkylbenzenes in wt% [50]
From olefin with HF
catalysis
From alkyl chloride with
AlCl3 catalysis
From olefin with
AlCl3 catalysis
Component (molar mass 240) (molar mass 241) (molar mass 240)
Sum of n-alkylbenzenes 93 88 98
2-Phenylalkanes 18 29 29
Dialkyltetralins 0.5 9 0.5
Phenyldecane 14 13 14
Phenylundecane 34 30 29
Phenyldodecane 31 30 32
Phenyltridecane 20 25 24
Phenyltetradecane 1 2 1
Isomer distribution of
phenyldodecanes
1-Phenyldodecane 0 trace 0
2-Phenyldodecane 18 28 29
3-Phenyldodecane 16 19 19
4-Phenyldodecane 17 17 18
5-Phenyldodecane 24 18 18
6-Phenyldodecane 25 18 17
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Processes for preparing linear industrial ‘‘dode-cylbenzene’’, a mixture of decylbenzene to tri-decylbenzene, from linear alkyl chlorides andfrom linear olefins are described hereinafter byway of example, the preparation of the alkylchlorides or olefins being integrated componentsof the processes.
Alkylation with Alkyl Chlorides (Fig. 12).The alkane, a homologous mixture of linearalkanes with average carbon number 12, is re-acted with gaseous chlorine at 100 – 140 �C in achlorination tower to a conversion of 30 mol%.To minimize backmixing and, thus, the propor-tion of higher chlorinated paraffins, the chlorina-tion can also be carried out in a cascade ofreactors. Suitable reactor materials are lead, sil-ver, or enamel; iron is not suitable.
The hydrogen chloride released in the reactionRH þ Cl2 ! RCl þ HCl escapes from the headof the chlorination tower and iswashed in counter-currentwithfreshalkanetotrapunreactedchlorine.The hydrogen chloride is of high purity and can beused for further chemical syntheses.The ‘‘chlorineoil’’, consisting of alkyl chlorides and unreactedparaffin, ispassed to the alkylation stage.Thealkylchlorides consist of positional isomers, secondaryalkyl chlorides predominating by far.
The active catalyst in the alkylation is a liquidcomplex compound formed in the reaction mix-
ture and containing up to 35 wt% aluminumchloride. This complex is only slightly solublein the reaction mixture and can, therefore, beseparated as a heavier phase and recycled. Lossescan be replenished by adding fresh aluminumchloride or aluminum, which reacts with hydro-gen chloride to form aluminum chloride.
The alkylation is preferentially carried out inglass-lined reaction towersat 80 �C.Tominimizemultiple alkylation, benzene is added in a many-fold molar excess. The hydrogen chloride re-leased in a stoichiometric amount according tothe equation RCl þ C6H6 ! RC6H5 þ HClmaintains the reactionmixture in a turbulent stateand is removed at the upper endof the reactor. It isalso washed with fresh paraffin, but still containslow boiling point hydrocarbons (e.g., isobutane)and accordingly cannot be reused directly.
The reaction mixture leaving the alkylationtower is passed to a separating vessel inwhich thecatalyst complex separates out. The organicphase is freed from residual catalyst by washingwith water and sodium hydroxide and is workedup in a distillation unit. First benzene, thenunreacted paraffin, and finally alkylbenzeneaccounts for only ca. 10% of the alkylationmixture are separated. More highly alkylatedbenzenes and diphenylalkanes formed by reac-tion with dichloroalkanes in the ‘‘chlorine oil’’can be isolated as a tail fraction.
Figure 12. Preparation of a linear alkylbenzene by the aluminum chloride processa) Storage vessel for fresh and recycled alkane; b) Chlorination tower; c) Wash towers for hydrogen chloride; d) Alkylationcolumn; e) Separating vessel for catalyst removal; f ) Water and sodium hydroxide wash; g) Dewatering column; h) Benzenecolumn; i) Intermediate cut column; j) Paraffin column; k) Alkylbenzene column; l) Tails column
452 Surfactants Vol. 35
The dichloroalkanes react also form bicycliccompounds such as 1,3-dialkyltetralins, whichduring distillation remain in the alkylbenzene inamounts of 5 – 10 wt%
Alkylation with Olefins. The UOP processfor producing alkylbenzenes, the mostwidely used process worldwide, is illustrated inFigure 13 [47]. The process consists of threestages: olefin production (Pacol ¼ paraffinconversion to olefins), selective hydrogenation(DeFine), and alkylation.
The dehydrogenation of n-alkanes takes placeat ca. 500 �C and with a slight hydrogen excesspressure of ca. 3 bar over a fixed-bed of modifiedplatinum catalyst on aluminum oxide.
The alkane conversion is held at 10 – 15% tominimize further dehydrogenation to diolefinsand aromatics. The hydrogen released containssmall amounts of lower alkanes; some of thehydrogen is recycled to the dehydrogenationreactor, while another part is used in a connectedreactor for the selective hydrogenation of diole-fins, formed in small amounts, to monoolefins(DeFine). The removal of diolefins is importantto suppress the formation of byproducts,bicyclichydrocarbons, and diphenylalkanes in the alkyl-ation. The dehydrogenation product, containing10 – 15 wt%monoolefins is passed to the alkyl-
ation reactor, in which it is intensively mixedwith benzene and hydrofluoric acid, which isonly slightly miscible with the hydrocarbons.The benzene is used in a manyfold molar excessto minimize multiple alkylation. The reaction iscarried out at < 50 �C with intensive cooling.The acid catalyst is removed in a separatingvessel connected to the reactor and is recycledto the alkylation reaction; a partial stream ispassed through a distillation column (not shownin Fig. 13) in which the hydrofluoric acid (bp19 �C) is freed from small amounts of dissolved,higher-boiling organic constituents, the so-called acid tar, a constant regeneration of thecatalyst thereby being ensured. Since water isexcluded from the whole alkylation section thelatter can be manufactured from normal steelwithout risk of corrosion.
After stripping residual dissolved hydrogenfluoride in a distillation unit, the upper, organicphase from the separating vessel is fractionatedinto benzene, alkane, alkylbenzene, and higher-boiling compounds, the so- called heavy alkyl-ate, which can be further fractionated. Benzeneand alkane are returned to the process.
The distillation is simplified if paraffin-freeolefins (e.g., tetrapropylene or linear alkenesfrom the SHOP process) are used for the alkyl-ation. In the preparation of higher alkylbenzenessuch as hexadecylbenzene, paraffin-free olefinsare preferably used as starting materials since thedistillation becomes more difficult as the boilingpoint rises.
Figure 13. Preparation of a linear alkylbenzene by the UOP processa) Heater; b) Heat exchanger; c) Dehydrogenation reactor; d) Gas – Liquid separator; e) Hydrogenation reactor; f ) Strippercolumn; g) Alkylation reactor; h) Acid settler; i) HF stripper; j) Benzene column; k) Paraffin column; l) Rerun column
Vol. 35 Surfactants 453
Formerly, hydrofluoric acid was the exclusivecatalyst for the alkylation of benzene with ole-fins; however, more recently a plant came onstream in which aluminum chloride is also usedfor this reaction [50]. The linear alkylbenzeneproduced in this way is reported to be particularlypure.
Sulfonation. The sulfonation of alkylben-zenes, which at low temperature mainly occursin the p-position, can be performed with concen-trated sulfuric acid, oleum, or sulfur trioxide.Sulfonation with sulfur trioxide is now the pre-dominant process. It has major advantages oversulfonation with sulfuric acid or oleum, includ-ing much faster reaction rate and higher space –time yield. Also, in sulfonation with sulfur triox-ide no waste sulfuric acid is formed, and accord-ingly there are no workup or disposal problems.
The high reaction rate, combined with strongevolution of heat (ca. 170 kJ/mol, correspondingto ca. 700 kJ/kg of dodecylbenzene), necessitatesintensivemixing and efficient cooling. The sulfurtrioxide used is diluted to 4 – 8% in air ornitrogen as carrier gas. This mixture is formedin sulfur combustion combined with the contactprocess; it can also be obtained by passing air ornitrogen over heated oleum or liquid sulfurtrioxide.
Bubble- column reactors are suitable for dis-continuous sulfonation with sulfur trioxide. Themechanical momentum of the inert gas producesa high turbulence, which can be enhanced bystirrers. The heat of reaction is dissipated throughthe jacket, through internal cooling tubes and, inthe case of rapid product circulation, by externalheat exchangers. The inert gas leaving the reactor
must be freed of entrained sulfur trioxide andacid droplets by washing with alkylbenzene, forexample. The course of the reaction is followedby acidimetric titration, and the addition of sulfurtrioxide is discontinued when the desired ‘‘titervalue’’ is reached.
The preferentially employed continuous sul-fonation with sulfur trioxide is carried out inspecial reactors in which backmixing of thereaction material is largely avoided and in whichthe reaction takes place in intensively cooled thinlayers. Many modifications of suitable reactors,namely falling film,multitube, or concentric tubereactors, are available [51], [52]. Such reactorsare used particularly for the sulfonation ofalkenes and for sulfation of ethoxylates, sincewith these substrates localized overheating andexcessive concentrations of sulfur trioxide canhave an even more disadvantageous effect on theproduct quality than in the case of alkylbenzenes.
The production of alkylbenzenesulfonate isillustrated in Figure 14. The sulfur trioxide/inertgas (air or nitrogen) mixture passes via a misteliminator, in which entrained sulfuric acid isremoved, to the continuous sulfonation reactor.The sulfur trioxide is used in a slight excess (up to10 mol%).
The residence time of the liquid and gas in thisreactor is a few minutes, and the reaction tem-perature is 40 – 50 �C. The reaction productpasses to a separator, where the inert gas isremoved. This waste gas is washed with liquid(e.g., alkylbenzene) to remove entrained aciddroplets. The degassed sulfonation mixturepasses through a digester where small amountsof pyrosulfonic acid present react with unreactedalkylbenzene:
Figure 14. Continuous production of alkylbenzenesulfonatea) Mist separator; b) Sulfonation reactor; c) Gas separator; d) Digestor; e) Hydrolyzer; f ) Mixing pump; g) Intermediate vessel
454 Surfactants Vol. 35
Acid anhydride, also present in small amounts, ishydrolyzed at 80 �C in a following hydrolyzer byadding 1 – 2 wt% of water:
The sulfonic acid obtained in thisway can then beneutralized directly, but as with sulfonic acidproduced batchwise it can also be transported tothe consumption site and neutralized there.
Anhydrous alkylbenzenesulfonic acids do notattack iron if water (and also atmospheric mois-ture) are excluded. Dodecylbenzenesulfonic acidcan therefore be stored and dispatched in steelvessels. It can easily be poured, transferred, andmetered as a liquid. Typical characteristic data ofa high-quality dodecylbenzenesulfonic acid aregiven below:
Content of alkylbenzenesulfonic acid 97 – 98%
Content of sulfuric acid ca. 0.5%
Content of water 0%
Content of unsulfonated compounds 1.5 – 2%
Titer value (mL 1 N NaOH/10 g acid) 31 – 32 mL
g Sodium salt/100 g acid 103 – 105 g
Density at 50 �C ca. 1.03 g/mL
Viscosity at 50 �C ca. 300 mPa � sColor yellow to brown
Neutralization. In the neutralization of thealkylbenzenesulfonic acid, in general only somuch water is added as is necessary to form aflowable 50% paste. The heat of neutralization isca. 180 kJ/kg; a temperature of 60 �C is main-tained by cooling. During neutralization the re-action mixture must be intensively mixed toavoid locally excessive sulfonic acid concentra-tions, since sulfonic acid containing 10 – 50%water forms highly viscous gels. It has provedmost convenient to add acid and sodium hydrox-ide continuously, while monitoring the pH, to analready neutralized mixture. In the continuousprocess illustrated in Figure 14 this is effected by
circulating a large stream of already neutralizedproduct and adding sulfonic acid as well assodium hydroxide to this product stream, just infront of the mixing pump.
The brown to dark brown sulfonic acidbecomes considerably lighter in color duringneutralization. The light brown sulfonates ob-tained can be lightened by adding 1 – 3 wt% ofsodium hypochlorite solution (containing 100 –150 g of active chlorine per liter). The productthen contains small amounts of chloride ions,which can cause stress corrosion cracking in steel,especially at high temperature. The apparatus forthe bleaching stage must therefore be claddedwith tantalum, silver, hastelloy, or earthenware.
Storage and Dispatch. Linear alkylbenze-nesulfonates are water-soluble at 20 �C in anamount of up to 25 wt% to form clear solutions,and at higher concentrations pastes are formedthat in the concentration range from 30% to 60%separate into two phases after a few hours. Suchpastes must therefore be stirred during storage.The 75% pastes do not decompose into twophases. The 65 – 75% pastes can be dispatchedonly in the hot state on account of their highviscosity, and at room temperature they solidifyto form rigid gels. The pastes, which can readilybe pumped at 60 – 80 �C, can be stored andtransported in stainless steel or glass fiber rein-forced polyester vessels. They can be conveyedthrough heated stainless steel pipelines by usingrotary piston pumps.
Anhydrous alkylbenzenesulfonates arehygroscopic and tend to agglomerate due toabsorption of atmospheric moisture. Additivessuch as sodium sulfate or sodium toluenesulfo-nate counteract agglomeration; alkylbenzenesul-fonates containing such additives can be workedup directly into pulverulent formulations. Anhy-drous alkylbenzene sulfonate is obtained bydrying hydrated pastes, e.g., on a drum dryer; itcan be packed in plastic bags and palleted. Sometypical data of an industrial sodium alkylbenzenesulfonate in various concentrations are shown inTable 6.
CAS Numbers. C10 – C13 alkylbenzene-sulfonic acid [85536-14-7], its sodium salt[68411-30-3], its diethanolamine salt [84989-15-1], its triethanolamine salt [68411-31-4];C10 – C16 alkylbenzenesulfonic acid [68584-
Vol. 35 Surfactants 455
22-5], its sodium salt [68081-81-2], its potassiumsalt [68584-27-0], its ammonium salt [68910-31-6], its calcium salt [68584-23-6], its magnesiumsalt [68584-26-9].
Producers. Major producers of alkylbenze-nesulfonic acids and alkylbenzenesulfonates inEurope include Albright &Wilson, Berol-Nobel,Harcros-Chemicals, H€uls, ICI, Kao, Manro, Pul-cra, Rhone-Poulenc, Stepan, Unger, Wibarco,Witco; in the United States, A. Harrison, Ard-more, Cromton & Knowles, Eastern Color &Chemical, Emkay Chemical, Exxon, Harcros,Henkel, ICI America, Pilot Chemicals, Rhone-Poulenc Surfactants, Stepan, Unger, Witco; in .Taiwan, Taiwan Surfactant; and in Israel, Zohar.
Whereas the sulfonation of alkylbenzene andthe neutralization of the sulfonic acid takes placein relatively simple plants and is accordinglyeven carried out by many producers for captiveuse (e.g., for detergents), production of the al-kylbenzene is more complex. The number ofproducers of alkylbenzene is therefore limited.Producers of linear alkylbenzenes by hydrogenfluoride catalysis include H€uls, Enichem, andPetresa in Europa; Vista and Monsanto in NorthAmerica; YPF La Plata and Deten in SouthAmerica; Al Astra in Saudi Arabia; EgyptianGeneralpet in Egypt; Mitsubishi Petrochemicaland Nippon Petroleum in Japan; Isu Chemical inSouth Korea; and Formosan Union in Taiwan.Enichem and Wibarco in Europe, Vista in theUnited States, and Nalken in Japan also producealkylbenzene using aluminum chloride ascatalyst.
Producers of tetrapropylenebenzene includeChevron in France, Monsanto in North America,Pemex in Mexico, Mitsubishi Petrochemical andNippon Petroleum in Japan, and FormosanUnionin Taiwan.
6.2.2. Alkylnaphthalenesulfonates
The first synthetic surfactants based on fossil rawmaterials were the alkylnaphthalenesulfonatessuch as dipropylnaphthalene- and dibutylnaphtha-lenesulfonates, which still play a role as wettingagents in the textile industry. These substancesare obtained in a one-pot reaction by alkylatingnaphthalene with isopropanol, butanol, or otheralcohols in the presence of concentrated sulfuricacid as catalyst andwater binder, and immediatelytreating the reaction product with oleum to form(di)-alkylnaphthalenesulfonic acid, in a similarmanner to the sulfonation of alkylbenzene, andthen separating the ‘‘waste sulfuric acid’’, andfinally neutralizing with sodium hydroxide.
The salts of naphthalenesulfonic acid condensedwith formaldehyde are chemically related to thealkylnaphthalenesulfonates, and can also be ob-tained in a one-pot reaction. For this purposenaphthalene is first of all sulfonated and the acidsulfonation mixture is then reacted withformaldehyde.
Table 6. Typical characteristic data of an industrial sodium alkylbenzenesulfonate
After the condensation the reaction product isneutralized. The molar ratio is chosen so that n isbetween 2 and 3. The salts of these condensationproducts are outstanding dispersants for finelydivided solids, and are used as pigment disper-sants and cement plasticizers.
Producers. of both types of products in-clude BASF, Hoechst, Auschem, Rhone-Poulenc, and ICI in Europe; American Cyana-mid, Du Pont, Emkay, Henkel, and Witco in theUnited States; TakemotoOil & Fat in Japan; andTaiwan Surfactant in Taiwan.
6.2.3. Alkanesulfonates [53–56]
Of the large number of possible ways of syn-thesizing alkanesulfonates (! Sulfonic Acids,Aliphatic), only sulfochlorination (the reactionof alkanes with sulfur dioxide and chlorine toform alkane sulfonyl chlorides and their sapon-ification with sodium hydroxide) and sulfoxi-dation (the reaction of alkanes with sulfurdioxide and oxygen and neutralization of thesulfonic acids) are of industrial importance.The radical addition of sodium hydrogensulfite,which proceeds with satisfactory yields only inthe case of terminal olefins, gives terminalalkanesulfonates, which are only slightlysoluble in water and are therefore of no indus-trial importance [57].
In sulfochlorination and sulfoxidation, mix-tures of straight- chain alkanes with 12 to 18,preferably 13 to 17 carbon atoms, are used asstarting materials. Both reactions proceed appro-priate by a radical- chain mechanism. In sulfox-idation a sulfoperoxy acid first of all beingformed:
The reaction can be started by radical initia-tors (e.g., peroxides) or by irradiation.Ultravioletra-diation is used industrially. In sulfochlorina-tion chlorine radicals are formed initially andreact with an alkane:
RHþCl.!R
.þHCl
In sulfoxidation the mechanism of the UV initia-tion has not been fully clarified. It is assumed thatactivated sulfur dioxide is formed, which reactswith the alkane:
RHþSO�2!R
.þHSO.
2
Aromatics, olefins, branched alkanes, and manyother substances (e.g., amines) inhibit the radi-cal- chain reaction; the paraffins must thereforebe of high purity and linearity for both processes.With high-purity alkanes quantum yields of10 000 Einstein are achieved in sulfochlorina-tion; termination reactions necessitate constantre-initiation (irradiation). In sulfoxidation, alka-nesulfoperoxyacids are formed initially butare unstable under the reaction conditions, anddecompose with the formation of additional ra-dicals:
RSO2O2H!RSO.
3þ.
OH
RSO.
3þRH!RSO3HþR�.
OHþRH!H2OþR.
These radicals compensate the deficit of radicalsdue to chain termination; being a radical- chainreaction with degenerative branching, in purealkanes sulfoxidation accordingly takes placeafter a single initiation, without further radical-producing measures. The decompositionproducts of the peroxyacid result in a number ofbyproducts (esters, alcohols, discoloring sub-stances), which adversely affect the productquality. Sulfoxidation is therefore carried out
Vol. 35 Surfactants 457
industrially with the addition of water, whichtogether with the sulfur dioxide present in thereaction medium traps the sulfoperoxy acid be-fore its radical-type decomposition begins:
RSO2O2HþSO2!R SO3HþH2SO4
A radical donor is lacking if the decomposition ofthe peroxyacid occurs by an ionic mechanism,and sulfoxidation in the presence of water there-fore requires constant initiation; in the case ofpure alkanes the quantum yield is ca. 10 Einstein.
In both processes substitution mainly takesplace at the internal, secondary carbon atoms ofthe alkane. The oxysulfonyl groups are distrib-uted randomly over the molecule. The degree ofpolysubstitution increases with increasing con-version. Since the interfacial activity of the sul-fonates obtained after saponification decreaseswith increasing proportions of polysulfonates,the paraffin conversion is restricted.
Alkanesulfonates below C8 are not surfaceactive. C12 – C18 sodium alkanesulfonates arereadily soluble in water; the solubility is in-creased still further by the disulfonate fractionthat is always present in industrial products. Onaccount of their proportion of species with cen-tral sulfonate groups (‘‘effective surfactants’’)the alkane sulfonates are good wetting agents.They are preferably used in liquid formulationsfor detergents and cleansing agents, but can alsobe incorporated in washing powders. Alkanesul-fonates are widely used as emulsifiers, for exam-ple, in the polymerization of vinyl chloride.
Sulfochlorination Processes. Alkanesulfo-nates obtained from sulfochlorination processesare known as Mersolates. They are obtained intwo steps:
RHþSO2þCl2!RSO2ClþHCl
RSO2Clþ2 NaOH!RSO3NaþNaCl
In the first stage the liquid paraffin is reacted at20 – 40 �C in chlorine- and acid-resistant reac-tors (earthenware, enamel, PVC)with an approx-imately equimolar mixture of sulfur dioxideand chlorine under irradiation from mercuryvapor lamps. The waste gas consists of hydrogenchloride contaminated with sulfur dioxide;the former can be recovered as aqueous hydro-chloric acid.
The paraffin conversion is adjusted to ca.30%; the sulfonyl chlorides then contained inthe reaction mixture (Mersol 30) in amountsof ca. 30% consist of 94% monosulfochloridesand 6% disulfochlorides. Mersol H, with 45%sulfonyl chlorides in the reactionmixture alreadycontains 16% disulfonyl chlorides and polysul-fonyl chlorides, while Mersol D, with 80% sul-fochlorides, contains ca. 40% disulfonyl chlor-ides and polysulfonyl chlorides.
