Dielectric spectroscopy of cosurfactant facilitated percolation in reverse microemulsions

Percolation in reverse microemulsion systems can be driven by various field variables, including temperature and cosurfactant concentration. We use dielectric spectroscopy and a macroscopic dipole correlation function (DCF) derived therefrom to examine mesoscale structural aspects of charge transport in a water, AOT, toluene reverse microemulsion that is driven into percolation by cosurfactant (acrylamide). A multiexponential fitting of the DCF data gives firm support to the importance of a parameter marking the onset of percolation, as distinguished from the percolation threshold. A stretched exponential fitting of the DCF data reveals microstructural and mesoscale similarities and differences between this case of cosurfactant-induced percolation and a previously examined case of temperature-induced percolation. This cosurfactant-driven system appears to exhibit a critical slowing down on approach to the percolation threshold, as does the temperature-driven case, but a much shorter relaxation time suggests the development of much less fractal structure in this cosurfactant case. The effective fractal dimensionality and number of self-similarity stages of the fractal structure are only weak functions of the reduced field variable in the case of cosurfactant-driven percolation, and contrast sharply with the temperature case.