Dynamics of Spontaneous Emulsification for Fabricationof Oil in Water Emulsions
Noushine Shahidzadeh,* Daniel Bonn, and Jacques Meunier
Laboratoire de Physique Statistique de l’ENS, 24 rue Lhomond, 75 231 Paris Cedex 05, France
Minou Nabavi, Marc Airiau, and Mikel Morvan
Rhodia-CRA, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France
Received April 3, 2000. In Final Form: August 23, 2000
We present an experimental study of the dynamics of spontaneous emulsification when a surfactantsolution in oil is brought into contact with pure water. Direct visualization using phase contrast microscopyshows that vesicles (closed bilayer structures) form in the oil phase near the interface with the water andexplode, thereby pulverizing oil droplets into the aqueous phase. The results thus show the importanceof the presence of bilayer structures in the spontaneous emulsification process for the formation of oil-in-water emulsions. Measurement of the droplet size distribution shows that an optimal ratio of surfactantto cosurfactant exist. It is shown that both the microscopy observations and the size of the droplets canbe related to the nonequilibrium water/oil interfacial tension just after the two phases have been broughtinto contact.
1. Introduction
Emulsions are mixtures of two liquids that are im-miscible, generally oil and water, one of the two liquidsbeing dispersed in the other in the form of droplets. Thesize of the droplets is usually on the order of 1 µm, whichgives emulsions their characteristic milky-white color. Theformation of such a dispersion thus entails the formationof a large amount of interfacial area between the twophases. To reduce the work necessary to create the largeinterfacial area, one usually employs surfactants to reducethe energy cost due to the interfacial tension associatedwith the creation of the emulsion.
A particularly important problem from an industrialpoint of view is the spontaneous formation of emulsionswhen the two immiscible phases are brought into contact.In such spontaneous emulsification the entire energyrequired for the emulsification comes from the redistribu-tion of material within the system; that is, no externalenergy of agitation is supplied.1
Although the static properties of emulsion-formingsystems are by now fairly well understood, the dynamicsof emulsion formation are not. Some mechanisms havebeen suggested in the literature for the spontaneousemulsification of oils in water.1-6 It has been establishedthat, for sufficiently high surfactant concentration, emul-sion formation can occur due to diffusion of a cosurfactant(generally an alcohol) in the other phase, leading to theformation of intermediate lamellar liquid crystalline or
microemulsion phases which are supersaturated withoil.5-8 The subsequent destabilization of these swollenbilayer phases then leads to the formation of the emulsion.
In this paper, we present a study of the spontaneousformation of oil-in-water emulsions, in systems in whichthe surfactant is dissolved in the oil. This system is studiedin order to obtain a better insight into the dynamics ofemulsion formation in emulsifiable concentrates of forinstance agriculture chemicals, which need to spontane-ously form oil-in-water dispersions when brought incontact with water. The results show the importance ofthe presence of bilayer structures (vesicles) in the spon-taneous emulsification process and demonstrate that thelocal formation of vesicles is sufficient to obtain a goodemulsification. This contrasts with earlier ideas thatlamellar phases had to form throughout the entire system,which are subsequently swollen with oil, after which theemulsion forms.5,7,8
2. Materials and Methods2.1. Materials. The system consists of a standard nonionic
surfactant C12E5 (Nikko) and a long-chain alcohol as cosurfactantC12H25OH (Aldrich), which has the same chain length as that of(1) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic:
New York, 1963.(2) Miller, C. A. Tenside Surf. Det. 1996, 33, 191-196.(3) Hackett, J.; Miller, C. A. SPE Res. Eng. 1988, 3, 791.(4) Miller, C. A.; Raney, K. H. Colloids Surf. A 1993, 74, 169.(5) Rang, M. J.; Miller, C. A. J. Colloid Interface Sci. 1999, 209, 179.(6) Rang, M. J.; Miller, C. A. Prog. Colloid Polym. Sci. 1998, 109,
101.
