Polymer–Surfactant Interactions

C H A P T E R

26 PolymereSurfactant Interactions B. Lindman1, T. Nylander1,2,3 1

Lund University, Lund, Sweden; 2Mid Sweden University, Sundsvall, Sweden; 3Nanyang Technological University, Singapore, Singapore

26.1 INTRODUCTION Mixed polymeresurfactant systems have broad applications, ranging from detergents, paints, pharmaceuticals, cosmetics, to biotechnologies. Formulations are typically complex multicomponent mixtures and polymers, and surfactants are ubiquitous in them. Thus in formulations like those for cosmetics and personal care products, there are always one or more macromolecular species and one or more low molecular weight amphiphiles, like a surfactant or a polar lipid. Depending on the identity of the components the macroscopic properties can vary widely. The properties have their basis in the intermolecular interactions between the different components. In this treatise, a broad overview of these interactions will be provided and, with this as a background, different aspects such as phase separation phenomena, rheological properties, and interfacial behavior will be described. The combined action of polymer and surfactant in formulations can sometimes be complementary, with surfactant giving cleansing effects and the polymer stabilization, thickening, or prevention of redeposition. In other cases, the effect can be termed synergistic; for example, strong thickening effects or phase separation effects are used in important formulations. In aqueous solutions there are two important attractive interactions: • electrostatic interactions of solutes with an opposite net charge and • hydrophobic interactions The electrostatic interactions can be repulsive or attractive depending on whether the cosolutes have opposite or similar charges and constitute dominantly an entropic force related to the counterions; coulombic interactions have less significance in an aqueous system. The counterion entropy effect is related to the fact that any highly charged system (macroscopic surface, polyion, surfactant aggregate, etc.) will attract counterions. There will thus be an accumulation of counterions in the vicinity of the highly charged species, which corresponds to a negative entropy contribution. If a positively charged polyelectrolyte is approaching a negatively charged surface or a negatively charged polyelectrolyte, the counterions can be released and there is an entropy gain. As we will see, this association is typically quite important and often leads to phase separation. Approaching a positively charged polyelectrolyte to a positively charged surface or to a positively charged polyelectrolyte, on the other hand, leads to a further accumulation of counterions and a further decrease of counterion entropy; it is therefore repulsive. The counterion entropy effect is directly related to the difference in counterion concentration at the highly charged surface/aggregate/molecule and in the bulk solution. A large difference due to attraction to the surface results in a negative entropy contribution. If an electrolyte is added to the system, the bulk counterion concentration increases, thus corresponding to an entropy increase. Therefore, electrostatic interactions are weakened by electrolyte addition; we talking about a “screening” effect.

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For aqueous systems there is one more strongly attractive interaction, the hydrophobic interaction. This leads to an attraction between molecules, which are nonpolar or contain nonpolar groups, i.e., are amphiphilic. The hydrophobic interactions have their basis in the reduced waterewater hydrogen bonding in the presence of the solute.

26.2 HOMOPOLYMEReIONIC SURFACTANT SYSTEMS SHOW ASSOCIATION In a solution of a polyelectrolyte and an oppositely charged surfactant there is typically a complex formation, and the same commonly applies in mixed solutions of a nonionic polymer and an ionic surfactant. It is commonly referred to a binding of the surfactant molecules to the polymer molecules, but as we will see later an alternative picture normally gives a better rationalization. To discuss this we need to consider the basis of surfactant self-assembly, which is driven by hydrophobic interactions. Surfactant molecules can aggregate into a large number of different structures and show very rich phase behavior in water. For a polar water-soluble surfactant, which we mainly consider here, the surfactant starts to self-assemble into micelles at a quite well-defined concentration, the critical micelle concentration (CMC). The micelles can be roughly spherical or elongated, “thread-like”; we will now focus on the spherical micelles. For an ionic surfactant, self-assembly leads to highly charged entities; it is thus accompanied by a large decrease of counterion entropy and therefore the CMC is high. Nonionic surfactants have roughly two orders of magnitude lower CMC values. A cosolute that reduces this entropic penalty will facilitate micelle formation and lower the CMC. A simple case is that of addition of simple electrolyte. Regarding polymer addition, it is clear that an oppositely charged polyelectrolyte will have a dramatic effect because binding of the polyion releases a large number of small counterions from the micelles; the result is a large increase in entropy and a dramatic lowering of the CMC. As expected, the effect is reduced if the solution contains simple electrolyte. In addition, a nonionic polymer will normally reduce the CMC, more so if it is has lower polarity, like poly(ethylene glycol) or poly(vinyl pyrrolidone). These molecules will be located at the micelle surface and reduce the charge density. Rather than binding of surfactant or micelle to the polymer molecule, it is thus more correct to consider polymerinduced surfactant self-assembly: it is energetically more favorable for the surfactant to form micelles at the polymer molecule. The micelles are quite similar to those formed by the surfactant alone. For the case of spherical micelles, we will have a “pearl-necklace” structure with discrete micelles along the polymer chain (Fig. 26.1) The surfactantepolymer association can be inferred from many physicochemical parameters, like surfactant activity, electric conductivity, solubilization, spectroscopy [nuclear magnetic resonance (NMR), etc.], calorimetry, scattering, etc. An important characteristic of a surfactant is the surface activity, and in Fig. 26.2 we compare schematically the surface tension of surfactant alone and that in the presence of a polymer. As can be seen there is a two-step lowering, rather than one such as for surfactant alone, in the presence of a polymer; the first break point indicates the onset of association and the second the saturation level after which the free surfactant concentration increases. One important feature shown in Fig. 26.2 is that in a wide range of surfactant concentration the surface tension is higher as polymer is added to a surfactant solution; thus the surfactant has lower tendency to adsorb at the airewater interface. A second feature is that at high enough surfactant concentrations the same surface tension is reached as for surfactant alone.

FIGURE 26.1

“Pearl-necklace model” of surfactantepolymer association. From Kronberg B, Holmberg K, Lindman B. Surface chemistry of surfactants and polymers. Chichester, UK: John Wiley & Sons; 2014.

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FIGURE 26.2

A schematic plot of the concentration dependence of the surface tension for mixed polymeresurfactant solutions. The corresponding curve for the system with only surfactant is also shown. For the case of surfactant alone, there is a single-step decrease before the CMC, whereas for the mixed system there is a decrease until the onset of polymeresurfactant association (an example of critical association concentration), then a plateau before a second-step decrease. From Kronberg B, Holmberg K, Lindman B. Surface chemistry of surfactants and polymers. Chichester, UK: John Wiley & Sons; 2014.

