Hydration and Distribution of Ions at the Mica-Water Interface

Molecular-scale structures of mica surfaces in electrolyte solutions reveal how ion and interfacial hydration control cation adsorption. Key differences are obtained for ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$ using resonant anomalous x-ray reflectivity: ${\mathrm{Rb}}^{+}$ adsorbs in a partially hydrated state and incompletely compensates the surface charge, but ${\mathrm{Sr}}^{2+}$ 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.

Figure 1

High-resolution XR as a function of momentum transfer, $q=4\pi /\lambda ·\sin (\alpha )$, where $\alpha$ is the incident and outgoing angle of x rays reflected by the surface in a specular geometry (inset). The Sr data are shifted by factor of 0.01 for clarity.

Figure 2

Selected RAXR spectra of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$ on the muscovite (001) surface, measured near the $K$-absorption edges of each ion (15.211 and 16.113 keV, respectively). The spectra are expressed in normalized form $|F(q,E)/F_{\mathrm{NR}}(q){|}^2=1+|F_{\mathrm{R}}(q,E)/F_{\mathrm{NR}}(q){|}^2+2|F_{\mathrm{R}}(q,E)/F_{\mathrm{NR}}(q)|\cos [{\Phi }_{\mathrm{NR}}(q)-{\Phi }_{\mathrm{R}}(q,E)]$, where $F(q,E)$ is the total structure factor, $F_{\mathrm{NR}}(q)$ is the nonresonant (NR) structure factor, $F_{\mathrm{R}}(q)$ the resonant structure factor, and $\Phi$ is the phase [13]. Each spectrum is shifted by 0.25, and data are shown over a limited $E$ range for better visibility.

Figure 3

Total and ion-specific electron density profiles along the surface-normal direction above the muscovite (001) surface for (a) ${\mathrm{Rb}}^{+}$ and (b) ${\mathrm{Sr}}^{2+}$ at concentrations of 0.01 M and $p\mathrm{H}\sim 5.5$. Net interfacial hydration profiles (blue dash-dot lines) obtained by subtracting the derived cation adsorption profiles (red dashed lines) from the total interfacial electron density profiles (black solid lines) are compared to the original interfacial hydration structure in deionized water (thin black dashed line) [14]. The origin is chosen at the mean position of the (001) surface oxygen layer in the muscovite. Pictorial models of (c) ${\mathrm{Rb}}^{+}$ and (d) ${\mathrm{Sr}}^{2+}$ aqueous complexes on the muscovite surface based on the measured ion adsorption profiles and known ion-hydration structures in bulk solution. Magenta: ${\mathrm{Rb}}^{+}$, green: ${\mathrm{Sr}}^{2+}$, orange: water, red: surface oxygen. The dashed lines represent cation-oxygen bonds in the coordination shell (indicated by the large spheres). Additional water in the surface hydration layer is not included in these pictorial models.