Enthalpies of Dilution of Penicillamines in <i>N</i>,<i>N</i>‑Dimethylformamide + Water Mixtures at 298.15 K

Dilution enthalpies of two penicillamine enantiomers, namely, d-penicillamine, l-penicillamine, and their racemate d,l-penicillamine, in <i>N</i>,<i>N</i>-dimethylformamide (DMF) + water mixtures of various compositions have been determined at 298.15 K respectively, using an isothermal titration calorimeter (MicroCal ITC200). According to the McMillan–Mayer theory, homochiral enthalpic pairwise interaction coefficients (<i>h</i>_XX) of the three penicillamines in DMF + water mixtures of different mass fractions (<i>w</i>_DMF = 0 to 0.3) have been calculated regressively. It is found that <i>h</i>_XX coefficients of each enantiomer and their racemate change increasingly with <i>w</i>_DMF to the maxima at <i>w</i>_DMF = 0.20 and then decrease relatively rapidly, which follows the order <i>h</i>_LL > <i>h</i>_DD ≈ <i>h</i>_RR > 0 (R represents the racemate mixture of penicillamine). The results are discussed from the point of view of competitive equilibrium between hydrophobic and hydrophilic interactions in solutions