Science 11 (2006) 337–344www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface

Polyelectrolyte/surfactant mixtures at the air–solution interface

J. Penfold a,⁎, R.K. Thomas b, D.J.F. Taylor b

a ISIS, CCLRC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, OX11 0QX, UKb Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OX1 3QZ, UK

Available online 6 October 2006

Abstract

This review presents some of the recent developments in our understanding of the behaviour of polyelectrolyte/surfactant mixtures at the air–solution interface. The existence of a strong surface polyelectrolyte/surfactant interaction results in a complex pattern of surface adsorption. Recentstudies, using a range of surface sensitive techniques, which include ellipsometry, neutron and X-ray reflectivity, surface tension and interfacialrheology, have considerably enhanced the understanding of their surface behaviour, which can be rationalized in terms of the competition betweenthe formation of surface active polymer/surfactant complexes and solution polymer/surfactant micelle complexes.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Polyelectrolytes; Surfactants; Adsorption; Air–solution interface

1. Introduction

Understanding the behaviour of polymer/surfactant mixturesin solution and at interfaces is important in the context of theirwidespread industrial, technological and domestic applications.Their widespread use and application has stimulated extensiveresearch activity. However, the emphasis has been predomi-nantly on the more weakly interacting polymer/surfactantmixtures and the bulk solution behaviour. Much of this ismore than adequately covered in a number of comprehensivereviews (see for example, [1•,2•]). Continuing in that theme,phase behaviour was the emphasis of the recent review by LaMesa [3]. However, there has been less emphasis in the past onthe strongly interacting polyelectrolyte/surfactant mixtures andtheir surface behaviour. Most recently, this was addressed byGoddard [4••], who discussed specifically the interfacial aspectsof polymer/surfactant interactions. The more complex surfacebehaviour that results from a strong surface polymer/surfactantinteraction was highlighted. Importantly, this can result in acomplex surface tension behaviour that can no longer bedescribed by the classical response, where Jones [5] describedthe two surface tension transitions, T1 and T2, in terms of the

⁎ Corresponding author.E-mail address: J.Penfold@rl.ac.uk (J. Penfold).

1359-0294/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.cocis.2006.08.003

critical aggregation concentration, cac, and the critical micellarconcentration, cmc.

In more recent years, the application of a range of surfacesensitive experimental techniques, such as ellipsometry, neutronand X-ray reflectivity, atomic force microscopy, AFM andinterfacial rheology, has provided an important complement totraditional techniques, such as surface tension. This hasconsiderably enhanced our understanding of the surface/interfacial behaviour in strongly interacting polyelectrolyte/surfactant mixtures. Some of that progress was reviewed in arecent article by Langevin [6]. However, since then, a moreextensive range of studies have been reported and a moreprofound understanding is emerging. The focus of this article isto review that recent progress on adsorption at the air–solutioninterface.

2. Polyelectrolyte/ionic surfactants of opposite charge

The combination of surface tension and neutron reflectivityhas proved a powerful combination for the study of polyelec-trolyte/ionic surfactant adsorption at the air–solution interfaceand we highlight some of the progress and key results from thatapproach. Neutron reflectivity provides a direct measure of thesurface composition, adsorbed amounts and structure of theadsorbed layer, which can be correlated with the surface tensionresponse. This has been exploited by Staples et al. [7••,8] and

338 J. Penfold et al. / Current Opinion in Colloid & Interface Science 11 (2006) 337–344

Taylor et al. [9••–11] in the study of two different types ofmixtures, which represent the two extremes of surface be-haviour, for the cationic polyelectrolyte poly(dimethyl-diallylammonium chloride) (polyDMDAAC) and the anionic surfac-tant sodium dodecyl sulfate (SDS) [7••,8], and for the anionicpolyelectrolyte/cationic surfactant mixture of poly(styrenesulfonate) (PSS)/dodecyl trimethyl ammonium bromide(C12TAB) [9••–11]. The different surface tension behaviourfor the two systems is shown in Fig. 1.

