Water vapor clustering in the field of a chlorine anion occurring in a planar nanopore with structureless walls

The Monte Carlo bicanonical statistical ensemble method has been employed to calculate the free energy, entropy, and work of Cl^? ion hydration in model planar pores 0.5 and 0.7 nm wide at 298 and 400 K. A detailed model of many-body interactions with the ion has been used, the model being matched to experimental data with respect to the free energy and enthalpy of attachment reaction in water vapor. Under the conditions of a restricted volume, the equilibrium size of a hydration shell substantially decreases, with the effect becoming stronger in the range of moderate and large sizes. In moderately supersaturated vapors, under the conditions of a nanopore, the ion loses its hydration shell as the temperature is decreased. In supersaturated vapors, the hydration shell formed on the ion is thermodynamically stable, while the stability crisis shifts to the region of larger sizes. The enhancement of the thermodynamic stability in the pore results from a rise in the chemical potential of molecules due to the deficiency of closet neighbors and a reduction in the entropy under the conditions of the restricted volume. As the temperature is elevated, the effect of ion displacement out of its hydration shell is leveled. The regularities derived in terms of the estimation model based on the capillary approximation are in qualitative agreement with the results of computer simulation.