Ion specific hydration of mica-electrolyte interfaces

Hydration forces play a crucial role for a wide range of phenomena in physics, chemistry, and biology. They control the formation or prevention of direct contact between any kind of dissolved molecules or suspended particles in water. Here we study the hydration of mica surfaces in contact with aqueous solutions of various alkali chloride salts over a wide range of concentrations and pH values. Using a combination of Atomic Force Microscopy and Molecular Dynamics simulations, we demonstrate that hydration forces generally consist of a superposition of a monotonically decaying and an oscillatory part, each with a unique dependence on the specific type of cation. The monotonic hydration force gradually decreases in strength with decreasing bulk hydration energy leading to a transition from an overall repulsive (𝐿𝑖+,𝑁𝑎+) to an overall attractive (𝑅𝑏+,𝐶𝑠+) total force. The oscillatory part, in contrast, displays a binary character, being hardly affected by the presence of strongly hydrated cations, but becoming completely suppressed in presence of weakly hydrated cations (𝑅𝑏+, 𝐶𝑠+), in agreement with a much less pronounced water structure in simulations.