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

Area of Science:

• Surface Science
• Physical Chemistry
• Biophysics

Background:

• Hydration forces are fundamental to physical, chemical, and biological processes.
• Understanding ion-surface interactions is key to controlling interfacial phenomena.

Purpose of the Study:

• To investigate the cation-specific hydration forces at mica-water interfaces.
• To determine the influence of alkali chloride concentration and pH on these forces.

Main Methods:

• Atomic Force Microscopy (AFM) for force measurements.
• Molecular Dynamics (MD) simulations for atomic-level insights.
• Systematic variation of alkali chloride solutions (Li+, Na+, K+, Rb+, Cs+) and pH.

Main Results:

• Hydration forces comprise monotonic and oscillatory components, both cation-dependent.
• Monotonic force transitions from repulsive (Li+, Na+) to attractive (Rb+, Cs+) with decreasing hydration energy.
• Oscillatory force is unaffected by strongly hydrated cations but suppressed by weakly hydrated ones, correlating with water structure.

Conclusions:

• Cation identity significantly dictates hydration forces at mica surfaces.
• The interplay between monotonic and oscillatory forces governs interfacial behavior.
• pH variations mimic effects of weakly hydrated cations, suggesting protonation effects on water structure.