PHASE-BEHAVIOR AND STRUCTURE IN A NONIONIC SURFACTANT-OIL-WATER MIXTURE

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 sph eres to planar bilayers, via cylinders, is observed as a function of d ecreasing water concentration at fixed temperature. As a function of i ncreasing temperature at fixed surfactant and oil concentration, we no te the commonly observed reversal in mean curvature of the polar/apola r interface. The mean curvature progressively alters from positive val ues in the low-temperature microemulsion phase to negative mean curvat ure in the high-temperature L(3) 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 syst em. The average area per surfactant at the polar/apolar interface is 4 6 +/- 2 Angstrom(2). A micellar cubic phase is observed between the mi croemulsion and the hexagonal phase while no bicontinuous cubic phase is observed.