Phases and structures of a polyion–surfactant ion complex salt in aqueous mixtures: cationic hydroxyethyl cellulose with dodecylsulfate counterions

Abstract

A polyion–surfactant ion complex salt, cat-HECDS, has been synthesised, consisting of cationic hydroxyethyl cellulose with dodecylsulfate counterions. Ternary phase diagrams have been established for aqueous mixtures of cat-HECDS with either the conventional surfactant NaDS or the polyelectrolyte cat-HECCl. Such mixtures represent the simplest possible aqueous mixtures of oppositely charged polymer and surfactant where the proportions of polyion and surfactant ion may be varied. Phases and structures were investigated by visual inspection through crossed polarisers and by small-angle X-ray scattering (SAXS). The pure complex salt cat-HECDS was insoluble in water, but it could absorb up to 60 wt.% water. No crystalline order was detected at any water content. Dissolution of the complex salt in water occurred if sufficient amounts of either surfactant or polyelectrolyte was added. A smaller excess of surfactant (50% by charge) than polyelectrolyte (100% by charge) was needed to dissolve the complex salt in the dilute region. The efficiency of excess NaDS to dissolve the complex salt was attributed to hydrophobic interactions between the surfactant and the cat-HEC backbone. The free surfactant concentration at the onset of dissolution was in good agreement with the critical association concentration observed in previous gel experiments. The investigated phase diagrams were dominated by a large isotropic phase containing micellar surfactant aggregates without long-range order. The micellar aggregation number varied from ∼20 for aqueous cat-HECDS to ∼80 for mixtures rich in NaDS. A small fraction of added complex salt was enough to destroy the hexagonal phase formed in binary mixtures of NaDS and water. The absence of liquid crystalline phases containing significant amounts of complex salt was attributed to the stiff character of the polyion and its low charge density. In a ternary complex salt–surfactant–water system studied previously, where the complex salt contained a flexible polyion with a high charge density, liquid crystalline phases were found at all proportions of complex salt.