D2O–H2O solvent isotope effects on the enthalpy of 1,1,3,3-tetramethyl-2-thiourea hydration at temperatures from (278.15 to 313.15) K and ambient pressure

Abstract The enthalpies of solution of 1,1,3,3-tetramethyl-2-thiourea (TMTU) in ordinary (H 2 O) and heavy (D 2 O) water were measured at (278.15, 283.15, 288.15, 298.15, and 313.15) K and atmospheric pressure. Standard molar enthalpies and heat capacities of solution and hydration, together with D 2 O–H 2 O solvent isotope effects (IEs) on these quantities, were calculated. It was established that, unlike the process of forming aqueous 1,1,3,3-tetramethyl-2-urea (TMU), the dissolution of TMTU in both H 2 O and D 2 O is an endothermic effect over the whole temperature range studied, and the standard enthalpy-isotopic effect undergoes a negative-to-positive sign inversion nearby of T  = 304 K. Going from TMTU to TMU, the standard heat capacity of solution (hydration) and corresponding IE become less positive.