H2O–D2O solvent isotope effects in adiabatic compressibility and volume of electrolytes and non-electrolytes: relation to specificities of ionic solvation

Abstract

Adiabatic compressibilities and molar volumes of a number of simple inorganic salts and tetraalkylammonium salts have been measured in H₂O and D₂O. The infinite dilution values, ϕ°_K,S, have been obtained by extrapolations based on results determined down to 0.001–0.002 mol kg^–1, i.e., towards the Debye–Hückel limiting law region. With these ϕ°_K,S values, hydration effects determining the solvent isotope difference of ϕ°_K,S values in D₂O and H₂O are discussed in terms of electrostriction and differences of local rigidity or relaxation characteristics of the solvent lattice.

The solvent isotope effects in ϕ°_K,S are related to accessibility of the N⁺ centre in the organic ions investigated. Comparison with the isotope effects in the infinite dilution partial molal volumes indicate that the series of salts investigated can be classified into three groups according to the hydration behaviour of the cations.

The behaviour of salts is compared with that of some neutral molecules, HOD and also some D-labelled acetic acids for which a secondary isotope effect is investigated.