Thermodynamic stability of finely dispersed Na+Cl−(H2O) n aerosol particles in water vapor

Abstract

Four qualitatively different thermodynamic states have been identified for hydrated sodium chlo ride ion pairs in water vapor. Two of the states are stable, and the other two are unstable. The stable states of a contact ion pair (CIP) and a solvent separated ion pair (SSIP) represent sites of heterogeneous nucleation in supersaturated vapors. The unstable states form free energy barriers that separate the stable states. As the temperature is elevated, the SSIP states become more stable than the CIP states already in unsaturated water vapors. Hydrated ion pairs that contain hydroxonium ions instead of alkali metal cations are absolutely stable in the SSIP state already in the range of unsaturated vapors and low temperatures. Computer assisted calcu lations of the free energy and work of hydration have been carried out by the Monte Carlo bicanonical statis tical ensemble method in terms of a detailed model of interactions comprising polarization, many particle covalent interactions, and charge transfer phenomena.