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

Abstract

Based on molecular dynamics simulations of eight ions (Na, K, Rb, Cs, Mg, Ca, Sr, and Ba) 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 Ca and Ba was explained. We find that Na, Ca, Sr, and Ba can form both IS1 and IS2; K, Rb, and Cs can form only IS1; and Mg 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).

Lubricating properties of single metal ions at interfaces

“…This is consistent with the hydration structures of Rb + and K + ions, with both ions adsorbing mainly in a single hydration state to the mica substrate. In contrast, Na + ions can adopt different hydration states (inner and outer shell coordination), 64 rendering the AFM images less uniform (Fig. 1d).…”

“…Divalent Ca 2+ ions can also adopt multiple hydration states and adsorb at both the centre of the ditrigonal cavities ( position 1 the cartoon in Fig. 1f ) and at interstitial sites 64 ( position 2 in Fig. 1f ).…”

“…Domains similar to those induced by K + are visible, but Ca 2+ ions are bound more strongly to the mica and cannot be easily removed. 64…”

“…Order can exist both at the molecular (single-ion) level and at the mesoscale, inducing variations in the hydration landscape of the interface. To single out the impact of single ions on the lubrication of the interface, we conducted shear-force spectroscopy measurements 64 with a shearing amplitude A t ∼ 0.5 nm, significantly smaller than mesoscale structures. A t is also comparable to the smallest distance ions would diffuse when moving between adjacent lattice sites of the mica.…”

“…Na + exhibits a slightly anomalous behaviour that partly challenges this simple description due to its complex adsorption profile. 24,64,73,74 The contribution of anions can be neglected here given the important negative surface charge of both confining surfaces. Anions may gather in the upper hydration layers, but X-ray scattering and AFM show a rapid decrease of the degree of ordering in this region, indicating limited impact.…”