Electrophoretic mobility of latex spheres in the presence of divalent ions: experiments and modeling

Electrophoretic mobilities (EPM) of negatively charged latex spheres were measured as a function of salt type and salt concentration. The measured values of EPM were analyzed using a standard electrokinetic model that includes double layer relaxation and the Poisson-Boltzmann model of diffuse double layer. Calculated values of EPM were in good agreement with experimental data taken in simple 1:1 (KCl) and 1:2 (Na2SO4) electrolyte solutions without using any fit parameters. For 2:1 electrolytes (CaCl2 and MgCl2), however, the magnitude of EPM calculated by the model was higher than the measured values of EPM at higher electrolyte concentrations. The difference between measured and calculated EPM was reduced by assuming the distance of slipping plane x(s) = 0.25 nm or by assuming the decrease of the magnitude of surface charge density from -0.07 to -0.025 C/m(2). These are probably due to the accumulation of divalent counterions in the vicinity of a particle's surface.