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.