Electrokinetic properties of colloids of variable charge. II. Electric birefringence versus dielectric properties

Abstract

We present a detailed comparison between the low frequency behavior of the electric birefringence (as measured by the Kerr constant, B) and of the real part of the dielectric increment Δε′, in aqueous colloidal dispersions of nearly monodisperse, elongated, polytetrafluoroethylene (PTFE) particles with adjustable surface charge. The electric charge and, consequently, the ζ-potential of the particles, is modified by adding a nonionic surfactant to the dispersion, as discussed in the first paper of this series [J. Chem. Phys. 103, 8228 (1995)]. We find that both B and Δε′ decrease upon decreasing the absolute value of the ζ-potential. Upon increasing the ionic strength, B and Δε′ display opposite behavior: B decreases while Δε′ increases. We propose an expression which connects B to the standard electrokinetic model and compare the experimental data to a calculation based on spheroids with thin double layers. If we calculate B and Δε′ using the particle ζ-potential obtained from electrophoretic mobility measurements, the theory qualitatively reproduces the experimental results, but systematically underestimates the values of both B and Δε′. If, instead, we take the ζ-potential as an adjustable parameter, we can reconcile both B and Δε′, although the values of ζ are larger than those derived from electrophoretic mobility measurements.