POLYMER/SURFACTANT INTERACTION 27 60 .-o. 50 40 PVP-CONCENTRATION •o• 0 g/I •o I g/I ß 3 g/I ß 10 g/I I I I 10-3 10-2 10-1 CT [10-3mol/I] Figure 3. Surface tension (•/)/concentration plot of SDS in the presence of PVP at various concentrations (8) "T" assignments follow Jones (6). ditionally used for protein binding studies, has been applied by a number of investi- gators to synthetic polymer/surfactant combinations. With this technique, one must add indifferent electrolyte (e.g., 0.1 M NaCI) to avoid Donnan membrane effects. As discussed later, the presence of salt can influence the oberved binding ratio. Combina- tions of SDS and a variety of nonionic polymers [PEO PVP polyvinyl acetate (PVAc) polyvinyl alcohol (PVOH) methyl cellulose (MeC)] have been examined by this method. Typical data for the PEO system (14) are given in Figure 5. It is seen that below a certain (monomer) concentration of SDS no binding occurs. The steepness of the uptake curve suggests a highly cooperative process, and this is followed by a pla- teau. Clearly, this behavior is fully compatible with the surface tension results. Ohbu et al. (15) carried out dialysis studies to study the binding of SDS by a series of cellulosic polymers of varying cationic substitution (CS). Binding was strong and coop- erative in all but the lowest CS case (0.05). The strength of binding was indicated by the fact that it occurred at very low concentrations of SDS at V2oth the CMC, the degree of binding [3 had already reached the value of 0.5 ([3 = 1 corresponds to one bound DS ion for each positive site on the polymer). As noted above, the reason for the strong interaction in such oppositely charge pairs is that potent electrostatic forces are brought into play. Specific ion electrodes. The development of "membrane" electrodes, specific for long-chain surfactant ions, has provided a very useful tool for studying the interaction of such ions with polymers. Illustrative data of Krescheck and Constantinidis (16) on binding of

28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 70 50 30 SDS + 0.1% POLYMER S ALONE SU R FACTANT/PO LYM E R Wt. RATIO f t f f f f _ 1:8 1:4 1:2 1:1 2:1 APPEARANCE c t pp sp c I I I -4 -3 -2 -1 LOG SURFACTANT CONCENTRATION (m/I) Figure 4. Surface tension/concentration curves of SDS with and without polyquaternium 10. The terms c, t, p, and sp refer to clear, turbid, precipitate, and slight precipitate, respectively (12). sodium decyl sulfate (SDecS) by PVP are presented in Figure 6 in the form of emf./ SDecS concentration plots at various levels of added polymer. Note that at zero polymer level the slope of the emf/log [SDecS] plot was linear until the CMC and was Nernstian, i.e., had a value of 59 mv. The presence of polymer caused a break in the linear slope (at "T•" concentration) and a shift of the apparent CMC, the shift increasing with increasing amount of polymer. For estimates of the degree of binding, these authors did not use the function T2-T • (see Figure 2) rather, a function T2-CMC' was used, where CMC' is a hypothetical or effective CMC in the presence of polymer. This ap- proach, strictly speaking, should be used when estimating degrees of binding from surface tension data. Thus in Figure 2 the amount of bound surfactant is given by T2'-T•, or, what should be equivalent, T2-CMC', rather than T2-T•, as used by Jones (6). Values of the binding obtained by these authors were 0.3 mols SDecS/base