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Review date: July 24, 2003
PNNL-SA-27883

Dramatic Density-Induced Structural Changes in Microemulsions Formed in Near-Critical and Supercritical Solvents

KW Hutchenson and NR Foster, Eds. ACS Symposium Series Vol. 608, 111-125 (1995).

Abstract: We demonstrate that the secondary structure of a microemulsion can be altered by changing the density of the continuous phase solvent. These changes in the microemulsion structure lead to dramatic changes in the physical properties of the system, such as the solution viscosity or electrical conductivity. For giant, rod-like micelles formed using the surfactant L-a-phosphatidlchloline (L-a-lecithin) in propylene, the system viscosity changes by three orders-of-magnitude with small changes in the system pressure. For the system of spherical micelles formed with didodecyldimethylammonium bromide (DDAB), the micelle clustering at low density increases electrical conductivity of the solution by three orders-of-magnitude.

The microemulsion systems in near- or supercritical fluids are of importance for understanding reaction mechanisms and for modeling polymers. We also report results for a sodium dodecyl sulfate (SDS) aqueous solution forming normal micelles that can incorporate a small amount of fluid in the micellar core. For this system, changes in the primary structure can be induced by altering the amount of supercritical fluid in the microemulsion core by changing the fluid density. We establish correlations between the measured physical properties and the spectroscopic results.