Vapour pressure isotope effect studies on ionic solutions and the structure of electrolyte solutions

Abstract

A comparison of vapour pressure isotope effect (VPIE) in single ionic solutions, i.e., hydrogen isotope effects in the dehydration of alkali metal ionic forms of resins (Li⁺, Na⁺, K⁺ and Cs⁺- forms) and corresponding aqueous alkali metal chloride solutions at various concentrations and temperatures has been made. Theory of VPIE for cationic solutions having 3 water sub-systems and anionic solutions having 2 water sub-systems has been developed. Single ion solutions (Li⁺ at 303K, Na ⁺ at 318K, K⁺ at 333K and Cs ⁺ at 353K) show all the features arising from the hydration shell models and predicted by the theory of VPIE in their a (separation factor) versus <i>m </i> (molality) plots. The absence of these features in the a versus <i>m </i> plots in low concentration region (~2<i>m) </i> for the corresponding chloride solutions (namely, LiCI, NaCI, KCI and CsCI) is primarily due to the separate contributions from various water sub-systems to overall separation factor. At high concentrations and at all temperatures, in alkali metal chloride solutions the various water sub-systems get disturbed because of sharing of water molecules and α versus <i>m </i> plots in this region differ markedly from the corresponding single ion plots. The separation factors, α(M⁺ + CI^-) (M⁺ =alkali metal ion) for hypothetical MCI solutions (where cation cosphere-anion cosphere interactions are absent) have been computed using single cation and chloride ion separation factors at different concentrations and temperatures. At low temperature, at or below 303K, where water structure is strong and the hydration shell structure is comparatively weak, the cosphere-cosphere interaction is constructive, {α(M⁺+CI^-) values < α(MCI) values}. At higher temperatures (for example, at 353 K), where hydration shell structure is stronger than water structure, the cosphere-cosphere interaction arising from sharing of water molecules is destructive {α(M⁺ + Cl^-) values > α(MCl) values}. This study shows that hydration shell models based on the water sub-systems are real and contribute proportionately to thermodynamic property of an electrolyte solution, like VPIE.