Solution structure and Kirkwood-Buff theory: Informativity and sensitivity to specific interactions

A comparative significance of the contribution from density and excess Gibbs energy into Kirkwood-Buff integrals G_ij has been considered exemplified by H₂O-sulfolane, H₂O-THF and model mixtures. It is shown that some salient features of the volume properties which are generally thought to be the sign of strongly hydrophobic behavior of solutes, do not clearly manifest themselves in the concentration dependences of these integrals. Literature data for G_ij have been analyzed and the structural information inherent to G_ij is assessed. Contributions from the first and the following solvation spheres to G_ij have been evaluated from the distribution functions of model mixtures computed by the Born-Green equation and it is shown that with no new chemical bonds, the contribution from the first solvation sphere does not govern the whole of G_ij and, consequently, G_ij as calculated from the thermodynamic properties contain no direct information as far as the details of short-range interactions are concerned.