Electrolyte dependent phase separation in aqueous mixtures of a polyelectrolyte and an ionic surfactant

Abstract

A mixture of a polyelectrolyte and an oppositely charged ionic surfactant generally phase separates from an aqueous solution due to the strong attractive interaction between the two solutes. Under certain conditions the concentrated phase is a transparent gel. Redissolution can be achieved by electrolyte addition or by a high surfactant concentration. Over a wide range of electrolyte concentrations, there is no phase separation. However, at high electrolyte concentrations, separation into two isotropic phases occurs. While phase separation at low electrolyte contents results in one dilute solution and one phase concentrated in polymer and surfactant, phase separation at high electrolyte concentrations is of a different nature and results in one solution rich in surfactant and one rich in polymer. The phenomenon is related to, but different from that displayed by two polymers in a common solvent; called "polymer incompatibility", and can be referred to the elimination of electrostatic interactions. The phase diagrams can be modelled in Flory-Huggins type calculations with reasonable assumptions of the intermolecular interactions.