New mechanisms for phase separation in polymer-surfactant mixtures

The cooperative association of ionic surfactants with polymer chains leads to quite novel features in the phase behaviour of polymer solutions. Using an analytic mean-field model, we analyze phase equilibria in solutions of neutral polymers mixed with ionic surfactants. We predict the possibilities of co-existence between dilute and semi-dilute phases or that between two semi-dilute phases with different degrees of loading of polymer chains by adsorbed micelles. The phase separation may be induced either by the variation of the concentration of the surfactants, or by increasing the ionic strength of the solution or decreasing the solvent strength. Due to coupling of self-assembly of surfactants on the polymer chains and the -transition, the latter is shifted to the range of poor solvent conditions. A critical temperature is identified below which collapse-swelling transition in an individual chain acquires the character of a first-order phase transition.