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Open statistical ensemble simulations are used to study the mechanism of nucleation of atmospheric water on sodium-chloride ion pair in a wide range of temperature and relative humidity values. The extended simple point-charge model is used for water molecules. Ions-water nonadditive interactions are taken into account by introducing the mutual polarization of ions and water in the field of each other. Gibbs free-energy variations are calculated from Na+-Cl- pair-correlation function and used as a criterion for determining the possible stable states of the cluster. In this relation, it was found that the dissociation of ion pairs in water clusters occurs even at vapor pressures of only a few millibars. In the conditions under consideration solvent-separated ion-pair states are found to be more probable than contact ion-pair configurations. The susceptibilities of water and ions are found to play an essential role in the stabilization of ions at large separations. The structure of ion-induced clusters is analyzed in terms of binary correlation functions. The non-pair interactions influence essentially the structure of ion solvation shells. The results of simulation show that the separation of the charges in water clusters containing simple ions can take place under atmospheric conditions.

The formation of water clusters on Li(+), Na(+), K(+), Cl(-), and I(-) ions from water vapor at atmospheric conditions have been studied using Monte Carlo simulations. The extended simple point charge model has been employed for water molecules. The polarization of ions in the field of molecules and the polarization of molecules in the field of ions have been considered explicitly in the total Hamiltonian of the molecular system. The cluster formation work and the Gibbs free energy and enthalpy of attachment reactions of one water molecule to the cluster have been calculated via the bicanonical ensemble method. Our results reveal the formation of stable clusters in equilibrium with the moist atmosphere in a wide range of vapor pressure values, with largest clusters are formed around cations. Decreasing the temperature, from 293 K to 253 K, leads to the formation of larger equilibrium clusters, and enhances the stability of systems as whole. According to clusters' molecular structures, negative ions are expected to be more active in atmospheric processes, including chemical reactions and cloud formation, than positive ones.

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