For the polyDMDAAC/SDS mixture [7••], the initial sharpdecrease in surface tension at low SDS concentrations (which isindependent of polymer concentration) is followed by a peak inthe surface tension at higher SDS concentrations, before theeventual plateau at SDS concentrations Ncmc (see Fig. 1b).From the neutron reflectivity data, the adsorption is of the formof a monolayer of polymer/surfactant complex, where thesurface complexation results in an enhanced SDS adsorption atlow surfactant concentrations. At the SDS concentrationscoincident with the peak in the surface tension, this peak isassociated with a partial depletion of polymer/surfactantcomplex from the surface in favour of solution polymer/surfactant micelle complexes. The subsequent decrease in thesurface tension at higher SDS concentrations is associated withan increased SDS adsorption as the SDS monomer concentra-tion in solution increases, as shown in Fig. 2a.

Staples et al. [8] also considered the effect of a nonionic co-surfactant, hexaethylene glycol monododecyl ether, C12E6, on

Fig. 1. Variation in surface tension with surfactant concentration, for differentpolymer concentrations for (a) PSS/C12TAB and (b) SDS/polyDMDAAC;reproduced from Refs. [7

••,9••].

Fig. 2. Variation in amount of SDS adsorbed for (a) SDS/20 ppmpolyDMDAAC in 0.1 M NaCl, (b) 80/20 SDS/C12E6/20 ppm polyDMDAACin 0.1 M NaCl, (•) SDS, (○) C12E6 and (▾) total surfactant adsorption;reproduced from Refs. [7

••,8].

the surface tension behaviour of the SDS/polyDMDAACmixtures. The addition of the nonionic is one of theexperimental approaches to diluting the effect of the electro-static interaction between the polymer and surfactant. Forsolutions progressively rich in the nonionic surfactant, the peakin the surface tension observed for SDS/polyDMDAAC [7••] issystematically reduced. The neutron reflectivity data, whichprovides direct information about the amount of polymer, SDS,and C12E6 at the interface, show that this is because thedepletion of SDS and polymer is compensated by an increasedadsorption of the C12E6, such that the total amount of adsorptionis roughly constant, see Fig. 2b.

Vaknin et al. [12] also studied the structure of SDS/polyDMDAAC complexes at the air–solution interface by surfacetension, X-ray reflectivity and grazing incidence diffraction,GIXD. It is difficult to make a direct comparison with the resultsof Staples et al. as the data ofVaknin et al. was obtained for polymerconcentrations at least 1000 times larger. However, the surfacetension data are consistent with strong complexation at the surface,and the surface structure was consistent with a much higher degreeof ordering which was attributed to surface crystallization.

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The results of Staples et al. [7••,8] for SDS/polyDMDAACcontrasts markedly with the surface behaviour for the PSS/C12TAB [9••–11] mixture, see Fig. 1a. For PSS/C12TAB, the initialpronounced decrease in surface tension at low surfactantconcentration is dependent upon polymer concentration, and thisis followed by a broad plateau in the surface tension, before thefinal decrease at the cmc. The neutron reflectivity results confirmthat this significant reduction in the surface tension at lowsurfactant concentrations is due to polymer/surfactant complexformation at the interface, and this is initially in the form of amonolayer similar to that of SDS/polyDMDAAC. However, withincreasing surfactant concentration, the thickness of the adsorbedlayer increases substantially in discrete steps, see Fig. 3.

Detailed analysis of neutron reflectivity profiles from differentisotopically labeled combinations of surfactant, polymer andsolvent [9••] reveal the details of a layered structure at theinterface, which comprises either three or five layers. The broadregion of low surface tension is attributed to a consequence of thatsurface layering. Taylor et al. [10,11] subsequently investigatedthe effect of alkyl chain length on this surface structure andsurface tension response.With increasing alkyl chain length, fromC10 to C16, the surface behaviour evolves into a pattern moreconsistent with SDS/polyDMDAAC. A similar behaviour wasreported for poly(vinyl pyridinium chloride, PVPmCl/SDS in thepresence of electrolyte [10].

