On the dissolution processes of Na₂I⁺ and Na₃I₂⁺ with the association of water molecules: Mechanistic and energetic details

The sequential association energies for one through six water molecules clustering to Na₂I⁺, as well as one and two water molecules clustering to Na₃I₂⁺, are measured. The association energies show a pairwise behavior, indicating a symmetric association of water molecules to the linear Na₂I⁺ and Na₃I₂⁺ ions. This pairwise behavior is well reproduced by Density Functional Theory (DFT) calculations. DFT calculations also suggest that a significant separation of charge for the Na-I ion pair occurs when four or more water molecules cluster to a single sodium center. Two different solvent-separated ion pairs have been identified with the DFT calculations. Experiments also show that the dissolution processes, loss of a neutral NaI unit, occurs when six or more water molecules have been added to Na₂I⁺ cluster. However, one or two water molecules are able to detach an NaI unit from the Na₃I₂⁺ cluster. The difference in solubility of the Na₂I⁺ and Na₃I₂⁺ ions is due to the difference in the energies required to lose an NaI unit from these two species. The experiment also confirms that the loss of a neutral NaI unit, instead of an Na+ ion, occurs during the dissolution processes of Na₃I₂⁺. The microsolvation schemes proposed to explain our experimental observations are supported by DFT and phase space theory (PST) calculations.