POLYMER/SURFACTANT INTERACTION SURFACE ACTIVITY AND ADSORPTION CHARACTERISTICS It has been pointed out that nonionized polymers may reduce the surface activity (air/ water interface) of ionic surfactants by binding the latter in the form of weakly surface- active complexes. On the other hand, the formation of highly surface-active complexes between polyelectrolytes and oppositely charged surfactants provides a synergistic en- hancement of the surface activity of the two components. Consequences of this enhance- ment could include improved emulsifying ability and, especially, foaming. [Instances of the latter have been reported (13,62).] At the same time, in the literature several references can be found to improvement of the foamability and foam quality of ionic surfactants by the addition of nonionized polymers, such as PVOH, PEO, modified starches, and cellulosics. Such implied alteration of surface properties can be understood if one considers the adsorbed layer of surfactant to resemble a surfactant micelie (in this case a "semi-inifinite hemi-micelle") able to interact with the polymer in a way similar to that depicted in Figure ! 1. In other words, in this case also, surfactant and polymer could influence each other's adsorption characteristics hence surface properties, in- cluding foaming, would be affected, even though the energy of association of the sur- factant and the base monomer may be weak. As regards solid surfaces, since both polymer and surfactant can adsorb on such surfaces, there has been much interest and considerable work done to determine what effect each has on the extent of adsorption of the other. Most of the work done has involved mineral (or latex) solids (3) and will not be detailed here. Suffice it to say that positive and negative effects have been found for both adsorbing components, depending on condi- tions, i.e., the acutal components themselves, addition sequence, the solid surface, the pH, and so on. As the adsorption energy per monomer unit of (especially) a nonionic polymer can be quite weak, it is not surprising that its adsorption overall can be af- fected by an added surfactant. The clear implication is that opportunities exist to modify the surface characteristics of chosen solids by appropriate choice of surfactants and polymers. Since the solid introduces a new phase (as in conditioning) and since other ingredients may also be present, determination of improved adsorption character- istics has to be done empirically in most cases. We point out that extensive studies have been carried out on the influence of SDS on the adsorption of radiotagged polyquaternium 10 on keratin substrates, which are nega- tively charged and water-swellable. Small additions of surfactant progessively reduced the adsorption of polymer owing to reduction of its positive charge density (63). How- ever, at high levels of SDS (in the post-precipitation zone), adsorption of this polymer was fully restored (64). POLYMERIC SURFACTANTS There is at present a tremendous renewal of interest in "polymeric surfactants" or so- called "associative thickeners." These materials are, in effect, conventional water-sol- uble polymers that have been modified by inclusion of hydrophobic rnoieties such as alkyl groups (65). They combine the properties of a surfactant/polymer mixture in one molecule and therefore display some of the properties of such mixtures. Thus, when dissolved in water, they tend to self-associate, generating association structures of high molecular weight, and hence substantial increases in viscosity of their solutions result. A depiction of possible association structures that might form is given in Figure 16. Several recent papers on the subject of associative thickening polymers may be found in

46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 16. Depiction of association structures of hydrophobically substituted polymers: (a) end-substitu- tion. (b) "comb" structure (Courtesy D. R. Bassett). reference 66. Other consequences of the associative tendency of molecules with this type of structure are the development of solubilizing properties for water-insoluble materials, including dyes, and also a strong tendency to produce foams, especially foams of un- usual stability: Goddard and Braun, in fact, showed that a hydrophobically modified cationic cellulosic polymer, polyquaternium 24, itself could form the basis of an aerosol mousse (67). On a historical note, it should be pointed out that the structure of the above-mentioned associative thickeners bears a strong resemblance to the so-called "polysoaps" that were investigated in detail in the 1960s.