Preferential solvation in mixed solvents

In a binary mixture of solvents A and B, the surroundings of a molecule of A (or of B) generally differ in terms of the relative amounts of A and B molecules from the bulk composition due to preferential solvation. An eminent method for studying this situation is the use of fluctuation theory, in terms of the Kirkwood-Buff integrals derived from thermodynamic data, provided the latter are sufficiently accurate. The interactions among the components are obtained from this approach when their relative sizes are taken into account. This method has been applied to a large number of binary aqueous-organic solvent mixtures as well as to nonaqueous mixtures and to a small number of ternary solvent mixtures. In some cases, the preferential solvation occurs beyond the first solvation shell, for example, in aqueous mixtures of tetrahydrofuran or acetonitrile, where self-interactions of the water molecules far outweigh those between water and organic cosolvent molecules. The merits and disadvantages of using the Kirkwood-Buff integral approach are briefly compared with those of other approaches, such as the quasi-lattice, quasi-chemical one or the use of solvatochromic probes and other spectroscopic methods.