PERFLUOROPOLYETHER WATER OIL MICROEMULSIONS - A H-1-NMR SELF-DIFFUSION STUDY OF WATER

The water self-diffusion coefficients were determined by means of the Fourier transform pulsed gradient spin-echo NMR technique in three- and four-component W/O systems based upon perfluoropolyether (PFPE) oils and an anionic surfactant with a PFPE hydrophobic chain. In order to investigate the structural changes associated with microemulsion formation, the study has been performed by varying the water content within the monophasic isotropic region of the phase diagram, which is located at relatively low water content (< 30% by weight). The addition of water to oil-surfactant mixtures, at constant mass ratio O/S, was found to cause the transition to a water-in-oil droplet structure in most of the systems, in agreement with light scattering and conductivity results recently reported. The effects of composition (O/S ratio), molecular weight of the oil, and ionic strength of the water phase have been examined. In particular the effect of the oil chain length on the droplet formation has been clarified. The effects of water-soluble (ethanol) and oil-soluble (long chain PFPE) alcohols were also investigated. The cosolvent effect of ethanol, well-known in hydrocarbon systems, and the cosurfactant behavior of the PFPE alcohol have been ascertained in PFPE systems. A quantitative interpretation of NMR self-diffusion coefficients enabled the evaluation of the amount of water in droplets in the range of composition where continuous water and water in droplets coexist. Good agreement was found between our NMR results and those obtained by applying the hydration/partial dissociation concept to conductivity data.