Unusual behavior of temperature-dependent solvent H/D isotope effects in the enthalpy and heat capacity of hexamethylenetetramine (urotropine) hydration

Abstract Based on the identified inconsistencies in the existing literature data on the enthalpies and heat capacities of dissolution (solvation) of hexamethylenetetramine (HMTA) in ordinary (H 2 O) and heavy (D 2 O) water, we carried out carefully the calorimetric study of these two binary systems at seven temperatures from (278.15 to 318.15) K. It is established that the considered D 2 O–H 2 O enthalpy-isotopic effect exhibits a pronounced exothermic maximum between two sign-inversion points near 293 K and 318 K. The same goes the negative (up to ~305 K) solvent isotope effect (IE) in the partial molar heat capacity of HMTA in water. Such an unusual temperature-dependent behavior of the thermodynamic IEs points out the dualistic character of the HMTA hydration process which is dependent on the initial structure state of the solvent.