(7) Groves, M. J. Chem. Ind. 1978, 17, 417.(8) Warkley, M.; Pouton, C. W.; Meakin, B. J.; Morton, F. S. In
Phenomena inmixedsurfactant systems; Scamehorn, J. F., Ed.;AmericanChemical Society Symposium Series; American Chemical Society:Washington, DC, 1986; Vol. 311, p 242.
Figure 1. Injection of oil/surfactant systems, under themicroscope, in the water-filled microcopy cell using the droptechnique.
the hydrocarbon tail. Two different oils are used: a long-chainalkane, hexadecane n-C16H34 (Aldrich), and a short-chain alkane,octane n-C8H18 (Aldrich). For the experiments, we prepareddifferent cosurfactant/surfactant ratios (0 to 0.9) for differenttotal concentrations of surfactant (surfactant plus cosurfactant)in the oil: that is, 5, 10, and 15 wt % These solutions are broughtinto contact with ultrapure water from a Milli-Q-Plus apparatus.
2.2. Phase Contrast Microscopy Observations. Thespontaneous emulsification was followed using both phasecontrast and polarized microscopy. The samples were studiedusing the drop technique: a droplet of the oil/surfactant mixtureis injected gently into contact with an excess of pure water in arectangular 30 × 5 mm2 glass cell of 0.5 mm thickness directlyunder the microscope. The injection of the oil/surfactant mixtureis done using a micropipet that was inserted through one of theopen sides of the water-filled microscopy cell (Figure 1).
2.3. Dynamic Surface Tension Measurements. The lowdynamic interfacial tensions that are characteristic for thesespontaneously emulsifying systems were measured using thespinning drop technique. In such an experiment, a small dropletof the oil containing the surfactant is injected in a rotatingcapillary filled with pure water. Note that this experimentalconfiguration is practically the same as the one used for themicroscopy observations (Figure 2). The form of the droplet isconsequently determined by a balance between the centrifugalforces and the surface tension; by measuring the radius R of thedroplet, the interfacial tension can be determined from
where γ is the interfacial tension (N/m), ∆F the density differencebetween the two phases (kg/m3), R the radius (m), and ω theangular speed (rad/s). In view of the fact that the systems weconsider undergo spontaneous emulsification after the two phaseshave been brought into contact, the interfacial tension changesin time (Figure 3). Consequently, the practically instantaneousvalue of the surface tension, that is, 15 s after injection of theoil drop, is taken as a characteristic surface tension for theexperiments.
2.4. Determination of the Droplet Size Distribution. Astandard light diffraction technique was used to determine thesize distribution of the emulsion droplets. For the experiments,a small quantity of oil containing surfactants (5 wt %) is broughtin contact with a large quantity of water (95 wt %) in a test tube.
Figure 2. Principe of the spinning drop technique used for thedynamic interfacial tension measurements.
Figure 3. Time evolution of the dynamic interfacial tension.Total surfactant: 5%. Co/S: 0.3. Oil: C16H34.
Figure 4. Phase contrast microscope observations near the oil droplet (C12E5-dodecanol-hexadecane)/water interface for a totalsurfactant concentration of 10% and Co/S ) 0.3. (a) Formation of bright structures (vesicles) in the system and presence of smallemulsion droplets (black points). Picture size: 200 × 120 µm2. (b-e) Mechanism of spontaneous emulsification: explosion of a brightstructure (vesicle) and pulverization of small emulsion droplets into the aqueous phase. Picture sizes: 24 × 16 µm2.
γ ) 1/4R3∆Fω2
9704 Langmuir, Vol. 16, No. 25, 2000 Shahidzadeh et al.
This was done as gently as possible, to avoid the introduction ofany external mechanical energy into the system. Once thespontaneous emulsification has begun and some quantity ofemulsion has formed (approximately 30 min after the contactbetween the two phases), a small amount is taken from theemulsified part and analyzed by the light-scattering setup. Thedilute solution of droplets (the emulsion), illuminated with alaser beam, scatters the laser light. The angular distribution ofthe light intensity then allows us to determine the droplet sizedistribution. The analysis is consequently performed simulta-neouslyontheentiredropletdistribution in thescatteringvolume.