The patterns of behaviour which range between the extremestypified by the SDS/polyDMDAAC and PSS/C12TAB mixtureswere rationalized in terms of the competition between theformation of surface active polymer/surfactant monomer com-plexes, PSs, and solution polymer/surfactant micelle complexes,PSm, and their relative stabilities [7••,10,11]. If the free energygap between PSs and PSm is small, then the SDS/poly-DMDAAC type behaviour is observed. If the free energy gapbetween PSs and PSm is large, then surface complex formation,and the formation of sub-surface layers, PSs′, to form layeredstructures at the interface, is favoured.

A number of other studies on a range of similar or relatedsystems and using a range of techniques have been reported,and we highlight some of the important and common features of

Fig. 3. Variation in adsorbed amount (equivalent area/molecule) (○) andadsorbed layer thickness (•) for C12TAB/140 ppm PSS.

those studies [13–24]. Asnacios et al. [13] studied a range ofnon surface active polyelectrolytes (poly(2-acrylamido-2-pro-pane sulfonate), PAMPS, PSS) with different surfactants(mainly cationic surfactants), by surface tension, ellipsometry,bulk and surface rheology. The measurements were generallymade at high polymer concentrations relative to those used byTaylor et al. [9••–11] on related systems. This makes directcomparison difficult, but nevertheless has revealed someinteresting common features. For polyelectrolyte/ionic surfac-tants of opposite charge, they observe the same synergisticlowering of the surface tension, which is now extensivelyreported for a wide range of systems. They used the approach ofBuckingham et al. [25] to evaluate the surfactant adsorption atlow surfactant concentrations, but the neutron reflectivityresults of Taylor et al. [9••–11] show that this approach shouldbe used with some caution. Bulk and surface viscositymeasurements show differences consistent with highly surfaceactive polymer/surfactant complexes forming at the interface atlow surfactant concentrations. For the more hydrophilicpolymer, PAMPS, a different surface structure is implied.Ritacco et al. [14] extended this work to consider the effects ofpolymer flexibility. They used surface tension and X-rayreflectivity to study two different polyelectrolyte/surfactantmixtures, PAMPS/C12TAB, and Xanthan/C12TAB. Changingfrom PAMPS to Xanthan explored the effect of a rigidcompared to a flexible polymer backbone, but rigidity wasnot found to have a major impact. Stubenrauch et al. [15] usedthe same approach to study the surface properties of C12TABmixed with a wider range of anionic polyelectrolytes, PAMPS,PSS, Xanthan and DNA. X-ray reflectivity was used tocharacterize the structure of the surface adsorbed polymer/surfactant complexes that gave rise to the synergistic loweringof the surface tension. Although thin adsorbed layers broadlyconsistent with those reported from related neutron reflectivitystudies [7••,10,11] are measured, detailed comparison is againdifficult as Stubenrauch et al. mostly used much higher polymerconcentrations. Although X-ray reflectivity does not have theselectivity associated with neutron reflectivity in multi-component systems, the authors were able to correlate anincrease in the thickness of the surface layer and an increase inthe polymer volume fraction at the interface with increasingpolymer rigidity, for PSSbPAMPSbDNAbXanthan. Notably,multilayer formation, as seen in PSS/C12TAB mixtures byTaylor et al. [9••–11], was not observed. Indeed, Guillot et al.[16] have interpreted the plateaux in surface tension observed indifferent C12TAB/polyelectrolyte mixtures as implying that thesurface aggregation remains unchanged. The neutron reflectiv-ity results of Taylor et al. [9••–11] have shown that this is not thecase. Jain et al. [17] have subsequently reported multilayerformation in the PAMPS/hexadecyl trimethyl ammoniumbromide, C16TAB, mixture from X-ray reflectivity, surfacerheology and Brewster angle microscopy, BAM, measurementsduring compression in a Langmuir trough.

