Ion Distribution and Hydration Structure in the Stern Layer on Muscovite Surface

Area of Science:

• Surface Chemistry
• Computational Materials Science
• Environmental Geochemistry

Background:

• Understanding ion adsorption on mineral surfaces is crucial for environmental and geochemical processes.
• Muscovite mica is a common phyllosilicate mineral with significant surface reactivity.
• Previous studies have shown varied cation adsorption behaviors, lacking a unified explanation.

Purpose of the Study:

• To rationalize experimental data on cation adsorption on muscovite mica surfaces using molecular dynamics simulations.
• To elucidate the adsorption structures, hydration states, and stability of eight different cations.
• To categorize and explain the formation of different inner-sphere surface complexes.

Main Methods:

• Molecular dynamics simulations of eight cations (Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+) on muscovite mica in water.
• Analysis of adsorption structures, including cation heights and hydration.
• Categorization of inner-sphere surface complexes into ditrigonal cavity (IS1) and on-top-of-Al (IS2) species.

Main Results:

• Two distinct inner-sphere surface complexes (IS1 and IS2) were identified and characterized.
• The formation of IS1 and IS2 was found to be dependent on ion charge density.
• Simulation results successfully explained experimental observations of cation height dependence on ionic radius.
• Differences in hydration structures between monovalent and divalent cations were observed.

Conclusions:

• A unified picture of cation adsorption on muscovite mica surfaces was established, rationalizing experimental data.
• The study provides a detailed understanding of ion-surface interactions, distinguishing between IS1 and IS2 complexes.
• Ion charge density is proposed as the governing factor for IS1 and IS2 formation, offering insights into mineral-surface chemistry.