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, polytetrafluoroethylen e (PTFE) particles with adjustable surface charge. The electric charge and, consequently, the zeta-potential of the particles, is modified b y 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 eps ilon' increases. We propose an expression which connects B to the stan dard electrokinetic model and compare the experimental data to a calcu lation based on spheroids with thin double layers. If we calculate B a nd Delta epsilon' using the particle zeta-potential obtained from elec trophoretic: mobility measurements, the theory qualitatively reproduce s the experimental results, but systematically underestimates the valu es of both B and Delta epsilon'. If, instead, we take the zeta-potenti al as an adjustable parameter, we can reconcile both B and Delta epsil on', although the values of zeta are larger than those derived from el ectrophoretic mobility measurements. (C) 1998 American Institute of Ph ysics. [S0021-9606(98)70240-5].