3. Results
3.1. Phase Contrast Microscopy Observations.With hexadecane as the oil, when the oil/surfactantmixture is brought into contact with the water, theformation of bright structures near the oil/water interfaceisobserved.Thesestructuressubsequentlyexplode (burst),thereby pulverizing small emulsion droplets into theaqueous phase (Figure 4). These droplets have a size thatis close to the limit of detection of the microscope, roughly1 µm, as will be confirmed below by the determination ofthe droplet size distribution.
Counting the number of microdroplets after severalexplosions shows that the total volume of emulsiondroplets (of roughly 1 µm size) left after the bursting ofone of the bright structures is much smaller than that ofthe structure itself. The counting shows that the volumeof droplets is about 10% of that of the initial structure.The droplet size distributions discussed below show, inaddition, that it is unlikely that there is an additionalpopulation of much smaller droplets that is so small thatit is not observable under the microscope, and thus thatwe would have missed in the counting.
An additional observation that has to be taken intoaccount is that, although the oil/surfactant mixture isinitially transparent, if a very small amount of water isadded to the surfactant mixture in the oil, the formationof vesicles is observed under the microscope (Figure 5). Itshould be noted that the observation of vesicles uponaddition of water does not exclude the possibility thatthese already exist also in the absence of water: they maysimply be too small to be observed.
The observation of vesicles implies that, in the presence
Figure 5. Phase contrast microscopy: vesicle formed in theoil/surfactant mixture after addition of a tiny amount of water.Total size is 32 × 32 µm2.
Figure 6. Phase contrast micrographs for a total surfactant concentration of 10% and a Co/S ratio of 0.3 (a) Large oil (C12E5-dodecanol-hexadecane) droplet in pure water. Formation of oil microdroplets in the aqueous phase around the large droplet. (band c) Picture sizes: 130 × 80 µm2. Zoom on the side of the oil droplet/water (b) bursting process at the oil/water interface. (c) Ejectionof microdroplets after the explosion in the aqueous phase. No bright structures are observed in the aqueous phase.
of water, oil-filled vesicles form in the oil-rich phase. Itthus seems likely that these vesicles are the brightstructures that eject part of their oil into the aqueousphase when they explode. These vesicles burst when theyreach the oil/water interface; the water will penetraterapidly into the bilayer, destabilizing the vesicle, whichsubsequently explodes. Upon the “explosion”, a part ofthe interior of the vesicle is expelled in the form of oilmicrodroplets, that disperse in the aqueous phase. If onelooks in a plane below the oil droplet, sometimes small oildroplets of an adjacent explosion are observed simulta-neously with the bright structures (Figure 4a). If one looksat the side of the large oil droplet, no bright structures areobserved in the aqueous phase, although many oil mi-crodroplets resulting from explosions at the oil/waterinterface can be observed (Figure 6). It thus seems clearthat the formation of vesicles in the oil phase is importantfor spontaneous emulsification.
Using octane as the oil, the findings are very different(Figure 7). After bringing into contact the surfactant/oilmixture with the water, the oil droplet undergoes asequence of “fingering” instabilities. The unstable tonguesof oil that result in the aqueous phase break up very rapidlyinto a very large amount of droplets. These droplets appearto be linked to each other by small filaments which are
formed of surfactant bilayers. This can be concluded fromthe behavior at later times: the filaments are observedto shrink very rapidly, leading to the coalescence of thesmall droplets, and at later times again, the formation ofa large droplet is observed. This large droplet is probablya mixture of oil, water, and surfactant. Consequently, noevidence is obtained for the formation of a dispersion ofthe small oil droplets into the aqueous phase in the absenceof shear: there is no spontaneous formation of an oil-in-water emulsion.
