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

We present a detailed comparison between the low frequency behavior of the electric birefringence las measured by the Kerr constant, B) and of the real part of the dielectric increment as', in aqueous colloidal dispersions of nearly monodisperse, elongated, polytetrafluoroethylene (PTFE) particles with adjustable surface charge. The electric charge and, consequently, the zeta-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 Delta epsilon' decrease upon decreasing the absolute value of the zeta-potential. Upon increasing the ionic strength, B and Delta epsilon' display opposite behavior: B decreases while Delta epsilon' 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 Delta epsilon' using the particle zeta-potential obtained from electrophoretic: mobility measurements, the theory qualitatively reproduces the experimental results, but systematically underestimates the values of both B and Delta epsilon'. If, instead, we take the zeta-potential as an adjustable parameter, we can reconcile both B and Delta epsilon', although the values of zeta are larger than those derived from electrophoretic mobility measurements. (C) 1998 American Institute of Physics. [S0021-9606(98)70240-5].