INTERACTIONS IN AQUEOUS MIXTURES OF HYDROPHOBICALLY-MODIFIED POLYELECTROLYTE AND OPPOSITELY CHARGED SURFACTANT - MIXED MICELLE FORMATION AND ASSOCIATIVE PHASE-SEPARATION

Aqueous mixtures of sodium dodecyl sulfate (SDS) with QUATRISOFT LM200 , a cellulose derivative substituted with cationic hydrophobic side ch ains, have been investigated by various techniques, in the absence and in the presence of added salt. Steady state fluorescence measurements show that hydrophobic microdomains are formed in aqueous solutions of LM200 already at low concentrations (much less than 1%). On adding SD S to a solution of LM200 in the range 0.02-1%, Liquid-liquid phase sep aration occurs near charge neutralization (for the same amount of poly mer and surfactant charges) for the salt-free mixture and earlier in t he presence of salt. In both cases, redissolution occurs upon further SDS addition. The total SDS concentration at redissolution increases l inearly with polymer concentration, from a Limiting value, at vanishin g polymer content, close to the cmc of the polymer-free solution. Visc osity measurements show that SDS associates to LM200 already at very l ow SDS concentrations (10(-5) M) and that the binding continues even a fter redissolution up to the highest investigated ratios of SDS to LM2 00. In a 1% solution, a very high viscosity is found on both sides of the two-phase area, as previously shown by Goddard and Leung (Colloids Surf. 1992, 65, 211). The results are interpreted in terms of a bindi ng isotherm of surfactant to polymer, analogous to isotherms observed for surfactants binding to proteins or to micelles of other surfactant s. The first stages of the isotherm involve binding of individual surf actant molecules to the mixed micelles, and the last stage, occurring when the free surfactant concentration approaches cmc, is a strong and cooperative binding related to the self-association of the surfactant . High SDS/LM200 binding ratios seem required for redissolution and ev en higher for breaking the micellar cross-links responsible for the en hanced viscosity. Such high binding ratios are only obtained near or w ithin the cooperative binding region, i.e. when the free surfactant co ncentration is close to the cmc.