Electric Birefringence of Dispersions of Platelets

This paper describes the electro-optic response of a suspension of disk-like colloids. We have considered aqueous suspensions of Gibbsite platelets and measured the eletrically induced birefringence in the broad frequency range 10(2)-10(8) Hz. When simply dispersed in an electrolyte solution, these particles orient with their mojor axis parallel to the electric field at the frequencies. The spectral dependence of their kerr coefficient features three regimes an electrokinetic alpha-relaxation within the kHz range, a conductive Maxwell - Wagner - O konski (MWO) relaxation having characteristic frequency in the 1-10 MHz range, and a nonzero high frequency asymptote. We quantitatively analyze the MWO spectral compenent and the high-frequency asymptote on the basis of a model developed for oblate particles. This analysis enables us to obtain the relevant particle properties : surface conductivity, zeta pontential and intrinsic Gibbsite birefringence. When the prticles are dispersed in a mixtuer that also contains smaller spherical particles that have a charge of the same sign, their electric birefringence becomes negative at low frequency. This anomalous orientation of the platelets is analogous to that observed in mixtures of prolate and spherical particles, and demonstrates the anomalous birefringence as a universal property of suspensions of nonspherical particles when surrounded by smaller charged particles.