The review addresses the influence of polyelectrolytes on the stabilisation of free-standing liquid foam
films, which affects the stability of a whole macroscopic foam. Both the composition of the film surface
and the stratification of the film bulk drives the drainage and the interfacial forces within a foam film.
Beside synthetic polyelectrolytes also natural polyelectrolytes like cellulose, proteins and DNA are
considered.
Introduction
Polyelectrolyte–surfactant mixtures are essential components ofmany products such as detergents, paints and shampoos.1 It isimportant to understand the interactions of such species bothin the bulk and at the surfaces to predict their physicochemicalbehavior and to optimize their performance. According to theapplication different strengths of interactions between poly-electrolytes and surfactants are required. For instance fordecalcication processes strong attraction between poly-electrolytes and surfactant is necessary in order to remove thepolyelectrolyte–calcium complexes. In contrast for cleaningproducts including a polymer for surface protection attraction
eiko Fauser studied chemistryt the University of Stuttgartnd the Tokyo Institute of Tech-ology. From 2010 to 2014, heas been a PhD student in theroup of Prof. Dr Regine vonlitzing at the Stranski Labo-atorium of Physical and Theo-etical Chemistry at theechnical University of Berlin.is research interests have beenoam lm behavior and surfaceomplexation of surfactant ands. He is now working for Pan-any.
Theoretische Chemie, Institut fr Chemie,
17.Juni 124, D-10623 Berlin, Germany.
9-30-3142-6602; Tel: +49-30-3142-3476
hemistry 2014
between polymers and surfactants has to be avoided. However,the interactions between polyelectrolytes and surfactants arequite manifold and include always hydrophobic interactions.Depending on the system electrostatic interaction or hydrogenbonding can be predominant. The performance of these poly-mer surfactant systems is oen based on the foamingproperties.
The stability of a foam depends on the stability of single thinlms formed by the continuous liquid phase. They separate airbubbles which present the dispersed phase. During foaming thedynamics within the foam lm, i.e. drainage plays a decisiverole. This is strongly related to rheological properties of the lmsurface and the lm bulk.2,3 In the last period of drainageinteractions between the opposing surfaces become more andmore important. They are summarized by the disjoining pres-sure P and are mainly affected by the composition of the lmsurface and the structuring of complex uids in the lm bulk.
Regine von Klitzing studiedphysics at Technical Universityof Braunschweig and Universityof Gottingen. Aerwards shespecialized in physical chemistryat Institute of Physical Chem-istry, Mainz, and nished herPhD in 1996. From 1996 to 1997she was a post-doc at Centre deRecherche Paul Pascal (Pessac/Bordeaux), then assistantresearcher and lecturer atStranski Laboratorium of Phys-
ical and Theoretical Chemistry, TU Berlin (1998–2003). In 2004she was group leader at Max-Planck-Institute for Colloids andInterfaces, Potsdam, then held a professorship in physical chem-istry at Kiel University (2004–2006). She is now full Professor ofapplied physical chemistry at TU Berlin.
Fig. 1 Scheme of a the structure of free-standing polymer–surfactantfoam film (a) with strong attraction between polyelectolyte andsurfactant leading to surfacae aggregates, (b) with no interaction oreven repulsion between both compounds and (c) formation of atransient polymer network. The mesh size x of the network in the filmbulk is related to stratification steps Dh.
Fig. 2 Scheme of a free-standing film in a Thin Film Pressure Balance(TFPB) (left) and of a film between two solid surfaces in a ColloidalProbe-AFM (CP-AFM) (right).
Soft Matter Review
Publ
ishe
d on
11
July
201
4. D
ownl
oade
d by
TU
Ber
lin -
Uni
vers
itaet
sbib
l on
24/0
2/20
16 1
6:18
:26.
View Article Online
The present review addresses foam lms containing poly-electrolytes. Their tendency to adsorb at the lm surface is oenmediated by additional surfactants (part (a) in Fig. 1). It isassumed that the adsorption of polyelectrolytes at the lmsurfaces has a strong effect on the stability of foam lms due toincrease in the elasticity and the change in charge of the lmsurface. Of course, depending on the charge combination ofsurfactant and polyelectrolyte also no interaction or evenrepulsion can take place leading to a pure surfactant layer at thelm surfaces (part (b) in Fig. 1) The rst part of the review dealswith the effect of the composition of the lm surfaces on thestabilisation of foam lms. Polyelectrolytes can form a type oftransient network above the overlap concentration c* inaqueous solutions which causes stratication of foam lms(part (c) in Fig. 1). This phenomenon is rather related to theproperties of the lm bulk and is described in the second part ofthe review. Again the surface properties are important since theformation of polymer–surfactant complexes at the lm surfacescan affect the velocity of stratication. The stratication hamperthe drainage and affects the stability of the foam lms.
1 Methods
The disjoining pressure is an excess pressure within the thinlm with respect to the pressure of the liquid in the meniscus. Itcan be measured with a Thin Film Pressure Balance (TFPB) independence of the lm thickness h resulting in a disjoiningpressure isotherm P(h). The TFPB with porous plate techniquewas developed by Mysels4 and Exerowa.5,6 It is mainly used tostudy foam lms, but recently it has been also extended to studywetting lms.7,8 Thereby the foam lm is formed over a 1 to 2mm hole that is drilled into a porous glass plate. It is enclosedwithin a pressure-controlled cell and connected to the outeratmospheric pressure. The lm thickness is measured inter-ferometrically. More detailed information about TFPB andinteraction in foam lms are given in former reviews.7,9,10
By increasing the pressure within the cell against the atmo-spheric pressure the lm starts to drain until repulsive inter-actions (positive disjoining pressure) between the opposingsurfaces prevents further thinning. In mechanical equilibrium
6904 | Soft Matter, 2014, 10, 6903–6916
the disjoining pressure compensates the capillary pressure.Typical repulsive interactions are electrostatic or steric ones, themain attractive contributions are based on van der Waalsforces. Structuring of mesoscopic objects like micelles ormacromolecules leads to oscillatory disjoining pressure curves.The mesoscopic object are expelled layer-wise from the lmwhich causes alternating attraction (depletion forces) andrepulsion, i.e. oscillatory force. Due to the fact that the TFPBmeasures only the repulsive parts of the force oscillation, onlyjumps in lm thickness are detected. This step-like thinning iscalled stratication of the foam lms.
Another method to study the structuring of complex uidsunder connement is a Colloidal Probe-AFM (CP-AFM).11
Instead of a tip, a several micrometer large Silica sphere(colloidal probe) is glued at a cantilever and the force ismeasured between the sphere and a planar Silicon waferthrough the complex uid. In contrast to a TFPB a CP-AFMallows measuring the full oscillation between two solid surfacesif a so cantilever is used. Therefore CP-AFM is preferred toTFPB for studies of the structuring of complex uids underconnement in thin liquid lms. Nevertheless, a TFPB can bebetter used to quantify the non-equilibrium dynamics/drainagebetween uid surfaces. Fig. 2 presents a scheme of the free-standing lm in a TFPB and of a lm between two solid surfacesin a CP-AFM.
2 Influence of surface compositionon foam film stability
Most of the polyelectrolytes in water do not form stable foamlms due to a missing amphiphilic character. Surfactants haveto be added to form stable lms. In this case the interactionbetween surfactant and polyelectrolyte plays a decisive role onthe drainage and stability of foam lms and therefore of themacroscopic foam. This chapter addresses the correlationbetween surface properties and the stability of foam lms. Thesurface properties of aqueous polymer–surfactant mixtures aremainly determined by tensiometry.12,13 Other methods are X-rayand neutron reectometry14,15 and surface rheologytechniques.16,17
Depending on the charge combination of the used poly-electrolytes and surfactants, either an electrostatically stabilizedcommon black lm (CBF) or a sterically (entropically) stabilized
Newton black lm (NBF) is formed as a nal state before lmrupture.10
2.1 Oppositely charged polyelectrolytes and surfactants:formation of surface aggregates
This chapter addresses aqueous mixtures containing oppositelycharged surfactants and polyelectrolytes.