Whereas ionic surfactants broadly associate with polymers, nonionic surfactants do not associate with homopolymers; they will only associate with polymers that have hydrophobic groups, i.e., are amphiphilic. In summary, we can distinguish between three types of polymeresurfactant systems that show association: 1. Systems of oppositely charged polymer and surfactant. 2. Systems of an ionic surfactant and a nonionic polymer. 3. Systems of an amphiphilic polymer, in which all types of surfactants show association.

26.3 POLYELECTROLYTEeSURFACTANT SYSTEMS MAY SHOW TWO-STEP ASSOCIATION The association between polymer and surfactant is thus mainly determined by two interactions, electrostatic and hydrophobic. As said, an oppositely charged polymer can strongly facilitate surfactant self-assembly. If the polymer is very polar, then binding of surfactant leads to partial charge neutralization of the polymer. This is illustrated for the case of mixtures of sodium polyacrylate and cationic surfactant in Fig. 26.3. The binding is strongly cooperative,

FIGURE 26.3 Binding isotherms for dodecyl trimethylammonium bromide (C12TAB) in 0.50 mM sodium poly(acrylate) (NaPA) at indicated NaBr concentations (mM). Here, b ¼ NsCm/Cp, in which Ns is the aggregation number, Cm is concentration of micelles and Cp is the concentration of polymer charges. Courtesy of Per Hansson.

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FIGURE 26.4 Typical surfactant-binding isotherm of ionic surfactant to oppositely charged polymer having some hydrophobic character. Courtesy of Lennart Piculell.

starting at a concentration termed the critical association concentration (CAC). The binding is best described by a micelle formation induced by the polyion, i.e., the driving force is the increased entropy of the counterions released from the polymer and the micelle. We note that it cannot be described as a binding of micelles to the polymer. The CAC is necessarily lower than the CMC, and, in the bulk solution, the free-surfactant concentration is below the CMC, and micelles do not exist. Fig. 26.3 also illustrates that in the presence of electrolyte the association is weakened and the CAC increased, a consequence of a screened electrostatic attraction. If the polyelectrolyte is less polar, hydrophobic interactions between surfactant and polymer can, as shown by Lennart Piculell, lead to a two-step binding isotherm as illustrated in Fig. 26.4 for the case of cationic hydroxyethyl cellulose. In a first step, the attraction between polymer and surfactant is dominated by electrostatic interaction. However, because cellulose is amphihilic, i.e., has distinct hydrophobic properties, there will also be attractive hydrophobic interactions between polymer and surfactant. At higher surfactant concentrations, there will be a second cooperative binding step, thus a second CAC. Although the first binding step leads to significant charge neutralization of the polymer, the second binding step will lead to charge reversal. As we will see, the two binding steps are often manifested in the phase behavior: The first step may lead to phase separation of an approximately charge neutral complex, whereas the second step leads to redissolution.

26.4 AMPHIPHILIC POLYMER SELF-ASSEMBLY For previous case 3, in which the polymer also has distinct hydrophobic groups, both components are self-assembling individually. In combination they give mixed micelles (or other aggregates). For the case of block copolymers, the aggregates show strong resemblances to mixed aggregates of two surfactants. For the case of hydrophobically modified water-soluble polymers, the picture is different and more complex. There are two common types of hydrophobically modified water-soluble polymers, referred to as associative thickeners, end-capped, or graft copolymers; the graft copolymers, which have a hydrophilic backbone and hydrophobic grafts (for example, alkyl chains) show the largest viscosity increases and are most sensitive to surfactant addition. Such polymers have a backbone that is water soluble, like polyacrylate or a cellulose derivative; onto this have been grafted a relatively low number (typically 1e5% of the monomer units modified) of hydrophobic groups, like C12eC18 alkyl chains. In Fig. 26.5 we illustrate the self-assembly of the polymer alone. The associated hydrophobic grafts act as physical cross-links and a physical gel is formed. Because surfactant association to hydrophobic sites is strong, there will, from low concentrations, be a binding of surfactant molecules to the hydrophobic microdomains (cross-links); these will grow and hence achieve longer lifetimes. In turn, this results in increased viscosity, typically by orders of magnitude. There is thus a dramatic synergistic effect in rheology. However, as more surfactant is added, the viscosity reaches a maximum and thereafter drops abruptly. The general behavior is schematically illustrated in Fig. 26.6.

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FIGURE 26.5

A hydrophobically modified water-soluble polymer associates with a three-dimensional network, giving physical gelation. Courtesy of Leif Karlson.

FIGURE 26.6 Association between a surfactant and a hydrophobically modified water-soluble polymer gives a strong thickening effect. Viscosity is shown as a function of surfactant concentration. Courtesy of Leif Karlson.

At higher surfactant concentrations there is as indicated a major loss of the thickening effect and the viscosity decreases to values well below that of the solution of the polymer alone. This occurs when the concentration of micellar aggregates reaches values similar to those of the polymer hydrophobe groups. For cross-linking at least two polymer molecules should be associated with the same micelles but at higher surfactant concentrations there is only one polymer hydrophobe in each micelle and there is a repulsion between polymer molecules. The stoichiometric aspects are further illustrated in Fig. 26.7. Here is shown the effect of addition of sodium dodecyl sulfate (SDS) to 1 wt% solutions of a hydrophobically modified nonionic cellulose derivative (ethyl hydroxyethyl cellulose). The viscosity is related to the concentration of micelles in the solution. As can be seen, addition of SDS over a wide range of concentration does not affect the micelle concentration. SDS addition instead leads to increase of the aggregation number of the micelles. As the optimal aggregation number (ca. 60 for SDS) is reached, the micelles cannot grow further and additional micelles are created; at this point the viscosity starts to decrease abruptly. The high sensitivity of rheology for relatively minor excesses of surfactant is a complication for several applications, because the surfactant concentration cannot always be controlled. The problem can be overcome by going away from spherical micelles, which are monodisperse and have a well-defined aggregation number. In the case of SDS, a transition from small spherical micelles to elongated thread-like micelles can be induced inter alia by addition of electrolyte or adding a long-chain alcohol or a relatively hydrophobic nonionic surfactant. Especially effective is the addition of relatively small amounts of an oppositely charged surfactant; this is illustrated in Fig. 26.8. For the case of Fig. 26.7, addition of cationic surfactant could lead to increase in viscosity by at least five orders of magnitude. These concepts of gel formation of mixed polymeresurfactant systems have been extended to other types of surfactant aggregates, like in vesicle gels and gelled microemulsions.

