Dilution Enthalpies and Enthalpic Pairwise Self-Interactions of Nicotinamide and Isonicotinamide in (Dimethylformamide + Water) and (Dimethyl Sulfoxide + Water) Mixed Solvents at 298.15 K

Abstract

The dilution enthalpies of nicotinamide (NA) and isonicotinamide (INA) in (dimethyl sulfoxide (DMSO) + water) and (dimethylformamide (DMF) + water) mixed solvents (M1 and M2) at 298.15 K have been determined, respectively, using an isothermal titration calorimeter (MicroCal ITC200). On the basis of the McMillan-Mayer theory, enthalpic pairwise self-interaction coefficients (hxx) of NA and INA in both M1 and M2 aqueous mixed solvents (mole fractions of cosolvents xcos = 0 to 0.10) have been calculated. It was found that the values of hxx are all large negative, with the relative magnitudes hxx(NA) hxx (in M2) for both NA and INA, and that all of them increase monotonously with the concentrations of cosolvents (both DMF and DMSO). It implies that an intense exothermic effect accompanies with the pairwise hydrophilic–hydrophilic self-interactions, in which the meso-isomer (NA) behaves as a stronger molecular associator than the para-isomer (INA).