NMR and SANS studies of aggregation and microemulsion formation by phosphorus fluorosurfactants in liquid and supercritical carbon dioxide.

H-1 NMR relaxation and diffusion studies were performed on water-in-CO2 (W/C) microemulsion systems formed with phosphorus fluorosurfactants of bis[2-(F-hexyl)ethyl] phosphate salts (DiF(8)), having different counterions (Na+, NH4+, N(CH3)(4)(+)) by means of high-pressure in situ NMR. Water has a low solubility in CO2 and is mainly solubilized by the microemulsion droplets formed with surfactants added to CO2 and water 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 is little "free" water in the core. Counterions entrapped by the droplets are mostly associated with the surfactant headgroups: diffusion measurements show that counterions and the surfactant molecules move together with a diffusion coefficient that is associated with the droplet. The outer shell of the microemulsion droplets consists of the surfactant tails with some associated CO2. For W/C microemulsions formed with the phosphate-based surfactant having the ammonia counterion (A-DiF(8)), the 1H NMR signal for NH4+ shows a much larger diffusion coefficient than that of the surfactant tails. This apparent paradox is explained on the basis of proton exchange between water and the ammonium ion. The observed dependence of the relaxation time (T-2) on W-0 (mole ratio of water to surfactant in the droplets) for water and NH4+ can also be explained by this exchange model. The average hydrodynamic radius of A-DiF(8) microemulsion droplets estimated from NMR diffusion measurements (25 degrees C, 206 bar, W-0 = 5) was Rh = 2.0 nm. Assuming the theoretical ratio of R-g/R-h = 0.775 for a solid sphere, where R-g is the radius of gyration, the equivalent hydrodynamic radius from SANS is R-h = 1.87 nm. The radii measured by the two techniques are in reasonable agreement, as the two techniques are weighted to measure somewhat different parts of the micelle structure.