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

Based on molecular dynamics simulations of eight ions (Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) on muscovite mica surfaces in water, we demonstrate that experimental data on the muscovite mica surface can be rationalized through a unified picture of adsorption structures including the hydration structure, cation heights from the muscovite surface, and state stability. These simulations enable us to categorize the inner-sphere surface complex into two different species: an inner-sphere surface complex in a ditrigonal cavity (IS1) and that on top of Al (IS2). By considering the presence of the two inner-sphere surface complexes, the experimental finding that the heights of adsorbed cations from the muscovite surface are proportional to the ionic radius for K⁺ and Cs⁺ but inversely proportional to the ionic radius for Ca²⁺ and Ba²⁺ was explained. We find that Na⁺, Ca²⁺, Sr²⁺, and Ba²⁺ can form both IS1 and IS2; K⁺, Rb⁺, and Cs⁺ can form only IS1; and Mg²⁺ can form only IS2. It is suggested that the formation of IS1 and IS2 is governed by the charge density of the ions. Among the eight ions, we also find that the hydration structure for the outer-sphere surface complexes of divalent cations differs from that of the monovalent cations by one adsorbed water molecule (i.e., a water molecule located in a ditrigonal cavity).