Surfactant–water interactions in lamellar phases. An equilibrium binding description of interbilayer forces

Abstract

The importance of ‘hydration forces’ in controlling short-range interactions in colloidal and biological systems has been recognised for some time, although their origin remains controversial. Equilibrium water vapour pressures and X-ray repeat spacings for a number of polyoxyethylene alkyl ether-type non-ionic surfactant–water liquid-crystalline phases as functions of both concentration and temperature have been measured in order to investigate hydration effects in simple systems where electrostatic interactions are absent. Measurements on the lamellar phases of C₁₂EO₃ and C₁₂EO₄, and on the hexagonal phase of C₁₂EO₈ are reported. The results have been analysed, along with published data on C₁₂EO₆, in terms of force–distance relationships. Although the net repulsive force decays approximately exponentially, the decay distances are significantly longer than those reported previously for phospholipid–water systems. An alternative treatment in terms of binding of water molecules to surfactant headgroups has been proposed. Using this simple approach, which does not require layers of ‘structured water’, the current thermodynamic data have been linked with earlier n.m.r. results for the non-ionic surfactant systems.