The decay curves of electro-optic transients are generally used to measure the rotary diffusion constants (D) of colloidal particles in suspension or biopolymers in solution. Analysis usually is made by plotting the logarithm of the decay as a function of time, t.
Curvature in such plots shows the presence of several values of D and hence a distribution of particle sizes. I n this case, analysis of the complete curve to yield the distribution is severely restricted by lack of precision in the experimental data. The most reliable and reproducible part of the decay curve is the initial (t = 0) slope.
This paper develops theory for the initial slope for dilute suspensions of long thin cylinders with various electricd properties. It is shown that the initial slope depends not only on the particlesize distribution and electric field strength, but also on the nature of the electrical properties of the molecules. Furthermore, for an identical polydisperse system, transient electric birefringence, electric optical rotation, and electric dichroism on the the one hand, and transient electric light scattering on the other, yield different initial slopes and hence apparently different values for D. These important conclusions have not been appreciated in previous studies and indicate the need for caution when comparing data from different experimental methods. Illustrative calculations are presented for a polydisperse system of long thin cylinders having a normal distribution in lengths, but of uniform diameter. Some preliminary measurements support the theory.