In the second step the degassed sulfochlorina-tion mixture is saponified with 10% sodiumhydroxide. Elevated temperatures must beavoided since the alkane sulfonyl chloride isdesulfonated to the alkyl chloride starting aboveca. 80 �C, with elimination of sulfur dioxide.While hot, the neutralized product depositspart of the unreacted alkane as upper phase, andwhen cold deposits most of the sodium chloridederived from the saponification as an aqueouslower phase. The remaining sulfonate ‘‘glue’’ isfreed from residual alkane in an evaporator,which together with the paraffin from the phaseseparation is returned to the sulfochlorination.The sulfonate is obtained as a melt flowing at150 – 175 �C, which solidifies on coolingdrums and can be removed as flakes. The flakesare hygroscopic and readily agglutinate – theycan be dispersed in water and processedinto pastes. The melt can, however, also betaken up directly under pressure in water. Table 7summarizes typical data for industrial commer-cial products of different concentrations.
Half the chlorine used in the process is con-verted to low-grade hydrochloric acid, andhalf into valueless sodium chloride solution.Thus the process is economically inefficient.This disadvantage is offset by the advantagethat the highly reactive alkanesulfonyl chlorideintermediates can be used in the preparationof a large number of derivatives besides alkane-sulfonates. For example, reaction of an alkane-sulfonyl chloride with glycine yields andalkylsulfamidocarboxylic acid, which can beused to prepare effective emulsifiers andcorrosion inhibitors. Esters, obtained with alco-hols and phenols, are used as plasticizers forplastics.
Storage. Alkanesulfonate flakes are hygro-scopic and should be processed in total. Thepastes undergo segregation on standing and must
458 Surfactants Vol. 35
be homogenized before use. The products can bestored for an unlimited time.
Sulfoxidation Processes. Figure 15 illus-trates a production process for alkanesulfonatesusing the following reactions
RHþ2 SO2þO2þH2O!RSO3HþH2SO4
RSO3HþNaOH!RSO3NaþH2O
When carried out under UV irradiation withaddition of water, this process is known as thelight – water process.
The reactors must be resistant to acids andoxidation, and are of large volume in order tomaximize the quantum yield. High-pressure10 – 40 kW mercury vapor lamps are used. Theprotective tubes are made of quartz and aredouble-walled to absorb the thermal radiationfrom the lamp,withwater cooling being providedbetween the lamp and reaction space. Sinceoxygen is substantially less soluble than sulfur
dioxide in alkanes the reaction gas must becirculated at a high rate with a blower tomaintaina sufficient supply of dissolved oxygen in thereaction mixture. This circulating gas also en-sures intensive mixing of the reactor contents,which, however, can also be achieved by power-ful stirrers. This is important because the aqueousand alkane phases must be constantly mixed sothat the alkanesulfoperoxyacid formed initiallycomes into immediate contact with water andsulfur dioxide, whereby it is decomposed to thealkanesulfonic acid. Furthermore, the aqueousphase constantly extracts sulfonic acids and sul-furic acid from the alkane phase. The unusuallyhigh degree of polysubstitution –with a 1%paraffin conversion the sulfonic acids formedalready contain 10% of disulfonic and polysul-fonic acids – is due to themultiphase nature of thesystem. To avoid a higher proportion of disulfo-nic acids, the alkane conversion is limited to ca.1%. The mixture leaving the reactor, whichconsists of about 25 parts of alkane and one partof aqueous phase, passes to a separator in which
Table 7. Typical data of industrial sodium alkanesulfonates [68037-49-01] obtained by the sulfochlorination process
Figure 15. Production process flow sheet for alkanesulfonates using sulfoxidationa) Photoreactor; b) Acid settler; c) Degassing column; d) Evaporation column; e) Separator for sulfuric acid; f ) Neutralizationvessel; g) Evaporator
Vol. 35 Surfactants 459
the alkane forms the upper phase. The alkane isrecycled through a cooler to the reactor, togetherwith fresh alkane and process water. At 1%conversion the amount of recycled alkane is verylarge, and can be used for external cooling of thereaction mixture. The reaction temperature ismaintained at 20 – 40 �C.
The acid phase that separates out in the settlercontains acids (sulfonic acids and sulfuric acid),water, and alkane in roughly equal amounts. Thisphase is freed from dissolved sulfur dioxide bydegassing and is concentrated by distilling offpart of the water under vacuum. The low-water-content bottom product from the evaporatingcolumn separates into two phases, the lowerphase consisting of 50 to 65% aqueous sulfuricacid, which is thereby largely removed from thesulfonic acid phase. Discolorations, which canoccur in this stage due to heating (60 – 120 �C)of the sulfonic acids, can be counteracted byaddition of hydrogen peroxide. The organicphase remaining after separating the sulfuric acidconsists of roughly equal parts of sulfonic acidsand alkane. It is neutralized with concentratedsodium hydroxide and freed from residual al-kane, e.g., in a thin-layer evaporator at 200 �C invacuo, which is recycled to the reaction. Thesulfonate melt that is formed can be cooled on arotating drum and converted to flakes, or pro-cessed with water into 60 – 65% pastes.
As an alternative to the thermal separation ofsulfuric acid, the sulfonic acids together with thealkane can be extracted from the mixture withweakly polar solvents such as alcohols, ketones,or ethers, leaving behind a 20% aqueous sulfuricacid. The solvent must then be separated after the
neutralization in an additional distillation col-umn [58].
Some data for commercial products of variousconcentrations are listed in Table 8.
Storage and Dispatch. The flake productcan be stored for an unlimited period withoutundergoing any change provided moisture isexcluded. The 60% paste tends to undergo phaseseparation, which is prevented by stirring orcirculation with a pump at 65 �C. The 30%solution remains homogeneous and is easilypumpable.
Stainless steel, aluminum, and glass fiber re-inforced polyester resin are suitable materials forstorage tanks and pipelines. Normal steel isunsuitable because even very small amounts ofrust seriously affect the quality.
Producers. Alkanesulfonates are producedby the sulfochlorination process by Bayer, Leunaand at Volgograd (Russia), and by the sulfoxida-tion process by Hoechst and H€uls.
6.2.4. a-Olefinsulfonates [59–63]
Olefinsulfonates are obtained by hydrolysis andneutralization of the sulfonation products of ole-fins. Suitable sulfonating agents include sulfurtrioxide and its complexes (e.g., with dioxan),and chlorosulfonic acid; gaseous sulfur trioxideis used exclusively in industry. The fact that onlya-olefins are used for the production of sulfo-nates is due primarily to the availability of C12 –C18 olefins of this type in the required degree of
Table 8. Typical characteristic data of industrial sodium alkanesulfonates [85711-69-9], [7757-82-6], and [7732-18-5] obtained by the
sulfoxidation process
Concentration
30% 60% 93%
Appearance at 25 �C clear, almost colorless liquid yellowish, soft paste yellowish, waxy flakes
Active substance, wt% 30 60 93
of which disulfonate
and polysulfonate,
wt% 4 7 11
Sodium sulfate, max.
wt% 2 4 6.5
Alkane, max. wt% 0.3 0.5 0.7
APHA color (10% active substance in water) 40 40 50
Bulk density, g/L 500
460 Surfactants Vol. 35
purity. Since the sulfonation takes place withoutdouble bond isomerization, a-sulfonates (a-ole-finsulfonates ¼ AOS) having good applicationproperties are obtained from a-olefins. If olefinscontaining a centrally situated double bond aresulfonated, sulfonates with a centrally bondedsulfonate group are obtained, which have not yetachieved any industrial importance [64], [65].
In the sulfonation ofa-olefins 1,2-sultones areprobably formed initially, which then isomerizerapidly to 1,3-sultones, and more slowly to 1,4-sultones:
In the reaction with sulfur trioxide diluted withinert gas, at temperatures up to 40 �C, almostexclusively sultones are formed at up to 60%olefin conversion, and at higher conversions thesultones isomerize to some extent to alkenesul-fonic acids:
With increasing conversion side reactions suchas disulfonation and oxidation also increase,leading to discoloration of the product. To con-vert the sultones completely to sulfonic acids orsulfonates, the sulfonation mixture must be hy-drolyzed. Hydrolysis can be performed underacid or alkaline conditions. Acid hydrolysis givespredominantly alkenesulfonic acids with doublebonds in the 2- and 3-positions, though com-pounds with more internal double bonds are alsoobtained; in alkaline hydrolysis about two-thirdsof the reaction product consists of salts of hydro-xyalkanesulfonic acids, and the remainder, saltsof alkenesulfonic acids, as isomer mixtures:
Industrial hydrolysis is carried out exclusivelyunder alkaline conditions, mixtures of 60 to 65%of alkenesulfonates and 40 – 35% of hydro-xyalkanesulfonates being obtained, which for thesake of simplicity are termed a-olefinsulfonates.
b-Branched a-olefins, formed by dimerizingshort- chain a-olefins, are sulfonated either assuch, but generally mixed with linear a-olefins.Due to the presence of a tertiary carbon atom,exclusively b-branched isomeric alkenesulfonicacids are obtained, e.g.,
Since they have a long- chain hydrophobic resi-due and a terminal hydrophilic group, a-olefin-sulfonates derived from linear a-olefins are effi-cient surfactants, with low CMCs, high detergentpower, and good foaming ability in water. Onaccount of their good water solubility sulfonatesobtained from 1-dodecene to 1-hexadecene arewidely used in liquid detergents and cleansingagents, in cosmetic preparations, and shampoos.Sulfonates obtained from a-hexadecene to a-oc-tadecene can be used in powder formulations.Sulfonates obtained fromb-branched a-olefinsare only slightly watersoluble; they are goodwetting agents, but are less suitable for detergentsand cleansing agents. In industrial products theyare used as a blendwith lineara-olefin sulfonatesin a maximum proportion of 30%.
Production. The reaction of sulfur trioxidewith olefins is rapid and strongly exothermic (ca.180 kJ/mole; i.e., for example, 860 kJ/kg hex-adecene). Local overheating, which leads tocarbonization and thus brown to black products,cannot be avoided in conventional sulfonationreactors, even with intensive cooling and thor-ough mixing, especially if high olefin conver-sions are used for economic reasons.
The sulfonation of a-olefins is nowadayscarried out industrially in continuous, short-timesulfonation reactors, like those used for sulfona-tion of alkylbenzene (see 6.2.1), and for sulfatingethoxylates [51], [52]. The principle of thesereactors, supplied by Allied, Ballestra, Chemi-thon, Lion, orMazzoni, is to bring the substrate inthe form of thin, fast-flowing and strongly cooledfilms or layers into contact with sulfur trioxide
Vol. 35 Surfactants 461
gas dilutedwith inert gas. Air or residual air fromthe combustion of sulfur dioxide to sulfur triox-ide serves as inert gas; the concentration of sulfurtrioxide is adjusted to 4 – 6 vol%.
Figure 16 illustrates the continuous productionofasodiuma-olefinsulfonate.Theolefinisreactedwith a 10 – 20% excess of sulfur trioxide in thereactor at as low a temperature as possible (40 �C;at lower temperature the high viscosity can causeproblems). The reaction product flows into a sepa-rator, in which liquid and gas separate. The wastegas, which contains traces of sulfur trioxide andsmall amounts of sulfur dioxide is washed withcaustic soda before being discharged to the atmo-sphere. The sulfonationmixture is introduced intothe circulating stream of neutralized sulfonic acidand mixed with caustic soda in a pump. A partialstreamischargedviaaboosterpumpandaheatertoa hydrolysis reactor where residual sultones aresaponified. Hydrolysis is carried out at 150� to250 �C with a residence time of < 1 h. If thesolution still contains fairly large amounts of un-sulfonated hydrocarbon, this separates as upperphase and can be removed. The hydrolysate iscooled and the pressure is released. The amountofwater added in the neutralization ismeasured sothat 30 to 40% solutions are formed, which arebleached with hydrogen peroxide.
The olefinsulfonates produced in this way con-tain, relative to total sulfonate, 10 – 15% of dis-ulfonates and small amounts of neutral oil (olefins,sulfones, oxidation products) and sodium sulfate.
Typical data of two industrial products basedon C14/C16 and C16/C18 a-olefins are summa-rized in Table 9.
Storage and Dispatch. Olefinsulfonatesolutions can be stored and dispatched in stain-less steel or plastic vessels. The solutions arepumpable. Olefinsulfonates are not sensitive toheat, though overheating should be avoided.Reversible precipitation can occur below 0 �C.
Producers. a-Olefinsulfonates are pro-duced by Akzo, Albright & Wilson, Hoechst,Rhone-Poulenc, and Witco in Europe; by PilotChemicals, Rhone-Poulenc Surfactant, Stepan,and Witco in the United States; and by Lion inJapan. Important producers of a-olefins, whichare produced on a large scale for variouspurposes such as sulfonation, hydroformylation,or alkylation, are Chevron in the United States,Ethyl in Belgium and the United States, Nizhne-kamsk in Russia, Shell in England and the UnitedStates, Spolana in Czechoslovakia, and IdimitsuPetrochemical in Japan.
Figure 16. Continuous preparation of sodium a-olefinsulfonatea) Sulfonation reactor; b) Gas separator; c) Mixing pump; d) Booster pump; e) Heater; f ) Hydrolysis reactor; g) Pressuremaintenance valve; h) Reactor for bleaching
Table 9. Typical data of industrial sodium a-olefinsulfonates
Olefin C14/C16 C16/C18
CAS no. [68439-57-6] [977067-81-4]
(RD)
Appearance at 25 �C clear, pale
yellow liquid
slightly turbid,
pale yellow
liquid
Active substance, wt% 37 33
of which disulfonate, wt% 4 3
Sodium sulfate, wt% 1 1
Sodium chloride trace trace
Unsulfonated substance, wt% 1.5 1.5
Klett color index
(5% in 5% butyl diglycol)
60 100
Viscosity, mPa � s ca. 1000 ca. 1000
462 Surfactants Vol. 35
6.2.5. a-Sulfo Fatty Acid Esters [63],[66–70]
The sodium salts of a-sulfo fatty acid methylesters based on coconut, palm kernel, or tallowfatty acids are far more soluble in water than thedisodium salts of a-sulfo fatty acids:
For this reason theestersarepreferred.Thea-sulfofatty acid esters are stable to hydrolysis in the pHrange 4 – 9 and are relatively insensitive to waterhardness. They have good emulsifying and limesoapdispersing properties, and their aqueous solu-tions foam well and have a good cleaning abilitywith respect to textiles. They are accordingly usedas components of soap bars and detergents.
The a-sulfo fatty acid esters are prepared bysulfonation of fatty acid methyl esters. The reac-tion proceeds in two stages [71]. Sulfur trioxide israpidly absorbed by the ester at< 50 �C to give a2 : 1 addition compound that rearranges to thea-sulfo fatty acid methyl ester at 70 – 90 �C.Sulfonation with 5 vol% of sulfur trioxide in aninert gas can be carried out in a stirred vesselcascade, in which the temperature increases fromvessel to vessel.With sulfonation in a continuousshort-time sulfonation reactor, a further reactormust be connected that ensures complete rear-rangement at elevated temperature. Sulfonationis carried out with a 10 – 20% excess of sulfurtrioxide. The sulfonation mixtures are darkcolored, and are bleached with hydrogen perox-ide at 60 to 80 �C and then neutralized withsodium hydroxide. The sodium salts of thea-sulfo fatty acid esters are handled as ca.40% pastes or slurries.
Producers. a-Sulfo fatty acid esters areproduced by Henkel and Lion.
6.2.6. Sulfosuccinates [72], [73]
The sodium salts of sulfosuccinate esters areobtained by reacting dialkyl maleates or salts ofmonoalkylmaleateswith sodiumhydrogensulfitein aqueous-alcoholic solution. Esterificationcomponents for dialkyl esters are mainly C5 –C8 alcohols and fatty acid ethanolamides, and formonoalkyl esters, fatty alcohols, fatty acid etha-
nolamides, and fatty acid ethoxylates. The sulfo-succinates thus offer a wide variety of molecularstructure, which can be adapted to numerousapplications. Thepure sulfo esters are also termedsulfosuccinates, while the amide-group- contain-ing products are known as sulfosuccinamates:
The dialkyl sulfosuccinates with short- chainalkyl groups R such as butyl, hexyl, or ethylhexylare readily soluble in water and have outstandingwetting power (fast wetters) and dispersing prop-erties, and are therefore used in textile processingand dyeing. They crystallize readily, like thesulfosuccinamates, and are therefore ideally suit-ed as components of dry cleaning agents. Withincreasing alkyl chain length the solubility andwetting ability of sulfosuccinates decrease, whiletheir detergent power and emulsifying powerincrease. The disodium salts of monosulfosucci-nates are becoming increasingly important ashardness-insensitive and dermatologically com-patible surfactants [74]. They are used in deter-gents and cleansing agents, especially for body-care products, and as emulsifiers in cosmetics.They are also used as polymerization emulsifiersand in ore flotation. Suitable alkyl residues inthese sulfosuccinates are fatty alkyl or nonylphe-nyl residues; the degree of ethoxylation n isbetween 3 and 12. Natural fatty acids are usedas starting products for sulfosuccinamates.
A common feature of all sulfosuccinates isthat they are readily saponified at the ester group.They can therefore be used only in approximatelyneutral media (pH 6 – 8).
Storage and Dispatch. The sodium salts ofthe diesterified sulfosuccinic acids can be ob-tained as powders by evaporating their solutions.They are generally transported as concentratedsolutions in plastic drums or stainless steel
Vol. 35 Surfactants 463
vessels. Table 10 gives some typical data forthree industrial products, which may contain afew percent of a solvent (isopropanol) in order torender them clear.
Producers include Albright & Wilson,Auschem, BASF, Cyanamid, Harcros, Henkel,H€uls, Manro, Rewo, Rhone-Poulenc, Stepan,Union Carbide, Witco, and Zschimmer &Schwarz in Europe; American Cyanamide,Croda, Enkay, Harcros, Hart, Henkel, TheMcIntyre Group, Mona Industries, Rhone-Poulenc, Sherex, Stepan, and Union Carbide inNorth America; Zohar in Israel; Toho and Sanyoin Japan; and Taiwan Surfactant in Taiwan.
Methods of preparation of alkoxyalkanesulfonicacid salts, whichwould be useful for synthesizingalkylpolyglycol ether sulfonates, which in con-trast to ether sulfates are resistant to hydrolysis,havenotyetbeencarriedoutonanindustrialscale.
The reaction of alcohols with epichlorohydrin toform alkoxyhydroxychloropropane,which reacts
with sodium sulfite to give an alkoxyhydroxy-propanesulfonate, has achieved some impor-tance:
Such sulfonates are stable to water hardness andhave good wetting and dispersing properties.They are dermatologically compatible and areused occasionally in the United States in bodycleansing agents. Their water solubility, howev-er, is low. Products containing C16 – C18 alkylgroups are only sparingly soluble.
Due to their insensitivity to water hardness,good foaming power, soil suspending power, anddermatological compatibility, the fatty acid-derived salts of acyloxyethanesulfonic andacylaminoethanesulfonic acids are industriallyfairly important (e.g., in the textile industry andin bodycare and cleansing agents). They are pre-pared by reacting the corresponding acid chlor-ides with sodium isethionate or N-methyltaurine.
Whereas the reaction of the isethionate with theacid chloride takes place at 100 – 120 �C with-out solvent and the reaction product occurs ina friable solid form after the hydrogen chloride,
Table 10. Typical data of some industrial sulfosuccinates
Sodium diethylhexyl
sulfosuccinate
Disodium lauryltriglycol
sulfosuccinate
Disodium lauroylaminoethyl
sulfosuccinate
CAS no. [577-11-7] [58450-52-5] [25882-44-4], [55101-78-5]
Apperance clear liquid clear liquid clear liquid
Active substance, wt% 68 – 70 34 35
Clear point, �C < 0 12 �2
pH (1% active substance) 6 – 7 6 – 7 6 – 7
Klett color index (5% active substance) 40 150 150
Density, g/cm3 1.0 1.1 1.1
Viscosity, mPa � s 300 50 25
Flash point, �C 45 >100
464 Surfactants Vol. 35
N-methyltaurine is reacted with the fatty acidchloride in aqueous solution at 20 – 30 �C. TheN-acyl-N-methyltauride is obtained as a 30 –40% solution, which can be converted into ananhydrous powder by drying.
Since the fatty acid chlorides must be synthe-sized from the fatty acids by reaction with phos-phorus trichloride or thionyl chloride, attemptshave been made to react the free acids directlywith isethionate or N-methyltaurine at elevatedtemperature. Although the desired derivatives ofethanesulfonic acid are formed in 80 – 90%yield, they are less pure than those obtained fromacid chlorides.
Producers. Isethionates are produced inEurope by Hoechst, in the United States byHenkel-Emery, Hoechst-Celanese, and Rhone-Poulenc Surfactant; taurates are produced inEurope by Croda, in the United States by Finitex,Hart, Henkel-Emery, Hoechst-Celanese, Sherex,and Union Carbide, in Israel by Zohar, and inJapan by Nikko Chemicals and Nippon Oil &Fats.
6.3. Sulfates
In surfactant chemistry the term ‘‘sulfates’’ refersto the salts of acidic sulfuric acid esters,R�OSO3M. In this class of compounds the sulfuratom of the hydrophilic sulfate group is bondedvia an oxygen atom to the hydrophobic residue.This ester group is susceptible to hydrolysis,more strongly in acid media than in alkalinemedia, and is less thermally stable than theanalogous sulfonates. Sulfates are prepared bysulfation of hydroxyl- containing substrates withsulfuric acid, oleum, chlorosulfonic acid, sulfu-ryl chloride, amidosulfonic acid, complexes ofsulfur trioxide, or sulfur trioxide itself, or byaddition of sulfuric acid to double bonds (!Sulfonic Acids, Aliphatic). A large variety ofsurfaceactive sulfates is thus available, includingthe sulfated unsaturated fatty acids and oils thatformed the starting point of surfactant chemistryand which still play a role in the textile industry[75], to the sulfates of fatty acid ethanolamidesand their ethoxylates, which on account of theirdermatological safety are used in cosmetics [76],and to the highly surface-active sulfated mono-glycerides of fatty acids [76].
