SPECTROSCOPIC INVESTIGATION OF AN L-ALPHA-PHOSPHATIDYLCHOLINE GEL FORMED IN NEAR-CRITICAL PROPYLENE

A gel phase can be formed in near-critical propylene (T/T-c = 0.83) by introducing a small volume fraction (phi(cis) = 0.06) of a mixture of the surfactant, L-alpha-phosphatidylcholine, with water. As the press ure of the system is increased from 200 to 400 bar al 30 degrees C, a dramatic decrease in the solution viscosity is observed corresponding to a structural transition from a solid-like gel to the low viscosity of a neat propylene phase. These viscosity effects are rationalized in terms of interchain interactions of long, flexible cylindrical aggreg ates. This interconnected network produces high viscosity even at a re latively low volume fraction of surfactant and water. We apply spectro scopic methods to probe the origin of the pressure- and water-induced structural changes that are responsible for large changes in dynamic v iscosity. In this paper, we determine how the lecithin microemulsion s tructure and viscosity affect intramolecular excimer formation process es using fluorescence probes and how continuous phase pressure affects micropolarity within microemulsions using fluorescence and vibrationa l (FT-IR) spectroscopies. These studies bring important new insights i nto the mechanisms of gel formation for this surfactant system because of the unique ability to use pressure to adjust the density of oillik e continuous phase solvents without changing the overall chemical comp osition of the system. These are also important model systems for stud ying the solvation of polymers in both near- and supercritical fluids.