Thermodynamics of Solvation of Urea and Some Monosubstituted N-Alkylureas in Water at 298.15 K

The enthalpies of solution, solHm, in water of urea, N-methylurea, N-ethylurea, N-propylurea, N-butylurea, N-isobutylurea, and N-tert-butylurea were measured by isothermal calorimetry at T = (296.84, 306.89, 316.95) K. The molar enthalpies of solution at infinite dilution, sol, at T = 298.15 K were derived and added to the molar enthalpies of sublimation, sub, at the same temperature to obtain the molar enthalpies of solvation at infinite dilution, solv. The partial molar heat capacities at infinite dilution at T = 298.15 K, , were also calculated by the summation of sol, derived from our experimental data, and molar heat capacities, Cp,m(cr), of urea and N-alkylureas from the literature. The contribution of the CH2 group to solvation enthalpy and partial molar heat capacity was -3.3 kJ·mol-1 and 90.8 J·K-1·mol-1, respectively, in very good agreement with data in the literature. Simple additive schemes were used to estimate the average molar enthalpy of solvation and partial molar heat capacity at infinite dilution of the HN-CO-NH functional group. Results are discussed in terms of current models for hydration and hydrogen bond formation for urea and its N-alkyl derivatives in water.