Interactions between hydrophobically modified polyelectrolyte and oppositely charged surfactant. Mixed micelle formation

Aqueous mixtures of sodium dodecyl sulphate (SDS) with QUATRISOFT™ LM200, a cellulose derivative substituted with cationic hydrophobic side chains, have been investigated in the absence and in the presence of added salt. On adding SDS to a solution of polymer in the range 0.02–1%, liquid-liquid phase separation occurs near charge neutralization (for the same amount of polymer and surfactant charges in solution) for the salt-free mixture, and earlier in the presence of salt. In both cases, redissolution occurs upon further SDS addition. The total SDS concentration at redissolution increases linearly with polymer concentration, from a limiting value close to the CMC of the polymer-free solution, at vanishing polymer content. In a 1% solution, a very high viscosity is found on both sides of the two-phase area, showing the formation of mixed micelles between the polymer alkyl side chains and the surfactant molecules. The results are interpreted in terms of a binding isotherm of surfactant to polymer. The first stages of the isotherm involve binding of individual surfactants 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/polymer binding ratios seem required for redissolution. Such high binding ratios are only obtained close to the cooperative binding region, i.e., when the free surfactant concentration is close to the CMC.