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 -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 PFPECOOremain 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.

Molecular Dynamics Simulation Studies of Polyether and Perfluoropolyether Surfactant Based Reverse Micelles in Supercritical Carbon Dioxide

“…Thus, the system contained a total of 23 775 particles. The values for the temperature, pressure, and W o are the same as the values present in the NMR studies of water transport in W/C microemulsions by Johnston and co-workers . The number of surfactant and water molecules were obtained by using the following formulas for the area of the aqueous core ( A c ) and from the experimental values for the area per headgroup ( A h = 76 Å 2 ), radius of the aqueous core ( R c = 20 Å), and W o = 8.4, where N h and N w are the number of surfactant headgroups and water molecules, respectively.…”

Zinc(II) Complexes of Ubiquitin: Speciation, Affinity and Binding Features

“…Room temperature ionic liquids (RTILs), along with supercritical carbon dioxide (scCO 2 ), have been receiving considerable attention from the industry and academia recently 1. scCO 2 is one of the well‐studied systems through experimental2, 3 and computational4–7 approaches. One of the interesting characteristics of scCO 2 is its liquid‐like density and gas‐like transport properties.…”

Molecular dynamics simulation studies of CO2 – [bmim][PF6] solutions: Effect of CO2 concentration

“…1 scCO 2 is one of the well-studied systems through experimental 2,3 and computational [4][5][6][7] approaches. One of the interesting characteristics of scCO 2 is its liquid-like density and gas-like transport properties.…”