Strong water-mediated friction asymmetry and surface dynamics of zwitterionic solids at ambient conditions: L-alanine as a case study

Water molecules strongly interact with freshly cleaved (011) surfaces of L-alanine single crystals at low relative humidity (below 10%) promoting diffusion of L-alanine molecules. Species mobility is enhanced above ~40% leading to the formation of two-dimensional islands with long-range order through Ostwald ripening. Scanning force microscopy experiments reveal that both, islands and terraces, are identical in nature (composition and crystallographic structure) but a relevant friction asymmetry appearing upon water-surface interaction evidences that orientation dependent properties exist between them at the molecular level. We interpret this observation as due to water incorporation in the topmost surface crystal structure. Eventually, for high humidity values, surface dissolution and roughening occur.