Temperature-dependent behavior of enthalpies and heat capacities of solution in water for thiourea and its N,N′-dimethylsubstituted derivative

Molar enthalpies of solution in water for thiourea (TU) and 1,3-dimethylthiourea (1,3-DMTU) were measured calorimetrically in the temperature range from 278.15 to 313.13 K at ambient pressure. Experimental results were used to derive limiting or standard (at infinite dilution) molar enthalpies of solution and hydration, changes in heat capacity due to the solution process and solute limiting molar heat capacities. Molar enthalpies for dilution of aqueous TU and 1,3-DMTU were also measured at T = 298.15 K. The enthalpic coefficients for pair-wise interactions between hydrated solute molecules were estimated using the McMillan–Mayer formalism. The latter quantity was found to be negative for both TU and 1,3-DMTU, decreasing in magnitude on going from the unsubstituted solute to its N,N′-methylated analog. Hydration behavior of both TU and 1,3-DMTU was discussed in comparison with that of urea and the corresponding dimethylsubstituted derivative.

► Enthalpies of solution of thiourea and 1,3-dimethylthiourea in water were measured.
► Enthalpies of dilution for solutions of TU and 1,3-DMTU in water were estimated.
► Heat capacities of solution of TU and 1,3-DMTU in water were computed.
► Enthalpic coefficients h22 for pairwise solute–solute interactions were derived.
► The h22 quantities were found to be negative and the more negative for aqueous TU.