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

Water-in-carbon dioxide (W/C) microemulsions stabilized by an ammonium carboxylate perfluoropolyether (PFPECOO - NH 4 + ) 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 CO 2 . 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 CO 2 density range of 0.88−1.00 g/mL at 25 °C. The fractional amounts of water diffusion in bulk CO 2, 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 ammonium ions and PFPECOO - remain approximately constant. The droplet clusters are formed with channels that permit water molecules to diffuse freely over distances on the order of microns. This detailed dynamic molecular description of these clusters complements, in a consistent manner, macroscopic studies of percolation by conductivity measurements and equilibrium measurements of correlation lengths by SANS.