Electrooptic birefringence and scattering of hectorite suspensions

A study is reported in which two suspensions of the rodlike clay mineral hectorite have been subjected to pulsed electric fields of up to 300 V cm(-1). As the hectorite particles were optically anisotropic, their partial alignment in the field rendered the over-all suspension birefringent. Under similar fields, the intensity of the light scattered from the suspensions also changed. The methods of detecting and recording the transient variations in both the birefringence and scattered light intensity have been outlined. By analyzing the transient responses in these optical properties as the pulsed electric field was applied and switched off, the rotary diffusion constant (D) and the ratio (r) of the permanent to the induced molecular dipole moments were obtained. Within the experimental errors, both methods gave identical results. The electrical parameters were indicated through the ratio 1/r = 4.7(+/-0.2). The negative sign indicated that the permanent and induced moments were associated with perpendicular axes of the particles, one along and the other across the major rod axis. Both this fact and the magnitude of r were in concord with previously published experiments on this mineral. Both of the suspensions were polydisperse. Particle sizes ranged from 1 microm to 1.3 microm and 1 microm to 2 microm for the two dispersions. Although the birefringence technique is well established, this is the first report in which the electrical parameters of large molecules have been determined using pulsed field light scattering. This method gave reliable molecular parameters, although the experimental data were associated with less sensitivity and greater experimental error than the birefringence measurements. A comparison was made between the two methods to indicate their relative merits.