6.3.1. Alkyl Sulfates [67], [77–79]
The primary alkyl sulfates developed in the1930s, based on natural alcohols obtained byhydrogenating fatty acid esters, were the firstsynthetic surfactants to be produced on an indus-trial scale. They give a neutral reaction in aque-ous solution. Their physicochemical and appli-cational properties have been investigated inten-sively, especially the influence of the structure ofthe hydrophobic group on the above-mentionedproperties has been described in detail, sinceindividual defined substances can readily beprepared [79].
The sodium salts of alkyl sulfates preparedfrom primary, linear alcohols up to C8 are scarce-ly surface-active. Industrially important sulfatesare derived from dodecyl and tetradecyl alcohols(coconut oil and palm kernel alcohol); thesesulfates are water-soluble and resistant to waterhardness, and form good detergents, foamingagents, and emulsifiers. Their good dispersantproperties are offset by a poor soil-suspendingpower. The sodium salts of the sulfates of hex-adecyl and octadecyl alcohols (tallow fat alco-hol) are fairly insoluble in water, especially inhard water; however, because the solubilityincreases markedly with increasing temperature,these sulfates are very active at elevated temper-ature (e.g., in textile washing).
In acidic aqueous solution (pH 3 – 4) alkylsulfates are stable in the cold but hydrolyze onheating. They are stable in hot neutral and alka-line solution.
The primary, linear alkyl sulfates have highthermal stability, and can be spray dried at hightemperature without decomposition. In contrast,the sulfates obtained from a-branched, primaryand from secondary alcohols are thermally lessstable, and readily decompose into sodiumhydrogensulfate and alkene.
Due to the more central position of their hy-drophilic groups the sulfates of highly branchedalcoholsobtainedby theoxoorGuerbetprocessesor of secondary alcohols are less suitable asdetergents and emulsifiers; however, in aqueoussolution they have higher wetting power than thesulfates of linear primary alcohols. Secondaryalkyl sulfates are no longer in use.
Production of Alkyl Sulfates. Since esteri-fication of alcohols with sulfuric acid, followed
Vol. 35 Surfactants 465
by neutralization
ROHþH2SO4�ROSO3HþH2O
is an equilibrium reaction the sulfuric acid mustbe used in excess to maximize alcohol conver-sion. Apart from corrosion problems caused bythewater formed in the reaction, the separation ofthe excess sulfuric acid from the reaction mix-ture is a problem. This can be avoided if a slightexcess of chlorosulfonic acid is used as sulfatingagent:
ROHþClSO3H!ROSO3HþHCl
The gaseous hydrogen chloride formed isabsorbed in water or sodium hydroxide. Thereaction with chlorosulfonic acid must be carriedout at the lowest possible temperature to avoidside reactions. Residual hydrogen chloride canbe removed from the reaction mixture almostcompletely by air sparging.
Although sulfation with chlorosulfonic acid isstill carried out in small production units, sulfa-tion with sulfur trioxide diluted with inert gas isthemethod of choice for large-scale plants. In thesame way as in the already discussed preparationof a-olefinsulfonates (see Section 6.2.4), thisreaction can be carried out in a continuouslyoperating short-time sulfonation reactor [51],[52]. Sulfation is performed with equimolaramounts of sulfur trioxide; an excess promotesside reactions and discoloration of the product.Since elevated temperatures have the sameeffect, the reaction temperature should be below50 �C. The reaction product leaving the reactor isdegassed and neutralized as quickly as possible,preferably with sodium hydroxide:
ROHþSO3!ROSO3H
ROSO3HþNaOH!ROSO3NaþH2O
Often, the product is bleached with hydrogenperoxide.
The fatty alkyl sulfates are generally commer-cially available as 30% pastes; above this con-centration highly viscous pastes form, which aredifficult to handle. The linear, primary alkylsulfates crystallize well and are therefore alsohandled as powders that have been dried onrollers or in the spray process. Characteristic dataof some industrial alkyl sulfates are listed inTable 11.
Storage and Dispatch conditions corre-spond to those of the alkylbenzenesulfonates (seeStorage and Dispatch) or alkanesulfonates. Theproducts must not be heated too strongly sincethey may be acidified further due to hydrolysis.
Producers of fatty alkyl sulfates includeAkzo, Albright & Wilson, Auschem, Berol No-bel, Chem-Y, Harcros, Henkel, H€uls, Manro,Rewo, Stepan, Unger, Witco, and Zschimmer& Schwarz in Europe; Du Pont, Emkay, Henkel,Lonza, Rhone-Poulenc Surfactant, Stepan, Un-ger, andWitco in theUnited States; Daiichi, Kao,Nippon Oil & Fats in Japan; and Zohar in Israel.
Leading producers of the fatty alcohol startingmaterials by fatty ester hydrogenation are: Hen-kel, H€uls, Marchon, Oleofina, Rodleben, RWE-DEA in Europe; Procter &Gamble and Sherex inthe United States; Kao and New Japan in Japan,and joint ventures of local and Western compa-nies in the Philippines and Malaysia. Ethylene-base Ziegler alcohols are produced by RWE-DEA in Germany and by Ethyl in the UnitedStates. Important producers of oxo alcohols byhydroformylation of olefins are BASF, Enichem,Exxon, ICI, Shell in Europe, and Shell and Vistain the United States.
6.3.2. Ether Sulfates [67], [77], [80–83]
Ether sulfates is the short name for salts ofsulfuric acid hemi-esters of alkyl or alkylaryloligoglycol ethers of general formula
RO½CH2CH2O�nSO3Na
Table 11. Typical data of some industrial fatty alkyl sulfates (sodium
salts)
CAS no. [97375-27-4] [68140-10-3] [68955-19-1]
Appearance paste paste powder
Active
substance, %
30 32 93
Na2SO4,
% max.
1.0 4.0 3.0
Nonsulfated
proportion,
% max.
0.5 1.8 1.0
pH 7 – 8 9 – 10 7 – 9
Color, APHA,
max.
50 100
Density, g/cm3 1.03 0.98 0.33*
*Bulk density.
466 Surfactants Vol. 35
The oligoglycol ethers, namely ethers of oli-gooxyethylenes or ethoxylates, on which theether sulfates are based are homologousmixtureswhere n ¼ 0, 1, 2 . . . , etc.; the number n in theabove formula specifying the mean degree ofethoxylation. Industrial ether sulfates are accord-ingly also homologous mixtures; with increasingdegree of ethoxylation the content of alkyl sul-fate (n ¼ 0) decreases.
The oligoglycol group of the ether sulfatesresults in better water solubility and higherstability to water hardness, compared to sulfatescontaining the same alkyl or alkylaryl group.The most important surfactants of this group arethe alkyl ether sulfates derived from ethoxylatesof dodecanol and tetradecanol (coconut andpalm kernel fatty alcohols) with a mean degreeof ethoxylation of 2 – 4. Instead of naturalalcohols, synthetic fatty alcohols are also usedin the corresponding C- cut range. These ethersulfates have achieved considerable importancedue to their good stability to hard water, der-matological compatibility, foaming and deter-gent power, their good emulsifying and limesoap dispersing power, their rheological behav-ior, and also on account of synergistic effectsthat they exhibit in conjunction with some otheranionic surfactants (sulfonates). They are usedin cosmetic preparations, foam baths, sham-poos, liquid detergents, cleansing agents, espe-cially fine detergents and rinsing agents, gener-ally in combination with other surfactants. Be-ing sulfuric acid hemi-esters, ether sulfates canbe saponified only with difficulty in alkalinemedia, but are readily saponified under acidconditions. Slow saponification also occurs inneutral media, but since sodium hydrogensul-fate is produced the solution becomes increas-ingly acidic and the saponification is therebyaccelerated autocatalytically:
RO½CH2CH2O�nSO3NaþH2O!RO½CH2CH2O�nHþHOSO3Na
Solutions of ether sulfates are protected againstthis autocatalytic decomposition by adding cit-rate, lactate, or phosphate buffers.
A well known property of ether sulfates is thethickening effect, i.e., the increase in viscosity byseveral orders of magnitude of even dilute solu-tions on adding electrolytes. The increase inviscosity passes through a maximum, dependingon the electrolyte concentration in the solution;
the effect also depends on the concentration andstructure of the ether sulfate (Fig. 17).
Production. Ether sulfates are obtained bysulfation of the corresponding ethoxylates andneutralization of the resulting sulfuric acidhemi-esters. Sulfuric acid is no longer usedfor sulfation, because it must be added inexcess and remains in the product as sodiumsulfate after neutralization. In smaller produc-tion units sulfation is performed with chlor-osulfonic acid, as described for the preparationof primary alkyl sulfates (see Production ofAlkyl Sulfates), and the neutralized productthen contains a few percent of sodium chlo-ride. Amidosulfonic acid is occasionally usedas sulfating agent, with the advantage that nobyproducts or waste products are formed. Inthis case, however, ammonium ether sulfatesare obtained:
RO½CH2CH2O�nHþHSO3NH2!RO½CH2CH2O�nSO3NH4
Amidosulfonic acid is preferably used in thesulfation of alkylphenyl ethoxylates, since inthis way sulfonation of the aromatic nucleus islargely avoided.
Figure 17. Thickening of 10% aqueous solutions of fattyalcohol ether sulfates containing 3 mol EO/mol, as a functionof the structure of the fatty alcohol residuea) C12/C14 100% linear; b) C12/C15 90% linear; c) C12/C15
50% linear; d) C12/C15 10% linear
Vol. 35 Surfactants 467
Ether sulfates are now mainly produced bycontinuous sulfation of ethoxylates in short-timesulfonation reactors using gaseous sulfurtrioxide, and continuous neutralization (seepreparation of alkylbenzenesulfonates and ole-finsulfonates, Sulfonation). The sulfation mix-ture should be neutralized as quickly as possiblesince decomposition and disproportionation re-actions occurring in the acidic reaction mixturelead to accumulation of byproducts, reduction ofthe content of active substance, and darkening ofthe color.
One of the decomposition products is 1,4-dioxane, a suspected carcinogen (! Dioxane).The dioxane content can be kept below50 ppm by precise control of the sulfationreaction (maintenance of low temperature,avoidance of excess sulfur trioxide, dilution ofthe sulfur trioxide gas, maintaining uniformthroughput with optimum mixing and minimumresidence time) as well as rapid degassing andneutralization [84], [85].
Even when the reaction is carefully con-trolled, the ether sulfates are not colorless. Theyexhibit a self-bleaching effect for several dayswhen allowed to stand in air, presumably due toautooxidation processes. The products are nor-mally rebleached with hydrogen peroxide orsodium hypochlorite.
Typical data of industrial sodium salts of ethersulfates are listed in Table 12.
Storage and Dispatch. Up to 30% aqueousether sulfate solutions are stored and transportedin plastic containers or in plastic-lined vessels.
Aluminum vessels are also suitable for short-term transportation. Phase separation may occurduring storage in the cold, which can be pre-vented by stirring.
Alkyl ether sulfates in aqueous solutions ofbetween25and30%concentrationexhibit a sharprise in viscosity from ca. 100 mPa � s to� 100 000 mPa � s.Thisviscosity reachesamax-imum value between 40 and 50%, thendecreases to10 000 mPa � s at60 – 70%concen-tration (Fig. 18). Since pastes of this viscosity arestill pumpable, 60 – 70% products are alsomarketed.
The concentration-dependent viscosity be-havior makes precautionary measures necessarywhen diluting concentrated pastes with water, soas to avoid the high-viscosity gel state at 40 –50% concentration. Dilute solutions are bestprepared by adding the concentrated paste towater.
Producers. include Akzo, Albright &Wilson, Auschem, Berol Nobel, Chem-Y,Harcros, Henkel, H€uls, Manro, Pulcra, Rewo,Rhone-Poulenc, Shell, Unger, Witco, andZschimmer & Schwarz in Europe; CloughChemical, Henkel, Niacet, Pilot Chemicals,Rhone-Poulenc Surfactant, Sandoz, Stepan, andWitco in North America; Kao, Lion, MarubishiOil Chemical, Nikko Chemicals, Nippon NyuKazai, Nippon Oil & Fats, and Sanyo in Japan;and Zohar in Israel.
Table 12. Typical characteristic data of some industrial ether sulfates
(sodium salts)
Fatty alcohol C12/C14 C13/C15
Degree of
ethoxylation
2 3
CAS no. [9004-82-4] [25446-78-0]
Active substance,
%
28 70 28 70
NaCl, % max. 0.3 1.2 2.0 1.2
Na2SO4, % max. 1.0 1.5 1.5 1.5
pH 6 – 8 6 – 8 6 – 8 6 – 8
Color, APHA,
max.
75 250 200 250
Viscosity, mPa � s 300 thixotropic 4000 thixotropic
Density, g/cm3 1.05 1.1 1.05 1.1
Figure 18. Viscosity of an alkyl ether sulfate as a function ofits concentration [80]
468 Surfactants Vol. 35
6.4. Alkyl Phosphates [86–88]
Acid phosphate esters and their salts are preparedby reacting alcohols or ethoxylates with phos-phorus pentoxide:
3 ROHþP2O5!ROPO3H2þðROÞ2PO2H
followed by neutralization, if required.Alkyl phosphates thus generally consist of
mixtures of monoalkyl and dialkyl esters ofphosphoric acid.Monoalkyl, dialkyl, and trialkylesters can specifically be prepared by stepwisereaction of a hydroxyl compound with phospho-ryl chloride, followed by saponification of theresulting phosphoric ester chlorides.
Phosphoric partial esters with short- chainalkyl groups (e.g., butyl phosphoric acids) arestrong acids, which have corrosion-inhibitingand bactericidal action. They are readily solublein hard water and act as wetting and dispersingagents, and are therefore used in acid-adjustedcleansing agents, e.g., for vehicles or swimmingbaths.
The water solubility and acid strength ofphosphoric partial esters decrease with increas-ing length of the alkyl chain. The sodium salts oflong- chain alkyl phosphoric acids readily dis-solve in water, have low sensitivity to waterhardness, and are resistant to saponification,especially in alkaline media. They are goodwetting agents and emulsifiers; the salts ofmono-alkyl phosphoric acids inhibit foam formation byother anionic or nonionic surfactants. Since thephosphates also have high dermatological com-patibility, they are used both in highly alkalinecleansing agents, and in general cleansing agents,including body cleansing agents (the latter espe-cially in Japan).
Producers. of alkyl phosphates includeAlb-right & Wilson, Auschem, BASF, Berol Nobel,Croda, Finetex, Harcros, Henkel, Hoechst, H€uls,Pulcra, Rewo, Rhone-Poulenc, Witco, andZschimmer & Schwarz in Europe; Climax Per-formance,Croda,Dexter,GradenChemical,Har-cros,Hart,Henkel, Intex,Lonza,MonaIndustries,Norman & Fox, Olin, PPG/Mazer, Reilly White-man, and Rhone-Poulenc Surfactant in NorthAmerica; Kao, Nikko Chemicals, Takemoto Oil& Fat, and Yoshimura Oil Chemicals in Japan;and Taiwan Surfactant in Taiwan.
7. Nonionic Surfactants [89–91]
The polarity of covalently bonded oxygen atomsin oligoethylene glycol ethers (also termed poly-glycol ethers) and oligohydroxy compounds or ofoxygen atoms bonded in a semipolar manner to aheteroatom imparts water solubility and interfa-cial activity to hydrophobic parent structures towhich such oxygen- containing hydrophilicgroups are bonded. The most important nonionicsurfactants are the ethoxylates, which are formal-ly condensation products of hydrophobic alco-hols, phenols, mercaptans, amines, carboxylicacids, carbonamides, etc., with oligoglycolethers, the fatty acid esters of glycerol, diglycer-ol, sugars, hydrogenated sugars such as sorbitol,and alkyl(poly)glucosides. Nonionic surfactantswith semipolar bonded oxygen as hydrophilicgroup include fatty amine oxides such as laur-yldimethylamine oxide, which is used to a limit-ed extent in liquid cleansing agents, as well as theindustrially unimportant sulfoxides and phos-phine oxides.
7.1. General Properties
Nonionic surfactants are colorless substances,but may be colored pale yellow to brown depend-ing on the production process. Low molecularmass products are liquid, and with increasingmolecularmass the products have a pasty towaxyconsistency. In the anhydrous state, especially inthe case of ethoxylates, the oligoglycol etherchains of short- chain ethoxylates are present ina stretched zig-zag form, while those of long-chain ethoxylates have a meandering structure,which is also formed in the micelles in aqueoussolution.
Nonionic surfactants are less sensitive towaterhardness than anionic surfactants. Their aqueoussolutions foam less strongly than those of manyanionic surfactants. The possible applications ofnonionic surfactants are universal, given the largevariability of their structure and thus of theirproperties. The differences between the individ-ual typesofnonionicsurfactantsareslight, and thechoice is primarily governed having regard to thecosts of special properties, e.g., effectiveness andefficiency, toxicity, dermatological compatibilityand biodegradability, or permission for use infoodstuffs. The solubility of nonionic surfactants
Vol. 35 Surfactants 469
in water results from hydration of the oxygengroups by hydrogen bonding. The degree ofhydration decreaseswith increasing temperature,and the water solubility of nonionic surfactantstherefore decreases with increasing temperature.In the case of surfactants that dissolve in water togive a clear solution, i.e., ethoxylateswith a fairlyhigh degree of ethoxylation, turbidity and sepa-rationof a surfactant phase that is immisciblewithwater occur at a specific temperature characteris-tic of the surfactant (cloud point).
The cloud point can be determined for water-insoluble ethoxylates by dissolving the surfactantin aqueous butyl diglycol. For highly water sol-uble ethoxylates whose solutions do not becometurbid even on boiling, the cloud point can bedetermined in aqueous sodium chloride solution.Figure 19 shows the cloud point of industrial fattyalcohol ethoxylates as a function of the degree ofethoxylation in 25% butyl diglycol solution, infully deionized water, and in 10% sodium chlo-ride solution.
Correlations exist between the cloud pointsand the hydrophilic – lipophilic balance (HLB)values or phase inversion temperatures of emul-sions with the corresponding surfactants as emul-sifiers [92].
The concept of an HLB is of practicalvalue [93].
The HLB system enables nonionic surfactantsto be arranged on a value scale from 0 to 20. In theideal case the HLB value of a surfactant is defined
astheratioofthemolecularmassofthehydrophilicfraction in the moleculeMh to the total molecularmassM of the surfactant, multiplied by 20:
HLB ¼ 20�Mh
M
The HLB scale allows rough classification ofnonionic surfactants according to their solubilityin water and their possible areas of application.Table 13 provides a summary.
The HLB values can be determined by emul-sification tests and comparison with emulsifiersof known HLB values (see also ! Emulsions).
7.2. Ethoxylates
Ethoxylates are generally obtained by addition ofethylene oxide to compounds containing disso-ciating protons.
Substrates used for ethoxylation are primarilylinear and branched, primary and secondaryC12 – C18 alcohols, i.e., natural and syntheticfatty alcohols, alkylphenols with branched octyl(butene dimer), nonly(propylene trimer) or do-decyl(propylene tetramer) groups, fatty acids,fatty acid ethanolamides, fatty amines, and fattyacid esters of polyhydroxyl compounds. Thedegree of ethoxylation, i.e., the molar ratio ofethylene oxide added per mole of substrate,varies within wide ranges, in general between3 and 40, and is chosen according to the intendeduse (HLB value, Table 13).
The addition of ethylene oxide to a substratecontaining acidic hydrogen is catalyzed by basesor (Lewis) acids.
Amphoteric catalysts, prepared in situand probably existing as finely dispersed solidshaving a large surface area [94], as well asheterogeneous catalysts [95], have also beendescribed.
The reaction mechanisms of base- catalyzedand acid- catalyzed ethoxylation differ, whichaffects the composition of the reaction products.In base- catalyzed ethoxylation an alcoholateanion formed initially by reaction with the cata-lyst (alkali metal, alkali-metal oxide, carbonate,hydroxide, or alkoxide), nucleophilically attacksethylene oxide. The resulting anion of the ethyl-ene oxide addition product can undergo an equi-librium reaction with the alcohol starting
Figure 19. Cloud point of ethoxylates of a C12/C14 fattyalcohol (linear) as a function of the degree of ethoxylation(mol EO/mol)a) 10% solution in 25% butyl diglycol; b) 2% solution indemineralized water; c) 2% solution in 10% NaCl
470 Surfactants Vol. 35
material or ethoxylate product, or can react fur-ther with ethylene oxide:
As this simplified reaction scheme illustrates, inalkaline- catalyzed ethoxylation several reac-tions proceed in parallel, the addition of ethyleneoxide to an anion with the formation of an etherbond being irreversible.
Proton exchange between various anions, oc-curring as an ionic reaction, is fast; the addition
reaction of ethylene oxide to an existing anion isthe rate-determining step. Thus in a reactionmixture, the more acidic species preferentiallyreact with ethylene oxide. If carboxylic acids orphenols are ethoxylated, the reaction proceedsexclusively via the left-hand path in the abovescheme, and the initially formed monoethyleneglycol ester or ether only reacts further when allthe starting material in the reaction mixture isconsumed. The fast equilibrium proton exchangereaction which precedes the addition of the eth-ylene oxide to the anionic species leads to apeculiarity of the reaction rate in the case ofstrongly acidic substrates such as carboxylicacids or phenols. On account of the lowernucleophilicity of their conjugate bases corre-sponding to the acidic parent substances, thesebases react relatively slowlywith ethylene oxide;thus, the reaction proceeds slowly until the start-ing material is consumed. Thereafter the reactionrate increases sharply with further supply of
Table 13. HLB values of nonionic compounds
HLB range Behavior in water Example of use Example HLB value
wetting agent Isotridecanol ethoxylate with 3 EO* 8.3
Isotridecanol ethoxylate with 4 EO 10.0
Isotridecanol ethoxylate with 5 EO 11.2
10 – 13 Soluble giving
translucent to
clear solutions
oil-in-water emulsions,
detergents
and cleansing agents
Isononylphenol ethoxylate with 5 EO 10.7
Isononylphenol ethoxylate with 6 EO 11.5
Isononylphenol ethoxylate with 7 EO 12.3
13 – 15 Soluble, giving
clear solution
oil-in-water emulsions,
detergents and cleansing
agents
Octadecanol ethoxylate with 10 EO 13
Octadecanol ethoxylate with 12 EO 14
Octadecanol ethoxylate with 16 EO 15
> 15 Soluble, giving clear solubilizer, cleansing agent Dodecanol/tetradecanol ethoxylate with 13 EO 15
solutions Dodecanol/tetradecanol ethoxylate with 17 EO 16
Dodecanol/tetradecanol ethoxylate with 25 EO 17
Dodecanol/tetradecanol ethoxylate with 38 EO 18
*Moles ethylene oxide per mole.