In a series of studies on PSS/C12TAB, Monteux et al. [18••,19•,20•] have interpreted the data from a range of techniques,including surface tension, ellipsometry and foam film drainageexperiments, in terms of the formation of interfacial gels.

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Furthermore, they have explored systematically the influence ofpolymer molecular weight, surfactant chain length and thepolymer/surfactant molar ratio. Although the measurementswere done predominantly at relatively high polymer concentra-tions, the evolution of the surface structure proposed bears somesimilarities to that observed by neutron reflectivity by Tayloret al. [7••,8,9••–11] on the same system, see Fig. 4. They ob-served a stronger tendency for complex formation with increas-ing polymer molecular weight, and the polymer/surfactantmolar ratio was seen as an important parameter in the determi-nation of surface active polymer/surfactant complexes.

Merta and Stenius [22] reported strong synergistic effects onthe surface tension due to the interaction between a cationicstarch and different anionic surfactants, with an emphasis onmixed anionic surfactants. The sharp change in the surfacetension behaviour was attributed to a cac, which was dependentupon the surfactant mixture. Phase separation was observed athigher surfactant concentrations. Their measurements were alsomade at relatively high polymer concentrations, and the surfacetension behaviour was more reminiscent of that observed innonionic polymer/ionic surfactant mixtures [5]. Strong surfacecomplexation, inferred from the significant suppression of thesurface tension at low surfactant concentrations, was observedwith PSS and a range of Gemini surfactants [23], broadlysimilar to that observed with other cationic surfactants [9••–11,13–20•]. The interaction between C12TACl and λ-carra-geenan were measured using a variety of techniques, includingsurface tension, conductivity, microscopy, light and X-rayscattering [24]. The rigidity of the carrageenan was associatedwith the formation of planar structures, involving lamellarordering and giant vesicle formation, and it was inferred thatthese structures existed in bulk and at the interface. Changes inthese structures were associated with the surface tensionbehavior, which contained many of the features seen in othersystems. A notable difference was that at low surfactantconcentrations the surface tension values were much higherthan observed in systems such as PSS/C12TAB.

3. Manipulation of interaction by pH

A number of experimental strategies exist for manipulating thestrength of the electrostatic interaction between the polyelectro-

Fig. 4. Possible structures for adsorbed layers of C12TAB/PSS with i

lyte and ionic surfactant in order to provide access to the relativecontributions of the electrostatic and hydrophobic interactions.These include the addition of electrolyte, or a nonioniccosurfactant, or dilution of the charge density by synthesizing ablock or random copolymer. An alternative approach has been touse a pH sensitive polymer, such as poly(ethyleneimine) (PEI) orpoly-acrylic acid (PAA). PEI is a highly charged polyelectrolyte atlow pH due to the protonation of the nitrogen groups, and PAA isneutral at low pH and highly charged at high pH.

Penfold et al. [26•] have used a combination of surfacetension and neutron reflectivity to investigate the effects of pHand polymer architecture on the adsorption of SDS/PEImixtures at the air–solution interface. The surface tensionbehaviour and adsorption patterns show a strong dependenceupon the solution pH. The adsorption is substantially enhanceddown to very low SDS concentrations (∼ 10−6 M) due tosurface SDS/PEI complexation, see Fig. 5. The adsorption ismost pronounced at high pH and for the branched PEI, and atpH 7 and 10 multilayer adsorption is observed over a wide rangeof SDS concentrations. In contrast, for the linear PEI, onlymonolayer adsorption is observed, and although the surfacetension shows a significant variation with pH the adsorption doesnot. These results imply that in addition to the electrostatic inter-action between the SDS and PEI there is a strong hydrophobiccontribution to the polymer/surfactant interaction.

