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

Supplementary information: 1. Supplementary Figures 1-7, Supplementary Methods and Supplementary References [pdf file].-- 2. Supplementary Movie 2: 3D AFM image of the mical-KCl aqueous solution (~ 4 M) was obtained by exciting the cantilever at 838 kHz (2nd eigenmode); k= 921 N/m, Q= 24; A0= 0.11 nm. Data points= 1984000 [movie].-- 3. Supplementary Movie 3: 3D AFM image of the mical-NaCl aqueous solution (~ 5 M) was obtained by exciting the cantilever at 159 kHz (1st eigenmode), k= 24 N/m, Q= 7; A0= 0.23 nm. Data points = 2400000 [movie].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.This work was funded by the European Research Council ERC-AdG-340177 (3DNanoMech) and Spanish Ministry of Science and Innovation (CSD2010-00024, FIS2013-47350-C5, MDM-2014-0377).Peer reviewe