The Physisorbate H2O-NaCl(001)
The adsorbate H2O-NaCl(001) dominated by physical interactions is of importance as a strongly polar, H-bonded system in a highly non-uniform electrostatic field. Water not only physically adsorbs on the surface to form an adlayer, but also reacts chemically at defects on the NaCl(001) surface. The cases from the isolated admolecule up to the monolayer collective behavior are of basic interest. The adsorption of water on salt is significant for numerous phenomena ranging from interstellar dust grains to biochemistry, from sea salt spray, heterogeneous nucleaction of clouds to industrial applications, such as catalysis, nuclear waste deposition in geologically stable rock salt domes. In particular, it is estimated that 10^9 to 10^11 tons of salt are injected annually as aerosols into the atmosphere from the oceans. The tropospheric reactions of the salt aerosols with NO2 produce NaNO3 and HCl. The role of adsorbed water in this transformation process is not understood. Another topic of scientific, economic and social relevance especially in northern Germany is the industrial processing of raw mineral salts. In a new electrostatic process, named ESTA, NaCl, KCl, MgSO4 are separated by contact charging after adsorption of water and proper acid vapor from air of defined humidity. Presently some 10^7 tons of mineral salt per year are separated in the ESTA process. More work needs to be done to understand the role of adsorbed water on the salt surface in order to further improve the process. Systematic studies under well-defined conditions are being carried out on the interaction between water vapor and the clean, monocrystalline NaCl(001) surface under ultrahigh vacuum (UHV) conditions. The surfaces are prepared either by cleaving single crystals insitu under UHV or under nitrogen gas. Perfect terraces, as far as can be monitored, extending up to 0.5 μm and mono- and poly-atomic steps, color centers and other defects are generated. This short paper only allows a partial report of the results. A more complete report will be published later.