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FIGURE 26.7 Viscosity of solutions of a hydrophobically modified cellulose derivative as a function of the concentration of added ionic surfactant. For comparison the concentration of micelles is shown (triangles, right axis). Courtesy of Krister Thuresson.

FIGURE 26.8 Micelle growth can be achieved, for example, by adding an oppositely charged surfactant. The corresponding reduction in micelle concentration can induce major increases in viscosity (see text). Courtesy of Susanne Nilsson.

26.5 PHASE SEPARATION IS COMMON FOR POLYMEReSURFACTANT MIXTURES Polymer molecules have low translational entropy and, therefore, the solubility is typically limited. The situation is different for polyelectrolytes because of the large entropy contribution from the counterions. The general rule is that nonionic polymers have relatively low solubility whereas polyelectrolytes are highly soluble. On addition of electrolyte, however, the counterion entropy effect is lost and many polyelectrolytes lose their solubility. In mixed systems of two nonionic polymers, there is typically a segregative phase separation; i.e., there are two solutions at equilibrium, each enriched in one of the polymers. The same applies to mixed solutions of two polyelectrolytes bearing the same charge. In mixed solutions of one nonionic and one ionic polymer there is typically miscibility, driven by the counterion entropy. Finally, if we mix two oppositely charged polyelectrolytes, there is a strong attraction that leads to associative phase separation, often denoted complex coacervation; the driving force is the release of the counterions of the two polyelectrolytes. Because surfactant aggregates are large and have a high effective molecular weight, and then low translational entropy, the same simple principles of phase behavior apply for mixed polymeresurfactant systems as for polymere polymer mixtures. Thus, in the absence of an attractive interaction (which for aqueous systems is either of electrostatic or hydrophobic origin) between polymer molecules and micelles, we expect a segregation. This is indeed observed when we mix in aqueous solution a nonionic polymer with a nonionic surfactant or mix a polyelectrolyte with an ionic surfactant of similar charge. An illustration is given in Fig. 26.9. This concerns an anionic surfactant (SDS) and an anionic polysaccharide (sodium hyaluronate). On salt addition, phase separation is enhanced due to micellar growth, thus reducing entropy of mixing. An intriguing case with several important applications is that of oppositely charged systems. As mentioned, a mixture of two oppositely charged polyelectrolytes shows strongly associative behavior, as demonstrated by a strong tendency to phase separation. A mixture of a polyelectrolyte and an oppositely charged surfactant will also associate strongly. We discussed above the considerable lowering of the CMC. In addition, phase separation

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FIGURE 26.9

Phase separation in mixtures of a polyelectrolyte and a similarly charged surfactant is typically segregative and may be enhanced on salt addition if there is an electrolyte-induced micellar growth. The example refers to mixtures of sodium dodecyl sulfate and the anionic polysaccharide sodium hyaluronate. From Thalberg K, Lindman, B. Segregation in aqueous systems of polyelectrolyte and ionic surfactant. Colloids Surf 1993;76:283e8.

takes place in a broad range of concentrations. For example, if an ionic surfactant is progressively added to a solution of an oppositely charged polymer, turbidity is encountered for certain concentrations. As surfactant starts to bind to the polymer, at the CAC, neutral insoluble complexes come out of solution. In many cases there is as mentioned a redissolution, as seen from the solutions becoming clear at higher surfactant concentrations. This will occur if the polymer has a certain hydrophobicity and thus can bind the surfactant above charge stoichiometry; when the complexes become increasingly charged, their solubility increases. This redissolution and charge reversal will not occur for entirely hydrophilic polymers. The strong associative phase separation of such systems can be characterized in the conventional phase diagram for three-component systems as exemplified in Fig. 26.10. As shown, an aqueous mixture of a polyelectrolyte and an oppositely charged surfactant phase separates into one dilute phase and one, typically highly viscous, phase, concentrated in both polymer and surfactant. The extent of phase separation in the system of Fig. 26.10 increases strongly with the surfactant alkyl chain length and the polymer molecular weight. On addition of an electrolyte, the phase separation is reduced and then

FIGURE 26.10 A mixture of an ionic surfactant and an oppositely charged polyelectrolyte typically gives an associative phase separation, as here exemplified by tetradecyltrimethyl ammonium bromide and sodium hyaluronate. From Thalberg K, Lindman B, Karlstro¨m G. Phase diagram of a system of cationic surfactant and anionic polyelectrolyte: tetradecyltrimethylammonium bromideehyaluronanewater. J Phys Chem 1990;94:4289e95.

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FIGURE 26.11 Phase separation in mixtures of a polyelectrolyte and an oppositely charged surfactant changes from associative, to no phase separation, and finally to segregative as electrolyte is added. The example shows mixtures of a cationic surfactant, tetradecyltrimethylammonium bromide, and an anionic polysaccharide, sodium hyaluronate. From Thalberg K, Lindman B, Karlstro¨m G. Phase behavior of a system of cationic surfactant and anionic polyelectrolyte: the effect of salt. J Phys Chem 1991;95:6004e11.

eliminated, but at higher electrolyte content there is a phase separation again. However, as we can see in Fig. 26.11, this is of a different nature. This behavior, which is very similar to what is observed for mixtures of two oppositely charged polymers, can best be understood from a combination of polymer incompatibility and electrostatic effects. We note that the concentration of counterions is very high and, therefore, unlike for the nonionic polymers, phase separation with a polyelectrolyte in one phase leads to confinement of the counterions and a very significant entropy loss. At high electrolyte concentrations, this entropy contribution is eliminated and the phase separation will be similar to that of uncharged polymer systems. In the case of Fig. 26.11, we have an intrinsically segregating system, as can be seen from the phase diagram at high electrolyte concentrations. The associative phase separation, occurring for mixtures of both anionic and cationic polymers with oppositely charged surfactants at low salt contents, is thus understood from the entropy of the counterion distribution. The highly charged micelles and polyelectrolyte molecules are enriched with counterions at their surface due to the coulombic attraction. On association, counterions of both cosolutes are transferred into the bulk with a concomitant entropy gain; therefore, there is a strong tendency to associative phase separation in the absence of added salt. When the surfactant and the polymer have similar charges, the entropic loss is absent and a segregative phase separation is the rule, as illustrated earlier in Fig. 26.9. Although the presentations of Figs. 26.10 and 26.11 are a useful starting point for considering the behavior of polyelectrolyte-oppositely charged surfactant systems, the actual phase behavior is more complex as it is for any system of two electrolytes without a common ion. Thus, we have to consider two additional electrolytes, which

