Mechanisms for ion retention in molecular water clusters in a planar nanopore against the background of thermal fluctuations

The Monte Carlo method has been used to calculate the potential of mean force for Na⁺ and Cl^? ions interacting in model planar nanopores with structureless walls under the conditions of the material contact with water vapor at room temperature and above water boiling point. The interactions have been described using a detailed many-body model calibrated with respect to experimental data on the free energy of attachment reactions and the results of quantum-chemical calculations. Dissociation becomes possible when the vapor density increases as a sufficient number of molecules are pulled into the field of the ions. The dissociation proceeds sooner under the conditions of the nanopore than in bulk water vapor. Hydration decreases the energy of the dissociated state; however, the entropy component of the free energy partly compensates for the decrease in the internal energy, thereby increasing the stability of a contact ion pair. After the dissociation of a contact ion pair (CIP), ions are retained within a cluster in the state of a solvent-separated ion pair (SSIP). Fluctuations in the number of pulled-in vapor molecules, which are correlated with fluctuations in the interionic distance, stabilize the SSIP states with respect to recombination, while a decrease in the screening of the field of ions under the conditions of the nanopore stabilize the SSIP states with respect to cluster decay. The conditions of the nanopore stimulate the passage of an ion pair from the CIP to the SSIP state due to the rearrangement of the statistical weights in favor of molecules being located in the interionic gap. Thus, under the conditions of the nanopore, the stability of the SSIP states increases with respect to both the recombination of the ions and the decay of the ion-molecular associate.