Spontaneous emergence of Cl- anions from NaCl(100) at low relative humidity

The emergence of Cl- ions from the surface of alkali halide salts at low relative humidity (RH) is predicted by density functional theory (DFT) calculations and supported by contact potential measurements. We find from DFT that, in the presence of water at the regime of one monolayer coverage on the (100) cleavage plane of NaCl, Cl- ions are displaced at very low energetic cost from their crystal lattice positions toward the plane of water molecules. Starting from plausible low-temperature layered structures, we use total energy DFT to calculate the energy cost to bring Cl- ions to the height of the water layer. We show the importance of the screening of the electrostatic interactions by the water layer in order to explain our findings and to determine the value of the surface dipole as a function of the chloride position. The theoretical surface dipole is used to estimate the concentration of raised Cl- ions at the surface. As dissolved chloride ions are the reactive component of the sea salt aerosols at high RH, we propose that the facile destabilization of Cl- is one of the mechanisms behind the catalytic activity of NaCl at low RH on reactions involving atmospheric gases.Financial support by UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish M.E.C. (Grant No. FIS2004-06490-C3-00), the EU network of excellence FP6-NoE “NANOQUANTA” (Grant No. 500198-2), and the Basque Government projects “NANOMATERIALES” and “NANOTRON” within the ETORTEK programme is gratefully acknowledged. A.V. thanks financial support from the Ramon y Cajal program of the Spanish M.E.C. M.S. was supported by the Office of Basic Energy Science, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231Peer reviewe