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FIGURE 26.12 Polyelectrolyteeionic surfactant systems require a three-dimensional diagram for complete illustration of the phase behavior. From Thalberg K, Lindman B, Karlstro¨m G. Phase diagram of a system of cationic surfactant and anionic polyelectrolyte: tetradecyltrimethylammonium bromideehyaluronanewater. J Phys Chem 1990;94:4289e95.

are the combination of polyion and surfactant ion and the combination of the counterions. Therefore, a threedimensional representation is needed as shown in Fig. 26.12. Lennart Piculell in Lund has elaborated on these aspects in various ways. He has inter alia demonstrated the usefulness of basing phase diagram work on the “complex salt,” i.e., the combination of polyion and surfactant ion. Aqueous mixtures of the complex salt combined with either surfactant or polymer are true three-component systems and can be accurately represented in the normal Gibbs triangle. Piculell has also pointed out the role of the combination of the counterions, the “simple salt,” as a “hidden variable.” In this way, the fact that polyelectrolyteesurfactant systems may be phase separated, and concentrated on adding water, is easily illustrated. Thus, the effect of adding water leads to a dilution of the simple salt and thus to a strengthened association between polyion and surfactant and then a larger propensity to phase separation (cf. Fig. 26.11). On dilution, the attraction becomes stronger, inducing phase separation, an effect used, for example, in hair-care formulations. The system illustrated previously concerns a rather hydrophilic polyion, hyaluronate. If the polyion is less polar, then there will be a strengthened association of surfactant and thus a larger tendency toward charge reversal of the complexes. Therefore, phase separation occurs in a more limited range, and the two-phase region is narrower. This has consequences for using polymeresurfactant systems for rheology control. If we mix a hydrophilic polyelectrolyte with an oppositely charged surfactant, it has limited use to increase the viscosity because phase separation occurs over a wide range. Hydrophobically modified water-soluble polymers, broadly used as associative thickeners, behave differently because they easily bind surfactants in excess of charge neutrality, leading to soluble complexes. Because of the combination of electrostatic and hydrophobic association such mixed systems are very useful for thickening purposes. An even larger effect may be obtained for mixtures of two oppositely charged hydrophobically modified polyelectrolytes (cf. Fig. 26.13). Again, although two hydrophilic polyelectrolytes will phase separate over most of the mixing range, the hydrophobic modification allows for the facile formation of nonstoichiometric complexes with significant net charge. Because of gene therapy applications, the interaction between DNA and cationic surfactants or lipids has received extensive attention. DNA is a polyelectrolyte with a particularly high linear charge density at the same time as it has strong hydrophobic groups. It is thus a distinctly amphiphilic polymer which can self-assemble driven by the hydrophobic interactions between the bases, the most well-known structure being the double helix. The CAC values of cationic surfactanteDNA systems are quite low as expected from the high charge density. Interestingly, single-stranded DNA (ssDNA) gives stronger association than double-stranded DNA (dsDNA), showing the importance of the exposed bases promoting hydrophobic interactions. The strong interaction in DNA systems is clearly reflected in phase diagrams, showing a strongly associative behavior. This is illustrated in Fig. 26.14 on a logarithmical concentration scale for three surfactants with different chain lengths.

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FIGURE 26.13

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Illustration of the association in mixed solutions of oppositely charged hydrophobically modified polyelectrolytes. Courtesy of

Filipe Antunes.

FIGURE 26.14 Phase diagrams of dsDNAecationic surfactant systems showing a strong associative phase separation, which is strongly dependent on surfactant alkyl chain length. Dias R, Mel’nikov SM, Lindman B, Miguel MG. DNA phase behavior in the presence of oppositely charged surfactants. Langmuir 2000;16:9577e83.

An unexpected observation in the phase diagram work on DNA was that excess surfactant was not observed to give redissolution, which would be expected for such a hydrophobic polyelectrolyte as DNA. Furthermore, unlike other systems (see earlier discussion), even high amounts of electrolyte did not lead to redissolution of the precipitate. The early work was performed in the conventional way by adding surfactant progressively to a solution of the polyelectrolyte. By introducing other mixing strategies, it could, however, be demonstrated that solutions with an excess of surfactant are indeed homogeneous solutions. This could be shown by adding DNA to concentrated surfactant solutions. The associative phase separation occurring for systems of oppositely charged polymer and surfactant leads to one dilute and one concentrated phase. The concentrated phase can be solid or liquid in nature depending on the system but, typically, it has a liquid crystalline nature. Lamellar, hexagonal, and cubic structures have been identified. We illustrate this by structures identified for mixed systems of dsDNA, cationic surfactant, and lipid in Fig. 26.15.

26.6 GELS: THERMAL GELATION, CHEMICAL GELS, AND MICROGEL PARTICLES In conjunction with gels and gelation, there are several aspects to polymeresurfactant systems. Mixed solutions can give rise to gels, and here we will illustrate with thermal gelation. Furthermore, surfactants can strongly

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FIGURE 26.15 The most important liquid crystalline structures occurring in mixtures of a (rigid) polyelectrolyte (in blue) and an oppositely charged surfactant (in yellow). After Bilalov A, Olsson U, Lindman B. Complexation between DNA and surfactants and lipids: phase behavior and molecular organization. Soft Matter 2012;8:11022e33.

influence the swelling behavior of both macroscopic gels and microgel particles, both of which have important applications. In addition, a controlled phase separation can efficiently produce gel particles of different sizes. The interactions of a surfactant with a polymer are as described above very much controlled by hydrophobic interactions. Several nonionic polymers like poly(ethylene glycol) and nonionic cellulose ethers change their polarity with temperature as manifested by clouding and phase separation as temperature is increased. For a mixed solution of such a polymer and an ionic surfactant the surfactant binding is induced by a temperature increase, as seen in reduced values of the CAC. This can be used to achieve a temperature-controlled gelation as illustrated in Fig. 26.16. Thus, we can design systems that are liquid solutions at room temperature and gels at body temperature, which can have pharmaceutical and other applications. The associative phase separation just described can be the basis for formation of polymer gels by physical crosslinking. By adding small drops of a concentrated solution of a polyelectrolyte into a surfactant solution, gel particles in the size range of 100 nm to mm can be formed; based on the associative phase separation, core-shell, or homogeneous particles can be prepared.