Vol. 35 Surfactants 471
ethylene oxide since the ethoxylate anions nowpresent react substantially more quickly withethylene oxide than do the anions of the carbox-ylic acids or phenols [96].
The situation is different in the ethoxylation ofalcohols. The ether oxygen atoms in alkyl (oligo)glycol ethers increase the acidity of the terminalprimary hydroxyl group compared to the initialalcohol; glycol ethers once formed thus reactpreferentially with ethylene oxide and lead tothe formation of a mixture of homologous oli-goglycol ethers, and unreacted starting alcoholremains in the reaction mixture up to high de-grees of ethoxylation. This is particularly true forthe ethoxylation of secondary alcohols.
Assuming the same acidity of the startingalcohol and all (oligo)glycol ethers present inthe mixture, a Poisson distribution of the indi-vidual species must be expected in the ethoxyla-tion, with a maximum corresponding to thatoligoglycol ether in which the number of addedethylene oxide units corresponds to the molarratio of ethylene oxide to starting alcohol. How-ever, on account of the aforementioned differentacidities of the individual species in the reactionmixture the homologue distribution that is actu-ally observed in an ethoxylation mixture differsfrom the Poisson distribution (Fig. 20).
This is true for all alkaline catalysts, althoughthe deviations in the case of alkaline-earth com-pounds are less strongly pronounced than in thecase of sodium hydroxide or sodium methoxide[97]. The distribution pattern of the homologouspolyethylene glycol ethers in an ethoxylateobtained by alkaline catalysis is independent of
the temperature, pressure, and catalyst concen-tration [98], [99]. The occasionally observeddependence of the homologue distribution onthe stirring rate can be attributed to an insuffi-ciently complete mixing of the reactor contents.
When Lewis acids such boron trifluoride, tintetrachloride, or antimony pentachloride are usedas catalysts, homologue distributions approxi-mating to the Poisson distribution are obtained(Fig. 20), because here it is not the proton activi-ty but the nucleophilicity of the substrate thatdetermines the reaction pathway. Lewis acidsactivate ethylene oxide and not the alcohol:
Lewis acids have not become established ascatalysts since they must be laboriously removedfrom the reaction product and because they leadto the formation of polyethylene glycol [the so-called polydiol, HO(CH2CH2O)nH], methyl-dioxolane and dioxane, due to side reactions anddecomposition reactions.
Since ethoxylates with a narrow, Poisson-likehomologue distribution, so- called narrow rangeethoxylates (NRE), have advantages in someapplications, amphoteric catalysts have beendeveloped that give distributions similar to thoseproduced by Lewis acids [94].
Among these catalysts a calcined hydrotalciteof idealized empirical formula Mg6Al2O5(OH)2is of interest since it can readily be handled as apneumatically conveyable powder and can easilybe separated as an insoluble solid from thereaction medium [95].
Advantages of narrow range ethoxylates,which also have a substantially lower content ofstarting alcohol, include: less odor, better solu-bility, lower volatility (reduced pluming on spraydrying) and better thickening properties of boththe ethoxylates and the ether sulfates derivedtherefrom [100–102]. However, broad distribu-tions may also be advantageous, for example, inemulsification processes [103].
The large variability of the hydrophobicgroup – straight- chain and branched fatty alco-hols, alkylphenols, fatty amines, fatty acids and
Figure 20. Homologue distribution of a lauryl ethoxylatecontaining 6 mol EO/mol [97]a) Acid catalysis; b) Base catalysis
472 Surfactants Vol. 35
fatty acid (alkanol)amides – as well as the optionto achieve any desired degree of ethoxylation,make the ethoxylates an extremely versatile classof surfactants. The ethoxylates of fatty alcoholsand alkylphenols as such or in combination withanionic, occasionally also cationic surfactants,play a major role in detergents and cleansingagents. Objections have been raised against theuse of alkylphenol ethoxylates on the basis ofecotoxicological findings, which has led to theirdiscrimination in some European countries[104].When adjusted to the necessaryHLBvalueby means of the degree of ethoxylation, fattyalcohol ethoxylates are used as emulsifiers incosmetics formulations, dry cleaning, crop pro-tection agents, metal working and processing,and textile auxiliaries, paints and coating com-positions. Like the alkylphenol ethoxylates, theyare also used as dispersants in petroleum produc-tion and in ores flotation. Fatty acid polyglycolesters have numerous uses as emulsifiers in body-care products and in textile treatment. The sameis also true of fatty acid alkanolamides and theirethoxylates, which moreover are used as anti-statics in plastics processing and as emulsifiersand corrosion inhibitors in metal working andprocessing. Finally, fatty amine ethoxylates areused as finishing agents and antistatics in textiletreatment and leather processing, and as emulsi-fiers in the petroleum industry and bitumenproduction.
General Production Processes. Molecu-larly uniform ethoxylates can be prepared by theWilliamson ether synthesis. Industrial ethoxy-lates, obtained by ethylene oxide addition aremixtures of homologues. These mixtures gener-ally have superior application properties tomolecularly uniform substances.
Special safety precautions must be adopted inindustrial ethoxylation. Ethylene oxide, bp10.7 �C, is stored, transported, and metered as aliquid under pressure. The flash point is� 57 �C,the ignition temperature in air is 429 �C, and thedecomposition temperature ofpure ethyleneoxidevapor is 571 �C. The heat of polymerization ofethylene oxide, which is released during ethox-ylation, is 2090 kJ/kg; 1900 kJ/kg is released inthe decomposition of ethylene oxide vapor. Inethoxylation processes the reaction can run outof control if there is overheating and a consequentrise in pressure; ethylene oxide tends to ignite or
undergo self-decomposition (e.g., on overheatedstirrer or pump shafts). Ethoxylation must there-fore be carried out under strict pressure and tem-perature control. The reactors must be safe-guard-ed against excessive pressure by means of safetyvalves or rupture disks. The excess of ethyleneoxide in the reactionmixturemust be kept low.Onaccount of the limited solubility of ethylene oxidein the reaction medium the presence of a second,liquid ethylene oxide phase in the reaction vesselmust be avoided. The storage vessel from whichthe ethylene oxide is metered into the reactionvessel must be protected against backflow ofreactants containing polymerization-initiatingcatalyst, e.g., by nonreturn valves and intermedi-ate vessels.
Industrial ethoxylations are carried outmainlyin a batchwise manner. Stainless steel pressurevessels are used as reactors. The substrate togeth-er with the catalyst, generally sodium hydroxide,is first placed in the vessel. The water introducedwith the sodiumhydroxide solution and thewaterformed in the reaction
ROHþNaOH�RONaþH2O
is removed by heating and applying a vacuum orby passing nitrogen through the reactionmixture,to avoid formation of polyglycols (‘‘polydiols’’).Polydiols do not, in general, affect the propertiesof ethoxylates provided their concentration doesnot exceed a few percent, but not being surface-active they result in a wastage of ethylene oxide.The catalyst content is usually 0.1 – 1.0%. Theethoxylation is carried out between 130� and180 �C, and the substrate first must be heatedup to this temperature. The reactor contents mustbe cooled during the addition of ethylene oxide.Heating and cooling can be performed by exter-nal jackets, internal tubes or by heat exchangersthroughwhich the reactor contents are constantlypumped.
The gaseous phase is inertized by diluting itwith nitrogen. The reaction is carried out at 1 –5 bar gauge pressure. The liquefied ethyleneoxide is metered into the reaction medium withintensive mixing. The temperature and pressureare monitored so that the ethylene oxide supplycan be adjusted if the reaction becomes quiescentor goes out of control. The desired degree ofethoxylation is adjusted via the amounts of sub-strate and ethylene oxide.
Vol. 35 Surfactants 473
The product specifications can quickly bechecked by determining the cloud point.
The reaction time depends on the effective-ness of cooling and amount of ethylene oxide tobe added; the production of an ethoxylate havinga medium degree of ethoxylation takes a fewhours in conventional stirred vessels providedwith internal cooling.
The catalyst is neutralized with carbon diox-ide, acetic acid, or citric acid. Phosphoric acidgives a precipitate that can be filtered off, but inmost cases the neutralized catalyst remains dis-solved in the product. Any slight discoloration ofthe products can be removed by bleaching withhydrogen peroxide.
The products are discharged hot into stainlesssteel storage tanks or vats, or are cooled in thereactor and discharged into plastics vessels.
Discoloration and polydiol concentration in-crease with increasing reaction temperature. Theformation of polydiols is due to increasing elimi-nation of water from ethoxylates as the tempera-ture rises:
Further impurities can be formed by reactions ofother compounds introduced with the ethyleneoxide (e.g., formaldehyde and acetaldehyde).
Although continuous ethoxylation processesare also described in the literature [105], indus-trial ethoxylations, like other alkoxylations, aremainly carried out as batch operations.
The Pressindustria alkoxylation process foraddition of ethylene oxide and/or propylene ox-ideoperatesaccording toadifferentprinciple thanthe stirred vessel method (Fig. 21). The liquidreaction mixture, circulated at high speed, isdispersed by a special device in a gas phaseconsisting of ethylene oxide (possibly also pro-pylene oxide), ensuring rapid absorptionof ethylene oxide [106], [107]. Catalyst and sub-strate are mixed in a heated receiving vessel andfreed from water by heating under vacuum. Thismixture is passed to the reactor and circulated bymeans of a pump through the special mixing unit(c) and the reactor. The mixture is heated toreaction temperature in the heat exchanger (e);residual moisture is removed by applying a vacu-um or passing nitrogen through the mixture. Theadditionofethyleneoxide thenstarts, theethyleneoxidebeingflashed into thegas phaseof themixer(c).Here the liquid reactionmixture introduced infinely dispersed form adsorbs the ethylene oxide.The reaction proceeds in the liquid phase in themixer and reactor, into which the liquid phaseflows from the mixer. The heat of reaction isdissipated by the cooler (f). When the desireddegree of ethoxylation is reached the product ispassed to theneutralizationvessel. The reaction iscarried out at 180 – 190 �C, and a maximumpressure of 5 bar.
The process has a high reaction rate anduniform product quality. The safety risks in-volved with gaseous ethylene oxide are mini-mized by keeping the gas phase as small aspossible. Since this part of the reactor containsno moving parts, ignition of the gas phase byshafts, for example, overheating, is ruled out.This process can also be operated continuously.
Figure 21. Pressindustria alkoxylation apparatus [106]a) Receiving vessel; b) Reactor; c) Mixer for gas and liquid; d) Circulating pump; e) Heater; f ) Cooler; g) Neutralization vessel
474 Surfactants Vol. 35
High reaction rates and low formation ofbyproducts can also be achieved in a jet nozzlereactor or jet suction reaction mixer [108].
Production of Ethoxylates of Fatty Acids,Fatty Acid Amides, and Fatty Amides. Allindustrial products based on primary alcohols oralkylphenols can be ethoxylated by the aboveprocesses.
In the ethoxylation of fatty acids the transes-terification of the initially formed oligoglycolesters that is unavoidable with alkaline catalysisleads to the formation of diesters and free oli-goglycols, which in the further course of theethoxylation lead to undesirably high polydiolcontents in the end product:
As carboxylic esters, fatty acid ethoxylates aresensitive to hydrolysis.
Ethoxylates of fatty acid amides are obtainednot by ethoxylation of amides but by alkaline-catalyzed ethoxylation of fatty acid monoetha-nolamides or diethanolamides, which are ob-tained from the reaction of fatty acids or theirmethyl esters with ethanolamine or diethanola-mine, which themselves are valuable products:
RCO��NH��CH2CH2OH
RCO��NðCH2CH2OHÞ2In the ethoxylation of primary amines, in thepresence of basic catalysts, only one hydrogenatom bonded to nitrogen reacts, and the initiallyformed N-alkylethanolamine reacts further withethylene oxide at the hydroxyl group, with theretention of the secondary amino group. Aminesdoubly ethoxylated on the nitrogen atom areobtained if the amines are first converted to theN-alkyldiethanolamine under proton catalysis,for which purpose small amounts of water aregenerally sufficient, and this diethanolamine isthen ethoxylated further with alkaline catalysis.
Properties. Ethoxylates are describedaccording to the state of aggregation (liquid,pasty, waxy, solid) and color (iodine no. < 2),and the density (ca. 1 g/cm3), refractive index,and viscosity are specified as physical data.Characteristic data also include the cloud point
(according to DIN 53 917) and the hydroxylvalue. The content of active substance is gener-ally given as 100%, despite the fact that ethox-ylates contain amounts of polydiols that increasewith increasing degree of ethoxylation. An in-dustrial lauryl alcohol ethoxylate containing 5mol of EO/mol contains, e.g., ca. 0.5%, anethoxylate with 15 mol EO/mol contains1.0%, and one with 30 mol EO/mol contains3.0% of polydiol.
Storage and Dispatch. Ethoxylates arestored and transported in stainless steel or plasticvessels. Tank wagons and storage tanks for high-er ethoxylated products that are solid at ambienttemperature must be thermally insulated andheatable.
Whenethoxylates aredissolvedordilutedwithwater the possibility of gel formation in specificconcentration ranges must be borne in mind.
CAS Numbers. of some important ethoxy-lates are listed below. Irrespective of the degreeof ethoxylation, ethoxylates with identicalhydrophobic groups have the same CASnumbers.
Hydrophobic starting material CAS number
100% linear C10-prim. alcohol [26183-52-8]
100% linear C10/C12-prim. alcohol [66455-15-0]
100% linear C12-prim. alcohol [9002-92-0]
100% linear C12/C14-prim. alcohol [68439-50-9]
100% linear C12/C14/C16/C18-alcohol [68213-23-0]
100% linear C16/C18-prim. alcohol [68439-49-6]
100% linear oleyl alcohol [9004-98-2]
[68920-66-1]
80% linear C9– C11 prim. alcohol, monobranched [68439-45-2]
80% linear C12 – C13 prim. alcohol, monobranched [66455-14-9]
80% linear C12 – C15 prim. alcohol, monobranched [68131-39-5]
80% linear C14 – C15 prim. alcohol, monobranched [68002-97-1]
60% linear C13 – C15 prim. alcohol, monobranched [64425-86-1]
50% linear C12 – C13 prim. alcohol, monobranched [66455-14-9]
50% linear C12 – C15 prim. alcohol, monobranched [68131-39-5]
50% linear C13 – C15 prim. alcohol, monobranched [68002-97-1]
Branched C10-alcohol [61827-42-7]
Branched C13-alcohol [69011-36-5]
p-Isooctylphenol [9004-87-9]
p-Isononylphenol [37205-87-1]
[68412-54-4]
Lauric acid [9004-81-3]
Coco fatty acid [61791-29-5]
Oleic acid [112-80-1]
Stearic acid [57-11-4]
Ricinoleic acid [61791-12-6]
Tallow fatty amine [61791-26-2]
Vol. 35 Surfactants 475
Oleylamine [112-90-3]
Stearylamine [124-30-1]
Alkanolamides
Coco fatty acid monoethanolamide [68140-00-1]
Coco fatty acid diethanolamide [61790-63-4]
Tallow fatty acid monoethanolamide [68153-63-9]
Producers. Fatty alcohol ethoxylates areproduced in Europe, e. g., by Akzo, Albright &Wilson, Amerchol Europe, Auschem, BASF,Berol Nobel, Chem-Y, Croda, Harcros, Henkel,Hoechst, H€uls, ICI, Kolb, Pulcra, Rewo, Rhone-Poulenc, Seppic, Shell, Stepan, Witco, andZschimmer & Schwarz; in North America, e. g.,by Amerchol, BASF, Chemax, Croda, Exxon,Hart, Henkel/Emery, Hoechst Celanese, ICIAmericas, Lonza, Olin, Procter & Gamble,Rhone-Poulenc Surfactant, Sandoz, Shell, Sher-ex, Stepan, Texaco, Union Carbide, Vista, andWitco; in Japan by Dai-Ichi, Nikko Chemicals,Nippon Nyukazai, Nippon Oil & Fat, SanyoChemical, and Takemoto Oil & Fat; in Australiaby ICI; and in Taiwan by Sino-Japan Chemicaland Taiwan Surfactant.
Alkylphenol ethoxylates are produced inEurope by Akzo, Auschem, BASF, Berol Nobel,Chem-Y,Dow,Harcros,Hefti,Hoechst,H€uls, ICI,Kolb,Pulcra,Stepan,Witco,andUnionCarbide; inNorth America by Chemax, Clough Chemical,Exxon, Hart, Henkel/Emery, Hoechst Celanese,Norman/Fox, Rhone-Poulenc Surfactant, Sandoz,Stepan, Texaco, Union Carbide, and Witco; inJapan by Dai-Ichi, Kao, Lion, Marubeni Oil,NipponNyukazai,NipponOil&Fat,SanyoChem-ical, and Toho Chemical Industry; in Australiaby ICI; and in Taiwan by Sino-Japan Chemicaland Taiwan Surfactant.
Fatty acid ethoxylates are produced in Europeby BASF, Croda, Fina Oleochemicals, Harcros,Hefti, Henkel, H€uls, ICI, Stepan, Witco, andZschimmer & Schwarz; in North America byChemax, Emkay, Hart, Henkel, Hodag Chemi-cal, ICI Americas, Lipo Chemicals, Lonza, PPG/Mazer, Rhone-Poulenc Surfactant, and Reilly-Whiteman; in Japan by Dai-Ichi, Kao, NikkoChemicals, Nippon Nyukazai, Nippon Oil & Fat,and TohoChemical Industry; in Australia by ICI;in Taiwan by Sino-Japan Chemical and TaiwanSurfactant; and in Israel by Zohar.
Special ethoxylated fatty esters and oils arealso produced by companies such as Auschem,Croda, ICI, Th. Goldschmidt and Westbrook
Lanolin in Europe, and Heterene Chemical, ICIAmericas, Lonza, Rhone-Poulenc Surfactants,and Sherex in the United States.
Ethoxylated amines and/or fatty acid amidesare produced in Europe by Albright & Wilson,Akzo, BASF, Berol Nobel, Harcros, Henkel,Hoechst, H€uls, Rewo, Rhone-Poulenc, andZschimmer & Schwarz; in the United States byAkzo, Chemax, Ethox Chemicals, Hart, Henkel,Heterene Chemical, Karlshanns, PPG/Mazer,Rhone-Poulenc Surfactant, Sandoz, Sherex,Union Carbide andWitco; in Japan by Dai-Ichi,Kao, Nippon Nyukazai, Nippon Oil & Fats,and Sanyo; in Australia by ICI; and inTaiwan by Taiwan Surfactant and Sino-JapanChemical.
7.3. Terminally Blocked Ethoxylates[109], [110]
By substituting the hydrogen atom of the termi-nal hydroxyl group of an ethoxylate by hydro-phobic residues such as benzyl, butyl, or methylgroups, terminally blocked ethoxylates areobtained that are chemically more resistant,especially in alkaline media, than the corre-sponding ethoxylates with a free hydroxylgroup. Since blocked ethoxylates also foam lessin aqueous solution than their starting ethoxy-lates, they have a certain value in (alkaline)cleaning processes involving strong mechanicalaction.
The addition of higher alkylene oxides, inparticular propylene oxide, may also be regardedas a terminal blocking:
However, since such products also contain freehydroxyl groups they are no more stable thantheir starting ethoxylates against alkalis, but inaqueous solution foam less than the pure ethox-ylates. Due to the blocking of the terminalhydroxyl groups by hydrophobic residues suchethoxylates to some extent lose their surfactantcharacter, and become less water-soluble and
476 Surfactants Vol. 35
less amphiphilic. This is already noticeable inpropoxylated products, the propoxy group itselfbeing hydrophobic. This property can be utilizedto enlarge a hydrophobic residue before ethox-ylation by addition of propylene oxide. Finally,alkoxylation can be performed with mixtures ofethylene oxide and propylene oxide and variouseffects can be achieved by appropriately choos-ing the ratio of the two epoxides, by alternatingthe order of addition, etc.
7.4. Fatty Acid Esters of PolyhydroxyCompounds
Polyhydroxy compounds such as glycerol, digly-cerol, polyglycerol, erythritol, pentaerythritol,xylitol, sorbitol, mannitol, sucrose, and otherglucosides that are partially esterified with fattyacids have surface-active properties. The hydro-philicity (HLB value) of such esters can beincreased and matched to the desired applicationby ethoxylating the unesterified hydroxyl groups.Esters of glycerol, sorbitol, and sucrose are dis-cussed hereinafter by way of example, as are theso- called alkyl polyglucosides, which have re-cently attracted increased attention.
Fatty Acid Esters of Glycerol [111]. Dis-proportionating transesterification of fats andoils, the fatty acid triesters of glycerol (triglycer-ides), with glycerol in the presence of acid, butpreferably alkaline catalysts, yields, dependingon the glycerol excess, equilibrium reaction mix-tures containing 35 – 60% of monoglycerides,35 –50% of diglycerides, 1 – 20% of triglycer-ides, 1 – 10%of fatty acids and their alkalimetalsalts, and 1 – 10% of glycerol after ca. 1 h at
200 –250 �C. Such so- called mono-diglycer-ides, which can also be obtained by direct reac-tion of free fatty acids with glycerol under acidcatalysis, are marketed as mixtures. They areinsoluble in water, and when they contain alkalimetal salts of fatty acids, they are self-emulsify-ing. The fatty acids that are used are predomi-nantly from the tallow fat range (palmitic andstearic acids). The above-mentionedmixtures aregenerally marketed under the name ‘‘monogly-cerides’’, e.g., ‘‘glycerol monostearates’’ (GMS).
