Water adsorption and diffusion on NaCl(100)

At low coverage and temperature the water−surface interaction determines the adsorption geometry of the water molecule on the NaCl(100) surface. However, at room temperature the molecules are also able to move on the surface and form islands where the water molecules are held together by hydrogen bonds. As a step toward the description of such complex phenomenology, in this work we have used density functional theory calculations to study the most favorable adsorption geometry of an isolated water molecule and the energy barriers associated with different hopping mechanisms between equivalent adsorption configurations on this surface. We propose different hopping processes that can be classified as translations, if the molecule moves from one adsorption site to the adjacent one, or reorientations, if the molecule only changes its orientation on the surface and remains in the same adsorption site. The straightforward parallel translation of the water molecule along the surface exhibits the highest barrier. All other processes, either translations or reorientations, involve the rotation of the water molecule around certain axes and present much smaller barriers (at least 50% smaller). To obtain a net movement of the molecule along the surface it is always necessary to combine one of these translational and reorientational processes. Such combinations provide favorable and plausible pathways for the diffusion of the water molecule on the NaCl(100) substrate.This work has been supported by the Basque Departamento de Educación, the UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2004-06490-C3-00), the European Network of Excellence FP6-NoE “NANOQUANTA” (Grant No. 500198-2), and the projects “NANOMATERIALES” and “NANOTRON” funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK program and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipúzcoa.Peer reviewe