FIGURE 26.16 For a mixed solution of a nonionic polymer and an ionic surfactant the surfactant binding is induced by a temperature increase. This can be used to achieve a temperature-controlled gelation. Courtesy of Anders Carlsson.

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This can be illustrated with double-stranded DNA, which as described is a highly charged and stiff polyanion (Figs. 26.17 and 26.18). Because of its high charge density, DNA interacts strongly with cationic surfactants. As also noted the surfactant binding isotherms show a strongly cooperative association, and the phase diagrams display a very strong associative phase separation. Because DNA generally has a very high molecular weight, it is possible to directly monitor the interactions on a single molecular level by using microscopy (Fig. 26.19). As a cationic surfactant is added to a dilute DNA solution, the DNA molecules change their conformation from an extended “coil” state to compact “globules.” The DNA molecules are compacted individually, and, over a wide concentration range, there is a coexistence of coils and globules. DNA induces self-assembly of a cationic surfactant and DNA compaction by a surfactant can be viewed as an associative phase separation at the single molecular level. DNA-Gel particles Associative Phase Separation Oppositely Charged Surfactant / Polyelectrolyte Complex

Gelation at WATER/WATER emulsion type interfaces INTERNAL POLYELECTROL YTE (DNA)

Main Advantages High content DNA reservoir Without adding any kind of cross-linker or organic solvent

FIGURE 26.17

Scheme outlining the formulation of DNA-Gel core shell particles that utilizes the associative phase separation between the DNA and an oppositely charged surfactant. Courtesy of Carmen Mora´n.

FIGURE 26.18

Gel particles can be prepared of different sizes from a few 100 nm. Courtesy of Carmen Mora´n.

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FIGURE 26.19 Adding cationic surfactant to a dilute solution of dsDNA induces compaction of DNA. This can be reversed by adding an anionic or nonionic surfactant. Courtesy of Rita Dias.

Many diseases have a genetic origin and may be cured by modifying the DNA sequence. However, to allow DNA to be transferred into cells, it needs to be compacted. Cationic cosolutes, such as surfactants and lipids, are efficient transfection agents of great interest for developments in gene therapy. Compaction can be used as means to regulate the transcription as well as to protect the DNA against degradation. Addition of ionic surfactants to chemical polymer gels dramatically changes the swelling of the gels. A nonionic polymer gel has a low swelling capacity in water by itself; but when an ionic surfactant binds (from the CAC), a major increase in volume takes place. Thus, surfactant binding effectively transforms the polymer into a polyelectrolyte gel. An ionic polymer gel has, on the other hand, a large volume by itself. As an oppositely charged surfactant starts to bind (from the CAC), there is charge neutralization and shrinking to low volumes. There may be a reswelling at higher surfactant contents, which is due to surfactant binding in excess of charge neutralization (cf. earlier), leading to a charge reversal of the gel. We also note that such gel-swelling experiments provide a simple and accurate way of obtaining CAC values. We illustrate the interactions of surfactants with polymer gels by the case of covalently cross-linked DNA and cationic surfactants (see Fig. 26.20). As seen, a surfactant has no effect on the gel volume until the CAC is reached; thereafter, there is a dramatic shrinkage with DNA concentration. The concentration for onset of shrinkage is as expected, lowered strongly as the surfactant alkyl chain length is increased. From comparison of ssDNA and dsDNA gels, a stronger interaction is inferred with ssDNA. If after shrinking, an anionic surfactant is added, there is a

FIGURE 26.20 Cross-linked DNA gels shrink by cationic surfactant, CnTAB, (A), the efficiency being strongly dependent on alkyl chain length of the surfactant. The subsequent addition of an anionic surfactant, SDS, leads to reswelling (B). Courtesy of Diana Costa.

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Viscosity vs C SDS 1.00E+05

Viscosity (Pa.s)

1.00E+04 1.00E+03 1.00E+02 1.00E+01 1.00E+00 1.00E-01 1.00E-02 1.00E-03 0.00

0.60

1.20

1.70

2.30

2.80

3.30

C SDS ( wt%)

FIGURE 26.21 Ionization of microgel particles leads to swelling, which in turn produces major effects on viscosity. Ionization by deprotonation and binding of ionic surfactant (as shown here) gives similar effects. After Antunes F, Alves L, Duarte C, Lindman B, Klotz B, Boettcher A, Haake H-M. Ionization by pH and anionic surfactant binding gives the same thickening effects of cross-linked polyacrylic acid derivatives. J Disp Sci Technol 2012;33:1368e72.

reswelling to essentially the same volume as for the initial polymer gel. The reason for this is that the association between anionic and cationic surfactant is stronger than DNAesurfactant interaction and therefore the cationic surfactant is released from the polymer gel. A corresponding behavior is seen for small cross-linked polymer gel particles or microgels. In Fig. 26.9, we illustrated the case of segregative phase separation between an anionic polymer and an anionic surfactant. If the polymer were modified by hydrophobic groups, then the repulsive interaction can be overcome and the surfactant associates with the polymer. If the polymer, furthermore, is cross-linked, intriguing swelling effects will arise. With ionizable groups, like particles containing polyacrylate, the particle size will vary strongly with pH. This is very useful for many formulations because in a wide range of volume fractions the changes in ionization will cause major changes in viscosity due to particle overlap. Ionization by increasing the pH can induce a change in viscosity by seven orders of magnitude; this is due to particle swelling and overlap. Addition of an ionic surfactant to uncharged microgel particles gives a similar increase in viscosity (Fig. 26.21), where ionization by binding the surfactant causes a similar effect as ionization by deprotonation. In both cases, the solution clarifies on ionization because the dramatic swelling leads to very water-rich particles with negligible optical contrast to water.