2.1.1 Surface tension. Firstly, a short overview is givenabout general aspects of the effect of polyelectrolytes on thesurface tension. Fig. 3 shows a characteristic surface tensionisotherm for a mixture of oppositely charged polyelectrolytesand surfactants. Surface tension curves are usually depictedwith xed polyelectrolyte concentration and varied surfactantconcentration. A typical concentration range is from 10�4 to10�2 (mono)mol l�1 (polyelectrolyte concentration refers to theconcentration of monomer units) for the polyelectrolyte and10�6 to 10�1 mol l�1 for the surfactant. The general features of asurface tension curve of oppositely charged polyelectrolytes andsurfactant are well explained in the book of Goddard andAnanthapadmanabhan:19 already at low surfactant concentra-tions, the addition of polyelectrolytes leads to the formation ofsurface active complexes that lower the surface tensioncompared to the one of the pure surfactant solution. A plateauin surface tension starts close to the critical aggregationconcentration in bulk (cac). In this concentration regime, addedsurfactant is incorporated into bulk aggregates and it does notadsorb at the surface. Related to this the solution is oen turbidand no homogeneous foam lms can be formed above the cac.Therefore, the concentrations for polyelectrolyte and surfactantshould be kept below the cac for quantitative analysis of dis-joining pressure isotherms. Upon further addition of surfac-tant, the surface tension decreases until the critical micelleconcentration (cmc) is reached. In this surfactant concentrationregime the slope of the surface tension curve is almost the sameas for the pure surfactant system which leads to the conclusionthat no polymer is adsorbed at the surface anymore. An expla-nation could be that the surface aggregates become
Fig. 3 Surface tension isotherm for a pure cationic surfactant versusan oppositely charged surfactant-polyelectrolyte mixture. At theabscissa the surfactant concentration. “PAMPS25” refers to 25%charged PAMPS. The PAMPS25 concentration of 750 ppm corre-sponds to a concentration of 3.5 � 10�3 (mono)mol l�1. The graph istaken from ref. 18.
hydrophobic, they desorb and precipitate leading to a puresurfactant surface layer.
So far, the general aspects of the surface tension curves aredescribed. Depending on the specic system, there are severalspecic features in the surface tension curves in terms of cac,width of the surface tension plateau and cmc.13 Examples aregiven in the following.
A strong synergistic lowering of the surface tension is foundfor mixtures of the cationic surfactant dodecyltrimethylammonium bromide (C12TAB) and the anionic polymer poly-(acrylamidomethylpropanesulfonate) sodium salt (PAMPS)12,18
as shown in Fig. 3. In presence of PAMPS the surface tension isnot affected by the amount of polymer for the concentrationrange studied. This is explained by polymer stretching at theair–water interface in order to form a neutral complex with thesurfactant. Also, the cmc is not affected by the addition ofpolyelectrolytes.
In contrast to this, for C12TAB–polystyrene sulfonate (PSS)mixtures the plateau region (i.e. cac) is very sensitive to the ratiobetween surfactant and polyelectrolyte concentration and to theabsolute concentrations (Fig. 4). With increasing polyelectrolyteconcentration the surface tension decreases, the region of theplateau becomes broader and the plateau is shied to highersurfactant concentrations.10 This is explained by highersurfactant concentration that is required to form hydrophobicbulk complexes at a higher polyelectrolyte concentration.
The differences to PAMPS (where no effect of the PAMPSconcentration could be detected) is explained by the strongerhydrophobicity and a more bulky molecular structure of PSS incomparison to PAMPS. Hydrophobic effects become even moreevident for mixtures with C16TAB–PSS. PSS even increases thesurface tension of the C16TAB solution, since the aliphaticsurfactant chain interacts with the hydrophobic PSS backbone.Hydrophilic complexes are formed which reduces the adsorbedamount with respect to pure C16TAB.20
Beside the polyanion concentration also the polyanioncharge affects the plateau in the surface tension. As an examplea mixture of the cationic C12TAB and the anionic polysaccharidecarboxymethylcellulose (carboxyCM)21–24 is given. CarboxyMC isa water-soluble random block copolymer derivative of cellulose.
Fig. 4 Surface tension measurements of PSS/C12TAB solutions withdifferent PSS concentrations. The graph is taken from ref. 10.
Fig. 5 Stability of PAMPS/C14TAB foam films for systems with differentIEP. C IEP ¼ 3 � 10�5 mol l�1; D IEP ¼ 1.6 � 10�4 mol l�1; B IEP ¼10�4 mol l�1. The graph is taken from ref. 17.
Soft Matter Review
Publ
ishe
d on
11
July
201
4. D
ownl
oade
d by
TU
Ber
lin -
Uni
vers
itaet
sbib
l on
24/0
2/20
16 1
6:18
:26.
View Article Online
It can be obtained from cellulose (consisting of b-D-glucoseunits) by substituting the hydroxyl-groups with sodiumcarboxyl. By degree of substitution the charge density can bevaried. With increasing degree of charge the plateau is extendedover a broader surfactant concentration regime, since moresurfactant is needed to hydrophobise the polyanion–surfactantcomplexes.
In presence of the oppositely charged polyelectrolyte the cmccan be shied as shown in Fig. 4. On the one hand poly-electrolytes can act as a salt which leads to a decrease in cmcdue to electrostatic screening of the surfactant charges. On theother hand the aggregation of polyelectrolytes and surfactantscauses a decrease in effective concentration of free surfactantmolecules which increases the cmc.
Most of the studies investigate the combination of cationicsurfactant and polyanion. The addition of polycations to ananionic surfactant was also studied and show basically the samegeneral features as for the opposite charge combination.7,25
The different examples for oppositely charged poly-electrolytes and polyanions show that beside general featuresthere are many system specic effects. The most dominantfactors affecting the surface tension are the ratio betweensurfactant and polyelectrolyte concentration, the all overallconcentration and molecular parameters like charge densityand bulkiness of the polyelectrolyte. Furthermore the hydro-philic/hydrophobic balance of polyelectrolytes and surfactantplay an important role since it determines whether electrostaticor hydrophobic interaction dominate the formation ofcomplexes. This in turn decides about the hydrophobicity of thecomplexes.
2.1.2 Foam lms. The addition of polyanions (e.g. PAMPS,PSS) to C12TAB solutions has a strong effect on the foam lmstability. Although no foam lms can be stabilized from pureC12TAB solutions at concentrations below its critical micellarconcentration (cmc: 1.5 � 10�2 mol l�1),26 the addition ofPAMPS or PSS to even very low concentrations of C12TAB resultsin stable foam lms.16,27,28 This stabilization results from the co-adsorption of polymer–surfactant complexes at the interface,which reduces the surface tension. On the other hand, thesurface tension itself is not always a measure for the stability.For instance sodium dodecyl sulfonate (SDS) solutions with asurface tension of about 70 mNm�1 i.e. close to the one of purewater can form stable foam lms, while C12TAB cannot. Thisphenomenon hasn't been fully claried, yet. An important sizewhich decides about foam and foam lm stability is the surfaceelasticity. A high elasticity is assumed to suppress surfaceundulations and leads to more stable lms.29 So far, most of thesurface rheology experiments are carried out for pure surfactantsystems and only for one type of surfactant, e.g. CnTAB.30,31 Asour knowledge a comparison between surface rheology data ofdifferent surfactant systems and their impact on foam lmstability is still missing. It is even more complex for surfacerheology data of polymer–surfactant mixtures. Only a fewstudies exist (e.g.16,17,32) and there is still a lack of understandingthe correlation with lm stability as shown below.
