A study of the structure of water and its dependence on solutes, based on the isotope effects on solvation thermodynamics in water

Abstract

The solvation of a particle in water, whether a ‘‘foreign’’ particle or a molecule of water, is expressed in terms of a non-hydrogen-bonding contribution to the pair potential and a contribution from hydrogen bonding. Only the latter depends on whether the water is H2O or D2O, since the hydrogen bond energy differs in these kinds of water. The effect of the isotopic constitution of the water on the solvation is examined for the cases of an inert solute, a water molecule, and a hydrogen-bond-forming solute. The average number of hydrogen bonds in which any water molecule in pure water participates is deduced from the Gibbs free energy and the enthalpy of condensation of H2O, D2O, and T2O molecules into the respective liquids. This number decreases from about 0.95 at 0 °C to about 0.35 at 100 °C, being a few percent higher in D2O than in H2O. The Gibbs free energies of transfer of gaseous solutes, nonelectrolytes, and ions from light to heavy water are interpreted in terms of the average number of hydrogen bonds broken or made in the water per particle of solute.