26.7 SURFACTANTePOLYELECTROLYTE MIXTURES AT INTERFACES Many applications, e.g., within consumer products, rest on the ability to control the deposition of active components on solideliquid and liquideair interfaces. Here, the associative phase separation of oppositely charged systems described earlier can be used to control the interfacial behavior. We will illustrate the interplay between bulk and surface behavior with examples for the airewater interface and for a solid surface in contact with water. Some of the typical scenarios we can expect, if we add a surfactant to a preadsorbed polymer layer at a surface, are illustrated in Fig. 26.22: 1. The polymeresurfactant interaction is so strong that the surfactant combines with the adsorbed polymer layer. This can have two consequences: a. An increase of adsorbed amount at the interface and formation of a mixed polymeresurfactant layer. b. Adsorption of excess surfactant and swelling of the polymer layer and eventual detachment of the formed polymeresurfactant complexes. 2. The surfactant adsorbs so strongly to the surface that it competes with the preadsorbed polymer. This leads to displacement and desorption of the polymer, which often occurs at the aireaqueous interface as well as on hydrophobic surfaces. This points to two important aspects of the situation in which a surfactant combines with a polymer. The first is that the surfactantepolymer association changes the solubility of the polymer, which depending on the polymere surfactant ratio either can increase or lower the solubility compared to the polymer alone. Because solvency often controls adsorption, this can in turn either decrease or increase polymer adsorption upon association with a

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FIGURE 26.22

463

Illustration of surfactantepolymer interactions at a surface and in bulk solution. Courtesy of Fredrik Joabsson.

surfactant. The second aspect relates to the kinetics of forming the complexes, which is very slow. Therefore, polymer desorption can take a long time, and the order of addition of the components can be crucial. In fact, the relevant timescale in application of a product containing polymer(s) and surfactant(s) is in many cases such that it rarely occurs under equilibrium conditions. This can be utilized to tune the adsorbed layer properties by accessing nonequilibrium trapped states. We will illustrate this by considering a case of the behavior of oppositely charged polyelectrolyteesurfactant systems at the airewater interface. First, we will note that one of the most significant aspects of a surfactant is its ability to lower the interfacial tension between an aqueous solution and air. In particular for an ionic surfactant, this is modified by the presence of a polymer in the solution. Equilibrium surface tension versus the surfactant concentration curve recorded in the presence of polymer (of constant concentration) usually features a decrease in surface tension far below what is observed for surfactant alone. After this decrease, more or less constant surface tension is attained. The concentration range over which this is observed is roughly proportional to the polymer concentration. Finally, at even higher surfactant concentrations there is a decrease toward the value obtained in the absence of polymer. If we consider the time effects, a very different behavior is observed in terms of bulk and surface properties as shown in Fig. 26.23. Here, we see a dramatic increase in surface tension on further addition of surfactant (B), as observed in a number of other studies. Because of the shape of the surface tension curve, this was referred to as the “cliff-edge effect.” Previously, the phenomenon was referred to a complex process involving different polymeresurfactant complexes. However, this effect is only observed after the sample has been equilibrated for some time (filled symbols in B). We also note that the corresponding behavior is observed in bulk solution, i.e., the turbidity decreases with time within the precipitation regime (A). This can in turn be related to sedimentation of formed complexes. It is, therefore, clear that this effect can be related to associative phase separation. Campbell et al. (52, 56) showed that the time effects for some common polymeresurfactant systems could be rationalized in terms of nonequilibrium effects and bulk aggregation phenomena. Fig. 26.23C and D shows that there are almost equal amounts of polymer and surfactant at the liquideair interface at low surfactant concentrations. Both the surface tension and the amounts are independent of the polymer concentration in this regime. This indicates the formation of a surfactantepolymer complex at the interface, which also leads to a drastic lowering of the surface tension compared to the situation with polymer alone. At higher surfactant concentrations, in the phase separation regime, the systems initially show constant surface tension because precipitation is not immediate. However, when the system is given sufficient time to undergo associative phase separation with subsequent precipitation, the solution becomes depleted of surface-active material. Therefore, a sharp increase in surface tension is observed. The surface concentrations of surfactant and polymer decrease (Fig. 26.23C and D). Above this point the surfactant dominates the liquideair interface. The precipitation is a time-consuming process and the settling process can take several days to reach completion. This is clearly an important phenomenon, perhaps practically even more significant for oilewater interfaces. Polyelectrolyteesurfactant adsorption at solid surfaces is of broad scientific interest as it is crucial in many different applications. Adsorption is as discussed earlier, typically controlled by solvency effects. This in turn is

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FIGURE 26.23 A comparison of the effect of time after mixing poly(diallyldimethylammonium chloride) (PDADMAC) and sodium dodecyl sulfate (SDS) on the bulk and aireaqueous interfacial behavior. Mixed just before measurements is shown as open symbols and stored and settled for 72 h is indicated as closed symbols. The gray shaded area shows the two-phase precipitation region. (A) Shows the bulk turbidity versus surfactant concentration shown as optical density at 450 nm with 100 ppm PDADMAC. (B) Surface tension of these PDADMAC/SDS solutions. (C) Shows the amount of polymer and (D) the amount of surfactant at the aireaqueous interface as determined by neutron reflectometry. From Campbell RA, Arteta MY, Angus-Smyth A, Nylander T, Varga I. Effects of bulk colloidal stability on adsorption layers of poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate at the airewater interface studied by neutron reflectometry. J Phys Chem B 2011;115:15202e13.

governed by the strength and the nature of the interaction between the surfactant and the polymer. A strong attraction between the surfactant and the polymer will lead to associative phase separation. Because of the decreased solvency, this can enhance adsorption of the surfactantepolymer complex. In Fig. 26.24 is shown what happens when surfactant is progressively added to a hydrophilic surface with preadsorbed oppositely charged polymer in the presence of the polymer solution. No change in adsorbed amount is generally observed at low surfactant concentrations. At the CAC, i.e., at the same concentration at which

FIGURE 26.24 Adsorption on silica from a solution of cationic hydroxyethyl celluloses (JR-400) at 100 ppm with sequentially increased amount of added SDS. The data are compared with the corresponding gel-swelling data. 0.1 mM SDS corresponds to the critical aggregation concentration (cac) of JR-400/SDS. Modified after Sjo¨stro¨m J, Piculell L. Colloids Surf A: Physicochem Eng Asp 2001;183:429e48.