The stability of foam lms is very sensitive to the hydro-philic/hydrophobic balance of the added surfactant. Stable
6906 | Soft Matter, 2014, 10, 6903–6916
foam lms for mixtures beyond 3.8 � 10�3 (mono)mol l�1 car-boxyMC with 10�4 mol l�1 C12TAB were reported.33 In case ofC14TAB and C16TAB a signicantly lower surfactant concentra-tion of 10�5 mol l�1 was sufficient to obtain stable lms. Filmsmade with C12TAB were less stable than those with C14 orC16TAB. For all combinations only electrostatically stabilizedCBFs were formed. But in all cases stratication occurred.Remarkably, trends exhibited by the stratication kinetics wereopposite to the trends in stability. A deeper insight into thisphenomena is given in Section 3.
2.1.3 Role of the isoelectric point (IEP). In case of CnTAB–polyanionmixtures a CBF is formed and stable up tomoderatelyhigh pressures. Since CBFs are stabilized by electrostaticrepulsion between the two opposing interfaces, either thesurfactant or the polyelectrolyte should determine the sign ofsurface charge. In order to study the origin of surface chargeKristen et al.32 measured disjoining pressure isotherms fordifferent combinations of C14TAB and fully charged PAMPS100.In contrast to the more common protocol in which the polymerconcentration is xed and the amount of surfactant is varied,foam lms from solutions of a xed surfactant concentration(10�4 mol l�1) and variable polyelectrolyte concentration wereinvestigated. Before the studies started the working hypothesiswas the following: for concentrations with excess of C14TAB itwas assumed that the surface is positively charged, whereas foran excess of PAMPS the surfaces should be negatively charged.When both concentrations are equal the charges shouldcompensate each other what is referred to as the nominalisoelectric point (IEP). At the nominal IEP, where electrostaticrepulsion between the opposing lm surfaces should bereduced, two scenarios are possible: formation of a NBF ordestabilization of the lm. The surface charge can be tuned bythe variation of the added polyelectrolyte and as a consequencethe stability of the foam lm may be affected. Fig. 5 displays thestability of the foam lms. In this graph the maximum appli-cable disjoining pressure before lm rupture versus the poly-electrolyte concentration is shown. The nominal IEP for themixture with 10�4 mol l�1 C14TABmixture (B) is at 10�4 (mono)mol l�1 PAMPS100. The graph shows that stable lms areformed below the IEP. With increasing polyelectrolyte concen-tration the stability decreases up to a minimum close to the IEP,
Fig. 6 Effect of PAMPS25 on AOT surface tension isotherm with andwithout added salt. B pure AOT; C AOT + 3.5 � 10�3 (mono)mol l�1
PAMPS25;> AOT + 10�1 mol l�1 NaCl; black diamond: AOT + 10�1 mNaCl + 3.5 � 10�3 (mono)mol l�1 PAMPS25. The graph is taken fromref. 12.
Review Soft Matter
Publ
ishe
d on
11
July
201
4. D
ownl
oade
d by
TU
Ber
lin -
Uni
vers
itaet
sbib
l on
24/0
2/20
16 1
6:18
:26.
View Article Online
where no lm can be formed. The reason for the fact that theminimum in stability is not exactly at the nominal IEP could bethat the surfactant is not completely dissociated or that thesurfactant–polyelectrolyte mixing ratio at the surface is differentfrom that in the bulk. Beyond the IEP the lm stability increasesagain. All lms formed were CBF and no NBF formation wasobserved.
Obviously, the concentration ratio between surfactant andoppositely charged surfactant plays a decisive role. Furtherstudies investigated the inuence of the surfactant concentra-tion and the charge degree of the polymer on foam lmsstabilized by C14TAB and PAMPS.17 Decreasing the degree ofpolymer charge results in a shi of the stability minimum tohigher polyelectrolyte concentrations as shown in Fig. 5. Adecrease in surfactant concentration leads to a shi in thestability minimum towards lower polyelectrolyteconcentrations.
From the stability measurements it seems reasonable thatthe foam lms are stabilized by cationic net charges below andby negative net charges above the IEP due to charge reversal.This would imply a monotonous increase in adsorbed amountof polyanion with increasing polyanion concentration. Addi-tional experiments investigating surface tension and surfacedilatational elasticity revealed that this image is much toosimple :16,17 close to the IEP the amount of adsorbed materialseems to be the highest in the measured concentration regimebut is decreases towards lower and higher concentrations.Obviously, polymer concentration regimes with high surfacecoverage but low net charge show low stability, whereas inregions of low surface coverage very stable foam lms wereobserved. This result indicates that the net charge within thefoam lm plays an important role in foam lm stabilization.
It has to be taken into account that the nominal IEP of thesystems can deviate from the IEP of the surface. Unfortunately,the IEP of the lm surface is difficult to access.
Fig. 7 Disjoining pressure isotherms for (a) 5 � 10�3 mol l�1 AOTsolutions, with and without 10�1 mol l�1 NaCl added, and (b) AOTsolutions, with and without 3.5� 10�3 (mono)mol l�1 PAMPS25 added.750 ppm correspond to 3.5 � 10�3 (mono)mol l�1. The graph is takenfrom ref. 18.
2.2 Mixtures of non-aggregating polyelectrolytes andsurfactants
Mixtures of polyelectrolytes with equally charged or nonionicsurfactants are supposed to interact weakly with each other.Surprisingly, for both cases stable foam lms could beobserved.34,35 Foam lms formed from CnTAB–carboxyMCmixtures are less stable compared to mixtures with the anionicsurfactant dioctyl sulfosuccinate (AOT). This leads to theconclusion that the presence of surface complexes of surfac-tants and polyelectrolytes is not the only reason for a stabili-sation of foam lms of polyelectrolyte–surfactant mixtures.
2.2.1 Equally charged polylectrolytes and surfactants.Langevin and coworkers intensively studied mixtures of theanionic surfactant AOT with the anionic polyelectrolytePAMPS25.18,12 Fig. 6 and 7 show that the addition of PAMPS25 tothe equally charged AOT has an almost negligible effect onsurface tension, and thickness and stability of the foam lm.The surface tension curves show clearly that no surfacecomplexes of AOT and PAMPS are formed, since the plateau ismissing, but that the polyelectrolyte acts rather like a simple
salt. The addition of 0.1 M NaCl has a much stronger effect onsurface tension and lm thickness than the addition of 3.5 �10�3 (mono)mol l�1 PAMPS. The cmc is shied to lowersurfactant concentrations and the foam lm becomes thinnerdue to electrostatic screening.
Fig. 9 Disjoining pressure isotherms of a free-standing pure C16TABfilm versus a film stabilized by the mixed system C16TAB/PDADMAC.C16TAB: 10
�2 mol l�1 and PDADMAC: 5 � 10�3 (mono)mol l�1. Thegraph is taken from ref. 10.
Fig. 10 Disjoining pressure isotherms of a free-standing pure C12G2
film versus a film stabilized by themixed systemC12G2/PDADMAC. TheC12G2 concentration is 5 � 10�5 mol l�1 and the PDADMAC concen-tration 5 � 10�3 (mono)mol l�1. The graph is taken from ref. 10.
Soft Matter Review
Publ
ishe
d on
11
July
201
4. D
ownl
oade
d by
TU
Ber
lin -
Uni
vers
itaet
sbib
l on
24/0
2/20
16 1
6:18
:26.
View Article Online
In all cases the disjoining pressure isotherms reveal CBFsstabilized by an electrostatic double layer.
2.2.2 Equally charged vs. non-ionic surfactants.Mixtures ofthe cationic polyelectrolyte poly(diallyldimethylammoniumchloride) (PDADMAC) with either equally charged cationicsurfactant (C16TAB) or nonionic surfactant dodecyl-a-maltoside(C12G2) show a completely different stability and disjoiningpressure as for thin foam lms stabilized by the respective puresurfactants.36–38
On one hand, the addition of PDADMAC to one of thesurfactants has no signicant inuence on the surface tensionisotherms (Fig. 8). No surface active complexes are formed inboth cases.