This was reinforced by the work of Zhang et al. [27], usingneutron reflectivity and surface tension, to study the adsorptionof sodium poly(acrylic acid) (NaPAA)–dodecyltrimethyl am-monium bromide mixtures at the air–water interface. At bothpH 4.2 and 9.2, the addition of NaPAA results in a strongreduction in the surface tension, consistent with adsorption ofpolymer/surfactant complexes at the interface. At low surfactantconcentrations the adsorbed layer is a monolayer and theadsorption is independent of pH. At high pH, the surface layerthickens to a layered structure, similar to that observed for SDS/PEI and for C12TAB/PSS. This implies that at high pH thesurface interaction is predominantly hydrophobic at the lowersurfactant concentrations and electrostatic at the higherconcentration: whereas at low pH the dominant interaction ishydrophobic in nature.

The hydrophobic nature of the polyelectrolyte/ionic surfac-tant interaction was exploited by Edler et al. [28] in the

ncreasing surfactant concentration; reproduced from Ref. [18••].

Fig. 5. Comparison of variation in SDS adsorption with surfactant concentration forSDS/PEI for different polymer/surfactant mixtures; reproduced from Ref. [26

•].

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formation of surfactant templated polyelectrolyte/surfactantself-assembled films at the air–water interface. Highly struc-tured film growth was demonstrated and characterized byBAM, GIXD and neutron reflectivity for PEI/C16TAB mixturesat high pH.

Modifying the PEI by ethoxylation had a significant effect onthe pH dependence of the surface tension and adsorptionbehaviour for PEI EO7/SDS mixtures, as demonstrated byPenfold et al. [29]. Although the surface tension and adsorptionnow show behaviour with pH that is consistent with anincreasing electrostatic interaction with decreasing pH, thesurface interaction is much less pronounced. Although theadsorption of the SDS increases with decreasing pH, it is notsignificantly greater than in the absence of PEI. However, theneutron reflectivity data shows that PEI is present at theinterface, except at high SDS concentrations, where theadsorption is independent of pH. The surface complexation isless pronounced and the polymer volume fraction adsorbedcorrespondingly less.

4. Hydrophobically modified polyelectrolytes

An alternative strategy used to manipulate the interactionbetween the polyelectrolyte and the surfactant is to hydro-phobically modify the polyelectrolyte. This introduces aninteresting additional complication, in that the polymer is thenoften surface active in the absence of surfactant, and a greaterrange of polymer/surfactant mixtures can be explored.

Bromberg et al. and Colby et al. [30,31] have studied theinteraction between a range of alkyl sulfate surfactants with aassociative polymer, poly(ethylene oxide)-b-poly(propyleneoxide)-b-poly(ethylene oxide)-b-poly(acrylic acid), pluronic-PAA. Surface tension, viscosity and fluorescence measurementswere focused primarily on the bulk behaviour and the binding ofthe alkyl sulfate to the pluronic-PAA copolymer micelles. Thesurface tension at low surfactant concentrations is highlyreduced due to the intrinsic surface activity of the copolymerand the coadsorption of the surfactant. At higher surfactantconcentrations, there is a peak in the surface tension which

shifts to lower surfactant concentrations with increasing alkylchain length, from C8 to C18. The behaviour is reminiscent ofthat observed for SDS/polyDMDAAC and PSS/CnTAB for thelonger alkyl chain length surfactants. This surface tensionbehaviour is attributed by the authors to the onset of pluronic-PAA micelle/surfactant complex formation and the incorpora-tion of the surfactant into the solution pluronic-PAA micelles.The variation with alkyl chain length reflects the relative surfaceactivity of the surfactant.

In contrast, Deo et al. and Somasundaram et al. [32,33] haveinvestigated the interaction between the hydrophobicallymodified polymer, poly(maleic acid/octyl vinyl ether),PMAOVE, with SDS and heptaethylene glycol monododecylether, C12E5. For the same charge interaction, PMAOVE/SDS[32], the surface tension, although reduced at low SDSconcentrations due to the PMAOVE surface activity, shows aresponse similar to that exhibited with neutral polymers [5].With C12E5 the surface tension shows a response similar to thoseexhibiting a peak at higher surfactant concentrations, and this isattributed to the formation of mixed PMAOVE/C12E5 micelles.

