Microstructure Determination in a Self Assembled Surfacant System Formed by AOT/Lecithin/Isooctane and Water

Abstract

We have observed a novel transformation from a microemulsion to a gel phase by increasing the water content of a system consisting of bis(2-ethylhexyl) sodium sulfosuccinate (AOT), phosphatidycholine(lecithin), and 2,2,4 -trimethylpentane (isooctane). The viscosity increases by six orders of magnitude and a rigid gel forms as the water content is increased above a specific threshold. Small angle neutron scattering (SANS) experiments on this surfactant mesophase reveal the microstructure as columnar hexagonal at lower water contents and temperatures, and lamellar at higher water contents and temperatures. SANS is also able to capture the transition between these two structures. These structures are thermally reversible. Differential scanning calorimetery results show no abrupt phase transitions in the gel system with increasing temperature. For low water and surfactant contents (W0=20, total surfactant concentration in isooctane = 0.01M), microemulsion droplets are formed. SANS experimental data fits well with the polydisperse sphere morphology and a gradual increase in the droplet size is observed as the lecithin/ AOT ratio is increased. The different packing parameters for AOT and lecithin drive the observed changes in droplet sizes. For lecithin/ AOT molar ratios approaching unity, both SANS and static light scattering measurements reveal an attractive interaction develops between these droplets, leading to the formation of clusters. Hydrodynamic radii obtained from dynamic light scattering measurements show results that are consistent with the SANS data.