Electric field‐induced structure changes in water‐in‐oil microemulsions: Time‐resolved Kerr effect measurements

Abstract

A percolation transition in microemulsions can be induced by high electric fields. We report on the dynamics of the processes. Comparison of time‐resolved conductivity, birefringence and light scattering measurements gives information on the structure changes. The differences of ionic and nonionic microemulsions are emphasized.

Electric Birefringence Dynamics of Crystallites and Reverse Micelles of Sodium Bis(2-ethylhexyl)phosphate in Benzene

“…The induced polarization of a rod segment most likely involves a structural change, i.e., the displacement of Na+ ions along the rod's major axis, which contributes to the anisotropy and hence to the birefringence amplitude. The fragmentation of the original crystallites and the polarization and reorientation of the resulting segments can be represented by the following equation S" -*(n/m)Sm' (m<n) (11) where the prime symbol indicates species polarized and/or aligned. Since no distribution of the relaxation times is observed, n must be approximately an integral multiple of m. The time constant associated with process 11 corresponds to the fast forward relaxation (Tf), which is responsible for about one-third of the total birefringence amplitude.…”

Dynamics and Energetics of Droplet Aggregation in Percolating AOT Water-in-Oil Microemulsions

“…By increasing the relative oil-to-water weight fraction α, it is possible to obtain a continuous structural inversion from a w/o to a o/w-microemulsion, with a disordered bicontinuous structure in-between. ,,, This transition is accompanied by changes in various physical properties of the microemulsion. Most remarkable is the dramatic increase of the electric conductivity over several orders of magnitude, which was interpreted in the frame of percolation theory. − Other microemulsion properties such as light scattering behavior or viscosity are strongly affected by the transition likewise. ,− Percolation can also be induced by other parameters. Decreasing the temperature or pressure for systems with the ionic DDAB , and nonionic surfactants ,, leads to a significant increase of the electric conductivity.…”

“…Most remarkable is the dramatic increase of the electric conductivity over several orders of magnitude, which was interpreted in the frame of percolation theory. − Other microemulsion properties such as light scattering behavior or viscosity are strongly affected by the transition likewise. ,− Percolation can also be induced by other parameters. Decreasing the temperature or pressure for systems with the ionic DDAB , and nonionic surfactants ,, leads to a significant increase of the electric conductivity. In the case of AOT, a temperature or pressure increase, application of high electric fields, , or the addition of gelatin 20 leads to the transition.…”

“…Decreasing the temperature or pressure for systems with the ionic DDAB , and nonionic surfactants ,, leads to a significant increase of the electric conductivity. In the case of AOT, a temperature or pressure increase, application of high electric fields, , or the addition of gelatin 20 leads to the transition. The addition of water-soluble polymers , or enzymes , was observed to counteract percolation.…”

“…The first model, static percolation, attributes the observed phenomena to the transformation into a bicontinuous structure, whereas the dynamic percolation model postulates attractive water globule interactions leading to cluster formation. Especially AOT seems to form very stable reversed micelles in the L 2 -phase, which dominates the phase behavior over a wide range of composition and temperature. ,, The cluster model was therefore widely used for such systems in the recent years, ,,,,− but also for nonionic microemulsions. ,, A history of this was lately reviewed by Koper et al However, in recent publications the unusual behavior was attributed to dispersion inversion even for AOT. , A general disadvantage of most of the applied methods is the necessity to assume a model for data interpretation, making a clear decision for one model difficult. Direct imaging was obtained by freeze fracture electron microscopy .…”