Effect of ionic mobility on the enhanced dielectric and electro-optic susceptibility of suspensions: Theory and experiments

It is a well-known fact that the presence of charged dispersed solid particles in an electrolyte solution considerably modifies the dielectric permittivity and conductivity of the system as compared to that of the pure dispersing medium. The enhanced conductivity of the electrical double layer, and its polarization under the action of the external field are responsible for that fact. A related phenomenon, which is also a manifestation of large induced dipole moments, is the enhanced electric birefringence (Kerr effect), which measures the electric torque on charged nonspherical colloids. Measurements of the Kerr constant are significant because a direct relationship exists between electrically induced birefringence and the particle's electric polarizability. In this work we analyze, from the experimental and theoretical points of view, the effects of coion and counterion mobility on the enhancement of both dielectric and Kerr constants: we show that, quite unexpectedly, the diffusion coefficient of coions has a large effect on both dielectric response and electric birefringence of the suspensions. To our knowledge, this effect had never been described before. Experimental data have been obtained on suspensions of various polymer particles, in different concentrations of NaCl and Na-salicylate: since the particles are anionic, this choice enables to assess the effects of the mobility of coions. We find that both the dielectric response and the Kerr effect are smaller (beyond experimental errors) in the presence of salicylate solutions. Experimental results and physical reasons for this behavior are discussed, and it is concluded that the classical theory of the low-frequency dielectric dispersion of colloidal systems provides a quantitative explanation for the coion effect on the dielectric constant. In the case of the Kerr effect, only qualitative arguments can be given in the low-frequency regime. In contrast, the high-frequency behavior is better justified in terms of a Maxwell-Wagner model. (C) 2002 American Institute of Physics