5. Biopolymer/surfactant mixtures

The interactions between proteins and surfactants at an interfacehave important consequences in pharmaceutical and cosmeticapplications and in food technology. The polypeptide sequencesand secondary structure of protein gives rise to a more complexinteraction with surfactants and a potentially rich behaviour. Ionicsurfactants can interact with both the charged and hydrophobicregions of the protein. The net charge on the protein is highly pHdependent, and can contain both negative and positive chargeregions. Furthermore, the protein is often surface active in theabsence of surfactant. A range of different protein/surfactantsystems have been studied, as well as other important biopolymers.Some of thesewere discussed earlier in the context of awider study,where the biopolymer was part of a sequence of measurements.

Green et al. [34••,35] have used neutron reflectivity toinvestigate the adsorption of lysozyme/SDS and lysozyme/C12E5mixtures at the air–water interface. For the lysozyme/SDSmixtureat a pH where the protein is positively charged, there is a strongcomplexation between the SDS and lysozyme in solution and atthe surface. At low surfactant concentrations, there is a significantenhancement in the adsorption of the lysozyme and the SDS at theinterface, due to surface complex formation. This is responsible forthe substantial reduction in the surface tension. At higher SDSconcentrations, there is a decrease in the SDS and lysozymeadsorption due to the increased solubility of the complex. Beyondthat, the SDS adsorption increases again as the SDS monomerconcentration in solution increases. This is very similar to what isobserved in the SDS/polyDMDAACmixtures and related systemsthat were discussed earlier [7••,8]. This changing pattern ofadsorption is also accompanied by changes in the surface structure,associated with a surface denaturing of the lysozyme, as illustratedin Fig. 6.

This contrasts markedly with the observations of Green et al.[35] for lysozyme/C12E5 mixtures, where the neutron reflectiv-ity results are consistent with only a partial breakdown of the

Fig. 6. Schematic representation of the structural changes to the SDS/lysozymecomplexes in bulk solution and at the interface, with increasing surfactantconcentration; reproduced from Ref. [34

••].

Fig. 7. Schematic representation of the structure of the adsorbed layer for (a)SDS and (b) gelatin/SDS; reproduced from Ref. [36].

342 J. Penfold et al. / Current Opinion in Colloid & Interface Science 11 (2006) 337–344

globular structure of the lysozyme. The adsorption pattern isnow more consistent with competitive adsorption, similar to thatobserved in nonionic polymer/ionic surfactant mixtures, wherethe initially preferentially adsorbed lysozyme is progressivelyreplaced by the nonionic surfactant as the surfactant concen-tration increases.

Gelatin is an important polyelectrolyte which interacts stronglywith anionic surfactants, althoughnot as strongly asmanyproteins.Cooke et al. [36] have used neutron reflectivity and surface tensionto investigate the surface interaction between gelatin and SDS. Thesurface tension behaviour shows some similarities with nonionicpolymer/ionic surfactant mixtures [5] and PSS/CnTAB mixtures[9••–11], although its behaviour more closely resembles that ofcationic starch/anionic surfactant mixtures [22]. At low SDSconcentrations, the gelatin greatly enhances the SDS adsorption.At intermediate SDS concentrations, the adsorption is relativelyconstant and there is significant gelatin adsorbed up to the cmc.The presence of gelatin has a significant impact on the structure ofthe adsorbed layer, which is considerably thickened comparedwith a surfactant monolayer, see Fig. 7.

This is somewhat different to the layered structure reportedin other polyelectrolyte/surfactant mixtures, as discussed earlier.This adsorption pattern was also deduced from the interfacialrheology measurements on gelatin/SDS mixtures by Rao et al.

[37]. They observed a decrease in the surface dilational moduluswith increasing SDS concentration and, above the cmc, it wasindistinguishable from that for SDS alone. Where SDS/gelatincomplexes were at the interface this was attributed to adisruption of the gelatin networks, which could be the origin ofthe structural roughening observed in the neutron reflectivitymeasurements. A broadly similar pattern of behaviour wasobserved for SDS/gelatin adsorption onto a polystyrene surfaceby small angle neutron scattering [38].

