Ion-Specific and pH-Dependent Hydration of Mica-Electrolyte Interfaces.

Hydration forces play a crucial role in a wide range of phenomena in physics, chemistry, and biology. Here, we study the hydration of mica surfaces in contact with various alkali chloride solutions over a wide range of concentrations and pH values. Using atomic force microscopy and molecular dynamics simulations, we demonstrate that hydration forces 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 attractive (Rb, Cs) force. The oscillatory part, in contrast, displays a binary character, being hardly affected by the presence of strongly hydrated cations (Li, Na), but it becomes completely suppressed in the presence of weakly hydrated cations (Rb, Cs), in agreement with a less pronounced water structure in simulations. For both aspects, K plays an intermediate role, and decreasing pH follows the trend of increasing Rb and Cs concentrations.