On the other hand, signicant differences in disjoiningpressure isotherms are observed. Disjoining pressure isothermsof foam lms stabilized by 10�4 mol l�1 of pure C16TAB and by amixture of 10�2 mol l�1 C16TAB and 5 � 10�3 (mono)mol l�1
PDADMAC are shown in Fig. 9. Both pure surfactant and theC16TAB–PDADMAC mixture lead to a CBF and no transition to aNBF was observed. However, aer addition of PDADMAC theobserved foam lms rupture at lower pressure, although theelectrostatic repulsion should be stronger due to additionalpositive charges. It is assumed that the mobility and uctuationof polyelectrolyte chains might reduce the lm stability.
Foam lms stabilized by nonionic surfactants at concentra-tions well below its cmc (5 � 10�5 mol l�1) form a CBF (Fig. 10).The electrostatic stabilization is explained by negative chargesat the air–water interface resulting from OH� adsorption.8,39,40
Aer the addition of 5 � 10�3 monoM PDADMAC a CBF to NBFtransition is already induced at low pressures at about 800 Pa.In this state the lm is only a few nm thick. The NBF is not verystable and ruptures aer a fewminutes at 800 Pa. A formation ofa NBF is also observed for pure C12G2 solutions for highsurfactant concentrations.9 This is explained by the replace-ment of negative charges at the air–water interface by nonionicsurfactants. Another possibility to induce a CBF to NBF transi-tion is the addition of an electrolyte.41 In both cases theobserved NBF is much more stable than the one observed forthe PDADMAC–C12G2 mixture. This discrepancy is explained by
Fig. 8 Dependence of surface tension on the surfactant concentra-tion of the pure surfactant solution (open symbols) and after theaddition of 5 � 10�3 (mono)mol l�1 PDADMAC and PDADMAC/C12G2.Graph is taken from ref. 10.
6908 | Soft Matter, 2014, 10, 6903–6916
a higher ordering of the surfactant molecules at the lm surfacefor pure surfactant solutions. In this case the NBF is highlyordered and forms a crystalline thin lm.42 Since PDADMACdoes not adsorb at the surface the NBF formation is explainedby electrostatic screening of the negative charges at the surfaceby PDADMAC. Nevertheless, the resulting structure within aNBF seemed to be less ordered. This might be due to a lowerpacking density of surfactants or the uctuation of polymerchains which leads to a lower stability of the resulting NBF. Inboth cases stratication occurs for mixtures of PDADMAC withequally or uncharged surfactants at concentrations studied(see Chapter 3).
To summarize, this example clearly shows that the foam lmstability can be easily tuned by the charge combination ofpolyelectrolytes and surfactants. It also shows that the type ofblack lm (NBF or CBF) plays a decisive role for the lmstability. In contrast to pure surfactant lms the NBF is notstable in presence of polyelectrolytes.
2.3 Foam lms stabilized by proteins
Proteins are natural polymers consisting of charged hydrophilicsegments and neutral hydrophobic segments. In solution, they
frequently adopt a compact structure, which can unfold duringadsorption at interfaces. If the bulk concentration is highenough, stable foam lms can be formed.43
Foam lms formed from partially aggregated proteins areeither uid- or gel-like, depending on the aggregate size and theamount of aggregates.44,45
Fig. 11 shows photos of foam lms at a xed ratio betweenaggregated and non-aggregated proteins, but for differentaggregate sizes. The photos show a clear increase in heteroge-neity with increasing aggregate size. In ref. 45 a lm phasediagram is presented which accounts the different features offoam lms stabilized by b-lactoglobulin solutions with respectto the concentration and size of the protein aggregates. Thisphase diagram reveals the existence of a critical amount ofaggregates above which a gel-like network forms within thelm. This amount depends on the aggregate size and concen-tration of the protein. Below this concentration, foam lms arestill inhomogeneous but with isolated mobile structures at thelm surface. In all cases, gel-like lms are more stable thanuid lms. The stability of the foam lm highly correlates withthe stability of the resulting macroscopic foam.
The most simple explanation for the higher stability of gel-like lms is the higher viscosity, which reduces the drainagevelocity with respect to uid lms. In addition larger aggregatesor networks can block the Plateau borders slowing down thedrainage as well. For instance “blocking of Plateau borders” wasalso used to switch the foam stability by using thermosensitivevesicles, which can change the shape.46,47 In addition, network-like structure at the surface leads to higher surface elasticitywhich stabilizes the lm due to easier reduction of local uc-tuations. In addition, larger protein aggregates at the lmsurface contribute stronger to electrostatic repulsion andtherefore to lm stability than small aggregates.
Foam lms of sodium caseinate solutions48 show similarbehavior. The inhomogeneities are mobile at low concentra-tion, but they do not longer move when they become inter-connected above a certain concentration. Again, this leads to anincreased stability of foam lms and macroscopic foams.
Maldonado-Valderama et al. investigated foam lmscomposed of the protein casein and a neutral low molecularweight surfactant (Tween 20).49 In their study two different typesof proteins were investigated. One was the commercial casein
Fig. 11 Top view of foam films stabilized by b-lactoglobulin (bulk conaggregated proteins. The aggregate size for the respective foam is: (a) Rh
were pre-formed at different protein bulk concentrations. (1 g l�1, 2 g l�
(which is a mixture containing different casein proteins), theother one was isolated b-casein (the major component of thecommercial casein mixture). Foam lms stabilized with thecommercial casein are thinner than the ones stabilized withisolated b-casein. The lower lm thickness in case of commer-cial casein correlates with a signicantly lower half-lifetime ofthe respective macroscopic foam. The different behavior of bothcasein samples is explained by differences in displacement of b-casein by Tween 20. In all cases the thickness and stability forfoam lms of protein–Tween 20 mixtures were higher than thestability of foam lms formed from pure Tween 20 solutions atconcentrations above its cmc.
On the other hand pure proteins form brittle monolayers atthe air–water interface, that can easily break and leave uncov-ered area as it is the case for some oppositely charged surfac-tant–polyelectrolyte mixtures. Surfactants make the rigidprotein layer more exible andmobile, and the layer then mightbetter respond to applied stresses without rupturing.50
3 Stratification of thin liquid foamfilms
So far, systems with continuous disjoining pressure isothermswere considered that can be described with the DLVO theoryand steric interactions. The effect of the composition of the lmsurface is reected by the occurrence of a NBF or CBF. Beyond acertain concentration of micelles,51–54 particles55–58 or poly-electrolytes9,27,59 oscillatory forces can be measured, whichindicates a certain ordering of these mesoscopic objects withinthe lm.60 Oscillatory forces reect interactions betweenmesoscopic objects within the lm bulk and can be partiallyseparated from interfacial effects. The thin liquid foam lmscan be considered as a tool to study the effect of geometricalconnement on the ordering of the mesoscopic objects. Thisphenomenon will be shortly addressed at the end of thischapter. In the following the stratication of foam lms as aconsequence of oscillatory forces and the effect on the dynamicsin foam lms containing polyelectrolytes will be discussed.
To investigate stratication behavior in thin liquid lms alarge number of experiments was performed with poly-electrolyte solutions, oen containing small amounts ofsurfactants for stabilization purposes. The rst experiments on
centration 1 g l�1) containing 50% protein aggregates and 50% non-¼ 35 nm (b) Rh¼ 71 nm (c) Rh¼ 117 nm (d) Rh¼ 197 nm. The aggregates1, 8 g l�1 and 10 g l�1). The photo is taken from ref. 45.
thin lms made from aqueous polyelectrolyte solutions wereperformed using AFM and revealed the presence of oscillatoryforces, which were attributed to the presence of polymer coils59
acting similar as surfactant micelles. This interpretation has tobe treated carefully as explained in Section 3.1 below.