High- concentration monoglycerides are ob-tained from the mixtures by molecular distilla-tion. They consist mainly of a-glycerides, andcontain less than 10% of b-glycerides:
Since they crystallize in fine platelets, they im-part a pearl gloss to pasty liquid detergents andcleansing agents.
Glycerides are widely used as emulsifiers infoods and cosmetics, as they are physiologicallyharmless. Characteristic data of some commer-cial products are listed in Table 14.
The enzymatically catalyzed synthesis ofglycerides has not yet become industriallyimportant [112].
Anionic surfactants having properties similarto those of the ether sulfates are obtained bysulfatingmonoglycerides and diglycerides [113].
Producers of glycerides include Akzo,Croda, Fina Oleochemicals, Gatte-Fosse, Hefti,Henkel, H€uls, ICI, Quest, Rewo, and
Table 14. Typical data of some industrial glycerides
Glycerol
monostearate
Glycerol mono/
distearate
Glycerol mono/
distearate, self-emulsifying
Glycerol
monolaurate
CAS no. [61789-09-1] [68308-54-3] [68308-54-3] [27215-38-9]
[593-29-3]
Color, consistency white powder white powder or flakes whitish flakes white, crystalline
Th. Goldschmidt in Europe; GoldschmidtChemical, Henkel, Hodag, Hoechst-Celanese,Humko, Inolex, Karlshamns, Lonza, PPG/Ma-zer, and Witco in North America; Kao, NikkoChemicals, Nippon Oil & Fats, and Toho inJapan; Taiwan Surfactant and Sino-Japan Chem-ical in Taiwan; and Zohar in Israel.
Fatty Acid Esters of Sorbitol. [114]. Sor-bitol can be converted to monoesters or polye-sters by reaction with fatty acid chlorides undermild conditions in the presence of pyridine. Theesterification is performed industrially with thefree fatty acids or their methyl esters at elevatedtemperature; sorbitol readily undergoes dehydra-tion to sorbitan, which can lose a further mole-cule of water to give isosorbitol, especially in thepresence of acid catalysts:
Esterification is generally carried out at 200 –250 �C after the addition of small amounts ofalkali, e.g., 0.1% sodium hydroxide; dependingon the molar ratio of the starting materials, fattyacid monoesters, diesters, or triesters of sorbitanare obtained. First marketed by Atlas Powder,these are known as ‘‘Span’’ types. They arewater-insoluble products having lowHLB valuesthat are liquid at ambient temperature or meltbelow 100 �C. Ethoxylation of the Span typesyields products having better water solubility andhigher HLB values, namely the ‘‘Tweens’’,which are waxy to solid products.
Spans and Tweens are dispatched in plasticcontainers, plastic- coated cardboard containers,or bags.
Producers of sorbitan esters include Akzo,Auschem, Croda, Hefti, Henkel, and ICI in Eur-ope; Chemax, Croda, Henkel/Emery, Hodag, ICIAmericas, Karlshamns, Lipo Chemical, Lonza,Norman/Fox, PPG/Mazer, Stepan, and Witco inNorth America; Kao, Nikko Chemicals, NipponNyukazai, Nippon Oil & Fats, Sanyo, and Toho
in Japan; and Sino-Japan Chemical and TaiwanSurfactant in Taiwan.
Fatty Acid Esters of Sucrose. [115], [116].As a plentifully available natural raw material,sucrose is a suitable parent substance for thehydrophilic group of surfactants. Surfactantsbased on sucrose, especially their fatty acid esters,are nontoxic, do not irritate the skin, and are fullybiodegradable. However, they have not hithertogained wide acceptance industrially due to acertain instability of the sucrose glycoside bond,and the insolubility of sucrose in conventionalorganic solvents, which makes reactions involv-ing sucrose more difficult. Fatty acid esters ofsucrose are prepared by alkaline- catalyzed reac-tion with 1 – 2 mol of fatty acid methyl ester permole of sucrose, the methanol being distilled offunder vacuum.The temperature shouldnot exceed100 �C. Dimethylformamide or dimethyl sulfox-ide, both highly toxic substances, are used assolvents and must be removed completely fromthe reaction product, which is laborious sincesucrose esters are temperature sensitive and mustnot be heated too strongly.
Producers of sucrose esters include Croda,Gattefosse, and ICI in Europe; Amerchol, Croda,and Union Carbide in the United States; and Dai-Ichi in Japan.
Alkyl Polyglucosides. [117–120]. Acetal-bonded glucosides mixed with oligoglucosidesare obtained as a randomly distributed mixture ofhomologues by acetalation of glucose with fattyalcohols; the average number of glucose units permolecule being between 1 and 3, depending onthe reaction parameters. Strictly speaking theproducts should be called alkyl oligoglucosides,though the incorrect term alkyl polyglucosides(APG) has persisted.
Alkyl polyglucosides are waxy, soft to glassy-solid, and colored yellow due to impurities.
478 Surfactants Vol. 35
They are generally extremely soluble inwater andare handled as 50% aqueous solutions. The HLBvalues lie above 10, and can be varied in the range11 – 15bychanging the lengthof thehydrophobicgroup and the degree of glucosidation.The surfacetension of aqueous solutions is ca. 30 mN/mand the interfacial tension toward hydrocarbons1 mN/m. Alkyl polyglucosides are moderatefoaming agents and outstanding emulsifiers. Theyare suitable for washing textiles and hard surfaces,particularly in combinationwith other anionic andnonionic surfactants, with which they give syner-gistic effects.Alkyl polyglucosides decompose onheating; their aqueous solutions are stable in thepH range 3 –13. They are nontoxic, only slightlyirritating to the skin, and biodegrade rapidly.
Alkyl polyglucosides can be prepared bydirect reaction of anhydrous glucose with fattyalcohols. In another method, in which also aque-ous glucose solution can be used, the latter is firstreacted with butanol to form butyl glucoside,which is then reacted in a second stage with fattyalcohol to form the fatty alkyl oligoglucoside.Butanol and excess fatty alcohol are distilled off.Strong acids such as sulfuric acid are used ascatalysts in both methods.
The alkyl polyglucoside thus obtained as amelt is dissolved in water and bleached withhydrogen peroxide.
CAS Numbers C8/C10-Fatty alkyl polyglu-coside [128664-36-8], C12/C14-fatty alkyl poly-glucoside [136797-44-9].
Producers include Henkel in the UnitedStates, Kao in Japan, Seppic in France, andBASF, Henkel, and H€uls in Germany.
7.5. Amine Oxides [121], [122]
Reaction of tertiary amines such as fatty alkyl-dimethylamine or fatty amines doubly ethoxy-lated at the nitrogen atom with hydrogen perox-ide yields amine oxides:
Surfactant amine oxides such as lauryldimethy-lamine oxide are insensitive to water hardness.
They disperse lime soaps, foam satisfactorily andare mild to the skin. They are therefore widelyused as constituents of dishwasher detergents,shampoos, and soaps. Amine oxides in neutralaqueous solution should be regarded as nonionicsurfactants. They are protonated in acid solutionand thus represent the transition to cationicsurfactants.
Producers of amine oxides include Akzo,Albright & Wilson, Ceca, Croda, Pulcra, Rewo,and Th. Goldschmidt in Europe; Clough, Ethyl,Goldschmidt Chemical, Henkel, McIntyre, PPG/Mazer, Rhone-Poulenc Surfactant, Sherex, SherChemicals, Stepan, and Witco in the UnitedStates; Nippon Oil & Fats in Japan; and TaiwanSurfactant in Taiwan.
8. Cationic Surfactants [123–125]
Surfactants whose hydrophilic group is a cationare termed cationic surfactants. The cationicstructure may already be present in the surfactantmolecule, as in the case of quaternary ammoniumor phosphonium salts, or may be formed only inaqueous solution, as, for example, in ethoxylatedfatty amines. In acidic solution, nonionic surfac-tants can adopt a cationic character due to pro-tonation at a heteroatom; examples are fattyamine oxides and even ethoxylates, whose ethergroups can form oxonium structures. However,strictly speaking only those surfactants that donot require protonation to become cationic areregarded as cationic surfactants.
The most important cationic surfactants arethe quaternary nitrogen compounds: tetraalky-lammonium salts, N,N-dialkylimidazoliniumcompounds, and N-alkylpyridinium salts. Thepositive electrical charge of the hydrophilic moi-ety confers on cationic surfactants properties thatopen up areas of application for which anionicand nonionic surfactants are unsuitable or lesssuitable. Important uses of these cationic surfac-tants are as microbicides, herbicides, corrosioninhibitors, oxidation inhibitors, adhesives, finish-ing and sizing agents, water repellents, fabric
Vol. 35 Surfactants 479
softeners, flotation aids, dispersants, levellingagents, etc.
Other cationic surfactants, e.g., the quaternaryphosphonium salts or tertiary sulfonium salts areless important industrially, although they arevery useful as phase-transfer catalysts in synthe-sis [126].
8.1. Quaternary AmmoniumCompounds
Quaternary ammonium compounds are obtainedby alkylating tertiary amines. At least one of thealkyl residues must be a relatively long alkylgroup. In general, tertiary amines having one ortwo long alkyl residues are used as startingcompounds (! Amines, Aliphatic), to which areadded shorter alkyl groups in the form of methylchloride, ethyl chloride or bromide, or dimethylsulfate. The corresponding ammonium chlor-ides, ammonium bromides, or ammonium meth-yl sulfates are then obtained. The additionrequires a polar solvent (e.g., alcohol); in thecase of alkyl chlorides, which aremainly used forthe alkylation, temperatures of 50 – 100 �C andreaction times of several hours are required. Theprocess can be simplified by carrying out thereaction under pressure in excess alkylatingagent (e.g., methyl chloride as solvent). Theproduct is isolated by releasing the pressure andevaporating unreacted alkyl chloride [127].
Benzyl chloride reacts very readily as alkylat-ing agent; the disinfectant Zephirol is obtained byaddition to dimethyldodecylamine:
Another method of quaternizing tertiary amines,as well as ethoxylated fatty amines, is the reac-tion with ethylene oxide and water in the absenceof a catalyst:
For special applications quaternary compoundscan also be obtained from fatty acid derivativessuch as the amides of diamines or esters oftriethanolamine:
RCONHCH2CH2NðCH3Þ2RCOOCH2CH2NðCH2CH2OHÞ2
A further group of amines that is suitable forquaternization is obtained by hydrogenatingsubstituted propionitriles, which are obtained byaddition of fatty alcohols or fatty amines toacrylonitrile:
ROCH2CH2CH2NH2 and RNHCH2CH2CH2NH2
Distearyldimethylammonium chloride(DSDMAC) or, more correctly, since the longalkyl residues are tallow fat alkyl residues, di-tallow alkyl dimethyl ammonium chloride(DTDMAC), was for a long time the most im-portant component in fabric softeners. Since itproved to be relatively toxic to water-borneorganisms [128], it has been replaced by lesstoxic ammonium compounds, which contain es-ter group as cleavage sites which lead to rapidbreakdown of the compounds in water. These arethe so- called esterquats obtained from fatty acid,triethanolamine orN-methyldiethanolamine, anddimethyl sulfate [129], [130]:
and the diesterquats obtainable from epoxypro-pyltrimethylammonium chloride and fatty acids[131]:
Typical data of some simple quaternary ammo-nium chlorides that are stored and transported inthe form of aqueous or aqueous-alcoholic solu-tion in stainless steel or plastic drums are listed inTable 15.
Producers of quaternary ammonium com-pounds includeAkzo,BASF,Bayer, Berol, Ceca,Croda, Fina Oleochemicals, Hoechst, H€uls, ICI,Pulcra, Rewo, Rhone-Poulenc, Stepan, and Wit-co in Europe; Akzo, Emkay, Henkel, Hoechst,Humko, ICI, Karlshamns, Rhone-Poulenc,
480 Surfactants Vol. 35
Sherex, and Witco in North America; Kao,Marubishi Oil, Sanyo Chemical, Takemoto Oil,Tokai Seiyu, and Yoshimura Oil in Japan; andTaiwan Surfactant in Taiwan.
8.2. Imidazoline Derivatives
Condensation of fatty acids with ethylenedia-mine or substituted ethylenediamine yieldssubstituted imidazolines, e.g.:
Such compounds are strongly cationic in acidicsolution, but undergo slow hydrolysis [132]:
True cationic surfactants are obtained by alkylat-ing imidazoline derivatives, mainly with chlor-omethane:
Suchsurfactants haveproperties similar to thoseofacyclic quaternary ammonium salts; they are der-matologically extremely compatible, are antisep-tic, and are used aswetting agents, dispersants andcleansing agents, including bodycare products.
An imidazoline derivative obtained by react-ing 1 mol of N-aminoethylethylenediamine with2 mol of fatty acid, and quaternized with dimeth-yl sulfate, is an important fabric softener [86088-85-9] [133]:
9. Amphoteric Surfactants [134–136]
Amphoteric surfactants can be classified as am-pholytes and betaines. Ampholytes are com-pounds having at least one active proton, thebest-known example being aminocarboxylicacids, which at the isoelectric point exist as innersalts, at low pH values as cationic species, and athigher pH values as anionic species:
Table 15. Typical data of some industrial quaternary ammonium compounds
Stearyltrimethyl-
ammonium
chloride
Stearylbenzyldimethyl-
ammonium
chloride
Ditallowalkyl-
dimethylammonium
chloride
Ditallowacyloxyethyl
hydroxyethylmethyl
ammonium methyl sulfate
(‘‘Ester quat’’)
CAS no. [112-03-8] [122-19-0] [61789-80-8] [93334-15-7]
Active substance content, % 50 – 52 50 – 52 74 – 76 ca. 90
Content of free amine and
hydrochloride, % 1 1 2
Isopropanol content, % 30 0 16 10
Density, g/mL 0.89 0.95 0.90 0.96
Color index (Gardner) < 7 < 7 4 3
pH (1% aqueous solution) 5 – 7 6 – 8
Ash, % < 0.8 1.0 < 0.35 < 0.1
Flash point, �C 15 ca. 20 26 – 32
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Betaines have no mobile protons and are trueamphoteric ions which assume a cationic naturein strongly acid media:
Ampholytes are of minor industrial importance,largely on account of the marked dependence oftheir properties on pH. Ampholytes are preparedby reacting fatty amines with chloroacetic acidand sodium hydroxide, or by alkaline- catalyzedaddition of acrylic acid to fatty amines:
Betaines are prepared by reacting a tertiary aminewith chloroacetic acid and sodium hydroxide:
or
The sulfobetaines are now of only minor impor-tance. Of the various ways of preparing sulfobe-taines, the elegant synthesis from amines, alkylchloride, and sodium hydrogensulfite is men-tioned here [137]:
Nowadays only true betaines are of economicimportance, especially acid amide betaines andthe betaines derived from imidazolines:
The latter, which are prepared in aqueous solu-tion, probably exist in the open- chain form [138].
Betaines are insensitive to the water hardnessand pH value of industrial water, are only slightlytoxic, are compatible with the skin and mucousmembranes, and have antimicrobial properties.They have good washing and foaming perfor-mance and are highly compatible with otherclasses of surfactants, and are therefore ideallysuited for use in bodycare products.
Lecithin or phosphatidylcholine, a naturallyoccurring phospholipid also belongs to the classof betaines [139], [140]:
Producers of ampholytes and betaines in-cludeAkzo,Albright&Wilson,Auschem,Croda,Harcros, Henkel, H€uls, Manro, Pulcra, Rewo,Rhone-Poulenc, Seppic, Stepan, Th. Gold-schmidt, Union Carbide, and Zschimmer &Schwarz in Europe; Clough, Goldschmidt Che-micals, Henkel, Inolex, McIntyre, Mona, PPG/Mazer, Rhone-Poulenc Surfactant, Sher Chemi-cals, Sherex, Stepan, and Witco in the UnitedStates; Lion,NipponOil&Fats, Sanyo, andTohoin Japan;TaiwanSurfactant inTaiwan; andZoharin Israel.
10. Surfactants with Heteroatomsin the Hydrophobic Group
10.1. Block Copolymers of PropyleneOxide and Ethylene Oxide [141], [142]
The incorporation of propoxy groups in a surfac-tant increases its hydrophobicity. Hydrophobicparent substances of any desired molecular masscan be prepared by addition of propylene oxide toa low molecular mass starter molecule. Propox-ylation is performed analogously to ethoxylation,with alkaline catalysts at 120 –180 �C under aslight excess pressure:
482 Surfactants Vol. 35
Suitable starter molecules are monofunctional orpolyfunctional protic substances, for example,methanol, ethanol, propanol, or higher alcohols;diols such as butanediol; and triols such as gyl-cerol.Propyleneglycol itself isalsoasuitablediol,and ethylenediamine is a suitable tetrafunctionalstarter molecule. Subsequent ethoxylation ofhydrophobic parent substances obtained in thisway yields nonionic surfactants, which can bedescribedasblockcopolymersofpropyleneoxideand ethylene oxide. When using polyfunctionalstarters, branching occurs at the starter molecule.Derivatives of propylene glycol and ethylenedia-mine, first introduced by Wyandotte under thetrade names of Pluronics and Tetronics, are nowthe most important surfactants of this type:
As the structural formulas indicate, a wide rangeof block copolymers can be synthesized. Theproducts are not uniform, and a homologue dis-tribution range exists for each propoxylate chainand for each ethoxylate chain. Mean molecularmasses of 1000 – 5000 are preferred in the caseof hydrophobic propoxylate parent substances.Ethoxylation can also be varied over a widerange; depending on the desired application,HLB value and cloud point, between 10 and80 wt% of ethylene oxide is added.
The products are liquids, pastes, or waxysolids, depending on the molecular mass. Theyare water-soluble above a content of about25 wt% of bound ethylene oxide. They are alsorelatively stable to alkalis and acids.
Their outstanding property is the lack offoaming ability of their aqueous solutions; thepropylene oxide – ethylene oxide addition pro-ducts also suppress the foam of strongly foamingsolutions. The block copolymers are thereforepreferably used as wetting and cleaning agents inprocesses involving high mechanical stress and
in high-speed cleaning machines. They are alsoused as demulsifiers and dispersants. However,their unsatisfactory biodegradability preventstheir widespread use.
Producers include BASF, Berol, Nobel,Dow, Harcros, Hoechst, H€uls, ICI, and Rhone-Poulenc in Europe; BASF, Hart, Henkel, ICIAmericas, Norman/Fox, Olin, and PPG/Mazer inNorthAmerica;Dai-Ichi andNipponOil& Fats inJapan; ICI in Australia; and Sino-Japan Chemicalin Taiwan.
10.2. Silicone-Based Surfactants[143–148]
Silicone-based surfactants are mainly derivedfrom methylsilicones (polydimethylsiloxanes,! Silicones, Section 2.2.! Silicones! Sili-cones). They lower the surface tension of waterto a greater degree than hydrocarbon-based sur-
factants, and in this respect are inferior only tofluorosurfactants. In their molecular structuresilicone-based surfac-tants resemble surfactantscontaining polypropylene glycol as hydrophobicparent substance, since the hydrophobic siliconegroup also has a random molecular mass distri-bution. The following basic structures (linear,comb, and branched) occur, where A denoteshydrophilic groups:
Vol. 35 Surfactants 483
Polydimethylsiloxanes are prepared by selectivehydrolysis of dimethyldichlorosilane:
The terminal chlorine atoms may be substitutedin the presence of bases by polyethylene glycolresidues, resulting in nonionic surfactants of theethoxylate type.
A special feature of silicone chemistry is the‘‘equilibration’’ of the polymers, in whichmolecular mass distribution and distribution ofthe individual siloxane building blocks can bebrought into a reproducible equilibrium byredistribution of the siloxane bonds under acidcatalysis. The linear, comb, and branched sili-cones are formed in this way.
Equilibration enables substituents to be incor-porated in the silicone molecule. The silicon –hydrogen bonds of silanes can add to an aliphaticdouble bond in the presence of hexachloropla-tinic acid:
The group R can carry functional groups. Byaddition of allyl alcohol, for example, hydroxyl-containing dichlorosilanes can be produced,which are converted by selective hydrolysis topoly(methylhydroxypropyl)siloxane, which canbe equilibratedwith a polydimethylsiloxane. Thehydroxyl groups introduced in this way can bereacted further, for example by sulfation to givesulfuric hemi-esters, which yield anionicsurfactants.
The choice of the siloxane building blocks,selective hydrolysis, and equilibration enable awide variety of silicone surfactants to be synthe-sized. The polysiloxane residues of most siliconesurfactants have a polymeric character, in con-trast to the previously discussed hydrocarbon-based surfactants. Polar and nonpolar groupsalternate in the polymer chain, similar to the
polypropylene glycol derivatives or the polypep-tides. In the polysiloxanes discussed here,the alternating groups are oxygen bridges anddimethylsilyl groups. The oxygen bridges caninteract weakly with water, with the result that apolydimethylsiloxane residue forms a layeron the water surface, with oxygen atomsoriented towards the aqueous phase. The sameis true of pure, nonfunctionalized polydimethyl-siloxanes, which have a surfactant characterbecause they have a lower surface tension dueto the weak mutual cohesive forces of the silox-ane residues.
The layer structure of the polysiloxane bound-ary films, on the other hand, renders these films acertain strength. Polysiloxanes modified withpolyethers are therefore important as integratedconstituents of polyurethane foams, in whichthey act as foam stabilizers. The stability of filmsof silicone surfactants in aqueous foams makesthem useful as additives to aqueous fire-extin-guishing agents. Although silicone surfactantsform stable foams, they are able to destroy foamsof hydrocarbon-based surfactants, which theydisplace from the surface because of their highersurface activity.
