Effect of dispersion forces on the potential of charged interfaces

Abstract Ionspecific, or Hofmeister, effects are omnipresent in biological and colloidal systems. They arise from specific interactions between ions and charged interfaces. In an earlier work we have modelled these interactions in terms of a non-linear local-balance theory that takes into account the finite volumes, polarizabilities and activities of ions and solvent within an electrical double layer [S. Woelki, H.-H. Kohler, Chem. Phys. 261 (2000) 411, 421]. As an extension of this model, the present work investigates the additional influence of dispersion interactions between the ions and the charged wall. Dispersion coefficients are estimated from a simple Lifshitz model for halide anions and alkali cations for several wall materials. Numerical solutions show that the importance of dispersion interactions depends not only on the wall material and the ion type but also, strongly, on the interface charge density and the salt concentration. The ionspecific effect of dispersion interactions generally is strongest at low and medium interface charge densities and shifts to lower charge densities with increasing salt concentration.