Solvation phenomena of potassium thiocyanate in methanol-water mixtures

This paper reports the results of a variety of experiments carried out for understanding the solvation behavior of potassium thiocyanate in methanol - water mixtures. Electrical conductivity, speed of sound, viscosity, and FT-Raman spectra of potassium thiocyanate solutions in 5 and 10% methanol - water (w/w) mixtures were measured as functions of concentration and temperature. The conductivity and structural relaxation time suggest the ion - solvent and solvent-separated ion - ion associations increase as the salt concentration increases in the mixtures. The Raman band shifts due to the C-O stretching mode of methanol for the solvent mixtures reveal the formation of methanol - water complexes. The significant changes in the Raman bands for the C-N, C-S and O-H stretching modes indicate the presence of SCN- - solvent interactions through the N- end, "free" SCN- and the solvent-shared ion pairs as potassium thiocyanate is added to the methanol - water mixtures. The relative changes corresponding to H-O-H bending and C-O stretching frequencies indicate that K+ is preferentially solvated by water in these solvent mixtures. The appearance and increase of the intensity of a broad band at approximate to 940 cm(-1) upon salt addition was attributed to the SCN- - H2O- K+ solvent-shared ion pairs. No Raman spectral evidence for K+(H2O)(n) species was observed. The preferential solvation of K+ and SCN- in the methanol - water mixtures was verified by the application of the Kirkwood-Buff theory of solutions. This theory confirms that K+ is strongly preferentially solvated by water, whereas SCN- is preferentially solvated by the methanol component.