TRANSIENT ELECTRIC BIREFRINGENCE STUDY OF ROD-SHAPED WATER-IN-OIL MICROEMULSIONS

Transient electric birefringence (TEB) studies have been carried out o n water-in-oil w/o microemulsions stabilized by Ni(AOT)(2), the nickel salt of bis(ethylhexyl) sulfosuccinate. The system forms rod-shaped d roplets at low water contents which convert to more spherical aggregat es as the water content is increased. TEB data have been obtained as a function of microemulsion volume fraction, phi, water content, and te mperature. 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 volu me fraction is consistent with the assumption that the length probabil ity distribution is exponential. A model describing the Kerr response of the microemulsion droplets is developed. By using this model we der ive the specific Kerr constant as a function of the volume fraction, f inding 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 phi. Values for L-m obtained from TEB are in good agreement with those obtained from small-angle neutron scattering meas urements.