TEMPERATURE AND GAS-PRESSURE INDUCED MICROSTRUCTURAL CHANGES IN AOT WATER-IN-OIL MICROEMULSIONS - CHARACTERIZATION THROUGH ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

EPR spectra of a variety of nitroxide spin labels have been used to characterize microstructural changes in AOT water-in-oil microemulsions at conditions approaching critical phase transitions. Of particular interest is a pressure-induced cloud point, where the dissolution of a light hydrocarbon gas into the liquid solvent (isooctane) reduces the solvent density and leads to micellar instability at a critical density. The pressure approach to a cloud point is compared to the temperature approach. A variety of spin label probes are used to report changes in the aqueous core, the interfacial layer, the micellar tail region, and the intermicellar bulk isooctane region. Both temperature and gas pressure increases result in a decrease of the rigidity (ordering) in the immediate vicinity of the surfactant layer in the interfacial region. The surfactant tails region exhibits maximum sensitivity toward both temperature and gas pressure variations.