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surfactant starts to associate to the polymer in bulk solution, the adsorbed amount increases. Adsorption increases with surfactant concentration until approximate charge stoichiometry is reached. At some stage, precipitation occurs as also can be observed by turbidity measurements. It is interesting to note that data correspond to those obtained from a gel-swelling experiment, which shows that the gel formed from JR-400 contracts at the same SDS concentration as the surface deposition increases. This corroborates the strong correlation between bulk behavior and adsorption. It is clear that the surfactant alters the charge of the polymer at the interface and in the bulk phase, affecting gel properties as well as deposition. At higher surfactant concentrations, the gel, as well as the adsorbed layer, swells due to charge reversal upon additional association of surfactant. Simultaneously, the adsorbed amount decreases and, in some cases, no polymer or surfactant remains at the interface. Similar trends are observed on hydrophobic surfaces, with the difference that in this case noncooperative surfactant adsorption starts at low surfactant concentrations. When considering the general trends just reported, the question arises to what extent we can manipulate the enhanced deposition close to the CAC by changing the polymer architecture. Here, we will discuss how we can control the deposition when we increase the hydrophobicity of the polymer and change the charge density. The adsorption from solutions of different cationic polymers with constant content of cationic groups, but different hydrophobicity is illustrated in Fig. 26.25. As for the JR-400 system, a marked increase in both the adsorbed amount and the bulk turbidity is observed at certain SDS concentrations when the surfactant is added stepwise to the dilute polymer solution. Again, this is the result of the formation of polyionesurfactant complexes of decreasing solubility. Both the turbidity and the adsorption are maximized, which is related to the overcharging of the formed complexes that dissolve and desorb from the surface. The maxima in adsorption and turbidity occur at lower SDS concentration for a polymer with a higher hydrophobicity. This is an effect of the combined electrostatic and hydrophobic interactions, in which the SDS binding to the more hydrophobic polyions is facilitated. Consequently, the aggregates formed with the more hydrophobic polymers dissolved at a lower SDS concentration and hence the shift of the maxima. The charge density of cationic polymer mixed with an anionic surfactant is clearly an important factor. This is illustrated in Fig. 26.26 for the interaction of sodium dodecyl sulfate (SDS) with a series of cationic copolymers of vinylpyrrolidone and quaternized vinylimidazol, with different charge density, at the silicaeaqueous interface using in situ ellipsometry. The maxima in adsorption/deposition of polymerSDS complexes scale with the SDS concentration corresponding to charge equivalence in the phase separation region. The fact that the peak position is determined by polymer charge, i.e., the amount of SDS needed to form a neutral complex, suggests that the CAC is low and the deposited amount is determined by the net charge of the polymeresurfactant complexes. That electrostatics 10 9

= 500 nm

HPA/DMAM

8 7 6 5

AA/MAPTAC

HPA/DMAM

Absorbance at

Adsorbed amount (mg/m2)

0.3

AMP/MAPTAC

HEA/MAPTAC

4 3 2

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AMP/MAPTAC HEA/MAPTAC

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AA/MAPTAC

0.15 0.1 0.05

1 0 0.001

0.01

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1

SDS (mM)

10

100

0 0.001

0.01

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10

100

SDS (mM)

FIGURE 26.25 The effects of polyion hydrophobicity on adsorption and phase separation of oppositely charged polyionesurfactant complexes are shown. Adsorbed amount and turbidity (absorbance at 500 nm) are shown for 100 ppm solutions of acrylamide/methacrylamidopropyl trimethylammonium chloride (AA/MAPTAC), hydroxyethyl acrylate/methacrylamidopropyl trimethylammonium chloride (HEA/MAPTAC), acrylomorpholine/methacrylamidopropyl trimethylammonium chloride (AMP/MAPTAC), and hydroxypropyl acrylate/ dimethylaminoethyl methacrylate (HPA/DMAM) copolymers with increasing amounts of SDS. The hydrophobicity of the polymer increases in the order AA/MAPTAC < HEA/MAPTAC z AMP/MAPTAC < HPA/DMAM, but the content of cationic groups was constant at 20%. After Santos O, Johnson ES, Nylander T, Panandiker RK, Sivik MR, Piculell L. Surface adsorption and phase separation of oppositely charged polyion-surfactant ion complexes: 3. Effects of polyion hydrophobicity. Langmuir 2010;26:9357e67. III. PHYSICOCHEMICAL ASPECTS AND FORMULATIONS

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FIGURE 26.26 Effect of polymer charge density on the adsorption from mixtures of cationic polymers with SDS at the silicaeaqueous interface for (A) 10 mM NaCl and (B) 100 mM NaCl solutions. The amount of the complex adsorbed from premixed solutions of 75 vinylpyrrolidone (VP)/25 quaternized vinylimidazol (QVI) (0.12 mM cationic charges), 50VP/50QVI (0.33 mM cations), and 25VP/75QVI solutions (0.42 mM cations) with SDS is shown. VP here refers to vinylpyrrolidone, QVI to quaternized vinylimidazol and the preceding number to the content in weight percent. Mohr A, Nylander T, Piculell L, Lindman B, Boyko V, Wilko Bartels F, Liu Y, Kurkal-Siebert V. ACS Appl Mater Interfaces 2012;4:1500e11.

controls the interaction is apparent as increasing the ionic strength shifts the maxima to lower SDS concentrations, i.e., less SDS is needed to form a neutral complex. On the other hand, the maximal amount adsorbed decreases at high salt concentration. We have so far discussed how the molecular architecture of the polymer can be used to manipulate the adsorption/deposition on a surface. Here, it should be noted that in many applications involving cleaning, not only application of the cleaning agent is important but also the rinsing step can be crucial. This is the case for, e.g., hair-care formulations, in which anionic surfactants are used for soil removal and cationic polymers are used as conditioners. This type of formulation would contain a large excess (by charge) of anionic surfactant, i.e., well above the two-phase region, thus no adsorption will occur as shown in Figs. 26.24e26.26. The action of such a formulation can be a twostep process, in which the initial step is the action of the surfactant on the soil surface leading to solubilization of the soil. In the second step, which involves dilution with water (Fig. 26.27), deposition of material will occur. The reason for this is apparent when looking at Figs. 26.24e26.26, in which, in principle, dilution means that we move to the left

FIGURE 26.27 Dilution of solutions of anionic surfactant [sodium dodecyl sulfate (SDS) in charge excess] þ cationic polymer (cationic hydroxyethyl cellulose) may lead to deposition as studied by ellipsometry. Rinsing starts at time 1000 s and is indicated with an arrow. After Terada E, Samoshina Y, Nylander T, Lindman. B. Adsorption of cationic cellulose derivatives/anionic surfactant complexes onto solid surfaces. I. Silica surfaces. Langmuir 2004;20:1753e62.

