Atomically resolved interfacial water structures on crystalline hydrophilic and hydrophobic surfaces

[EN] Hydration layers are formed on hydrophilic crystalline surfaces immersed in water. Their existence has also been predicted for hydrophobic surfaces, yet the experimental evidence is controversial. Using 3D-AFM imaging, we probed the interfacial water structure of hydrophobic and hydrophilic surfaces with atomic-scale spatial resolution. We demonstrate that the atomic-scale structure of interfacial water on crystalline surfaces presents two antagonistic arrangements. On mica, a common hydrophilic crystalline surface, the interface is characterized by the formation of 2 to 3 hydration layers separated by approximately 0.3 nm. On hydrophobic surfaces such as graphite or hexagonal boron nitride (h-BN), the interface is characterized by the formation of 2 to 4 layers separated by about 0.5 nm. The latter interlayer distance indicates that water molecules are expelled from the vicinity of the surface and replaced by hydrocarbon molecules. This creates a new 1.5-2 nm thick interface between the hydrophobic surface and the bulk water. Molecular dynamics simulations reproduced the experimental data and confirmed the above interfacial water structures. This journal isEuropean Research Council ERC-AdG-340177, the Ministerio de Ciencia, Innovación y Universidades (PID2019- 106801GB-I00; MAT2016-76507-R) and European Commission Marie Sklodowska-Curie grant agreement No. 721874. J.C. acknowledges financial support by the US National Science Foundation under Grant No. CHE-1726332 and DMR-1945589.Peer reviewe