The interpretation of electrokinetic measurements using a dynamic model of the stern layer: II. Comparisons between theory and experiment

The mathematical model developed in Part I (C. F. Zukoski IV and D. A. Saville, J. Colloid Interface Sci.114, 32–44 (1986)), where the Stern layer is treated in a dynamic sense with ion transport by both diffusion and electromigration along the surface, has been used to interpret experimental data on particle mobility and suspension conductivity. In the model, particle charge arises from a balance between the effects of adsorbed cations, which neutralize covalently bound surface groups, and co-ions adsorbed to hydrophobic areas on the surface of the latex particles. Using the dynamic model of the Stern layer it is possible to depict the electrophoretic mobility of individual particles and electrical conductivity of a suspension over a wide range of electrolyte concentrations. According to the results ion motion parallel to the surface but within the Stern layer affects the bulk conductivity of the suspension more than the mobility of individual particles. Ion mobilities in the Stern layer are naturally smaller than in the bulk. The discrepancy between the ζ potentials (or surface charge densities) inferred from applying the standard theory (which omits the surface transport of ions) to mobility and conductivity measurements is due to the fact that lateral transport in the Stern layer acts so as to reduce the effective electric force on a small particle, which reduces the apparent charge, whereas such transport affects the conductivity in ways that increase the apparent charge.