Effect of Ions on Solution Structure and Hydration Forces at the Orthoclase–Water Interface

Abstract

The distribution of water molecules and ions comprising an electrical double layer at solid–liquid interfaces remains difficult to accurately predict, in part due to the limited ability to probe the interactions between surface sites, water molecules, and adsorbed ions from solution. We investigated the effect of solution composition on orthoclase–water interfaces using a combination of three-dimensional atomic force microscopy (AFM) and molecular dynamics (MD) simulations. In dilute solutions, water distribution is templated by the underlying lattice, with the first water layer adsorbing at cavity sites, followed by two layers of low and high degrees of structuring. At z = 0.6 nm, water molecules assemble into striped patterns with approximately 0.69 nm periodicity that connect the preferred packing locations above cavity sites. Corresponding MD simulations in combination with the solvent tip approximation yield three key oscillatory features separated by 0.21 and 0.27 nm in reasonable agreement with the AFM data. We further investigated the effect of specific ions; magnesium and calcium chloride showed oscillations with spacings of 0.41–0.45 nm, extending >1.5 nm from the surface even at dilute concentrations. High salt concentrations of 2 molal strongly influenced the solution structure and resulted in a larger “work of approach” as the incoming probe displaced the interfacial solution, which can be ascribed to a stronger hydration force. Our results demonstrate the effect of electrolyte type and concentration on the structure of the solution at the interface, with implications for a diversity of processes such as adsorption, dissolution/precipitation, particle aggregation, and self-assembly.