Pisarcik et al. [39] observed an unusual surface tensionbehaviour for a mixture of the anionic polysaccharide, sodiumhyaluronate, NaHa and a dimeric cationic surfactant (Gemini).The surface tension showed a parabolic form with a broadminimum which varied with polymer concentration. This wasattributed as due to the adsorption of NaHa/Gemini surfactantclusters and Gemini monomers at the interface. Chen et al. [40]have considered DNA/surfactant complex formation at the air–water and solution–solid interfaces, respectively. They showedthat the spacer length of the Gemini surfactant controlled theproperties of the monolayers of DNA–Gemini surfactantcomplex formed at the air–water interface.

6. Summary

In the last few years, the study of the behaviour of polyelec-trolyte/surfactant mixtures at interfaces has been an active field.The application of a range of surface sensitive techniques, such asSFA, AFM, ellipsometry, X-ray and neutron reflectometry, SFSand optical reflectometry, and surface rheological measurements,has given considerable impetus to those studies. The recent studieshave considerably advanced our understanding of the complexpattern of behaviour that can arise due to the strong surface

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interaction and complexation. A clearer understanding of the fac-tors which control the surface behaviour is now emerging, and thecompetition between the formation of solution polymer/surfactantand surface active polymer/surfactantmonomer complexes is a keyfactor. The ability to access both the surface structure andcomposition, and to relate that microscopic scale information tomore macroscopic phenomena, as illustrated by surface tensionand surface rheology, are the major contributing factors to thatprogress. In the near future, the major challenges are now toestablish this emerging understanding on a sound theoretical andthermodynamic basis, and to use that understanding to manipulateand tailor surface properties. Furthermore, there is a need toestablish and confirm the extent to which some of the phenomenaobserved are equilibrium properties, and to explore the kinetics ofthe changes in surface behaviour and response.

References and recommended readings

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Goddard ED, Ananthapadmanabhan KP, editors. Interactions of surfac-tants with polymers and protein. Boca Rotan: CRC Press; 1993.

Comprehensive compilation of studies on polymer/surfactant mixtures.[2]•

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Comprehensive review of polymer/surfactant mixtures.[3] La Mesa C. Polymer–surfactant and protein–surfactant interactions.

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Goddard ED. Polymer–surfactant interactions: interfacial aspects.J Colloid Interface Sci 2002;256:228–35.

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Staples E, Tucker I, Penfold J, Warren N, Thomas RK, Taylor DJF.Organisation of polymer–surfactant mixtures at the air–water interface:sodium dodecyl sulfate and poly(dimethyldiallyl ammonium chloride).Langmuir 2002;18:5147–53.

Clear demonstration of the competition between the formation of differentcomplexes, and their consequences for the surface tension and adsorption.[8] Staples E, Tucker I, Penfold J, Warren N, Thomas RK. Organisation of

polymer–surfactant mixtures at the air–water interface: poly(dimethyl-diallyl ammonium chloride), sodium dodecyl sulfate and hexaethyleneglycol monododecyl ether. Langmuir 2002;18:5139–46.

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Taylor DJF, Thomas RK, Penfold J. The adsorption of oppositely chargedpolyelectrolyte/surfactant mixtures: neutron reflection from dodecyltrimethyl ammonium bromide and sodium poly(styrene sulfonate) at theair/water interface. Langmuir 2002;18:4748–57.

Detailed description of the complex surface layering that can occur in polymer /surfactant mixtures t interfaces.[10] Taylor DJF, Thomas RK, Hines JD, Humphreys K, Penfold J. The adsorption

of oppositely charged polyelectrolyte/surfactant mixtures at the air/waterinterface: neutron reflection form dodecyl trimethyl ammonium bromide/sodium poly(styrene sulfonate) and sodium dodecyl sulfate/poly(vinylpyridinium chloride). Langmuir 2002;18:9783–91.

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Detailed discussion of surface rheology and other properties in terms of surfacestructure.[19]

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