The polysiloxane skeleton is biologically per-sistent, and is broken down only slowly in theenvironment by hydrolysis. However, polydi-methylsiloxanes are physiologically harmless.Silicone-based surfactants are used in a widerange of special applications, for example, inskincare and haircare preparations [145]; in theproduction of fibers and textiles [146]; as wettingagents, dispersants, and flow- control agents inthe production of paints and coatings [147]; asmold release agents in plastics processing; and asdefoaming agents, emulsifiers, demulsifiers, andwater repellents.
Producers. Silicone-based surfactants areproduced in Europe by Hoechst and Th. Gold-schmidt; in the United States by Dow Corning,Goldschmidt Chemical, Rhone-Poulenc Surfac-tant, Troy Chemical, and Union Carbide; and inJapan by Tosho and Yoshimura Oil Chemical.
10.3. Fluorosurfactants [149–151]
The fluorosurfactants used nowadays can beregarded as analogues of surfactants containing
484 Surfactants Vol. 35
aliphatic hydrocarbon groups, in which thehydrogen atoms have been wholly or partlyreplaced by fluorine atoms. The extreme hydro-phobicity of fluorinated hydrocarbons means,however, that amphiphilic fluorine compoundswith substantially shorter hydrophobic residuesthan their aliphatic analogs are already surface-active. Whereas a marked interfacial activity insalts of aliphatic carboxylic acids is observedstarting with 8 – 9 carbon atoms in the hydro-phobic group, perfluorobutyric acid and its saltsare already considerably surface-active. The verylow cohesive forces between the fluorinated alkylgroups result in extremely low surface tensions offluorosurfactants. Surface tensions � 20 mN/mcan be achieved only with fluorosurfactants.Fluorosurfactants have very low CMCs andare hence already active at very low concentra-tions, which to some extent compensates fortheir high cost. Fluorosurfactants have highchemical and thermal stability, and accordinglyare used for special applications (high-tempera-ture processes, electroplating techniques). Theydiffuse rapidly in aqueous solution, and aretherefore also suitable for high-speed industrialprocesses.
Perfluoroalkane carboxylic or perfluoroalka-nesulfonic acids are prepared by electrochemicalfluorination of the corresponding carboxylicacids or sulfonic acids. Telomerization of per-fluoroethylene with, for example, pentafluoroio-doethane as starter molecule yields homologousperfluoro-1-iodoalkanes, which can be convertedto carboxylic acids, sulfonic acids, phosphonicacids, or amines. Perfluoroalkenes, possiblybranched, are obtained by anionically catalyzedoligomerization of tetrafluoroethylene; a fluorineatom located at the double bond can be electro-philically replaced, in the presence of a base, byalcohol, phenol, ethylene glycol, or olgioethy-lene glycol to give nonionic surfactants of theethoxylate type.
Fluorosurfactants are used as wetting, emul-sifying, and flow- control agents; as absorptionagents for imparting water-repellent and soil-repellent properties to textiles, leather, and pa-per; in coating photocopying paper; as moldrelease agents for plastics; and as additives infire-extinguishing agents.
Producers of fluorosurfactants includeHoechst in Europe and 3 M and Du Pont in theUnited States.
11. Analysis [152–157]
Surface-active substances are identified qualita-tively by the fact that their solutions foam andwet fibers or solid surfacesmore quickly than purewater, and also by the reduction in surface tensionof water, which is recognized by the rapid sinkingof a small thinmetal or PVCplate carefully laid onthe surface of thewater,whichwould otherwise beheld on the surface in the absence of surfactants.These test methods are not unambiguous, howev-er, not all surface-active substances should beregarded as surfactants.
11.1. Identification of Surfactants
A number of group-specific detection reactionsare known for identifying different types ofsurfactants, which, however, have become lessimportant with improvements in instrumentalanalysis. The property of anionic and cationicsurfactants of forming sparingly soluble salts inwater can be used as a quick aid; an anionicsurfactant, for example a sulfonate, can be pre-cipitatedwith a cationic surfactant, for example aquaternary ammonium salt, and vice versa.Anionic surfactants produce a precipitate onaddition of aluminum acetate, and soaps areconverted to water-insoluble fatty acids on acid-ification. Nonionic surfactants of the ethoxylatetype and cationic surfactants form precipitatessparingly soluble in water with phosphotungsticand phosphomolybdic acids, potassium hexacya-noferrate, and Dragendorff’s reagent (bariumbismuth iodide). Amphoteric surfactants alsoform precipitates with phosphotungstic andphosphomolybdic acids; generally, they are spar-ingly soluble at their isoelectric point.
The modern method of choice for identifyingsurfactants is IR spectroscopy, for which manyreference spectra exist. This technique has beenrefined to such an extent that amounts of less than1 mg can be measured. UV spectroscopy allowsidentification of surfactants containing aromaticgroups.
Mass spectroscopy, which is generallyemployed in combination with gas chromatogra-phy and high-pressure liquid chromatography, hasalso become a valuable aid in surfactant analysis,especially after the introduction of soft ionizationmethods such as direct chemical ionization (DCI),
Vol. 35 Surfactants 485
fielddesorption(FD),and fastatombombardment,which can also be applied to nonvaporizable sub-stances. Although mass spectroscopy is complexand thereforeunsuitable for routine investigations,NMR spectroscopy, especially 1H NMR, hasbecome an extremely useful tool in surfactantanalysis, for example, in identifying hydrophobichydrocarbon parent substances and indeterminingthe number of added ethylene oxide or propyleneoxide molecules in alkoxylates.
Gas chromatography is now an importantmethod in surfactant analysis. Substances of lowvolatility are often modified chemically for GCanalysis. Ethoxylates, for example, are deter-mined as trimethylsilyl ethers; homologues con-taining up to seven ethylene oxide units per molecan be determined in this way. Fatty acids andsulfonic acids are subjected to GC in the form oftheirmethyl esters.Gas chromatography andmassspectroscopy are suitable for identifying hydro-phobic residues of surfactants after specific pyro-lytic decomposition. With concentrated phospho-ric acid at 215 �C the structurally unchangedalkylbenzene is obtained from the correspondingalkylbenzenesulfonate, while alkanes shortenedby the hydrophilic group are obtained from alkylsulfates, fatty acid esters, and fatty acid amides.Alkanesulfonates and a-sulfocarboxylic estersproduce nonuniform fragments in this type ofpyrolysis. In the pyrolysis of alkylbenzenesulfo-nate or alkanesulfonate in aKOH – NaOHmelt at290 �C, the parent hydrocarbons are obtained asalkylphenols or alkenes. Quaternary ammoniumcompounds are subjected toHofmanndegradationwith potassiumhydroxide or sodiumhydroxide, inwhich alkene and tertiary amines are formed.Decomposition andGC separation can be coupledon-line (pyrolysis – gas chromatography).Decomposition is often performed as Curie pointpyrolysis to ensure reproducible fragmentationunder controlled conditions.
Iodoalkane, ethylene, and iodine are formedwhen ethoxylates are reacted with boiling 58%hydroiodic acid:
The parent substances and their simple deriva-tives obtained from the reaction can also beidentified by gas chromatography or massspectrometry.
The decomposition of ethoxylateswith hydro-iodic acid is also utilized in the wet chemicaldetermination of the degree of ethoxylation n ofan ethoxylate; the iodine released is titrated withthiosulfate solution.
A modification of this Zeisel cleavage con-verts the ethoxy groups to ethyliodide and pro-poxy groups to propyliodide which again can bedetermined by GC.
11.2. Isolation and Separation
Surfactants often exist as a mixture of differenttypes of surfactant or mixed with nonsurfactantcomponents. A number of methods are availablefor separating and specifically isolating individ-ual surfactants and for removing byproducts.
Extraction. Nonpolar constituents (neutraloil) can be extracted with petroleum ether fromneutral or alkaline-adjusted aqueous or aqueous-alcoholic solutions of anionic surfactants. If thesolution is acidified, fatty acids also pass into thepetroleum ether phase.
Surfactant sulfonic acids are extracted fromaqueous hydrochloric acid sulutions with diethylether. Short- chain alkylbenzenesulfonic acids,disulfonic acids, etc., remain in the aqueous phase,from which they can be extracted with butanol.
Polyglycols (polydiols) can be extracted fromsolutions of ethoxylates in ethyl acetate by usingaqueous sodium chloride solution. Ethoxylatesare extracted from aqueous solution by chloro-form after saturating the solution with sodiumchloride.
Blowing Out. Blowing out is a nonspecificmethod for isolating traces of all surface-activesubstances from aqueous solutions. A stream ofnitrogen loadedwith ethyl acetate vapor is passedthrough a frit into the solution. Surfactants areadsorbed on the surfaces of the nitrogen bubblesand transported upwardly, where they areabsorbed in an ethyl acetate phase covering theaqueous solution. This method is used in traceanalysis of surfactants, especially in effluents andwastewaters.
Adsorption. Surfactants can be separated byadsorption on aluminum oxide, activated char-coal, polymer resins, or silica gel. Using solvents
486 Surfactants Vol. 35
of increasing polarity it is possible, by columnchromatography, to separate nonionic surfactantsfrom anionic surfactants, and nonionic surfactantsof different HLB value from one another.Reverse-phase cartridges have been used to re-cover traces of surfactant from aqueous solution.
Ion exchangers. are being used increasing-ly in surfactant separation and analysis, and aquantitative analysis procedure has been pro-posed [152]. Three ion-exchange columns areconnected in series: a strongly acidic cationexchanger in the Hþ form, in which quaternaryammonium compounds, betaines, and imidazo-line derivatives are retained from ethanolicsolution; a strongly basic anion exchanger inthe Cl� form, which retains sulfonates and sul-fates; and a strongly basic anion exchanger in theOH� form, where fatty acids and aminocar-boxylic acids are retained. The nonionic surfac-tants appear in the eluate. The surfactantsretained in the columns are eluted with aqueousethanolic hydrochloric acid or 2% aqueousisopropanolic sodium hydroxide and then sepa-rated further.
A procedure for separating surfactants fromheavy-duty detergents that do not contain anycationic surfactants is as follows [158]: theethanol-soluble material is redissolved in amixture of isopropanol and water and subjectedto a double ion exchange (cation exchanger andmacroporous anion exchanger). The eluate con-tains the nonionic surfactant. The anion exchangeris then charged with ethanol saturated with carbondioxide, whereby the soaps are eluted. The hydro-tropic compounds (e.g., cumenesulfonate) are thenisolated with a solution of 1 N ammonium bicar-bonate in 80 parts of water and 20 parts of iso-propanol, and finally the sulfonates and sulfates areisolated with 0.3 N ammonium bicarbonate solu-tion in60parts ofwater and40parts of isopropanol.
The two separation processes described abovedemonstrate the numerous possibilities providedby ion exchangers in surfactant analysis.
Chromatographic Methods. Besides gaschromatography (GC), thin-layer chromatogra-phy (TLC) is an important method [159], whichhas the advantage of being able to identify di-rectly individual surfactants even in mixtures.
High-performance liquid chromatography(HPLC) and gradient gel-permeation chromatog-
raphy (GPC) are being increasingly used. Notonly can homologous and analogous compoundsbe separated in this way, but trace constituentscan be also detected.
Supercritical fluid chromatography (SCF) hasrecently become important especially for higherethoxylates [160], and capillary electrophoresis,which is suitable for separating ionic compounds,is also occasionally used.
11.3. Quantitative Determination
The afore-described methods of identifying andisolating surfactants are often also suitable fortheir quantitative determination. HCLP has be-come a powerful tool for quantitative analysis inthe last few years. In chromatographic methods,quantiative determination is performed by cali-bration with reference substances. For extracts oreluates available in larger amounts, this is per-formed gravimetrically after evaporation or afterprecipitation with a specific reagent. Also, director indirect photometric determination in solutionusing calibration curves is often employed. Theterm indirect photometric determination refers tothe photometric determination of a substancecontaining a chromophor that forms a salt, anaddition compound or a complex of definedcomposition with the surfactant and which isquantitatively determined after reaction with thesurfactant.
Anionic and cationic surfactants can also bedetermined by volumetric analysis if the molec-ular mass is known or assumed. A solution of asurfactant having an opposite electricalcharge to that of the surfactant ion is used fortitration. Anionic and cationic surfactants gener-ally form sparingly soluble salts in water; thetitration can be performed turbidimetrically as aprecipitation titration. The determination of theendpoint can also be achieved potentiometricallywith tenside-selective electrodes, e. g., by meansof a graphite electrode coated with poly(vinylchloride) [161].
Two-phase Epton titration is universallyemployed, especially for the volumetricdetermination of sulfonates and sulfates. Thetitration of the anionic surfactant is performedin two mutually immiscible solvents (water andtrichloromethane) with a cationic surfactant suchas n- cetylpyridinium chloride or Hyamine 1622.
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The water-insoluble cation and anion surfactantsalt formed in the titration dissolves in the chlo-roform phase. The end point is indicated bydyes or dye mixtures which form salts with thesurfactants and pass from one phase to the otherat the end point.
Modified Epton Titration (CIA Method).The anionic surfactant is titrated in the two-phasesystem water –trichloromethane with Hyamine1622, a cationic surfactant. The indicator is amixture of the cationic dye dimidium bromideand the anionic dye Disulfin Blue VN. Theanionic surfactant forms a salt with the cationicdye, which dissolves in trichloromethane to givea pinkish-red color. At the end point theHyaminecation displaces the dimidium cation from thesalt and thus from the trichloromethane, the redcoloration migrating to the aqueous phase. Ex-cess Hyamine forms a salt with the dye DisulfinBlue, which dissolves in trichloromethane togive a blue color. At the end point the color ofthe trichloromethane phase thus changes fromred through colorless to blue.
Longwell – Maniece Methylene BlueMethod. Sulfonate and sulfate surfactantsform salts with the cationic dye methyleneblue that are insoluble in water but soluble intrichloromethane. Since methylene blue, whichis added as its hydrochloride, is practically insol-uble in trichloromethane, when the aqueous andtrichloromethane phases are mixed the dyepasses into the trichloromethane phase only inan amount equivalent to the quantity of anionicspecies. The surfactant concentration in the tri-chloromethane extract can then be determinedaccurately by photometric determination at650 nm.
The Longwell – Maniece methylene bluemethod can be combined with the blowing outprocess for trace analysis of residual surfactantsin biodegradation experiments and of surfactantsin effluents and wastewaters. As little as 20 mg ofalkylbenzenesulfonate can be determined.
The anionic surfactant determined with thismethod is termed methylene blue active sub-stance (MBAS).
Determination ofCationic SurfactantswithDisulfin Blue (KunkelMethod). This method isbased on the formation of a trichloromethanesoluble salt of a cationic surfactant with theanionic dye Disulfin Blue. A solution of thecationic surfactant in trichloromethane is thor-
oughly mixed with the aqueous ethanolic solu-tion of the dye. After separating the phases thetrichloromethane phase is evaporated, and theresidue dissolved in methanol. This solution isdetermined photometrically at 628 nm. Sub-stance-specific calibration curves are used forevaluation. This method can be used in combi-nation with the blowing out process for traceanalysis; 10 mg of cationic surfactant can bedetermined. The cationic surfactant determinedusing this method is termed Disulfin Blue sub-stance (DSBS).
Ethoxylates are determined not in a twophasetitration, but indirectly via a precipitationreaction.
Determination of Ethoxylates withDragendorff’s Reagent (Wickbold Method).With modified Dragendorff’s reagent (bariumchloride/potassium tetraiodobismuthate (III)in glacial acetic acid) ethoxylates containing� 6 mol of ethylene oxide per mole formsparingly soluble precipitates in water. The pre-cipitate is filtered, washed with glacial aceticacid, dissolved in ammonium tartrate solution,and acidified with sulfuric acid. The bismuthpresent in solution is titrated with ethylenedia-mine tetraacetate solution with xylenol orangeas indicator. To calculate the amount of ethox-ylate, substance-specific calibration factors arerequired,which can also be calculated if themeanmolecular mass of the surfactant is known suffi-ciently accurately.
In another type of titration, especially suitablefor trace analysis, the precipitate is dissolved inammonium tartrate and titrated with pyrrolidinedithiocarbamate solution; the end point is deter-mined potentiographically. Using this methodallows as little as 50 mg of nonylphenol ethox-ylate (10 mol EO/mol) to be detected.
The nonionic surfactants determined by thismethod are termed bismuth-active substances(BiAS).
The four methods of quantitative surfactantdetermination outlined above presuppose aknowledge of the mean molecular mass of theinvestigated surfactant. As calibration or refer-ence substance tetrapropylenebenzenesulfonateis used for anionic surfactants, nonylphenolethoxylate containing 10 mol of ethylene oxideper mole for nonionic surfactants, and dodecyl-dimethylbenzylammonium chloride for cationicsurfactants.
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11.4. Unspecific Additive Parameters
The titrimetric methods discussed above for theanalytical, in particular quantitative determina-tion of surfactants are group-specific and areassociated with the amphiphilic nature of thesurfactants. The methods fail as soon as polar,hydrophilic groups are introduced into thehydrophobic residue, for example, by enzymaticattack. However, especially in investigations onthe biodegradation of surfactants, not only is theloss of surface-active properties of interest butalso their further degradation, i.e., enzymaticoxidation to water, carbon dioxide, sulfate, ni-trate, etc. It is thus important to determine theintermediate products, the metabolites. Howev-er, due to the large number of possible metabo-lites, which in many cases occur only in verysmall amounts, the effort and cost involved intheir individual determination is disproportion-ately large. To obtain some information about theoverall biodegradability of surfactants, attemptswere already made early on to determine sub-stance-unspecific additive parameters that aregenerally employable for characterizing the pol-lution of rivers and waters by organic substances(BOD and COD values) and, more recently, theTOC and DOC values.
BOD Value. In the measurement of the bio-logical oxygen demand (BOD) the biologicaldecomposition (biodegradation) of a surfactant issimulated by adding it to a mineral nutrient saltsolution that has been inocculated with a broad-spectrum bacterial culture. The solutions areincubated at 20 �C in closed flasks; the oxygencontent in the initially air-saturated solution isdetermined after 5, 15, and 30 d. The oxygenconsumption calculated from these values is ex-pressed as a percentage BOD 5, BOD 15, or BOD30 value, relative to the amount of oxygenrequired for total oxidation of the test substance.
COD Value. In the determination of thechemical oxygen demand (COD) the organicwater-borne constituents are characterized bychemical oxidation with potassium dichromatein sulfuric acid solution in the presence of silverions. The COD values depend largely on thechemical structure. For example, the readilybiodegradable compound acetic acid is scarcelyattacked by chromic acid.However, in the case of
more complex compounds such as surfactants,correlations exist between COD and BOD valuesfor groups of similar surfactants. Thus the quick-ly and easily performed COD determination canbe used to give an indication of the BOD value.The COD is expressed as a percentage of theamount of oxygen calculated for total oxidation.
TOCValue. The total organic carbon (TOC)in an aqueous solution is themost suitable quantityfor expressing the extent of mineralization of asubstance in aqueous, bacterially infected solu-tion. The TOCvalue is determined by combustionof the organic substances to carbon dioxide in ahot reactor through which a stream of air is passedand into which a measured water sample isinjected. The amount carbon dioxide in the off-gas from the reactor is determined quantitativelyby IR spectroscopy and is used to calculate thecarbon content in the sample. With the develop-ment of analytical equipment that determinescarbon automatically in the mg/L range, the TOCmethod has become an important analytical tool.The degree of biodegradation of a substance canbe specified simply as a percentage by determin-ing theTOCvaluebefore and after the degradationexperiment if the investigated substance is the solesource of carbon for the microorganisms. Sinceinorganic carbon in the form of dissolved carbondioxide or carbonate may also be present in theinvestigated solutions, the sample is acidifiedbefore combustion and the inorganic carbon isexpelled as carbon dioxide. The TOC analysis canhowever also be carried out in two paralleldeterminations, one of which, performed at a hightemperature, determines the total dissolved car-bon, while the other, performed at a low tempera-ture, determines only the inorganically boundcarbon by acidification and sparging with air. TheTOC value is then the difference between the totalcarbon and inorganically bound carbon.
DOC Value. If the carbon determination iscarried out by the TOC method after filtering thesample, then the DOC value (dissolved organiccarbon) is obtained.
