Polyelectrolyte — Ionic Surfactant Systems: Phase Behavior and Interactions

Abstract

Systems containing a linear polyelectrolyte and an oppositely charged surfactant are considered with respect to interactions and phase behavior. The critical aggregation concentration, cac, i.e., the surfactant concentration at which surfactant binding to the polyelectrolyte starts, is highly dependent on the surfactant chain length; an increasing difference between log(cac) and log (cmc) is seen when the surfactant chain length is increased. The relation between the cac and the polyelectrolyte concentration is described. Surfactant binding to a polyelectrolyte leads to phase separation and the formation of two transparent isotropic phases in equilibrium. A typical phase diagram contains a droplet-shaped two-phase region, anchored in the water corner. The size of the two-phase region is reduced for a surfactant of shorter chain length and when salt is added, while the molecular weight of the polyelectrolyte has a minor influence on the phase behavior. Addition of moderate concentrations of salt may totally prevent phase separation, while at high salt concentrations phase separation may again occur, but is of a different kind and results in one surfactant-rich and one polymer-rich phase. The studied systems are four-component systems; in order to obtain a correct picture of the phase behavior, a new, complete way of representing the systems has been developed, in which the phase diagram has the shape of a pyramid.