Molar volumes of LiI in H2O and D2O solutions; Structural hydration interactions

According to a recent study of the H₂O and D₂O molar volume isotope effect (MVIE) of the alkali metal chloride solutions, neither the standard nor the excess MVIE of the LiCl corresponds to the usual hydrophilic hydration characteristics of the inorganic ions above room temperatures. This phenomenon can not be rationalized by electrostriction, with the collapse of the “loose” tetrahedral (“ice-like”) water structure due to the electrostatic (ion + dipole) interaction.It seemed possible that this unique hydration behaviour of the Li⁺ would be stronger and could reveal further structural information with a less hydrophilic anion than the chloride. Therefore we have determined the MVIE of the LiI as a function of temperature and concentration. The densities of normal and heavy water solutions of LiI have been measured with six-figure precision at T = (288.15, 298.15, and 308.15) K from (0.03 to 4) molal, m, using a vibrating-tube densitometer. The solvent isotope effect on the apparent molar volume, as well as on the solute and solvent partial molar volumes, was evaluated.As expected, with the rationalization of the MVIE of LiI instead of the geometric structural differences of the isotopic solvents, the energetic contributions have to be considered at all the temperatures investigated. At infinite dilution, a high degree of compensation between the reversed influences of the Li⁺ and I^? on the activities of the isotopic solvents determines the MVIE. By increasing concentration, the highly asymmetric energetic interactions of the Li⁺ and the I^? with the solvent apparently result in a “mutual salting-out” effect. At a concentration ≈0.7m, a uniquely abrupt structural rearrangement results in a “solvent-separated ion-pair” solution structure.