3.2. Dynamic Surface Tension Measurements.Dynamic interfacial tensions were measured as a functionof the ratio Co/S of cosurfactant (dodecanol) to surfactant(C12E5), and for three different total concentrations Co +S in oil. The results for hexadecane are shown in Figure8: a very pronounced decrease, leading to ultralowinterfacial tensions, followed by a similar very pronouncedincrease is observed. This is very similar to the behaviorof the equilibrium interfacial tension for microemulsion-forming systems,9 as will be discussed below. From themeasurements it follows that increasing the total con-centration Co + S leads to lower interfacial tensions at
(9) Sottman, T.; Strey, R. J. Chem. Phys. 1997, 106, 20.
Figure 7. Phase contrast micrographs of C12E5-dodecanol-octane (total surfactant concentration 5%; Co/S ) 0.4). (a) After contactwith pure water the oil droplet undergoes some instabilities. Tongues break up into a very large amount of droplets. Picture size:240 × 120 µm2. (b) The droplets are linked by small filaments consisting probably of surfactant bilayers. (c) Shrinking of thefilaments followed by the coalescence of the small droplets and the formation of a large droplet again. There is no spontaneousformation of oil in water emulsions. Picture sizes for parts b and c: 140 × 90 µm2.
9706 Langmuir, Vol. 16, No. 25, 2000 Shahidzadeh et al.
the tension minimum and a shift of the minimum to alower ratio Co/S.
With octane as the oil, results are obtained that arevery similar to those with hexadecane as the oil. Compar-ing with that system (Figure 9) leads to the conclusionthat for octane the tension minimum occurs at a lowerCo/S ratio than that for hexadecane.
3.3. Determination of the Droplet Size Distribu-tion. The droplet size distribution was measured for thesystem with hexadecane as the oil as a function of theratio Co/S of cosurfactant (dodecanol) to surfactant (C12E5).The distributions are shown in Figure 10. It is observedthat two clearly distinct populations of droplets exist fora low Co/S ratio, very small droplets of average radius 0.3µm and very large ones of average radius 29 µm. Whenincreasing the ratio Co/S, the number of very large dropletsdecreases. However, the size of the small droplets increasesa little from 0.3 µm for the lowest Co/S ratio to 1 µm forthe highest ratio. For the high ratios, the goal of havinga good spontaneous emulsification seems to be attained:a narrow distribution of small droplets is found and nolargedropletsappear tobe formed.Evenmore importantly,these emulsions have a rather good stability; one has towait for more than a day before a second populationappears again. The results show that, after 6 days, thepopulation of micrometer-sized droplets is replaced by two
populations, one with smaller (0.5 µm) droplets and asecond population of very large droplets (40 µm), this timeprobably formed by creaming (Figure 11). The observationof the formation of smaller droplets from larger ones ispuzzling but seems reproducible from experiment toexperiment. One possible explanation of this observationwould be the Oswald ripening of the emulsion droplets.
4. Discussion and ConclusionIt is useful to recall that the equilibrium phase behavior
of microemulsion-forming surfactant systems follows from
Figure 8. Dynamic interfacial tension measurements as afunction of dodecanol/C12E5 ratio for three different totalsurfactant concentrations (wt %) in hexadecane.
Figure 9. Comparison between the dynamic interfacial tensionmeasurements as a function of dodecanol/C12E5 ratio for twooils: hexadecane and octane at the same total concentration(5 wt %).
Figure 10. Droplet size distribution after spontaneous emul-sification as a function of dodecanol/C12E5 ratios: dodecanol/C12E5 ) (a) 0.1, (b) 0.2, and (c) 0.4 for a total surfactantconcentration of 10 wt % in hexadecane.
