  The dissolution of NaCl in water is one of the most common everyday
processes, yet it remains poorly understood at the molecular level. Here we
report the results of an extensive density functional theory study in which the
initial stages of NaCl dissolution have been examined at low water coverages.
Our specific approach is to study how the energetic cost of moving an ion or a
pair of ions to a less coordinated site at the surface of various NaCl crystals
varies with the number of water molecules adsorbed on the surface. This
"microsolvation" approach allows us to study the dependence of the defect
energies on the number of water molecules in the cluster and thus to establish
when and where dissolution becomes favorable. Moreover, this approach allows us
to understand the roles of the individual ions and water molecules in the
dissolution process. Consistent with previous work we identify a clear
preference for dissolution of Cl ions over Na ions. However, the detailed
information obtained here leads to the conclusion that the process is governed
by the higher affinity of the water molecules to Na ions than to Cl ions. The
Cl ions are released first as this exposes more Na ions at the surface creating
favorable adsorption sites for water. We discuss how this mechanism is likely
to be effective for other alkali halides.
