Oscillatory and ion-correlation forces observed in direct force measurements between silica surfaces in concentrated CaCl2 solutions

Abstract

The force between silica spheres and naturally oxidised silicon wafer has been measured in calcium chloride solutions at concentrations between 1 and 5 M using an atomic force microscope. An oscillatory force, consistent in periodicity with the expulsion of layers of ions, was found to overlay the expected van der Waals force. The extent and magnitude of the oscillations increased markedly with electrolyte concentration. Measured pull-off forces point to the oscillatory force minima being much larger in magnitude than the maxima, and appears to confirm the existence of ion correlation forces, possibly resulting from shared hydration waters forming an attractive network. Forces were also measured in 1 M NaCl solution. A monotonic repulsion was observed at short-range, in contrast with the ‘ hard-wall’ of Ca²⁺ ions observed in 1 M CaCl₂ before expulsion at a force of 3 mN m⁻¹. These observations suggest that calcium ions are attracted to the surface strongly enough to disrupt the hydration of the surface and/or the ions, whereas sodium ions are not. The results demonstrate a simple methodology for the direct investigation of ion-specific surface forces at high salt concentrations.