12. Utility Evaluation of Surfactants
Besides the purely analytical methods a largenumber of utility evaluation methods, in some
Vol. 35 Surfactants 489
cases standardized, have been introduced for thecharacterization and quality control of surfac-tants. A list of the relevant ISO and DIN regula-tions is given below:
ISO 304: Surface Active Agents –Determination of Surface
Tension by Drawing Up Liquid Films
ISO 607: Surface Active Agents and Detergents – Methods of
Sample Division
ISO 696: Surface Active Agents –Measurements of Foaming
Power –Modified Ross – Miles Method
ISO 697: Surface Active Agents –Washing Powders –Determi-
nation of Apparent Density –Method by Measuring the
Mass of a Given Volume
ISO 1063: Surface Active Agents –Determination of Stability in
Hard Water
ISO 1064: Surface Active Agents –Determination of Apparent
Density of Pastes on Filling
ISO 1065: Non-Ionic Surface active agents obtained from Ethylene
Oxide –Determination of Cloud Temperature (cloud
point)
ISO 2131: Surface Active Agents – Simplified Classification
ISO 2174: Surface Active Agents – Preparation of Water with
Known Calcium Hardness
ISO 2267: Surface Active Agents – Evaluation of Certain Effects of
Laundering –Methods of Preparation and Use of Un-
soiled Cotton Control Cloth
ISO 2456: Surface Active Agents –Water Used as a Solvent for
Tests – Specification and Test Methods
ISO 4198: Surface Active Agents –Detergents for Hand Dish-
washing –Guide for Comparative Testing of
Performance
ISO 4311: Anionic and Non-Ionic Surface Active Agents –Deter-
mination of the Critical Micellization Concentration –
Method by Measuring Surface Tension with a Plate,
Stirrup or Ring
ISO 4312: Surface Active Agents – Evaluation of Certain Effects of
Laundering –Methods ofAnalysis and Test for Unsoiled
Cotton Control Cloth
ISO 4316: Surface Active Agents –Determination of pH of
Aqueous Solutions – Potentiometric Method
ISO 4317: Surface Active Agents –Determination of Water Con-
tent –Karl Fischer Method
ISO 4319: Surface Active Agents –Detergents for Washing
Fabrics –Guide for Comparative Testing of
Performance
ISO 4320: Non-Ionic Surface Active Agents –Determination of
Cloud Point Index –Volumetric Method
ISO 4324: Surface Active Agents – Powder and Granules –Mea-
surement of the Angle of Repose
ISO 6387: Surface Active Agents –Determination of the Power to
DisperseCalciumSoap –AcidimetricMethod (Modified
Schoenfeldt Method)
ISO 6388: Surface Active Agents –Determination of Flow Prop-
erties Using a Rotational Viscometer
ISO 6836: Surface Active Agents –Mercerizing Agents – Evalua-
tion of the Activity of Wetting Products for Merceri-
zation by Determination of the Shrinkage Rate of
Cotton
ISO 6837: Surface Active Agents –Water Dispersing Power in Dry
Cleaning Solvents
ISO 6839: Anionic Surface Active Agents –Determination of
Solubility in Water
ISO 6840: Cationic Surface Agents (Hydrochlorides and Hydro-
bromides) –Determination of Critical Micellization
Concentration –Method byMeasurement ofCounter Ion
Activity
ISO 6889: Surface Active Agents –Determination of Interfacial
Tension by Drawing Up Liquid Film
ISO 7535: Surface Active Agents –Detergents for Domestic Ma-
chine Dishwashing –Guide for Comparative Testing of
Performance
ISO 8022: Surface Active Agents –Determination of Wetting
Power by Immersion
ISO 8212: Soaps and Detergents – Techniques of Sampling During
Manufacture
ISO 9101: Surface Active Agents –Determination of Interfacial
Tension –Drop Volume Method
DIN 59 900: Terms and Definitions
DIN 53 901: Determination of the Wetting Ability of Surfactant
Solutions by Measuring the Time required for a Small
Piece of Cotton Cloth Added to the Solution to Sink
DIN 53 902: Determinations of the Foaming Power of Surfactant
Solutions by the Perforated Disk impact Method or the
Ross – Miles Method
DIN 53 903: Determination of the Power to Disperse Calcium Soaps
for Pure Calcium Water Hardness
DIN 53 904: Determination of the Ability to Wash Out Lubricants
from Textiles
DIN 53 905: Determination of the Hard Water Resistance of
Surfactants
DIN 53 908: Determination of the Dispersive Power of Surfactants
with Respect to Dyes
DIN 53 910: Preparation of Hard Water having Adjusted Calcium
Hardness
DIN 53 911: Sampling of Powders and Preparation of Size-Reduced
Average Samples
DIN 53 912: Determination of the Bulk Density of Powders and
Granules
DIN 53 913: Determination of the Filling Density of Pastes, Oint-
ments, and Gelatinous Materials
DIN 53 914: Determination of the Surface Tension
DIN 53 916: Determination of the Flowability of Powders and
Granules
DIN 53 917: Determination of the Cloud Point of Ethoxylates
DIN 53 918: Determination of the Krafft Point and Solubility of Ionic
Surfactants
DIN 53 921:Measurement of the Flow Properties by means of a
Rotational Viscometer
DIN 53 988: Examination of the Action of Levelling Agents in Vat
Dyes
DIN 53 989: Determination of the Turbidity Titration Number
DIN 53 990: Principles of the Comparative Examination of Utility
Properties of Detergents for Washing Textiles
DIN 53 993: Determination of Interfacial Tension by the Stirrup or
Ring Method.
Manymethods used for testing the detergencyand cleansing power of surfactants and surfactantformulations in their solutions have not hithertobeen standardized. To evaluate the detergentpower in textiles reproducibly soiled test fabricsarewashed in the presence of the test substance inwashing machines or washing simulators. Afterthe test fabric has been dried and ironed its degree
490 Surfactants Vol. 35
of whiteness is measured as remission of whitelight (e.g., in a Zeiss Elrepho apparatus), andcompared with the remission values of the un-washed soiled test fabric, and of the unsoiledclean test fabric.
To test the cleaning effect on hard surfaces thelatter are dirtied with colored greases and treatedwith solutions of the test substances. The degreeof removal of the colorant from the surface is ameasure of the cleaning effect. In the plate-rinsing test greasy plates are washed in succes-sion in the solution of the product to be tested.The detergent power is determined on the basis ofthe number of plates that can be cleaned untilthe detergent solution no longer contains anyfoam and the dirt that has been washed offcollects visibly on the surface of the rinsingwater.
An important property of surfactants and theirformulations, mainly for aesthetic reasons, istheir color. Slightly colored or even colorlesssurfactants are preferred. Among the variousmethods for determining the color, those pre-dominate in which the yellow and brown shadesthat mainly occur in surfactants as also in otherorganic substances are determined. In the iodinecolor value (DIN 6162, standard method C IV 4aof the Deutsche Gesellschaft f€ur Fettwis-senschaft, DGF) the color of a solution of thetest substance is compared optically with thecolor of solutions of iodine of various concentra-tions in aqueous potassium iodide solution andspecified as mg iodine/100 mL. In the Hazencolor index (APHA, ASTM D-1209–69; A.O.C.S. Td 1b-64) solutions of platinum and cobaltsalts of different concentrations are used as ref-erence solution. In the Klett color index theextinction (E ¼ log I0/I ) of blue light of wave-length 420 –430 nm passing through a solutionof the test substance is directly measured photo-metrically and multiplied by the factor 1000;turbidities are also expressed as color indices inthis method.
The Lovibond and Gardner color indices pro-vide a differentiated expression of the color of asolution. In the Lovibond color index (standardmethod C IV 4b of the DGF) the color of a beamof light passing through the solution is comparedin the optical field of a colorimeter with a secondbeam of light passed through yellow, red, andblue filters of adjustable thickness. The colorindex is specified by three figures, corresponding
to the thicknesses of the three filters. The mea-surement of the Gardner color index (DIN – ISO4630; A.O.C.S. Td 1a-64), standardmethod C IVc of the DGF) is based on the same principle,though the reference colors are already standard-ized as mixed colors and are specified by meansof progressively numbered colored glasses.
The abovemethods have proved their worth inpractice even though they have the disadvantagethat they are based on a subjective and compara-tive perception or, as in the case of the Klettindex, are restricted to measurements within anarrow spectral range.
Although an objective determination of thecolor shade in the CIE color triangle is possibleby measuring the overall visible spectrum andconversion to tristimulus values, this method islaborious. A simplified method based on thelatter for the colorimetric characterization oftransparent liquids is proposed in [162].
13. Uses [1–3], [6]
The largest proportion of surfactants is used indetergents and cleansing agents for domestic andindustrial use (! Dry Cleaning, ! CleansingAgents, ! Laundry Detergents, ! Soaps) [4],[163–166]. In universal laundry detergentsmain-ly combinations of alkylbenzenesulfonate, fattyalcohol ethoxylate, and soaps, in addition to othercomponents, are used. In modern washingmachines the soap serves primarily not as asurfactant and cleansing agent, but as a foamregulator. Nowadays it is increasingly beingreplaced by, for example, the more effectivesilicone oils. Occasionally, tallow fat alkyl sulfateis used to partly or completely replace alkylben-zenesulfonate. Specialty detergents are similar insurfactant composition as regards the compo-nents; instead of tallow fat alkyl sulfates, coconutoil alkyl sulfates and coconut oil ether sulfates,which dissolve better in the cold are preferred.
In fabric softeners, cationic surfactants areemployed. On account of their good crystalliz-ability sulfosuccinates and olefinsulfonates areemployed besides alkylbenzenesulfonate andethoxylates in foam cleaning agents and drycleaning agents. Besides dodecylbenzenesulfo-nates, alkylbenzenes with shorter alkyl chains,C14 – C17 alkanesulfonates, C12 – C14 olefinsul-fonates, and fatty alcohol ether sulfates are
Vol. 35 Surfactants 491
preferably added to liquid rinsing agents andcleansing agents since they produce highly con-centrated, clear solutions. Due to their betterdermatological compatibility betaines are addedto domestic washing up liquids. Alternatively aproportion of the anionic surfactants is replacedby nonionic surfactants such as fatty alcoholethoxylates or alkyl polyglucosides.
In the industrial cleansing sector particularimportance is placed on the degreasing andemulsifying ability, foam behavior, electrolytecompatibility, and chemical stability of the pro-ducts. Besides alkylbenzenesulfonates, alkane-sulfonates, fatty alcohol ethoxylates, alkylphenolethoxylates, and fatty amine ethoxylates, phos-phate esters and low-foam nonionic surfactantssuch as propylene oxide/ethylene oxide adductsor terminally blocked ethoxylates are alsoimportant here.
Mild surfactants that are gentle to the skin playan important role in bodycare products, andespecially in cosmetic cleansing agents (! SkinCosmetics, Section 7.1.1.) [167–169]. Such sur-factants include ether sulfates and ether carbox-ylates, sulfosuccinic esters of fatty alcohol poly-glycol ethers, protein – fatty acid condensatesand the related sarcosinates and taurides, isethio-nates, as well as ampholytes and betaines, amineoxides, fatty acid polyglycol and sorbitan esters,and alkyl polyglucosides.
A broad field of application of surfactants isthe textile industry, historically the oldest sector,with its numerous processes such as washing,cleaning, lubricating, sizing, fulling, bleaching,mercerizing, carbonizing, and finishing (! Tex-tile Auxiliaries) [170], [171]. Almost exclusivelynonionic surfactants are used for washing wool.Cotton is washed alkaline, for which purposenonionic and anionic surfactants are suitable(alkylbenzenesulfonates, alkyl sulfates and sul-fosuccinates). In lubricating, a process in whichthe slip of the fibers is improved, mainly fattyacid and fatty alcohol ethoxylates are used. Sul-fonates, sulfates, or ethoxylates are used fordesizing, in which the mutual adhesion of thefibers is overcome. Soaps or ethoxylates are usedin fulling. Bleaching auxiliaries include alkylsulfates, alkylbenzenesulfonates, and alkylphe-nol ethoxylates. Mercerization, the treatment ofcotton with a concentrated lye, requires wettingagents such as alkanesulfonates, alkyl sulfates, oralkyl phosphates. Alkanesulfonates or alkyl sul-
fates are also used in carbonizing, i.e., treatmentwith sulfuric acid. Softening, which improves thehandle of textiles, is largely carried out withcationic surfactants.
Surfactants are also required as emulsifiers,dispersants, levelling agents, and swelling agentsin textile dyeing (! Textile Dyeing; ! Dis-perse Systems and Dispersants, Chap. 1.).
The leather industry uses nonionic andcationic surfactants as wetting and cleansingagents. Leather oils, which are used to greaseleather, contain alkane sulfonates or alkylsulfates. Cationic surfactants are used in theaftertreatment of leather to make it supple andwater-repellent.
Surfactants have a wide range of applicationas emulsifiers (! Emulsions) [172–175] anddispersants (! Disperse Systems and Disper-sants). The foods industry uses mainly naturalsubstances such as fatty acid salts, glycerides, andfatty acid esters of lactic acid, tartaric acid, andsorbitol (! Foods, 3. Food Additives, Section3.5.! Foods, 3. Food Additives, Section 3.6.).
Mainly fatty alcohol ethoxylates and fattyalcohol sulfates, fatty acid ethoxylates, and fattyacid esters of glycerol and sorbitol are used inpharmacy, where attention must be paid to phar-macological and toxicological innocuousness(! Pharmaceutical Dosage Forms, Chap. 5.)[176].
Emulsifiers are added to crop-protectionand pest- control agents since the active sub-stances are often only sparingly soluble inwater. Emulsifiers widely used here include thesodium, calcium, and diethanolammonium saltsof alkylbenzenesulfonic acids, ethoxylates ofalkylphenols and condensed phenols, and fattyacid esters of polyglycols, glycerol, and sorbitol.Short- chain alkylbenzene sulfonates and alkyl-naphthalene sulfonates are used as wettingagents.
Emulsifiers are required in numerous opera-tions involved in petroleum production. Thedrilling muds used in large amounts for coolingand rinsing in drilling technology contain, forexample, alkylbenzenesulfonate or lignin sulfo-nate to suspend the solids. Lignin sulfonateand naphthalenesulfonate are used as a cementadditive in the construction of boreholes.Demulsifiers, e.g., condensation products ofpropylene oxide and ethylene oxide, ethoxylatesof alcohols, fatty acids, alkylphenols, and
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condensed phenols, are indispensable in workingup the oil, which is initially obtained as awater –oil emul-sion.
Emulsions of water andmineral oil are widelyused inmetalworking andmachining. They act aslubricants and coolants in drilling, cutting, roll-ing, and drawing. Preferred emulsifiers includethe heavy metal salts of fatty acids and naphtenicacids, as well as salts of phosphoric esters orcarboxymethylated ethoxylates.
Quenching oils for steel hardening containethoxylates of fatty alcohols or alkylphenols.Emulsions prepared with ethoxylates or sulfo-nates are suitable as mold release agents incasting and founding. Alkaline-adjusted mineraloil emulsions, prepared using ethoxylates basedon alkylphenols or fatty amines, are used ascleansing agents. Emulsions prepared with phos-phates, quaternary ammonium salts or ethoxy-lated fatty acid ethanolamides are used as anti-corrosion agents.
In the mineral oil industry emulsifiers play asignificant role as additives for cleaning carbure-tors and as flow enhancers and corrosioninhibitors.
Emulsifiers used in petroleum exploration,production and processing [177], and for oil slickcontrol aremainly nonionic dispersants or demul-sifiers [178]. A potential use of ethoxylates ormodified ethoxylates is for transporting heavyoilsand bitumen as low-water emulsions [179], orcoal as concentrated coal – water slurries[180].
Almost exclusively cationic surfactants, morespecifically amine oxides, fatty amine salts andimidazoline salts, are used for bitumen applica-tion in road construction work.
In the cement industry alkylnaphthalenesul-fonates are used as wetting agents, phenol –formaldehyde – sodium bisulfite adducts or lig-nin sulfonates are used as flow enhancers, andethoxylates are used to form air pores.
The production of numerous plastics by emul-sion polymerization is only possible by usingemulsifiers (soaps, alkylbenzenesulfonates, alka-nesulfonates, alkyl sulfates, primary and quater-nary ammonium salts, pyridinium salts, fattyalcohol ethoxylates and alkylphenol ethoxylates,glycerides, sorbitol esters, etc.). Other uses ofsurfactants in the plastics industry include stabi-lization of polymer dispersions, production offoamed plastics, mold release agents, and pro-duction of microcapsules.
In the production of paints and coatings abroad range of emulsifiers is required to dispersepigments and emulsify oils and resins, and to actas flow- control and thickening agents. Suitabledispersants include ethoxylates of alkylphenols,fatty alcohols and fatty acids, sulfated oils andnaphthenates; ethoxylates and sulfonates areused as emulsifiers.
Adhesives contain mainly nonionic surfac-tants, which emulsify constituents of the adhe-sive, and ensure rapid wetting of the surfaces tobe bonded.
In pulp and paper production, ethoxylates,alkanesulfonates, alkylnaphthalenesulfonatesare used as wetting agents to remove resinousconstituents. Various ethoxylates and sulfonatesas well as cationic surfactants are used in papersizing and finishing.
Surfactants are used in electroplating to cleanmetal surfaces and also to accelerate hydrogenevolution at the cathode. Fatty alcohol ethoxy-lates and sulfates are often used, and in somecases also alkylbenzenesulfonates. Fluorosurfac-tants are particularly advantageous in electro-plating due to their chemical stability.
Fire-Extinguishing Foams are produced byadding alkyl or alkyl ether sulfates and fatty acidesters to water. Rapid wetting of the material tobe extinguished can be achieved by using fluorosurfactants.
Alkylphenol ethoxylates, sorbitol esters, gly-cerides, alkylbenzenesulfonates, and sulfosucci-nic esters are used in the photographic industry toproduce light-sensitive emulsions.
An increasingly important use of surfactants isore flotation (! Flotation) [181–183].
The applications listed above by no meansexhaust the possible uses of surfactants. Fur-ther applications are described in [183–186]. Afew applications of minor industrial impor-tance are: the catalysis of chemical reactionsin micelles and vesicles [17], [187]; phase-transfer catalysis [188]; extractions in multipleemulsions on liquid membranes [189]; ion-pairextraction [190] and coacervate extraction[191], which is preferably used in the separa-tion of biological materials from aqueous solu-tions; and admicellar chromatography [192], inwhich micelles adsorbed on a solid surface actas carrier phase.
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Finally, the detailed study of microemulsionshas opened up a broad application potential forthe latter [193–195].
14. Economic Aspects
World production of surfactants is ca. 15�106 t/a. Of this, soaps account for ca. 8�106 t/a; in many countries they are still the mostimportant surfactant for everyday use. Aftersoaps, the alkylbenzenesulfonates are quantita-tively the most important class of surfactants,accounting for ca. 2�106 t/a.
Surfactants are mainly marketed as constitu-ents of finished products, together with nonsur-factant components. In many cases their specificconsumption cannot be determined exactly onaccount of lack of knowledge of their content incommercial products, even when consumptiondata exist for products themselves. Also, mostmarket estimates are limited to specific applica-tion sectors. There are therefore no accurate dataon the total consumption. Estimated productionfigures in 103 t/a for some surfactants in 1990were as follows:
Linear alkylbenzenesulfonates 1700
Branched alkylbenzenesulfonates 300
Lignin sulfonates 600
Fatty alcohol ether sulfates 400
Fatty alcohol sulfates 300
Petroleum sulfonates 200
Alkanesulfonates 100
Fatty alcohol ethoxylates 700
Alkylphenol ethoxylates 500
Fatty acid esters 300
Fatty acid alkanolamides 100
Quaternary ammonium compounds 300
Other amine derivatives 100
Surfactant consumption (in 103 t/a) accordingto fields of application in the United States,Japan, and Western Europe in 1982 broke downas follows [196]:
Washing and cleaning 1900
Cosmetics and pharmacy 300
Textiles and fibers 200
Leather and furs 50
Colorants, coatings, and plastics 200
Cellulose and paper 100
Mining, flotation, and oil production 300
Metalworking 130
Building and construction 50
Crop protection and pest control 100
Foods 200
Miscellaneous 400
15. Toxicology and EnvironmentalAspects
15.1. Introduction
Surfactants are used in many areas of humanactivity, in the home and in commerce, in agri-culture, and in industry. Their effects on livingorganisms are therefore particularly important,even in cases where products, when used accord-ing to the instructions, do not come into directcontact with higher life forms. A large number ofsurfactants come into direct contact with humanskin, as constituents of detergents and cleansingagents; their accidental oral ingestion, even ifonly as residues on washed dishes, thereforecannot be ruled out. Surfactants used as auxili-aries in foods and beverages are therefore subjectto legal provisions that require them to be abso-lutely harmless. The special attention that isnowadays paid to the possible harmful effectsof chemicals has meant that some surfactantsintended for use in the foods sector are notpermitted for use as such in all countries. Acharacteristic feature or surfactants permitted foruse in the foodstuffs sector is that they areoverwhelmingly natural products or are obtainedfrom natural products by slight chemical modifi-cation. They are mainly derived from glycerides,fatty acids, lactic, tartaric, and citric acids, sugar,and sorbitol.
Irrespective of their intended use, productsafety – including environmental protection – isof major importance for all surfactants. The fateand effect of surfactants in rivers and waters areof particular importance in environmental riskassessment, since a large proportion of surfac-tants is discharged after use into effluent andsewage and ultimately flows into rivers, lakes,and oceans. Here, the degradation of surfactantsby microorganisms in natural waters and insewage plants, which ultimately leads to theircomplete mineralization is particularlyimportant.
494 Surfactants Vol. 35
15.2. Toxicology [197–199]
The majority of surfactants now in use – alkyl-benzenesulfonates, alkyl ether sulfates, alkylsulfates, alkanesulfonates, olefinsulfonates, alkylpolyglycol ethers, and nonylphenol polyglycolethers – are nontoxic or have only a slight acuteor chronic toxicity. Also, no toxic effects of themetabolites of these surfactants have been de-tected in mammals.
The oral LD50 values of both anionic andnonionic surfactants is generally in the rangefrom several hundred to several thousand milli-grams per kilogram of body weight (comparableto sodium chloride). Betaines, which are used inparticular in body cleansing preparations, arenontoxic. The quaternary ammonium com-pounds, which are known to be bactericidal andfungicidal, exhibit a range of behavior. Whereasdialkyldimethylammonium chlorides used asfabric softeners have LD50 values of ca. 5 g/kgbody weight and are nontoxic, alkylbenzyldi-methylammonium chloride used as a disinfectanthas a fairly low LD50 value of 0.35 g/kg.
There are few data on the toxicity of surfac-tants derived from polysiloxanes or fluoroalk-anes. Methylpolysiloxanes are physiologicallyinert, but this is not necessarily the case forphenyl-substituted polysiloxanes.
Nothing is known of the carcinogenic, muta-genic, or teratogenic effects of surfactants.
There is no danger of acute lethal toxicity byresorption of surfactants through the skin orby inhalation. Surfactants can cause skin irrita-tion and skin damage on prolonged action,since they destroy the water-lipid membranethat serves as the external protective layer of theskin, by dissolving individual constituents. Theimmediate results are swelling, drying, and chap-ping of the skin; prolonged action of surfactantscan lead to eczematous changes in the skin,which recede when the source of irritation isremoved.
There are a large number of standardizedmethods for testing irritation of the skin andmucous membranes. It is important to differenti-ate between tests under practically relevant con-ditions at realistic concentrations, and tests usedto determine the potential hazard of pure sub-stances, as required, for example, by the GermanChemikaliengesetz (Chemicals Act). Accordingto the latter most surfactants should be charac-
terized as ‘‘irritant’’, a few as ‘‘moderately tox-ic’’, while strongly acidic or basic products,including a number of amine derivatives andammonium salts, as ‘‘corrosive’’ [200]. Sucheffects detected at high concentration hardlyever occur if surfactants are used accordingto the instructions, either because such concen-trations are not employed, or because the timethe surfactant is in contact with the skin is tooshort to produce an irritant or indeed a corrosiveeffect.
