Formation of Complexes between Surfactants and Polymers

Mixtnres of snrfactants and water-solnble polymers are of both fimdamental and practical interest. If one excludes hydrophobically modified polymers, whieh have hydrocarbon chains attached periodically to rather hydrophilie baekbones, polymer-surfactant complexes oecur only for (a) ionie surfactants and nonionic polymers and (b) ionic snrfactants and poly electrolytes having opposite eharges. In the former simation, for instance, mixtnres of SDS and PEO, the EO segments can penetrate the micelle snrfaee and separate the charged surfactant ions in much the same way as described above for ionic-nonionic smfactant mixtures. The EO segments also prevent water molecules from coming into direct contact with any 

In these systems, viscosity typically increases rapidly as surfactant concentration increases above the CAC. As more micelles form along the polymer chains, they become longer and straighter to increase micelle separation and reduce electrical repulsion. The greater length leads to a higher viscosity. 

It should be noted that the interaction of nonionic polymers with cationic smfactants is much weaker than with anionic surfactants. The reason for this difference in behavior is not entirely clear but may stem from a small positive charge acquired by the EO chain in aqueous solution. 

The presence of both polymers and surfactants affects phase behavior as well. For instance, the presence of SDS increases the cloud point temperature of PEO. The negatively charged micelles produce an electrical repulsion between polymer chains that impedes their aggregation to form a polymer-rich phase. (SDS 

FIG U RE 4.16 Surface tension as a function of SDS concentration in solutions of a charged polymer (PVP). The two T points can be seen. Reprinted from Lange (1971) with permission from Springer (copyright). 

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