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

Abstract

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 Lm. It is found that Lm grows approximately as the square root of φ. Values for Lm obtained from TEB are in good agreement with those obtained from small-angle neutron scattering measurements.