Solvation of solutes with S=O or C=O groups in the methanol-acetonitrile solvent system: A combined calorimetric and spectroscopic study

Dimethyl sulfoxide, dimethyl sulfone, dimethyl sulfate, methyl acetate and acetic acid are studied, as solutes, in methanol-acetonitrile mixed solvent, using IR spectroscopy and calorimetry. IR measurements are also reported in a wider range of single-component solvents. It is shown that the S=O stretching band of the three solutes varies linearly with Reichardt's E(T) for the single-component solvents, with the variation being largest for dimethyl sulfoxide and smallest for dimethyl sulfate, in line with the basicities of the solutes. Analysis of the data in the mixed solvent systems shows that dimethyl sulfoxide and acetic acid are preferentially solvated by methanol while the others show random or slight preferential solvation by acetonitrile. Similarly, dimethyl sulfoxide and acetic acid, interact significantly more strongly with methanol than with acetonitrile, while the other solutes show relatively little difference in the strengths of their interactions with methanol and acetonitrile. It is also found that the solvation of dimethyl sulfoxide and acetic acid involves greater disruption of the solvent structure than those of the other solutes. Comparison of the calorimetric and IR results supports the view that, in these non-aqueous solvent systems, the solvation parameters recovered from analysis of the calorimetric data principally reflect the solvation of the polar functional groups.