Numerical Solution of the Electrokinetic Equations for Multi-ionic Electrolytes Including Different Ionic Size Related Effects

One of the main assumptions of the standard electrokinetic model is that ions behave as point-like entities. In a previous work (Lopez-Garcia, et al., 2015) we removed this assumption and analyzed the influence of finite ionic size on the dielectric and electrokinetic properties of colloidal suspensions using both the Bikerman and the Carnahan-Starling equations for the steric interactions. It was shown that these interactions improved upon the standard model predictions so that the surface potential, electrophoretic mobility, and the conductivity and permittivity increment values were increased. In the present study, we extend our preceding works to systems made of three or more ionic species with different ionic sizes. Under these conditions, the Bikerman and Carnahan-Starling expressions cease to be valid since they were deduced for single-size spheres. Fortunately, the Carnahan-Starling expression has been extended to mixtures of spheres of unequal size, namely the Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of state, making it possible to analyze the most general case. It is shown that the BMCSL expression leads to results that differ qualitatively and quantitatively from the standard electrokinetic model.