Adsorption of water molecules on selected charged sodium-chloride clusters

The adsorption of water molecules (H ₂O) on sodium chloride cluster cations and anions was studied at 298 K over a mass range of 100-1200 amu using a custom-built laser desorption ionization reactor and mass spectrometer. Under the conditions used, the cations Na ₃Cl ₂ ⁺ and Na ₄Cl ₃ ⁺ bind up to three water molecules, whereas the larger cations, Na ₅Cl ₄ ⁺ to Na ₁₉Cl ₁₈ ⁺, formed hydrates with one or two only. The overall trend is a decrease in hydration with increasing cluster size, with an abrupt drop occurring at the closed-shell Na ₁₄Cl ₁₃ ⁺. As compared to the cluster cations, the cluster anions showed almost no adsorption. Among smaller clusters, a weak adsorption of one water molecule was observed for the cluster anions Na ₆Cl ₇ ^- and Na ₇Cl ₈ ^-. In the higher mass region, a substantial adsorption of one water molecule was observed for Na ₁₄Cl ₁₅ ^-. Density functional theory (DFT) computations were carried out for the adsorption of one molecule of H ₂O on the cations Na _nCl _n-1 ⁺, for n = 2-8, and the anions Na _nCl _n+1 ^-, for n = 1-7. For each ion, the structure of the hydrate, the hydration energy, and the standard-state enthalpy, entropy, and Gibbs energy of hydration at 298 K were computed. In addition, it was useful to compute the distortion energy, defined as the electronic energy lost due to weakening of the Na-Cl bonds upon adsorption of H ₂O. The results show that strong adsorption of a H ₂O molecule occurs for the linear cations only at an end Na ion and for the nonlinear cations only at a corner Na ion bonded to two Cl ions. An unexpected result of the theoretical investigation for the anions is that certain low-energy isomers of Na ₆Cl ₇ ^- and Na ₇Cl ₈ ^- bind H ₂O strongly enough to produce the observed weak adsorption. The possible implications of these results for the initial hydration of extended NaCl surfaces are discussed.