Skin irritation generally increases in thesequence nonionic, amphoteric, anionic, cationicsurfactants. Aromatic rings in the hydrophobicgroup increase the irritant action. Good wettingagents have a higher irritant potential than poorwetting agents.With anionic surfactants contain-ing a linear alkyl group such as soaps, alkylsulfates, and olefinsulfates, maximum skin irri-tation is found for an alkyl chain length of 12carbon atoms. The incorporation of oligoglycolresidues between hydrophobic and hydrophilicgroups, as in fatty alcohol ether sulfates, fattyalcohol sulfosuccinates, or fatty alcohol carbox-ylates, reduces the irritant action of such anionicsurfactants.
Nitrogen- containing surfactants irritate theskin less, and the action of sulfonates or sulfatesis reduced by using their ammonium salts (e.g.,triethanolammonium salts).
The above comments regarding skin irritationalso apply to irritation of themucousmembranes,but the mucous membranes are much more sen-sitive than the skin.
Betaines and amine oxides are extremelyweakly irritating to the skin; they reduce theirritant action of anionic surfactants.
On account of their compatibilitywith the skinand mucous membranes, betaines, fatty acidamidoalkylaminobetaines, amphoteric N-alkyla-mino acids, and fatty acid alkylolamidoethersulfates are playing an increasing role as consti-tuents of body cleansing agents. Condensationproducts of fatty acids and protein degradationproducts (oligopeptides) have long been knownto be particularly gentle to the skin and mucousmembranes.
Allergies and sensitizations due to surfactantsare uncommon. If they do occur, it shouldbe checked whether they are caused by accom-panying substances or impurities in thesurfactants.
Vol. 35 Surfactants 495
15.3. Biological Degradation [201–203]
Organic substances can be degraded aerobicallyand anaerobically bymicroorganisms in the pres-ence of water. Since surfactants are dischargedafter usemainly into effluent and sewage and thuseventually into rivers and seas, their aerobic bio-degradation is of primary interest, which isalready reflected inEurope in a number of legisla-tive measures (! Laundry Detergents, Section10.4.). In the determination of the biodegradabili-tyadistinctionismadebetweentheprimaryattackon the surfactant molecule and total degradation.Primaryattackis the lossof interfacialactivitydueto a structural change in the surfactant molecule.Total degradation is the complete conversion ofthe surfactant into inorganic substances (e.g.,sodium sulfate, carbon dioxide, water) and theincorporation of its constituents into the cellularmaterial of microorganisms.
Primary attack, which has been shown to leadto total degradation, albeit after a long time, iseasy to determine (in contrast to total degrada-tion) by measuring analytically the residualsurfactant in a degradation test. Intact anionicsurfactants are detected as methylene blue-activesubstances (MBAS), nonionic surfactants as bis-muth-active substance (BiAS), and cationic sur-factants as DSBAS (see Section 11.3).
The investigation of the degradation pathwaysof a surfactant to total degradation is laboriousand time- consuming, and up to now has beenperformed only on a few, industrially particularlyimportant surfactants (e.g., linear alkylbenzenesulfonate; see below). To quickly obtain infor-mation on total degradation, additive parametersare nowadays preferably employed (see Section11.4), which can readily be determined in degra-dation experiments.
The test specifications recommended by theOECD have been adopted by the EC states aswell as by some other countries. These are thestatic OECD screening test, which simulates theconditions of surface waters, and the continuousOECD confirmatory test in which the processesoccurring in a sewage plant are approximatedusing activated sludge (! Laundry Detergents,
Whereas surfactant-specific analysis only pro-vides information on primary attack, the intro-duction of the DOC method (dissolved organiccarbon) provides further, but however only sum-mary information, on the degradation of a surfac-tant. If the surfactant is the sole source of carbonin the test solution, then the determination of theorganically bound carbon in the solution at thestart and end of the experiment provides a directmeasure of the degradation of the surfactant. TheDOC method can be extended to the OECDconfirmatory test if two units instead of one areoperated, surfactant being added to the inflow ofone unit but not to the other. A homogeneouscomposition of the bioceonosis in both batches isensured by daily exchange of sludge between theparallel units. Entrainment of surfactant, whichcould influence the DOC values, is taken intoaccount computationally in the evaluation of theresults. Reliable balances and thus information onthe degree of degradation of the surfactants can beobtained by this method by employing suitabletechnical and statistical measures [205].
Although it is complicated and costly, this so-called coupled units test has nowadays become astandard method (! Laundry Detergents, Sec-tion 10.4.2.2.). If the overflowing effluent in thiscontinuously operated test is recycled up to fiftytimes and replenished with fresh surfactant, ad-ditional information is obtained on difficultlydegradable catabolites or byproducts in the sur-factant used (catabolites test) [206].
In addition to these methods a large number ofother test methods are used, some of which havebeen adopted and numbered in the OECDGuide-lines as ready, inherent, and simulation tests:
Modified Zahn – Wellens Test302 BModified MITI (II) Test302 C
Simulation Test
Coupled Units Test 303 A
Inherent Biodegradability in Soil 304 A
496 Surfactants Vol. 35
Whereas most of the aforementionedmethodsdetermine the biodegradation via DOC analysis,in the closed bottle test the oxygen consumptionis measured, while in the MITI and STURM teststhe carbon dioxide evolution is measured. Infor-mation on widely used test methods is given inTable 16 [207]. In the ready tests such as theclosed bottle or Sturm tests, besides informationon the biodegradability of the substance infor-mation can also be obtained on the ready biode-gradability, which applies if a degree of degra-dation of at least 70% is achieved after a degreeof degradation of 10% is exceeded within 10 d(Fig. 22).
All the methods mentioned here refer to theaerobic biodegradation of surfactants. Since thesurfactants used nowadays in consumer productsundergo rapid aerobic degradation, their anaero-bic degradation plays only a minor role in eco-
logical considerations. Accordingly, at presentthere are still no validated methods for determin-ing anaerobic degradation. Occasionally, theECETOC test is used.
A further, very informative but complex andexpensive method is the use of radioactivelylabelled (generally 14C)material in a degradationexperiment and identification of the radioactiveend products.
A specialist report has summarized a largeamount of data on biogradation and other eco-logically relevant data concerning surfactants[208]; this information is reproduced in abbrevi-ated and supplemented form in Table 17.
15.3.2. Biodegradation Mechanisms
Surfactants whose hydrophobic group arederived from hydrocarbons can be oxidizedenzymatically under aerobic conditions andthus biodegraded. Surfactants derived fromhydrophobic parent substances such as perfluor-oalkanes, poly(propylene glycols), or polydi-methylsiloxanes are not biodegradable.
The enzymatic attack leading to biodegrada-tion occurs in most surfactants at the hydropho-bic group. For ionic surfactants the nature of thehydrophilic group has only a minor influence onthe rate of degradation. High biodegradationrates have been found even for bactericidal qua-ternary ammonium compounds such as cetyltri-methylammonium bromide, benzyldodecyldi-methylammonium chloride, and distearyldi-
Table 16. Test methods for determining total degradation (mineralization)
Test method Characteristics of the test Analysis Test duration, d
Degree of degradation
to be regarded as degradable
Modified OECD
screening test
static test (small amount of organisms) DOC 28 > 70%
Closed bottle test static test (small amount of organisms) O2 28 > 60%
Sturm test static test (small amount of organisms) CO2 28 > 60%
Zahn – Wellens test static test (activated sludge) DOC 28 > 70%
Coupled units test continuous test (simulation of a
communal clarification plant)
DOC � 30 > 70%
Catabolites test continuous test with recycling and
replenishment of the purified effuent
DOC � 30 the necessary degree of
degradation depends on
the carbon content of
the test substance
ECETOC test static test (digested sludge); anaerobic
conditions
CH4 and CO2 � 30 not specified
Figure 22. Curves illustrating the DOC decrease of tworeadily biodegradable substances [207]
Vol. 35 Surfactants 497
Table
17.Biodegradabilityandecotoxicityofim
portantsurfactants[208],supplemented
Surfactant
Confirm
atory
test
a,%
Coupled
units
test
b,%
Mod.OECD
screening
test
b,%
Closed
bottle
testc,%
Sturm
test
d,%
Fishtoxicity
LC50,mg/L
Daphnia
toxicity
EC50,mg/L
Algae
toxicitymg/L
LinearC10–C13alkylbenzenesulfonate
93–97
92
94
55–65
45–76
3–9
9–14
50e
C14–C17alkanesul-
fonate
97–98
99
88–96
63–95
56–91
3–24
9–13
96
C12–C18fattyalcoholsulfate
98–99
97
88–96
63–95
64–96
3–20
5–70
60
Dioctylsulfosuccinate
96
49
96
50
39
33
C14–C18a-olefinsulfonate
98
70–78
85
85
65–80
2–20
5–50
10–100
Soap
85–100
80–99
strongly
dependant
onwater
hardness
C12–C18fattyalcoholþ
10EO
93–98
95–96
94
69–86
54
3
C16–C18fattyalcoholþ
5EO
95
>80
65–75
30
2
C16–C18fattyalcoholþ
30EO
98
93
27
16
>1000
C13–C15oxoalcoholþ
10EO
95
>80
75
3
Isononylphenolþ
9EO
87–97
77–90
8–17
5–10
40
5–11
4–50
20–50
C16–C18fattyam
ineþ
10EO
97–98
6–33
C12–C18fattyacid
polyglycolester
þ5/29EO
92–96
71–92
100
60–80
35–>100
Dim
ethyldistearylammonium
chloride
94
108
05
1–6
0.1
–1
1-M
ethyl-1-octadecylamidoethyl-
2-heptadecylimidazolinium
methylsulfate
97
75–100
84
1.5
13
Ditallowacyloxyethylhydroxyethylm
ethyl
ammonium
methylsulfate
>90
93
>73
3.3
f87f
3f
aMBAS,BiAS,orDSB.
bDOC.
cBOD.
dCO2evolution.
eEC10
fEC0.
498 Surfactants Vol. 35
methylammonium chloride. With nonionic sur-factants of the ethoxylate type the degradationrate is influenced by the length of the poly(ethyl-ene glycol) chain (i.e., by the degree of ethox-ylation); with the same hydrophobic residue, thedegradation rate decreaseswith increasing lengthof the poly(ethylene glycol) chain.
The constitution of the hydrophobic hydro-carbon group of a surfactant has a decisiveinfluence on the degradation rate.
Branchings in an aliphatic group stronglydecelerates degradation. Phenylidene groupsbetween the aliphatic and hydrophilic groupscan make degradation slower. Linear alkyl-benzenesulfonates are degraded more slowlythan alkanesulfonates and olefinsulfonates,and the latter more slowly than linear alkylsulfates.
Natural fatty acids and their salts as well assome of their esters with glycerol, sorbitol orsugars are readily biodegradable, although in thecase of the esters a decrease in the degradationrate is already observed, which can be explainedby the need for a preceding hydrolysis. Theenzymatic oxidative degradation of linear fattyacids takes placemainly byb-oxidation, inwhichthe hydrocarbon chain is shortened each time bytwo carbon atoms.
Aerobic biodegradation of secondary alkane-sulfonates probably begins with oxidative desul-fonation; the resulting oxoalkanes are readilydegraded.
In the case of internal sulfonates, alkylbenze-nesulfonates, and alcohol ethoxylates,microbial degradation can be initiated by w-oxi-dation, i.e., by oxidative attack on both terminalmethyl groups. After introduction of the terminalcarboxylate groups further degradation then pro-ceeds as b-oxidation. In the degradation of al-kylbenzenesulfonates sulfophenylalkanecar-boxylic acids are formed as intermediates, whilein the degradation of fatty alcohol ethoxylatespolyglycol ethers ofw-hydroxyalkanecarboxylicacids are formed as intermediates.
The biodegradation of the industriallymost important surfactant, alkylbenzenesulfo-nate (also named dodecylbenzenesulfonate sincethe mean number of carbon atoms in the alkylgroup is 12) has been largely elucidated. Thedegradative pathway proceeds by w-oxidationand subsequent b-oxidation, followed by ringopening. The sulfonate group remains bound to
the carbon atom after opening of the aromaticring. The homologous and isomeric species con-tained in the alkylbenzenesulfonate are attackedat various rates. According to the ‘‘distanceprinciple’’, the degradation rate increaseswith increasing distance of the terminal methylgroup from the coupling site of thesulfophenyl group, or the length of the availableunsubstituted alkyl chain. For example, 2-dode-cylbenzenesulfonate is degraded morerapidly than 6-dodecylbenzenesulfonate, whichin turn is degraded more quickly than 6-undecylbenzenesulfonate.
That ca. 70% of the benzene nuclei in indus-trial alkylbenzenesulfonate are already degradedunder the conditions of the OECD confirmatorytest has been confirmed by examining the degra-dation of an alkylbenzenesulfonate labeled in thebenzene nucleus with 14C [209].
Tetrapropylenebenzenesulfonate is difficultlybiodegradable on account of its highly branchedalkyl group.
Nonionic surfactants of the ethoxylate typeare also preferentially degraded starting from thehydrophobic group by enzymatic attack on aterminal methyl group. The degradation ratedecreases sharply with increasing branching ofthe hydrophobic alkyl group. With simplebranching, which occurs for example in the caseof isomeric oxoalcohols, the distance principleapplies to the degradation of ethoxylates as itdoes for linear alkylbenzene sulfonates, wherebythe surfactant is degraded more quickly the lon-ger a linear alkyl residue starting from a branch-ing site is:
The biodegradation of linear ethoxylates pro-ceeds by two different mechanisms that seemsoccur simultaneously:
Vol. 35 Surfactants 499
1. The molecule is cleaved at the ether bridgebetween the alkyl group and the polyethyleneglycol ether group, following which bothfragments are degraded independently of oneanother
2. Microbial attack occurs simultaneously atboth ends of the molecule, i.e., at the terminalmethyl group of the alkyl chain and at thehydroxyl group of the poly(ethylene glycol)unit
The alkyl group of industrial alkylphenolethoxylates are attacked microbially with diffi-culty on account of their high degree of branch-ing. In this case the degradation proceeds fromthe polyglycol chain: The chain is shortenedstarting from the end of the free hydroxyl group,stepwise in each case by one ethylene glycol unit,glycolic acid presumably being formed as anintermediate:
Degradation becomes slower with decreasinglength of the polyglycol chain. Since the hydro-carbon skeleton is also degraded with difficulty,the total mineralization of alkylphenol ethoxy-lates proceeds more slowly than the fast primarydegradation.
15.3.3. Toxicity of Surfactants and theirMetabolites to Aquatic Organisms [210]
The toxicity of surfactants to aquatic organismscan only be evaluated if the rate and complete-ness of their biodegradation is taken into account.Thus substances with high toxicity will generallynot have any harmful effect on aquatic organismsif they are degraded sufficiently quickly.
The toxicity is tested on green algae (Chlorel-la), water fleas (Daphnia), fish (trout, orfe, gold-fish), and mixed bacterial cultures. These troutand young orfe are particularly sensitive to sur-factants; therefore, lethal and nonlethal concen-
trations for these fish are measured to providepreliminary information (screening). In Ger-many orfe are used for testing; the test method,in which the fish are exposed for 48 h to gradu-ated concentrations of the surfactant in the am-bient water, has been standardized. Nonlethalconcentrations (48-h LC0 in mg/L) of somecommon surfactants for orfe are listed below:
Sodium tetrapropylenebenzenesulfonate 8.0
Sodium dodecylbenzenesulfonate (linear) 2.3
Sodium alkanesulfonate (C14 – C17) 3.0
Sodium olefinsulfonate (C14 – C16) 2.0
Sodium tallow fat alcohol sulfate 6.0
Sodium tallow fat alkyl ether sulfate containing
3 EO 10.0
Nonylphenol polyglycol ether with 9 EO 5 – 6
Fatty alcohol polyglycol ether with 5 EO 1 – 2
Fatty alcohol polyglycol ether with 10 EO 2 – 3
Fatty alcohol polyglycol ether with 14 EO 3 – 4
Fish toxicity is strongly dependent on thestructure of the surfactant, as exemplified by thestructural isomers tetrapropylenebenzenesulfo-nate (branched side chain) and dodecylbenzene-sulfonate (linear side chain). In general, fishtoxicity increases with the effective length ofthe hydrophobic group; in anionic surfactants,branching and an internally located hydrophilicgroup reduce the toxicity. This is exemplified bythe fish toxicity (LC50) of alkylbenzenesulfonateisomers (for Carassius auratus [210]):
5-decylbenzenesulfonate 76
2-decylbenzenesulfonate 36
6-undecylbenzenesulfonate 46.5
2-undecylbenzenesulfonate 14.5
6-dodecylbenzenesulfonate 20.5
2-dodecylbenzenesulfonate 4.5
7-tridecylbenzenesulfonate 8.5
2-tridecylbenzenesulfonate 2.0
These toxicity data demonstrate clearly thatthe toxicity of surfactants cannot be consideredseparately from their biodegradation in naturalwaters, since the toxic isomers are degradedmostrapidly, (distance principle) and accordinglyreach the waters in far lower concentration, if atall, than the less toxic internal isomers.
Except for one striking exception, up to nownothing is known about the aquatic toxicity ofcatabolites produced in the biodegradation ofsurfactants. The exception are the isononylphe-
500 Surfactants Vol. 35
nol ethoxylates, which, because of their markeddegree of branching are degraded not from thealkyl residue but from the terminal hydroxylgroup of the poly(ethylene glycol) chain, where-by the degradation rate decreaseswith decreasingnumber of ethylene oxide units in the poly(eth-ylene glycol) chain. The diethylene glycol andmonoethylene glycol ethers that are formed asintermediates and degrade only slowly and ex-hibit, like the parent substance isononyl(or iso-octyl)phenol, a pronounced toxicity to aquaticorganisms, which is much higher than that ofalkylphenol ethoxylate surfactants with degreesof ethoxylation of 5 and above [104].
Anionic and nonionic surfactants are general-ly nontoxic to bacteria. However, reference ismade here to a remarkable connection betweenchemical structure and bacterial toxicity in thecase of nonionic surfactants, which in particularcases may result in an inhibition of the biodegra-dation of some nonionic surfactants. The toxicityof ethoxylates increases sharply with decreasinglength of the poly(ethylene glycol) chain (seeTable 17). Some toxicity threshold values (TLVin mg/L) for Pseudomonas are given below:
Coconut oil fatty alcohol polyglycol ether with 10 EO 1000
Coconut oil fatty alcohol polyglycol ether with 2 EO 10
Stearyl alcohol polyglycol ether with 25 EO 10 000
Stearyl alcohol polyglycol ether with 10 EO 100
Stearyl alcohol polyglycol ether with 2 EO 10
Nonylphenol polyglycol ether with 10 EO 1 000
Nonylphenol polyglycol ether with 6 EO 500
Nonylphenol polyglycol ether with 4 EO 50
A number of quaternary ammonium com-pounds and betaines have a substantial bacteri-cidal activity. However, if sufficiently dilutedthese compounds, too, are biodegraded if thechemical structure of the hydrophobic groupallows enzymatic attack to occur.
15.4. Preservation of Surfactants[211], [212]
The biodegradation of surfactants occurs notonly at high dilution in waters and rivers, but inthe presence of water can already start in theproduction process, in storage and transportationvessels, and in formulations. Bacteria cannotthrive on anhydrous surfactants or in highlyconcentrated solutions and pastes; however,
bacterial infection must already be expected in20 to 30% solutions of anionic or nonionic sur-factants.Also,more highly concentrated solutionsor pastes that tend to coacervate with the forma-tion of low concentration phases can act as nutri-ent media for bacteria and exhibit signs of putre-faction. Bacterial infection must also be expectedif highly concentrated surfactant formulations arehandled incorrectly; small residues of surfactantsin open vessels can be diluted by absorption ofatmospheric moisture, and dilute surfactant solu-tions can accumulate on the floor or in dead spacesof vessels that have not been thoroughly cleanedwith water. Cationic surfactants and amphotericsurfactants have a bactericidal action at and belowthe isoelectric point; except in highly dilutedsolutions (ppm range), a biodegradation of thesesurfactants can be ruled out.
Sterile conditions must be maintained to pre-vent premature biodegradation of surfactantsduring production. A sterile environment is gen-erally ensured by maintaing high concentrations,high temperatures, and by using chemicalbleaches. Transportation vessels can be filled inthe hot state and thus under sterile conditions; thevessels must then be sealed in an air-tight andgerm-proof manner. To protect surfactantsagainst putrefaction during processing and in thedilute aqueous state (e.g., washing up liquids orshampoos), preservatives are added. Formalde-hyde (0.1 – 0.3%) is an excellent preservative,but has been discredited in some countries onaccount of its alleged carcinogenicity. Formal-dehyde-releasing substances can also be used, forexample, 2-Nitro-2-bromopropane-1,3-diol. 2-Methyl-3-isothiazolone, 5- chloro-2-methyl-3-isothiazolone, and 1,3-dicyano-1,2-dibromobu-tane are used in ppm amounts. 2-Phenoxyetha-nol, methyl and ethyl p-hydroxybenzoates andsorbic acid are effective at higher concentrations(ca. 0.5%).
References
General References1 K. Lindner: Tenside-Textilhilfsmittel-Waschrohstoffe,
Wissenschaftl. Verlagsges., vols. 1 and 2, Stuttgart
1964; vol. 3, 1971.
2 G. Gawalek: Tenside, Akademie-Verlag, Berlin 1975.
3 H. Stache (ed.): Tensid-Taschenbuch, 2nd ed., Hanser
Verlag, M€unchen 1981.
Vol. 35 Surfactants 501
4 J. Falbe (ed.): Surfactants in Consumer Products, Theo-
ry, Technology and Application, Springer Verlag, Hei-
delberg 1987.
5 M. J. Rosen: Surfactants and Interfacial Phenomena,
2nd ed., J. Wiley & Sons, New York 1989.
6 K. Kosswig, H. Stache (eds.): Die Tenside, Physik, Che-