The rst TFPB experiments on foam lms containing poly-electrolytes evidenced similar oscillations.18 For disjoiningpressure isotherms of foam lms stabilized with C12TAB/PAMPS jumps in lm thickness (D(h)) were observed whenincreasing the applied pressure.
The jump in lm thickness is attributed to the transitionbetween two neighbored branches of a disjoining pressureisotherm as shown in Fig. 12. The transition is observed by theappearance of darker domains, corresponding to the newthinner lm thickness. The domains expand over the whole lmand form the new thinner lm. This stratication is notreversible. If the pressure is decreased the lm stays on thesame branch of the isotherm and the lm thickness does notincrease much.
The occurrence of oscillatory forces is explained by expulsionof quantized amounts of polymer chains as explained below.
3.1 Effect of the polymer system
Stratication is easily observed for lms containing ratherexible polyelectrolytes, i.e. polymers with a relatively lowpersistence length like PAMPS, PSS and carboxymethyl-chitin(CM-Chitin). If at all, stratication is much more difficult toobserve for stiff polyelectrolytes like DNA and xanthane.61 It isassumed that the time scale for polymer network relaxationincreases with increasing backbone rigidity. Stratication withstiffer polymers is only observed when the viscosity is largeenough so that the polymer network has time to adjust andremain in equilibrium while thinning is taking place. Similarly,stratication is only seen with lms conned between solidsurfaces when the velocity of approach is slow enough. This wasrealized with a CP-AFM which allows adjusting the velocity ofapproach of the opposing surfaces.61
Fig. 12 Stepwise thinning for a foam film stabilized by a mixture of 5�10�5 mol l�1 C12TAB and 8.6 � 10�3 (mono)mol l�1 PAMPS25. Thegraph is taken from ref. 27.
6910 | Soft Matter, 2014, 10, 6903–6916
The jump size Dh of the oscillation period depends on theproperties of the polymer like chemical structure, chargedensity and concentration. In general the stepwise thinning ofthe foam lm is observed in the semi-dilute concentrationregime above c* of the polyelectrolyte but below the cac.18
Stratication occurs due to the oscillation of the disjoiningpressure in the lm and is assumed to originate in a transientpolyelectrolyte network that is formed in the lm core above theoverlap concentration of the polymer c*.28,35,62,63 It was shownrst by Asnascios et al.27 that the oscillation period decreases asthe square root of the polymer concentration (Fig. 13) and isindependent of molecular weight.36
Depending on the geometry of the polymer backbone theobserved step size either scales with Dh proportional c�1/2 forlinear polyelectrolytes and Dh proportional c�1/3 for branchedpolyelectrolytes.37,64 Branched polymers can be considered ascharged spheres and show the typical scaling law of objects,ordered in 3 dimensions.58 In the case of linear poly-electrolytes the scaling law Dhf c�1/2 reects the formation ofa polyelectrolyte network with a mesh size x. The mesh size ofthe respective bulk solution was determined by small angleneutron or X-ray scattering (SANS, SAXS). It was shown that thedistance between the polyelectrolyte chains (x ¼ 2p/qmax)obtained from the position qmax of the structure peak is equalto the jump size Dh of the lm stratication.36 This means thatthere is no effect of geometrical connement on the averagedistance of polyelectrolyte chains (i.e. mesh size) within thenetwork. The similarity between jump size in the lm andnetwork mesh size in the bulk is explained by the fact that thenetwork is transient (breaks down and rebuilds) and ts intothe available volume. If the lm thickness becomes too smalldue to increase of the outer pressure for nmeshes the networkrebuilds with n � 1 meshes. The relation between the lmstratication and the pair correlation function is explained inSection 3.4.
The stratication is affected by the ionic strength. Withincreasing ionic strength, the number of visible oscillations andtheir amplitude decreases, until they disappear at high saltconcentrations (around 10�2 mol l�1).27,65 This is well correlatedwith SANS experiments, where a characteristic peak disappearsabove 10�2 mol l�1 salt.36
Fig. 13 Distances d between the branches of the disjoining pressurecurves versus polymer concentration. The graph is taken from ref. 27.
The degree of polymer charge has also a strong impact on thestratication behavior. With decreasing charge density theamplitude of oscillatory forces decreases.65 For highly chargedpolymers the disjoining pressure isotherm is divided intoseveral pronounced branches and multiple stratication occursstepwise at successive higher pressure (empty squares inFig. 14). On contrary, in lms stabilized with polyelectrolyteswith a low degree of charge all steps take place simultaneouslyat very low disjoining pressures (C and photo in Fig. 14).
Fig. 15 Disjoining pressure versus film thickness for mixed solutions of10�4 mol l�1 CnTAB and 3.8 � 10�3 (mono)mol l�1 carboxyMC.The graph is taken from ref. 33.
3.2 Effect of surfactant
In general, stratication phenomena are not affected by thechoice of surfactant with respect to the step size.33,38 Fig. 15displays disjoining pressure isotherms for solutions containing3.8 � 10�3 (mono)mol l�1 carboxyMC and 10�4 mol l�1
surfactant varying in chain length from 12 to 16 carbon atoms.Stratication was observed for all CnTAB systems, and the sizeof the thickness jump is almost the same (23–25 nm). The onlydifference is that foam lms stabilized from C12TAB mixturesare less stable than those with C14 or C16TAB. Also the inuenceof different surfactant types was investigated. The same stepsize was observed for a mixture of 3.8 � 10�3 (mono)mol l�1
carboxyMC with the anionic surfactant AOT. Similar observa-tions were made for the anionic PSS using either the cationicC12TAB or a neutral surfactant28 and mixtures of the cationicPDADMAC and either C16TAB or C12G2
38 (compare Fig. 9 and10). CP-AFM experiments on solutions containing poly-electrolytes with and without surfactants conrmed that thechoice of surfactant does not inuence the occurrence and thestep-size of stratication.61 This is a strong hint, that the strat-ication is a connement phenomenon and that the propertiesof the lm surfaces play a minor role. This point is addressedmore in detail in Section 3.4.
Although the surfactant has no effect on the step size, itinuences strongly the kinetics of stratication as shown in thefollowing section.
Fig. 14 Disjoining pressure as a function of film thickness for APG/PDADMAC films at two different degrees of PDADMAC charge: 100%(fully charged chain) and 24%. The graph is taken from.9 The photo istaken from ref. 65.
According to the Navier Stokes equation (creeping ow limit)the drainage velocity of liquid lms is proportional to theinverse viscosity of the respective bulk solution. This principalrelation was also found for the effective diffusion coefficient Dof the rim during wetting of stratied drops (D f 1/heff (ref. 66and 67)), but the viscosity of a thin lm (heff) can differ from thebulk viscosity of the respective bulk solution (h). The strati-cation of a foam lm is also considered as a dewettingphenomenon and the effective diffusion coefficient is68
D ¼ � hN3
12heff
vP
vh(1)
where hN is the equilibrium lm thickness far from the openingdomain. The opening velocity, related to the change in area withtime is directly measured by video microscopy
D0 ¼ vr2
vt(2)
and has the dimension of a diffusion coefficient. The relation-ship between D0 and D is described by,69 and D0 is only a fewpercent of D.70
There is a strong effect of the polyelectrolyte and surfactantcharge on the stratication kinetics. One has to distinguish thepolymer adsorbing and polymer non-adsorbing case. Further-more it is important if one considers the opening velocity or theeffective viscosity heff.70 According to eqn (1) heff takes the dis-
joining pressure gradientvP
vhof each branch during stratica-
tion into account.(1) Opening velocity of domains: in case of oppositely charged
surfactants and polyelectrolytes complexes are formed at thelm interface. With decreasing lm thickness the openingvelocity decreases. The transition from a lm containing 3layers of polyelectrolyte network to a 2–layer lm is faster thanfor a 2 / 1 layer transition. This was found for C12TAB/PAMPSand C12TAB/CMC lms.69,70 One reason for the decrease in
velocity might be dangling polyelectrolyte chain at the lminterface. In case of non-adsorbing polyelectrolytes the openingvelocity is much faster: factor 3 for the non-ionic surfactantC12G2
70 and factor 6 for the equally charged surfactant AOT.71
The higher velocity for AOT can be explained by a strong elec-trostatic repulsion. The repulsion might also lead to anincreasing velocity of stratication with decreasing lm thick-ness. In contrast to oppositely charged surfactants and poly-electrolytes, now the transition from a lm containing 2 layersof polyelectrolyte network to a 1–layer lm is faster than for a 3/ 2 layer transition71 (see 0.3 wt% in Fig. 16a).
