MICROSTRUCTURE ANALYSIS AT THE PERCOLATION-THRESHOLD IN REVERSE MICROEMULSIONS

Time domain dielectric spectroscopy of reverse water/acrylamide/Aeroso l-OT (AOT)/toluene microemulsions shows that percolation induced by in creasing 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 s tatic percolation limits. Self-diffusion measurements derived from nuc lear magnetic resonance pulsed-gradient spin-echo experiments reveal a n 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, cosurfa ctant, and surfactant (AOT) below threshold is modeled quantitatively laking into account the chemical partitioning equilibria between the m icroemulsion droplets and the toluene continuous pseudophase. Above th reshold, the apparent increasing water and cosurfactant partitioning i nto the toluene (continuous) pseudophase suggests facilitated transpor t through fractal aggregates. A dynamic partitioning model is used to estimate the volume of percolating fractal clusters, and yields an ord er parameter for water-in-oil to percolating cluster microstructural t ransitions. This same order parameter is also illustrated to derive fr om self-diffusion data wherein percolation and transformation to spong e phase microstructure are driven by increases in temperature and in d isperse phase volume fraction. For microstructural transitions driven by three different field variables, chemical potential, temperature, a nd disperse phase volume fraction, this order parameter shows that the onset of percolation corresponds to the onset of increasing water pro ton self-diffusion, and that the onset of increasing surfactant self-d iffusion corresponds to the formation of bicontinuous microstructures and the onset of transformation to middle phase microemulsion. (C) 199 7 Elsevier Science B.V.