Transient scattering changes induced by pulsed sinusoidal electric fields

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Abstract
The application of electric fields to solutions or suspensions of electrically anisotropic macromolecules results in changes in the angular distribution of the intensity of the light scattered from such solutions or suspensions. Recently, pulsed direct current fields have been used to determine the rotary diffusion constant D of the solute. In principle, the rate at which the intensity changes are established in a DC field should indicate the relative magnitudes of the permanent and induced dipole moments of the macromolecules. In practice, the method has not proved to be reliable. A new method is proposed in which pulsed sine-wave fields are used to give equivalent transient changes. By applying first a direct current pulse and then a sine-wave pulse of frequency much greater than the dipolar relaxation frequency, the amplitudes of the two traces enable the relative magnitudes of the permanent and induced dipole moments to be determined. Simultaneously, D and the directions of these moments are evaluated relative to the major molecular geometric axis. The method of applying the fields and recording the transient phenomena is outlined. Transient traces are presented from a study on Laponite clay, for which the above mentioned parameters were evaluated. Additional data were obtained from a conventional Zimm plot which enabled the apparent permanent and induced dipole moments to be completely evaluated. Two interesting effects were revealed from the pulsed sine-wave experiments on Laponite. Firstly, values of D from data obtained during and after the field application were not the same. Secondly, the transients in pulsed direct current fields exhibited overshoot effects as the field switched on and off. These were not present when high-frequency sine-wave pulses were employed.