Figure 11. Stability of the emulsion. Spontaneous emulsifica-tion of hexadecane with a dodecanol/C12E5 ratio of 0.4 and atotal surfactant concentration of 10 wt % leads to the followingsize distributions of the droplets: (a) after 1 h, one populationof average size 1 µm; (b) after 6 days, two populations, R1 ) 0.5µm and R2 ) 40 µm.
the preferential curvature of the surfactant monolayers(towards either the oil or the water) and is consequentlydetermined by the head and tail group area of thesurfactant molecules.10 If the head area is the larger ofthe two, an oil-in-water microemulsion is preferred; if thetail group is larger, a water-in-oil microemulsion is formed.If the two are nearly balanced, in the region of the phasediagram where the tension is ultralow, a third, interme-diate oil/water/surfactant phase is formed which can bean LR (lamellar) or bicontinuous microemulsion phase.9-11
The equilibrium interfacial tension in these systems9
behaves exactly like the dynamic interfacial tension inour system. It was established recently that there is adirect connection between the equilibrium interfacialtension and the way the spontaneous emulsificationproceeds.12 It is therefore tempting to see whether thedynamic interfacial tension can also be indicative of thespontaneous emulsification mechanism. To do this, weassociate the descending part of the dynamic interfacialtension curve with a preference for the formation of oil-in-water (micro-) emulsions and the ascending part witha preference for water-in-oil (micro-) emulsions.
If these identifications are made, the ratio Co/S usedfor the measurements on the octane and hexadecanesystems then shows us that for the former system a water-in oil emulsion would be preferred, whereas for the lattera oil-in-water emulsion would be favorable.
For the hexadecane system, upon contact of the sur-factant/oil mixture with water, vesicles are formed thatexplode. If we compare with the “preferred” situation (fromthe dynamic interfacial tension curve), the oil togetherwith the surfactant molecules has to migrate into the waterphase in order to form an oil-in-water (micro-) emulsion.In agreement with this, the microscopy observations thatare made for Co/S < 0.4, show that the formation of localbilayers structures (vesicles) and their bursting lead tothe dispersion of oil microdroplets containing the sur-factant in the aqueous solution: an oil-in water emulsionforms spontaneously.
For the octane system, as the “preferred” situation isexactly the opposite, a water-in-oil emulsion, water should
penetrate to the oil/ surfactant phase. This penetrationseems to happen by passing through an intermediate stepin which the surfactant reorganization as bilayers (fila-ments13,14) also plays an important role. However, no oil-in-water emulsion is formed, as could indeed be anticipatedfrom the dynamic interfacial tension curve.
The dynamic interfacial tension thus turns out to be agood diagnostic for the ability of a system to spontaneouslyform an oil-in-water emulsion. This was verified by lookingat the spontaneous emulsification process of the hexa-decane system at different ratios of surfactant to cosur-factant. In addition, the tension measurements allow usto control the droplet size distribution for the final oil-in-water emulsion. The droplet size distribution measure-ments show that, as the interfacial tension is too high, apopulation of large droplets exists. However, if the dynamictension is close to its ultralow value, one obtains a singlerather narrow population of emulsion droplets, of radiusaround 1 µm.
The visualization in addition provides us with detailedinformation on the emulsification mechanism. The resultsobtained here show that the local formation of vesicles issufficient to obtain a good spontaneous emulsification. Asalready mentioned before, this contrasts with earlier ideasthat the whole system had to form lamellar phases thatwere subsequently swollen with oil, after which theemulsion formed.5,7,8
In conclusion, the formation of bilayer structures thatburst seems to be an essential ingredient of spontaneousemulsification in general. In addition, an excellent indica-tion for the efficiency of spontaneous emulsion formationis provided by the dynamic interfacial tension betweenthe two phases, a short time after they have been broughtinto contact. Those measurements can be related to boththe droplet size distribution and emulsion stability andto the capability for a given system to form oil-in-wateremulsions rather than the inverse. We show that theability of a solution to form an oil-in-water emulsion canin fact be controlled by controlling the ratio of surfactantto cosurfactant. Therefore, this should allow for optimiza-tion of a number of spontaneous emulsification processes.
LA000493L(10) Israelachvili, J. N. Intermolecular and Surface Forces; Aca-demic: London, 1992.
(11) Hendrikx, Y.; Kellay, H.; Meunier, J. Europhys. Lett. 1994, 25,735.
