Microstructure analysis at the percolation threshold in reverse microemulsions

Abstract

Time domain dielectric spectroscopy of reverse water/acrylamide/Aerosol-OT (AOT)/toluene microemulsions shows that percolation induced by increasing cosurfactant concentration (increasing cosurfactant chemical potential) obeys scaling above and below a percolation threshold. This scaling analysis suggests that the observed percolation is close to static percolation limits. Self-diffusion measurements derived from nuclear magnetic resonance pulsed-gradient spin-echo experiments reveal an increase in water proton diffusion above the percolation threshold. This increase is assigned to water transport through fractally chained assemblies of microemulsion droplets. The diffusion of water, cosurfactant, and surfactant (AOT) below threshold is modeled quantitatively taking into account the chemical partitioning equilibria between the microemulsion droplets and the toluene continuous pseudophase. Above threshold, the apparent increasing water and cosurfactant partitioning into the toluene (continuous) pseudophase suggests facilitated transport through fractal aggregates. A dynamic partitioning model is used to estimate the volume of percolating fractal clusters, and yields an order parameter for water-in-oil to percolating cluster microstructural transitions. This same order parameter is also illustrated to derive from self-diffusion data wherein percolation and transformation to sponge phase microstructure are driven by increases in temperature and in disperse phase volume fraction. For microstructural transitions driven by three different field variables, chemical potential, temperature, and disperse phase volume fraction, this order parameter shows that the onset of percolation corresponds to the onset of increasing water proton self-diffusion, and that the onset of increasing surfactant self-diffusion corresponds to the formation of bicontinuous microstructures and the onset of transformation to middle phase microemulsion.