Enthalpic discrimination of position isomerism: Pairwise interaction of piperidinecarboxylic acid isomers in DMSO + H2O mixtures at 298.15 K

Dilution enthalpies of three piperidinecarboxylic acid isomers, namely 2-piperidinecarboxylic acid, 3-piperidinecarboxylic acid and 4-piperidinecarboxylic acid in dimethylsulfoxide (DMSO) + H2O mixtures (mass fractions of DMSO w = 0–0.3) have been determined respectively using an isothermal titration calorimeter (ITC200, MicroCal) at 298.15 K. According to the McMillan–Mayer theory, the corresponding homogeneous enthalpic pairwise interaction coefficients (hXX) of the three isomers have been calculated at each composition of mixed solvents. It is found that position isomerization effect of the three piperidinecarboxylic acid isomers can be discriminated completely by enthalpic effects from pairwise interactions between solute molecules. The results were interpreted from the point of view of solute–solute and solute–solvent interactions in these ternary aqueous systems, as well as competition equilibrium between hydrophobic–hydrophobic, hydrophobic–hydrophilic and hydrophilic–hydrophilic interactions.