| Research Abstract |
In a binary mixed solvent an ion usually has a solvent composition in the salvation shell different from that in the bulk due to different ion-solvent. It is called preferential solvation of ions, because the ion prefers one solvent than the other. Preferential solvation has been expected from various investigation such as ionic mobilities, heats of ionic solvation, etc. in mixed solvents. The aim of the present investigation is to elucidate reasons why preferential solvation occurs, what are the controlling factors of preferential solvation of ions. An attempt has been made for introducing a quantitative expression of describing the degree of preferential solvation of ions in order to discuss the phenomenon of preferential solvation in terms of thermodynamics, rather than simple phenomenological qualitative considerations so far done in previous studies.
We employed the solution X-ray diffraction method for the determination of the solvent composition in the first solvation shell of va
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rious ions in different binary mixtures. Then, we introduced the Preferential Solvation Quotient, K_<PSQ> = {[A]_<SS>/[B]_<SS>}/{[A]_B[B]_B> = [x^^-_A/(1 - x^^-_A)]/[x_A/(1-x_A], where []_<SS> and x^^- represent mole fraction of solvents in the solvation shell and []_B and x in the bulk of an A-B mixed solvent. -RTIn K_<PSQ> is related to the difference in the Gibbs energies of solvation of the relevant ion with A and with B for typical case of alkali and alkaline earth ions. When solvent molecules have bulky substituents, solvent-solvent repulsive interactions caused by the steric hindrance can be a cause of preferential solvation. From the values of -RTIn K_<PSQ> obtained from the present studies, the energies of the steric hindrance between molecules in the solvation shell can be estimated. Strong solvent-solvent interactions in the bulk can also be a reason why one solvent, which has a smaller coordinating property to the ion, preferably solvates the ion than the other with a larger coordinating property, probably because the concentration of "free" solvent molecules of the latter is significantly decreased in the bulk by the formation of associates of solvent molecules. Less
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