Effect of Oil Type on Liquid Crystalline Phase Behavior in

Sodium n -Dodecanoate-Water-Oil Mixtures

Peter K. Kilpatrick and Mark A. Bogard

Department of Chemical Engineering, North Carolina State University,

Raleigh, North Carolina 27695-7905

Received December 2, 1987. In Final Form: February 8, 1988

The liquid crystalline phase behavior of the binary system sodium n-dodecanoate (laurate)-water has been studied by ²H and ²³Na NMR quadrupole spectroscopy from 55 to 80 °C. Three lyotropic liquid crystal phases were identified: hexagonal and lamellar phases and a single intermediate phase with composition and temperature range falling between the composition and temperature range of the other two phases.

Complete high-resolution phase diagrams of the ternary systems sodium lauratewateyn-decane and sodium-laurate-water-toluene were determined at 60 °C by using optical birefringence and ²H and ²³Na NMR quadrupole spectroscopy. The ternary system with n-decane is similar to analogous sodium n-octanoate-water-nonamphiphilic oil phase diagrams studied by Ekwall and co-workers; little oil is solubilized by isotropic micellar (<3% w/w) and hexagonal (<5%) phases, and a small region of viscous isotropic phase exists at low decane content (ca. 11-13%). The ternary system with toluene is markedly different; toluene is extremely well solubilized by micellar (ca. 13%), hexagonal (29%), and viscous isotropic (25%) phases. Moreover, a large lamellar phase extends from about 3% to 27% toluene at surfactant concentrations of 46-60% and is stable at 60 °C only within the ternary diagram. The differences in n-decane versus toluene solubilization are likely attributable to the mode of molecular solubilization: n-decane is largely confined to the lipophilic cores of the microstructural subunits while toluene is significantly solubilized in the surfactant interfacial film.