Transient Electric Birefringence Study of Rod-Shaped Water-in-Oil Microemulsions

Transient electric birefringence (TEB) studies have been carried out on water-in-oil w/o microemulsions stabilized by Ni(AOT)2, the nickel salt of bis(ethylhexyl) sulfosuccinate. The system forms rod-shaped droplets at low water contents which convert to more spherical aggregates as the water content is increased. TEB data have been obtained as a function of microemulsion volume fraction, φ, water content, and temperature. Relaxation transients of the electric birefringence signal were found to be nonexponential, following asymptotically a stretched-exponential behavior. The value of the stretching exponent at low volume fraction is consistent with the assumption that the length probability distribution is exponential. A model describing the Kerr response of the microemulsion droplets is developed. By using this model we derive the specific Kerr constant as a function of the volume fraction, finding a good agreement with the experimentally observed behavior. We also use the model to derive, from the initial slope of the relaxation, the mean rod length L m. It is found that L m grows approximately as the square root of φ. Values for L m obtained from TEB are in good agreement with those obtained from small-angle neutron scattering measurements.