| Abstract
| - 1H NMR relaxation and diffusion studies were performed on water-in-CO2 (W/C) microemulsion systemsformed with phosphorus fluorosurfactants of bis[2-(F-hexyl)ethyl] phosphate salts (DiF8), having differentcounterions (Na+, NH4+, N(CH3)4+) by means of high-pressure in situ NMR. Water has a low solubility inCO2 and is mainly solubilized by the microemulsion droplets formed with surfactants added to CO2 andwater mixtures. There is rapid exchange of water between the bulk CO2 and the microemulsion droplets;however, NMR relaxation measurements show that the entrapped water has restricted motion, and there islittle “free” water in the core. Counterions entrapped by the droplets are mostly associated with the surfactantheadgroups: diffusion measurements show that counterions and the surfactant molecules move together witha diffusion coefficient that is associated with the droplet. The outer shell of the microemulsion droplets consistsof the surfactant tails with some associated CO2. For W/C microemulsions formed with the phosphate-basedsurfactant having the ammonia counterion (A-DiF8), the 1H NMR signal for NH4+ shows a much largerdiffusion coefficient than that of the surfactant tails. This apparent paradox is explained on the basis of protonexchange between water and the ammonium ion. The observed dependence of the relaxation time (T2) on W0(mole ratio of water to surfactant in the droplets) for water and NH4+ can also be explained by this exchangemodel. The average hydrodynamic radius of A−DiF8 microemulsion droplets estimated from NMR diffusionmeasurements (25 °C, 206 bar, W0 = 5) was Rh = 2.0 nm. Assuming the theoretical ratio of Rg/Rh = 0.775for a solid sphere, where Rg is the radius of gyration, the equivalent hydrodynamic radius from SANS is Rh= 1.87 nm. The radii measured by the two techniques are in reasonable agreement, as the two techniques areweighted to measure somewhat different parts of the micelle structure.
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