The formation of water-in-oil (W/O) microemulsions based on perfluoropolyether (PFPE) oil and a surfactant (S) with a similar PFPE hydrophobic chain has been investigated by conductivity and static light scattering. As previously reported, conductivity measurements across the isotropic oil-rich region (W < 30%) allowed the detection of the transition from solution of hydrated surfactant species to microdroplets of water in oil. The quantitative interpretation of the conductivity data was proposed. The hydration and partial dissociation of ionic surfactant molecules were found to be responsible for the variation of conductivity upon addition of water at a constant oil to surfactant ratio. The contribution of charge fluctuation among droplets was negligible over a wide range of water concentration and could explain only the relatively low conductivities at high W/S ratio. In turn, high conductivities, on the scale of percolative systems, were measured at lower water content. Light scattering results confirmed the onset of droplet formation at the compositions where the maxima of the conductivity curves were observed. The droplet formation was driven by the number of water molecules per polar head. Varying systematically the chain length of either surfactant or oil, an increase in oil penetration (higher packing parameter V/al) favored droplet formation over a wider composition range and decreased conductivity, while a decrease in oil penetration (lower V/al) inhibited droplet formation and increased conductivity, according to previous observations in hydrocarbon systems.
