NMR Studies of Water Transport and Proton Exchange in Water-in-Carbon Dioxide Microemulsions

Abstract

Water-in-carbon dioxide (W/C) microemulsions stabilized by an ammonium carboxylate perfluoropolyether (PFPECOO-NH4+) surfactant are studied with NMR diffusion and relaxation methods with the aim of obtaining information on the dynamics of this system, as well as aiding in the design of new surfactants that can form stable microemulsions in CO2. Short proton transverse relaxation times (3−10 ms) measured for water and ammonium ions are shown to agree with a simple proton exchange model. As the pressure is lowered below the phase boundary, the NMR spectra indicate that surfactant migrates to the new liquid phase along with the water. Diffusion coefficients are reported in the CO2 density range of 0.88−1.00 g/mL at 25 °C. The fractional amounts of water diffusion in bulk CO2, within the droplets, and through the water channels are delineated quantitatively. In decreasing the density from 0.96 to 0.88 g/mL, the water diffusion coefficient increases by a factor of 2 while the diffusion coefficients for ammoniu...