Atomically resolved three-dimensional structures of electrolyte aqueous solutions near a solid surface

Abstract

Interfacial liquid layers play a central role in a variety of phenomena ranging from friction to molecular recognition. Liquids near a solid surface form an interfacial layer where the molecular structure is different from that of the bulk. Here we report atomic resolution three-dimensional images of electrolyte solutions near a mica surface that demonstrate the existence of three types of interfacial structures. At low concentrations (0.01-1 M), cations are adsorbed onto the mica. The cation layer is topped by a few hydration layers. At higher concentrations, the interfacial layer extends several nanometres into the liquid. It involves the alternation of cation and anion planes. Fluid Density Functional calculations show that water molecules are a critical factor for stabilizing the structure of the interfacial layer. The interfacial layer stabilizes a crystal-like structure compatible with liquid-like ion and solvent mobilities. At saturation, some ions precipitate and small crystals are formed on the mica.

Figure 1. Fluid cell and 3D AFM scheme.

(a) AFM fluid cell. A thin mica substrate is glued onto a Teflon plate (white disk) while a transparent glass surface closes the top of the liquid cell. The thickness of the electrolyte solution is about 1–2 mm. The AFM cantilever is inside the liquid. (b) Scheme of the tip displacements, cantilever excitation and detection and observables used to acquire a 3D image of a solid-liquid interface. Scale bar, 5 mm (a).

Figure 2. Three-dimensional images of mica-electrolyte solution interfaces.

(a) 3D AFM image of a KCl (aq.) solution (0.2 M KCl). The image shows a monolayer of adsorbed K⁺ ions (light red) topped by two hydration layers (lighter stripes). The hydration layers (∼0.3 nm thick) follow the atomic corrugation of the surface. (b) 3D AFM image of a mica-KCl (aq.) interface (∼4 M). The interface is divided in two main regions, an ordered liquid layer extending up to 2 nm from the mica and the bulk solution above it. (c) 3D AFM image of a mica-NaCl (aq.) interface. Atomic scale order is seen in xy, xz and yz planes. These 3D maps show the variations of the phase shift of the tip's oscillation. Supplementary Movies 1 (52 s), 2 (105 s) and 3 (105 s) include the complete 3D data, respectively, of a–c. The 3D AFM experiments were performed at ∼300 K.

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).

Figure 4. Atomic scale structure of mica-KCl (aq.) interfaces at high molarities.

(a) xz frame (low pass filtered image). At high molarities the interface is characterized by the presence of an ordered liquid layer 2–3 nanometres thick. The inset shows a filtered image (FFT) of the bottom right corner of the xz frame. The ordered layers show atomic scale features along x and z axis. The xz frame shows the variations of the phase shift. (b) xz frame extending 5 nm above the mica surface (not shown). The side bar shows the variations of the oscillation amplitude during the z motion (A₀=169 pm). (c) The force curve taken along the horizontal dashed line in a show a mean periodicity of 0.5 nm. It follows the lattice structure of the mica(001). (d) Force curve along the perpendicular line plotted in a. The force oscillates with the distance from the mica. (e) CF-DFT simulations at high molarities. Number density profiles perpendicular to the mica surface for K⁺, Cl⁻ and water. The density profiles are normalized with respect to the bulk number density of water. (f) Net charge density profile (cation minus anion). Scale bars, 0.5 nm (a,b).