Bulk and surface behavior of cationic guars in solutions of oppositely charged surfactants

We study the bulk and surface behavior of aqueous mixtures of cationic guars and anionic sulfate surfactants. The phase diagrams are determined and the phases characterized with a variety of techniques. When the surfactant is gradually added, the polyelectrolyte is physically modified by ionic binding of the surfactant or more likely surfactant aggregates to the polymer chains. This results in the increase of the solution viscosity and leads to phase separation between a polymer- and surfactant-rich phase and a dilute supernatant; Phase separation always occurs for surfactant concentrations below the critical micelle concentration (cmc). A second one-phase region is obtained for surfactant concentration as low as twice the cmc, independent of the polymer charge density or of the alkyl chain length of the sulfate surfactant. The surface behavior is investigated with a surface forces apparatus. The force profiles between two layers of cationic guar preadsorbed onto mica surfaces are measured in water and in surfactant solutions for concentrations covering the three regions of the bulk-phase diagram. We conclude that the structure of the adsorbed layers is clearly related to the structure of the corresponding polymer/surfactant mixtures.