Effect of Ion-Specific Hydration Forces on the Stability of Water Films on Calcite Surfaces

The hydration force is indispensable for understanding short-range interfacial forces in aqueous systems. Perturbation of the hydration structure by ions generates an ion-specific hydration force. Surface-force measurements on calcite surfaces have suggested that Na⁺ decreases the repulsive hydration force by directly adsorbing the surface and disrupting the hydration layers. However, the influence of structural changes on the surface force remains unclear. We conducted molecular dynamics simulations for water films between calcite (104) surfaces and oil/water interfaces. Ion-specific hydration forces estimated by the simulations were consistent with the experimental results. Notably, the ion-specific hydration forces cannot be explained solely by the structure of water molecules because ions do not significantly change the structure of the hydration layers, such as density distributions and orientations. We propose a novel mechanism whereby ion-specific electrostatic potentials in the water films control the adhesive and repulsive nature of the interfaces. The directly adsorbed Na⁺ on the calcite causes the monotonically decreasing electrostatic potential from the calcite surface, thereby enhancing adhesion. Ca²⁺ results in a convex shape of the electrostatic potential curve, which enhances repulsion. Importantly, the shape of the electrostatic potential curve depends on the Stern layer structure and the perturbation between the surface and interfaces. This study offers important insight for interpreting surface-force measurements in aqueous systems.