High-resolution specular x-ray reflectivity of the mica(001)-water interface under ambient conditions reveals oscillations in water oxygen density in the surface-normal direction, giving evidence of interfacial water ordering. The spacings between neighboring water layers in the near-surface, strongly oscillatory region are 2.5(2)-2.7(2) A, approximately the size of the water molecule. The density oscillations extend to about 10 A above the surface and do not strictly maintain a solvent-size periodicity as that in interfacial liquid metal and hard-sphere molecular liquids. We interpret this oscillatory density profile of the interfacial water as due to the "hard-wall" effect of the molecularly smooth mica surface.

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The interfacial structure between the muscovite (001) surface and aqueous solutions containing monovalent cations (3 × 10(-3) m Li(+), Na(+), H(3)O(+), K(+), Rb(+), or Cs(+), or 3 × 10(-2) m Li(+) or Na(+)) was measured using in situ specular X-ray reflectivity. The element-specific distribution of Rb(+) was also obtained with resonant anomalous X-ray reflectivity. The results demonstrate complex interdependencies among adsorbed cation coverage and speciation, interfacial hydration structure, and muscovite surface relaxation. Electron-density profiles of the solution near the surface varied systematically and distinctly with each adsorbed cation. Observations include a broad profile for H(3)O(+), a more structured profile for Li(+) and Na(+), and increasing electron density near the surface because of the inner-sphere adsorption of K(+), Rb(+), and Cs(+) at 1.91 ± 0.12, 1.97 ± 0.01, and 2.26 ± 0.01 Å, respectively. Estimated inner-sphere coverages increased from ~0.6 to 0.78 ± 0.01 to ~0.9 per unit cell area with decreasing cation hydration strength for K(+), Rb(+), and Cs(+), respectively. Between 7 and 12% of the Rb(+) coverage occurred as an outer-sphere species. Systematic trends in the vertical displacement of the muscovite lattice were observed within ~40 Å of the surface. These include a <0.1 Å shift of the interlayer K(+) toward the interface that decays into the crystal and an expansion of the tetrahedral-octahedral-tetrahedral layers except for the top layer in contact with solution. The distortion of the top tetrahedral sheet depends on the adsorbed cation, ranging from an expansion (by ~0.05 Å vertically) in 3 × 10(-3)m H(3)O(+) to a contraction (by ~0.1 Å) in 3 × 10(-3) m Cs(+). The tetrahedral tilting angle in the top sheet increases by 1 to 4° in 3 × 10(-3) m Li(+) or Na(+), which is similar to that in deionized water where the adsorbed cation coverages are insufficient for full charge compensation.

Molecular-scale structures of mica surfaces in electrolyte solutions reveal how ion and interfacial hydration control cation adsorption. Key differences are obtained for Rb+and Sr2+ using resonant anomalous x-ray reflectivity: Rb+ adsorbs in a partially hydrated state and incompletely compensates the surface charge, but Sr2+ adsorbs in both fully and partially hydrated states while achieving full charge compensation. These differences are driven by balancing the energy cost of disrupting ion and interface hydration with the electrostatic attraction between the cation and charged surface.