MICELLAR AND BICONTINUOUS MICROEMULSIONS FORMED IN BOTH NEAR-CRITICAL AND SUPERCRITICAL PROPANE WITH DIDODECYLDIMETHYLAMMONIUM BROMIDE AND WATER

Bicontinuous microemulsions readily form in liquid propane at 25-degrees-C and pressures from 10 to 500 bar with the addition of the surfactant didodecyldimethylammonium bromide (DDAB) and water. The phase behavior of this system is much like that of the normal liquid alkanes, C6-C-10, but with unusual and dramatic effects due to pressure. When the pressure of the solution is increased from 80 to 400 bar with the addition of pure propane, the conductivity is observed to decrease by 3 orders of magnitude. In accord with existing structural models for conventional liquid microemulsion systems, these changes in the conductivity are ascribed to changes in the interface region as the propane solvent penetrates and solvates the hydrocarbon tails of the surfactant. The corresponding supercritical propane system studied at 100-degrees-C is best explained as a micellar microemulsion with an oil-continuous phase in which the structure is also affected by the amount of water or the pressure of the system.