Microstructure of perfluoropolyether water/oil microemulsions

It is shown that a geometric model accounts for the microstructure of microemulsions composed of perfluoropolyether (PFPE) oils and surfactant. These fluorocarbons are low molecular weight polymers with perfluoropropylene oxide group as the main repeating unit. Ammonium carboxylate is the polar group of the PFPE surfactant. The microstructure of the PFPE microemulsions is compared with that known for the didodecyldimethylammonium bromide (DDAB)/water/cyclohexane system. Fluorocarbons are expected to behave rather differently from hydrocarbons, but here the two systems are shown to be practically identical, indicating the generality of models for microstructure based on simple packing and geometric arguments. Water self-diffusion coefficients measured previously on these totally fluorinated systems (Langmuir 1992, 8, 1278), at constant oil to surfactant ratio and varying water to surfactant ratios, quantitatively agree with model predictions based on an effective medium approximation. The similarities in the physical behavior between DDAB and PFPE systems, previously pointed out only at a qualitative level, are thus demonstrated to arise from an effective packing parameter, v/al, close to unity for both surfactants.