More difficult to understand is the effect of the poly-electrolyte concentration on the opening velocity of thedomains. Oppositely charged surfactants and polyelectrolytesand mixtures with non-ionic surfactants show a decrease inopening velocity with increasing polyelectrolyte concentra-tion.70 In contrast, no systematic effect of polyelectrolyteconcentration on the opening velocity was detected for equallycharged surfactants and polyelectrolytes71 (see Fig. 16a). There,only the lm thickness seems to be decisive for the openingvelocity. So far, an explanation for the difference is still missing.The bulk viscosity does not change signicantly within thestudied polyelectrolyte concentration regime. Therefore, thechange in surface properties might be a reason, i.e. heff
Fig. 16 (a) AOT/carboxyMC foam film during stratification: openingvelocity of the domains as a function of the film thickness for differentcarboxyMC concentrations at 0.1 mM AOT. At 0.3 wt% 2 transitionsoccur, all other carboxyMC concentrations give one transition. Thegraph is taken from ref. 71. (b) PAMPS/C12TAB foam film during strat-ification: film viscosity (heff) normalized by the viscosity (h) of therespective bulk solution in dependence of PAMPS concentration fortwo different transitions. The graph is taken from ref. 70.
6912 | Soft Matter, 2014, 10, 6903–6916
increases. With increasing polyelectrolyte concentration thetendency for polyelectrolyte adsorption increases, mediated bysurfactants. This decreases the drainage velocity as mentionedabove for thin foam lms. In case of equally charged poly-electrolytes and surfactants the polyelectrolyte adsorption isprevented due to electrostatic repulsion.
(2) Effective viscosity: considering the effective viscosity thedifferences in stratication between oppositely chargedsurfactant and polyelectrolyte and mixtures with non-ionicsurfactants vanish.70 Interestingly, the ratio heff/h increases withincreasing polyelectrolyte concentration (see Fig. 16b). If therewere just bulk effects due to increasing polyelectrolyte concen-tration heff/h should be constant in Fig. 16b. The increase is astrong hint for increasing dissipation with increasing poly-electrolyte concentration. In addition, Fig. 16b shows foroppositely charged surfactants and polyelectrolytes that heff/hincreases with decreasing lm thickness (3/ 2 vs. 2/ 1 layerstransition) which is a strong hint for increasing dissipation withdecreasing lm thickness. The increase in heff/h leads to thedecrease in opening velocity as mentioned above. Unfortu-nately, there is no graph like Fig. 16b for the system AOT/car-boxyMC in literature. According to the considerations madeabove, one would expect that heff/h AOT/carboxyMC remainsconstant irrespective of the concentration and that heff/hdecreases with decreasing lm thickness.
In case of foam lms the choice of surfactant–polyelectrolytecomposition has an effect on the characteristic time behaviorfor the different systems and a strong relation to Rayleighinstabilities.
Fig. 17 shows that the kinetics of liquid lms containing apolyanion (carboxyMC) are very sensitive to the nature of thesurfactant. When cationic surfactants (CnTAB) are added tocarboxyMC, a slower stratication is found and the movementof the domain border shows diffusion-like behavior (r(t) f t1/2).This is a typical feature of domains with a smooth rim withoutany Rayleigh instabilities.72 The layer of mixed CnTAB/carbox-yMC complexes at the lm surfaces is assumed to slow down theopening of the stratication domains. In contrast, in presenceof the anionic surfactant AOT no complex layer with the equallycharged carboxyMC is formed at the lm surface. This leads to afaster transition and thicker droplets (Rayleigh instabilities) areformed at the domain border. The domain border moves withconstant velocity (r(t) f t).33
In further studies for AOT/carboxyMC a transition from adiffusion-like stratication kinetic to a linear one was found at aspecic domain radius rc.71 The transition was accompanied bythe occurrence of rim instabilities (Rayleigh instabilities).
To summarize, foam lms with a faster transition kinetics(equally charged surfactant and polyelectrolyte) show a transi-tion from r(t) f t1/2 to r(t) f t accompanied by Raleigh insta-bilities while more slowly expanding domains (oppositelycharged surfactant and polyelectrolyte) grow always as r(t)f t1/2.
What do the exponents r(t) f t1/2 and r(t) f t mean in termsof slip/no slip conditions? In case of no-slip conditions onewould expect a constant velocity (rf t) and for slip conditions rf t2/3.73,74 An increase of the domain radius r with t1/2 was foundfor stratication of foam lms containing micelles75 or for
Fig. 17 (a) Time dependence of the stratification domain radius r(t) forAOT/carbocyMC and CnTAB/carboxyMC systems. The solid linescorrespond to fits with r(t) � t for AOT and r(t) � t1/2 for CnTAB. Theconcentration of 1000 ppm (0.1 wt%) corresponds to 3.8 � 10�3
(mono)mol l�1 carboxyMC (DS ¼ 1.23). The graph is taken from ref. 33.(b) Typical time dependence of the radius of domains for carboxyMCconcentration of 0.15 wt% (1500 ppm ¼ 5.7 � 10�3 (mono)mol l�1).The graph is taken from ref. 71.
Review Soft Matter
Publ
ishe
d on
11
July
201
4. D
ownl
oade
d by
TU
Ber
lin -
Uni
vers
itaet
sbib
l on
24/0
2/20
16 1
6:18
:26.
View Article Online
stratied polymer lms.76 The exponent 1/2 as also found forfoam lms with oppositely charged surfactant and poly-electrolyte is closer to the exponent 2/3 for slip conditions.However, it is not very likely that a water layer can slip onanother water layer. Heinig69 extended a model, which wasoriginally used for the opposite phenomenon: spreading ofmicrocopic droplets68. In the model the domain expansion isassumed to be driven by capillary forces. The model useslubrication approximation with no-slip boundary conditions atthe surface of the lm and is summarized in ref. 70.
The trends exhibited by the stratication kinetic are oppositeto the trends found for lm stability: AOT/carboxyMC lms aremore stable than CnTAB/carboxyMC as mentioned in Chapter 2,but their transition kinetics is faster. For continuously draininglms a reduction in drainage velocity e.g. by increase in viscositycan increase the lm stability.2 An explanation for the counterintuitive behavior during stratication might be the strongelectrostatic repulsion between AOT and CarboxyMC thataccelerates the expulsion of polyelectrolyte chains from the bulkbut also stabilizes the lm.
3.4 Comparison with oscillatory forces throughpolyelectrolyte solutions conned between two solid surfaces
Due to the fact that a CP-AFM allows measuring more or less thecomplete oscillation, CP-AFM is a more versatile tool to char-acterize the oscillatory forces.58 In addition this method allows
adjusting the approach velocity of the two opposing surfaceswhich is of interest for kinetic studies61,70.
