Enthalpic pairwise self-interactions of urea and its four derivatives in (dimethylformamide + water) mixtures rich in water at T = 298.15 K

Abstract

Dilution enthalpies of urea (U) and its four derivatives {biuret (BU), methylurea (MU), 1,1-dimethylurea (1,1-DMU) and 1,3-dimethylurea (1,3-DMU)} in (dimethylformamide (DMF)+water) mixtures of various mole fractions (xDMF=0 to 0.10) at T=298.15K have been determined by isothermal titration calorimetry (ITC). In light of the approach of McMillan–Mayer’s theory, enthalpic pairwise self-interaction coefficients (h2) of each compound at the corresponding composition (xDMF) of mixed solvent (DMF+water) have been calculated. It is found that the h2 coefficients of each compound are all negative in pure water and increase incrementally with the concentration of DMF in the mixed solvent, following the order h2(BU)<h2(U)<h2(MU)<h2(1,1-DMU)<h2(1,3-DMU). It is concluded that (1) the addition of co-solvent DMF to solvent water is unfavourable to the pairwise self-interactions of these solutes from the point of view of enthalpy; (2) the substitution of methyl (Me) on N atom of urea enhances the component of hydrophobic interactions during the pairwise self-interactions of these solutes; (3) BU is a more hydrophilic molecule than U, and 1,3-DMU is a more hydrophobic molecule than 1,1-DMU.