Phase behaviour and structure in a non-ionic surfactant–oil–water mixture

Abstract

A section of the composition–temperature phase prism of a ternary non-ionic surfactant–water–oil system defined by a constant surfactant to oil ratio has been investigated. The structural sequence of normal spheres to planar bilayers, via cylinders, is observed as a function of decreasing water concentration at fixed temperature. As a function of increasing temperature at fixed surfactant and oil concentration, we note the commonly observed reversal in mean curvature of the polar/apolar interface. The mean curvature progressively alters from positive values in the low-temperature microemulsion phase to negative mean curvature in the high-temperature L₃ phase, with an intermediate lamellar phase where the mean curvature passes through zero. The phase diagram of this section is similar to that of the binary surfactant–water system. The average area per surfactant at the polar/apolar interface is 46 ± 2 Ų. A micellar cubic phase is observed between the microemulsion and the hexagonal phase while no bicontinuous cubic phase is observed.