Electrostatic interaction forces in aqueous salt solutions of variable concentration andvalency

Abstract

We use atomic force microscopy (AFM) to determine electrostatic interactions between Sitips and Si wafers in aqueous electrolytes of variable composition. We demonstrate thatdynamic force spectroscopy (DFS) in the frequency modulation (FM) mode with stiffcantilevers and sharp tips allows for a continuous detection of the tip–sample interactionswithout mechanical jump-to-contact instability and with substantially higher lateral resolutionthan standard colloidal probe measurements. For four different species of salt (NaCl, KCl,MgCl2,CaCl2) we find repulsive electrostatic forces at the lowest salt concentrations (1 mM) that becomeprogressively screened until they are dominated by attractive van der Waals forces atthe highest concentration (100 mM). For the divalent cations the crossover fromrepulsive to attractive forces occurs at lower concentrations than for monovalentcations. Surface charges extracted from fits to standard Poisson–Boltzmann doublelayer theory indicate a rather weak dependence of the surface charge on the ionconcentration. The high lateral resolution of our approach is illustrated by a 2D force fieldmeasurement over a patterned surface of a supported lipid bilayer on a mica substrate.