Structural transition in the OH−(H2O)n cluster in water vapors

The hydration of hydroxyl ion in water vapors at temperatures corresponding to seasonal variations in natural air medium is studied by the Monte Carlo simulation in grand canonical statistical ensemble using the detailed model of intermolecular forces that takes into account many-particle covalent interactions, polarization, and charge transfer. An increase in the number of water molecules in a cluster is accompanied by a structural transition from strongly asymmetric ion environment of water molecules to the formation of enveloping shell composed of these molecules. This transition is accompanied by an abrupt increase in cluster size and qualitative changes in its structural characteristics. The displacement of ion on the surface of clusters with extremely small sizes is an entropy effect. Results of simulation are compared with data on the hydration of hydroxonium at which similar structural transition is not observed, and with data of quantum-chemical calculations.