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

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.

the number density perpendicular to the mica surface (z direction), for several salt concentrations. In this case the same HS radii has been used for the three species (water, cations and anions). At each z point, the density is averaged over the corresponding xy plane. The density profiles are normalized with respect to the bulk number density of water. In these figures, a 50% of the coverage of the mica by Potassium at low salt concentration (0.2 M) has been considered. This is in contrast to the 90% coverage shown in Fig. 3e. However, the trend observed at high molarities (4.4 M) is independent of the initial cation coverage and of the approximation used for the HS radii.

Supplementary methods
Our theoretical Classical Fluid Density Functional (CF-DF) calculation is useful to explore the generic aspects in the molecular structure of the liquid solution near the substrate, which is observed in the 3D-AFM maps. We do not aim to a full quantitative description of the experimental results, nor to the most realistic description of the ionic solutions, but rather to reproduce the phenomena qualitatively, with the simplest possible model, as a test of its generality, and for a better understanding of its origin.
The ionic interactions are qualitatively important to set the equilibrium crystal structure, and it is included with a mean-field approximation 3,4 for an effective Yukawa pair potential φ(r) = ±u e −κr /(4πκr). This interaction potential acts beyond the HS core (r ≥ σ), with repulsion (+ sign) between equal ions and attraction (− sign) between unequal ion species. All the results presented here correspond to the choice σκ=4, which gives a reasonable ratio of 3.7 between the attraction from the six nearest neighbors (at distance σ) repulsion from the twelfth next-nearest neighbors.  Supplementary Fig. 6. The numerical minimization of the CF-DF grandpotential energy takes advantage of the mica symmetry, but still allowing for a possible symmetry breaking of an epitaxial crystal growing with the (111) face on the mica plane. At low molar fraction, = 0.05 , the interfacial structure is limited to a strongly structured absorbed monolayer of cations and water, followed by a few hydration layers (Fig. 3e). We use the results of the realistic model simulation in this regime 5 to tune the parameters of wall potential and the effective bulk water density, the later set to 3 = 0.6 to get a similar decay of the water layers.
The minimization of Ω[ ( ⃗), ( ⃗), ( ⃗)] (see the sketch in Supplementary Fig. 6) gives qualitatively similar results for different ionic strengths (over the range 225<u/kBT<350 explored here), and ratios between the semi-distance of the cation adsorbing sites on the mica and the HS diameter in the model (over the range1.05 ≤a/σ≤1.20 ). The growth of a thick epitaxial crystalline structure is always observed for large salt concentrations (Fig. 4f), while for very low concentrations the density profiles have an adsorbed cation monolayer followed by a few water layers, weakly structured. These two structures correspond to two separated minima of , this is to a first order wetting transition between the "direct" micaliquid interface and the epitaxial crystal layer. The strength of this first order transition depends on the parameter choice. All the relevant results presented in the main text, for the comparison with the 3D-AFM images, are within the "direct" liquid interface regime. This is the regime observed in our CF-DF results over a broad range of salt concentrations ( Supplementary Fig. 7), smoothly developing a deeper 3D structured region of cations and anions as the salt 9 concentration increases. In our CF-DF model, these structured-fluid surfaces are thermodynamically separated from the epitaxial crystal (depicted also in Supplementary Fig. 7), and they persist as a meta-stable local minima of Ω[ ( ⃗), ( ⃗), ( ⃗)] even for salt concentrations that make the epitaxial crystal layer to be stable state of the surface.
The calculations shown in the main text the interaction with the mica is modelled to have a nearly complete monolayer of cations at low salt concentrations (0.2 M) ( Fig. 3e-g), as indicated by the AFM images (Fig. 3c). The Supplementary Fig. 3 shows that the DFT predictions at high concentration are very robust with respect to a significant reduction of the cation coverage of the mica at low salt concentration. Those results emphasize that the observed phenomena does not require to break charge neutrality at the mica-cation interface. Besides the results of Supplementary Fig. 3 have been obtained using our simplified model with the same molecular diameter for all the species. The similarity of the results shows that the size differences between the ions, or other molecular details like the anisotropy of water, would certainly be important for a quantitative prediction, but these effects are not needed to understand the qualitative aspects of the experimental observations.