Ionic limiting partial molal volumes in various solvents

Abstract

The existing methods for the separation of the limiting partial molal volumes of solutes are analysed, especially for non-aqueous electrolyte solutions. The method of Conway, Verrall and Desnoyers has been used for the calculation of ionic partial molal volumes at infinite dilution in various solvents and a comparative study of the V⁰_ion values obtained has been attempted. The calculated ionic limiting partial molal volumes of tetra-alkylammonium cations are practically identical in all organic solvents at the temperatures examined and differ greatly from the values in water and heavy water solutions. This fact provides additional evidence of the unsolvation of large tetra-alkyl-ammonium cations in organic solvents and the specific hydrophobic hydration of these cations in water and heavy water. The V⁰_R4N+ values for solutions in water and heavy water are considerably smaller than their appropriate values in organic solvents, due to the phenomenon of hydrophobic hydration of these cations in water and heavy water, when the organic chains of large tetra-alkyl-ammonium cations occupy intermolecular cavities in the water structure. The temperature dependence of the limiting partial molal expansibilities of tetra-alkylammonium cations in water and heavy water shows that Me₄N⁺ and Et₄N⁺ cations are “structure-breaking” ions in water and heavy water, but large tetra-alkylammonium cations, starting from Pr₄N⁺, are “structure-making” ions. In accordance with the fact that ∂^–_ER4N+/∂T= 0 in organic solvents, these cations are neither “structure-makers” nor “structure-breakers” in organic solvents.