In the experiments described in the following the forces weremeasured between the colloidal silica microsphere and a planarsilicon wafer. The force curves are characterized by threeparameters: the period of the oscillation (x), the exponentialenvelope (decay length) l and the amplitude. These parametersare compared quantitatively with the measurable sizes obtainedfrom scattering experiments at the respective bulk solutions asfollowed: the period of the force curves is compared to theinverse position of the structure peak (2p/qmax) and indicates themesh size of the polyelectrolyte network. The decay length of theforce curve is compared to the inverse width of the structurepeak (1/Dq) and is attributed to the range of ordering. Theamplitudes of both methods indicate the strength of orderingbut can only be qualitatively compared. The effect of outer andinner parameters like ionic strength, polyelectrolyte chargedensity and molecular weight are the same as for foam lms.
It is worth noticing that there are some differences withrespect to the TFPB measurements:
(1) A CP-AFM allows measuring oscillatory forces in both thedilute regime (interchain distance f c�1/3, polyelectrolyte coils)and the semi-dilute regime (interchain distance f c�1/2, poly-electrolyte network), while with the TFPB oscillatory forces areonly accessible in the semi-dilute regime.
(2) In CP-AFM experiments a slight compression of thechains by a maximum 20% and an increase of counterioncondensation at the PSS chains could be detected due toconnement.
(3) Due to the fact that with the CP-AFM the full oscillation isaccessible, the decay length can be measured. The decay lengthis larger than the Debye length.77,78 It gives the same value as therange of ordering in the bulk solution.
Thereby it is unclear if the reason for the difference is thehigher precision of the CP-AFM or the fact that the measure-ments are carried out between two solid interfaces instead of touid interfaces. For Silica dispersions conned in a CP-AFM ithas been shown that the structural forces (which corresponds tostratication in foam lms) are an effect of connement. Theoscillation period is very robust against changes in elasticity ofthe surface (uid or solid),79 surface charge,80 surface rough-ness,81 ionic strength82 and against (non)presence of surfactantsof different charges.79 On the other hand, these parametersaffect the amplitude (strength of ordering) and the decay length(range of ordering).
In general it is worth noticing that the oscillatory curve that ismeasured by a CP-AFM correspond to the pair correlation func-tion of the polyelectrolyte solution and presents the inverseFourier transform of the structure factor of the solutionmeasured by SANS or SAXS. The branches measured by TFPBcorrespond to the repulsive parts of the pair correlation function.
4 Conclusion and open questions
The results presented in the review lead to the conclusion thatthe lm can be divided into the two interfacial regions with anexcess of surfactant and the lm core containing a geometrically
conned polyelectrolyte solution. Repulsion of the poly-electrolyte chains within the lm core from the lm surfaceleads to stable foam lms. This is the case for (a) equallycharged and (b) oppositely charged polyelectrolytes andsurfactants and (c) for the combination of polyanions and non-ionic surfactants. In case of oppositely charged compounds(case b) this simple consideration is only valid below the cac.Above the cac polyelectrolyte–surfactant bulk aggregates areformed which block the drainage and lead to ultrastable lms.For case c one has to take into account that the air–waterinterface is assumed to be negatively charged and only poly-anions are repelled. In contrast, in foam lms of polycationsand non-ionic surfactants, polycations are attracted by a lmsurface, which leads to less stable foam lms.
Replacing the synthetic polymers by proteins shows that notonly the adsorbed amount but also the lateral structuring ofproteins affects the stability. The formation of percolatednetworks of particles leads to higher stabilisation. This hasbeen recently found also for nanoparticles in Pickering foams,83
and proteins can be also considered as particles.In contrast to pure surfactant foam lms mixed poly-
electrolyte–surfactant foam lms show a lower stability inconcentration regimes around the isoelectric point (IEP) goingwith high elasticity and low surface tension. This indicates thatthe electrostatic repulsion between both surfaces determinesthe lm stability. The electrostatic repulsion is reduced if a lotof material is adsorbed due to charge compensation of poly-electrolyte and surfactant.
The minimum in foam lm stability with increasing poly-electrolyte implies a continuously increasing amount of adsor-bed polyelectrolyte leading to a charge reversal close to the IEP.Results of surface tension and surface elasticity measurementsdo not support this simple image. Close to the IEP the surfacetension shows a minimum and the elasticity a maximum whichindicates a maximum in adsorbed amount. So far, it is not clear,why the adsorption maximum occurs at low polyelectrolyteconcentrations and low surfactant concentrations. A specula-tion is the formation of two different types of aggregates: onearound the IEP with at adsorbed polyelectrolyte chains andone type at higher polyelectrolyte concentrations with long tailsdangling into the solution.
Above the overlap concentration c* the polyelectrolytechains overlap in the bulk solution and a transient network isformed. This leads to oscillatory forces under connement ina thin lm. The comparison with results of SAXS and SANSmeasurements show that the characteristic parameters likemesh size of the network and range of ordering do not changeduring connement. The mesh size scales with c�1/2 for linearpolyelectrolytes and with c�1/3 for branched polyelectrolytesand is very robust against changes in properties of the outersurfaces (charge, elasticity, roughness). This indicates thatthe oscillatory forces present properties of the lm coreindependent of the properties of the conning surfaces. Adecrease in polyelectrolyte charge or an increase in ionicstrength leads to a decrease in amplitude of the oscillatoryforces.
6914 | Soft Matter, 2014, 10, 6903–6916
While the surface properties have no effect on the struc-turing of the polyelectrolytes under connement they have astrong impact on the transition kinetics of the during lmstratication. In case of equally charged polyelectrolytes andsurfactant the kinetics is quite fast due to strong electrostaticrepulsion across the foam lm and due to smooth interfaces.The effective viscosity decreases with decreasing lm thickness.In case of oppositely charged polyelectrolytes and surfactantsthe lm surface is rather rough due to dangling ends directedtowards the lm core. This slows down the expulsion of poly-electrolytes and increases the effective viscosity with decreasinglm thickness.
The absolute values for heff are controversially discussed inliterature. Both, values more than an order of magnitudehigher69 and lower70 than the bulk viscosity are reported.Another weak point is that neither the Marangoni effect norsurface rheology is included within the model. All these effectsare combined in the effective viscosity heff, which does not allowto split up the different contributions.
Acknowledgements
We thank Adrian Carl and Sebastian Schoen for improving thequality of the gures. The German Research Council (DFG) isacknowledged for nancial support via SPP “Kolloidverfahren-stechnik” (Kl 1165/10).
References
1 P. Hansson and B. Lindman, Curr. Opin. Colloid Interface Sci.,1996, 1, 604–613.
2 I. Ivanov and D. Dimitrov, Thin Liquid Films, Marcel Dekker,New York, 1988, p. 379.
3 D. Exerova and P. M. Kruglykov, Foam and Foam Films Theory,Experiment, Application, Elsevier, Amsterdam, 1998.
4 K. J. Mysels and M. N. Jones, Discuss. Faraday Soc., 1966, 42,42–50.
5 D. Exerowa and A. Scheludko, Chim. Phys. Bulgarien, 1971,24, 47.
6 D. Exerowa, T. Kolarov and K. H. R. Khristov, Colloids Surf.,1987, 22, 161–169.
7 R. von Klitzing, Adv. Colloid Interface Sci., 2005, 114, 253–266.8 K. Ciunel, M. Armelin, G. H. Findenegg and R. von Klitzing,Langmuir, 2005, 21, 4790–4793.