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in the graph and eventually reach the two-phase region in the phase diagram. This in turn leads to deposition of the complexes on the surface as water is added. This general phenomenon so far has received relatively little attention. Such deposition is taken advantage of in formulation of, for example, shampoos, dishwashing compounds, and detergents. Here, the deposition of polymer on rinsing is of importance for product functionality. It should be noted that it can also have negative effects, like redeposition of soil, if the formulation is imperfect. Formulation of new cleaning agents, therefore, requires knowledge of the delicate balance of interactions between the surfactante polymer in solution and the surfactantepolymer at the surface, as well as the interaction of the individual components with the surface. Another important aspect is that because the adsorption is mainly governed by the solubility, similar effects occur irrespectively of the surface. Here we note however that surfactants always adsorb to a hydrophobic surface, so in this respect the layer composition will be different. Hair-care formulation may also contain a silicon oil emulsion; this will affect the interfacial behavior. Thus there may be a selective deposition from mixtures of cationic polymers and SDS with silicon oil emulsion. Although the adsorption from the mixtures in most cases features co-deposition of silicone oil droplets on both hydrophilic and hydrophobic surface, the effect of dilution is quite different on the two surfaces. Here, the amount of material deposited after dilution was found to be large on the hydrophilic silica, whereas almost no significant co-deposition of silicone oil was found on the hydrophobic surface when the initial SDS concentration was high. This can be understood from the presence of similar layers on the hydrophobic silicon oil droplets as on the hydrophobic surface; thus, there is no driving force for adsorption. Phase separation in such polyelectrolyteeionic surfactant systems is, as discussed earlier, reduced on addition of electrolyte. We also showed in Fig. 26.26 that this is valid for the adsorption from the polyionesurfactant mixtures at or close to phase separation. Fig. 26.28 shows the strong effect on the salinity of the solution used for rinsing on the deposition. We can conclude that deposition from polymeresurfactant mixtures is directly related to the phase behavior of the system and thus strongly dependent on the polyelectrolyteesurfactant interaction. This can be controlled by cosolutes like electrolyte, as well as polymer molecular weight and charge density, but also by polymer hydrophobicity. The functionality of the system is often limited to a narrow interval of molecular properties. For instance, if the polymer is not hydrophobic enough, the surfactant binding is too limited to ensure redissolution. On the other hand, if the polymer contains too many hydrophobic groups, surfactant binding will be too strong and the phase separation range too limited. Similarly, no phase separation might occur if the charge density is too low, but a too-high charge density will cause attractive interactions between surfactant and polymer so strong that deposition does not occur. It is important to bear in mind that during the timescale of the application of a formulation, nonequilibrium effects can be significant. This can be utilized to form a layer that is trapped in a nonequilibrium state, but gives the desired surface functionality.

4.0 a

Adsorbed amount [mg/m 2]

3.5 3.0 b

2.5 2.0 1.5 1.0

c

0.5 0.0 0

1000

2000

3000

4000

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Time [sec.]

FIGURE 26.28 Effect of rinsing on adsorbed layers of cationic hydroxyethyl cellulose and SDS on hydrophobized silica. The complexes adsorbed from mixed polymer (100 ppm)/surfactant (5 mM) solutions, and rinsing was started at t ¼ 1000 s. (A) Adsorption was carried out in water followed by rinsing with water; (B) adsorption was carried out in 10 mM NaCl followed by rinsing with water; (C) adsorption was carried out in 10 mM NaCl followed by rinsing with 10 mM NaCl. Terada E, Samoshina Y, Nylander T, Lindman B. Adsorption of cationic cellulose derivatives/ anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces. Langmuir 2004;20:6692e701.

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Bibliography 1. Kwak JCT, editor. Polymer-surfactant systems. (New York): Marcel Dekker; 1998. 2. Kabalnov A, Lindman B, Olsson U, Piculell L, Thuresson K, Wennerstro¨m H. Microemulsions in amphiphilic and polymer-surfactant systems. Colloid Polym Sci 1996;274:297e308. 3. Kronberg B, Holmberg K, Lindman B. Surface chemistry of surfactants and polymers. (Chichester, UK): John Wiley & Sons; 2014. 4. Lindman B, Thalberg K. Polymer-surfactant interactions e recent developments. In: Goddard ED, Ananthapadmanabhan KP, editors. Interactions of surfactants with polymers and proteins. (Boca Raton, FL): CRC Press; 1993. p. 203e76. Chapter 5. 5. Lindman B. Surfactant-polymer systems. In: Holmberg K, editor. Handbook of applied surface and colloid chemistry, vol. 1. (Chichester): John Wiley & Sons, Ltd; 2002. p. 445e64. Chapter 20. 6. Piculell L, Lindman B. Association and segregation in aqueous polymer/polymer, polymer/surfactant, and surfactant/surfactant mixtures: similarities and differences. 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37. Svensson AV, Huang L, Johnson ES, Nylander T, Piculell L. Surface deposition and phase behavior of oppositely charged polyion/surfactant ion complexes. 1. Cationic guar versus cationic hydroxyethylcellulose in mixtures with anionic surfactants. ACS Appl Mater Interfaces 2009;1: 2431e42. 38. Svensson AV, Johnson ES, Nylander T, Piculell L. Surface deposition and phase behavior of oppositely charged polyion-surfactant ion complexes. 2. A means to deliver silicone oil to hydrophilic surfaces. ACS Appl Mater Interfaces 2010;2:143e56. 39. Santos O, Johnson ES, Nylander T, Panandiker RK, Sivik MR, Piculell L. Surface adsorption and phase separation of oppositely charged polyion-surfactant ion complexes: 3. Effects of polyion hydrophobicity. Langmuir 2010;26:9357e67. 41. Clauzel M, Johnson ES, Nylander T, Panandiker RK, Sivik MR, Piculell L. Surface deposition and phase behavior of oppositely charged polyion-surfactant ion complexes. Delivery of silicone oil emulsions to hydrophobic and hydrophilic surfaces. ACS Appl Mater Interfaces 2011;3:2451e62.

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