Treating Electrostatic Shielding at the Surface of Silica as Discrete Siloxide‚Cation Interactions

This work examines the influence of ions in solution on electroosmosis inside a fused silica capillary using capillary electrophoresis; it thereby examines “shielding” at charged interfaces. Theories are reviewed that model ionic solutions as continuous dielectrics: the nonlinear form of the Poisson-Boltzmann equation gives rise to the simplified, more commonly used Debye-Hückel (DH) equation. Capillary electrophoresis (CE) is used to measure the rate of electroosmotic flow as a function of the concentration of different monovalent and divalent cations in aqueous solution. These data are used to test three specific predictions of DH theory: this theory does not describe these data adequately. The central reason behind the inadequacy of DH theory here is its inability to account for details at the level of individual ions other than by mean-field electrostatics: that is, chemical characteristics of ionssfor example, polarizability, hydrated size, energy of hydration, ability to coordinate other ions by chelationsare not accounted for. A model (the “dissociation model”) is described that treats the interactions between cations in solution and negatively charged groups on a surface in terms of discrete association equilibria with characteristic dissociation constants, Kd. CE is then used as a tool to measure values of Kd for different cations. These dissociation constants follow patterns that are consistent with ones that are familiar from studies in solution.