9 C. Stubenrauch and R. von Klitzing, J. Phys.: Condens. Matter,2003, 15, R1197–R1232.
10 N. Kristen and R. von Klitzing, So Matter, 2010, 6, 849.11 W. Ducker, T. Senden and R. Pashley, Nature, 1991, 353, 239.12 A. Asnacios, D. Langevin and J. F. Argillier, Macromolecules,
1996, 29, 7412–7417.13 A. Asnacios, R. von Klitzing, R. Klitzing and D. Langevin,
Colloids Surf., A, 2000, 167, 189–197.14 C. Stubenrauch, P. A. Albouy, R. von Klitzing and
D. Langevin, Langmuir, 2000, 16, 3206–3213.15 D. J. F. Taylor, R. K. Thomas and J. Penfold, Langmuir, 2002,
16 N. Kristen, A. Vullings, A. Laschewsky, R. Miller and R. vonKlitzing, Langmuir, 2010, 26, 9321–9327.
17 N. Kristen-Hochrein, R. Miller and R. von Klitzing, J. Phys.Chem. B, 2011, 115, 14475–14483.
18 V. Bergeron, D. Langevin and A. Asnacios, Langmuir, 1996,12, 1550–1556.
19 Interactions of Surfactants with Polymers and Proteins, ed. E.Goddard and K. Ananthapadmanabhan, CRC Press, 1993.
20 C. Monteux, C. E. Williams and V. Bergeron, Langmuir, 2004,20, 5367–5374.
21 S. Guillot, M. Delsanti, S. Desert and D. Langevin, Langmuir,2003, 19, 230–237.
22 S. Trabelsi and D. Langevin, Langmuir, 2007, 23, 1248–1252.
23 S. Trabelsi, E. Raspaud and D. Langevin, Langmuir, 2007, 23,10053–10062.
24 G. Espinosa and D. Langevin, Langmuir, 2009, 25, 12201–12207.
25 C. Bain, P. Claesson, D. Langevin, R. Meszaros, T. Nylander,C. Stubenrauch, R. Titmuss and S. von Klitzing, Adv. ColloidInterface Sci., 2010, 155, 32–49.
26 E. Carey and C. Stubenrauch, J. Colloid Interface Sci., 2010,343, 314–323.
27 A. Asnacios, A. Espert, A. Colin and D. Langevin, Phys. Rev.Lett., 1997, 78, 4974–4977.
28 R. von Klitzing, A. Espert, A. Asnacios, T. Hellweg, A. Colinand D. Langevin, Colloids Surf., A, 1999, 149, 131–140.
29 V. Bergeron, Langmuir, 1997, 13, 3474–3482.30 L. Bergstrm, Adv. Colloid Interface Sci., 1997, 70, 125–169.31 C. Stubenrauch, B. Fainerman, E. Aksenenko and R. Miller,
J. Phys. Chem. B, 2005, 109, 1505–1509.32 N. Kristen, V. Simulescu, A. Vllings, A. Laschewsky, R. Miller
and R. v. Klitzing, J. Phys. Chem. C, 2009, 133, 7986–7990.33 C. Beltran, S. Guillot and D. Langevin,Macromolecules, 2003,
36, 8506–8512.34 O. Theodoly, J. S. Tan, R. Ober, C. E. Williams and
V. Bergeron, Langmuir, 2001, 17, 4910–4918.35 J. L. Toca-Herrera and R. von Klitzing,Macromolecules, 2002,
35, 2861–2864.36 R. von Klitzing, B. Kolaric, W. Jaeger and A. Brandt, Phys.
Chem. Chem. Phys., 2002, 4, 1907–1914.37 R. von Klitzing and B. Kolaric, Prog. Colloid Polym. Sci., 2003,
122, 9.38 B. Kolaric, W. Jaeger, G. Hedicke and R. von Klitzing, J. Phys.
Chem. B, 2003, 107, 8152–8157.39 C. Stubenrauch, J. Schlarmann and R. Strey, Phys. Chem.
Chem. Phys., 2002, 4, 4504–4513.40 N. Schelero and R. von Klitzing, So Matter, 2011, 7, 2936–
2942.41 V. Bergeron, A. Waltermo and P. Claesson, Langmuir, 1996,
12, 1336.42 L. Evers, E. Nijman and G. Frens, Colloids Surf., A, 1999, 149,
521–527.43 T. Dimitrova, F. Leal-Calderon, T. Gurkov and B. Campbell,
Langmuir, 2001, 17, 8069–8077.44 B. Rullier, M. Axelos, D. Langevin and B. Novales, J. Colloid
45 B. Rullier, M. Axelos, D. Langevin and B. Novales, J. ColloidInterface Sci., 2010, 343, 330–337.
46 A. L. Fameau, A. Saint-Jalmes, F. Cousin, B. H. Houssou,B. Novales, L. Navailles, F. Nallet, C. Gaillard, F. Boue andJ.-P. Douliez, Angew. Chem., Int. Ed., 2011, 50, 8264.
47 A. Carl and R. von Klitzing, Angew. Chem., Int. Ed., 2011, 50,11290–11292.
48 A. Saint-Jalmes, M.-L. Peugeot, H. Ferraz and D. Langevin,Colloids Surf., A, 2005, 263, 219–225.
49 J. Maldonado-Valderrama and D. Langevin, J. Phys. Chem. B,2008, 112, 3989–3996.
50 M. Bos and T. van Vliet, Adv. Colloid Interface Sci., 2001, 91,437–471.
51 A. Nikolov and D. Wasan, J. Colloid Interface Sci., 1989, 133,1.
52 A. Nikolov, D. Wasan, N. Denkov, P. Kralchewsky andI. Ivanov, Prog. Colloid Polym. Sci., 1990, 82, 87.
53 V. Bergeron and C. Radke, Langmuir, 1992, 8, 3020.54 A. Sonin and D. Langevin, Europhys. Lett., 1993, 22, 271.55 E. Perez, J. Proust and L. Ter-Minassian-Saraga, Thin Liquid
lms, Marcel Dekker, 1988, p. 891.56 E. Basheva, A. Nikolov, P. Kralchevsy, I. Ivanov and
D. Wasan, Surfactants in Solution, Plenum Press New York,1991, vol. 11.
57 A. Nikolov and D. Wasan, Langmuir, 1992, 8, 2985.58 S. Klapp, Y. Zeng, D. Qu and R. von Klitzing, Phys. Rev. Lett.,
2008, 100, 118303.59 A. Milling, J. Phys. Chem., 1996, 100, 8986.60 R. von Klitzing, E. Thormann, T. Nylander, D. Langevin and
C. Stubenrauch, Adv. Colloid Interface Sci., 2010, 155, 1931.61 F. Kleinschmidt, C. Stubenrauch, J. Deacotte, R. v. Klitzing
and D. Langevin, J. Phys. Chem. B, 2009, 113, 3972–3980.62 R. von Klitzing, A. Espert, A. Colin and D. Langevin, Colloids
Surf., A, 2001, 176, 109–116.63 R. von Klitzing and H. Mueller, Curr. Opin. Colloid Interface
Sci., 2002, 7, 18.64 R. von Klitzing and B. Kolaric, Tenside, Surfactants, Deterg.,
2002, 39, 247–253.65 B. Kolaric, W. Jaeger and R. von Klitzing, J. Phys. Chem. B,
2000, 104, 5096–5101.66 P. de Gennes, C. R. Acad. Sci., Paris, Ser. II, 1984, 298, 475.67 P. de Gennes and A. Cazabat, C. R. Acad. Sci., Paris, Ser. II,
1990, 310, 1601.68 J. F. Joanny and P.-G. de Gennes, C. R. Acad. Sci., Paris, Ser. II,
1984, 299, 279–283.69 P. Heinig, C. Beltran and D. Langevin, Phys. Rev. E: Stat.,
Nonlinear, So Matter Phys., 2006, 73, 051607.70 J. Delacotte, E. Rio, F. Restagno, C. Uzum, R. von Klitzing
and D. Langevin, Langmuir, 2010, 26, 7819–7823.71 C. Beltran and D. Langevin, Phys. Rev. Lett., 2005, 94, 217803.72 A. Sonin, A. Bonllon and D. Langevin, Phys. Rev. Lett., 1993,
71, 2342–2345.73 C. Redon, J. B. Brzoska and F. Brochard-Wyart,
Macromolecules, 1994, 27, 468–471.74 G. Reiter and R. Khanna, Langmuir, 2000, 16, 6351.75 P. Kralchevsky, A. Nikolov, D. Wasan and I. Ivanov,
