A study on Kirkwood-Buff integrals and preferential solvation in mixtures with small deviations from ideality and/or with size mismatch of components. Importance of a proper reference system

It is shown that a reference state is necessary for the correct interpretation of the Kirkwood-Buff integrals, G(ij), of liquid mixtures in terms of interactions between the components and for their use in the calculation of the local composition. This state is identified with the symmetrical ideal system, and its G(ij)'s are taken as zero-level. The equations for calculating the G(ij) of the reference mixtures are derived and discussed, and those for calculating local composition and preferential solvation are reformulated with respect to the reference system. The mixtures of compounds with large size mismatch and/or small deviations from Raoult's law are those for which the new equations should be applied in order to get correct indications on the interactions that take place and influence the preferential solvation. A comparison with Flory-Huggins athermal mixtures (G(E) = -TSE) as reference systems is also made, allowing one to estimate the relative weights of the contributions to G(ij) and preferential solvation due to the excluded volume and to the entropy effect both originated by the size mismatch of the components. Seventeen binary mixtures chosen to cover a wide-range of deviation from Raoult's law and of the ratio and difference of the component molar volumes are examined; they comprise mixtures of apolar or low-polarity substances (rare gases, hydrocarbons, halocarbons), athermal mixtures, as well as systems involving strongly polar or associating compounds such as water, alcohols, glycols, dimethylformamide. For most mixtures, the present new treatment provides either different or new information, in regard to preferential solvation and molecular interactions, with respect to the usual way of examining G(ij) data.