Enthalpic pairwise interactions of isomers of 2,4-pentanediol and 2,5-hexanediol in dimethylsulfoxide + water mixtures at 298.15 K

The dilution enthalpies of four chiral diols, namely (2S,4S)-(+)-2,4-pentanediol vs (2R,4R)-(−)-2,4-pentanediol and (2S,5S)-(+)-2,5-hexanediol vs (2R,5R)-(−)-2,5-hexanediol in dimethylsulfoxide (DMSO) + water mixtures at 298.15 K have been determined respectively, using isothermal titration calorimeter (ITC). On the basis of the McMillan–Mayer theory, homochiral enthalpic pairwise interaction coefficients (hxx) of the four diols in the mixed solvents of various mass fractions (w = 0–0.3) have been calculated. It is found amazedly that across the whole studied composition range of mixed solvent values of hxx for S-enantiomer are almost higher than R-enantiomer for each diol. The results are discussed from the point of view of solute–solute interaction and solute–solvent interaction accompanied by chiral discrimination effect. The variations of hxx with w for the four chiral diols depend largely on the competition equilibrium between hydrophobic–hydrophobic, hydrophilic–hydrophilic and hydrophobic–hydrophilic interactions in the ternary solution systems.