Figure 3: Atomic scale structure of mica-KCl interfaces at low molarities.

(a) xz frame (raw data). At low to moderate salt concentrations (0.01–1 M), the ordered layer is very thin (below 1.0 nm). It is formed by a single cation monolayer on top of the mica lattice and a few hydration layers. (b) The force curve across the dashed line of a shows two hydration layers about 0.34 nm apart. (c) xy frame taken at z=0 nm. In the experimental images, the z=0 nm is chosen to be located at the interstitial position between two K⁺. This assumption facilitates the direct comparison between theory and CF-DFT data. The image shows the hexagonal structure of the K⁺ on mica. The inset shows the structure of mica (001); Oxygen (red), silicon or aluminium (yellow) and the adsorbed K ions (purple). Si and Al are in a 3 to 1 ratio. The plane of the K atoms lies above the O and Si planes. (d) xy frame acquired at z=0.34 nm. The image shows the structure of the water molecules in the 2^nd hydration layer. The origin of z is chosen at the mica surface (minima in a). (e) CF-DFT results of the number density perpendicular to the mica surface (z direction). At each z point, the density profile represents the average value over the corresponding xy plane. The density profiles are normalized with respect to the bulk number density of water. (f) CF-DFT xz total number density map of the mica-KCl (aq.) interface. This map is calculated for the x position marked with a dashed line in g. (g) CF-DFT xy map at z=0.34 nm. At this position the map mainly reflects the arrangement of the water molecules. Scale bar, 1 nm (c).