Some interaction-related thermodynamic properties of aqueous tetramethylenediethylenetetramine (drug teotropine) solutions: Effect of the solvent H/D isotope substitution and temperature

The molar enthalpies of dissolution of 1,3,6,8-tetraazatricyclo[4.4.1.13,8]dodecane or TMDETA (tetramethylenediethylenetetramine: the teotropine pharmaceutical) in ordinary (H2O) and heavy (D2O) water were measured calorimetrically at T = (278.15, 288.15, 298.15, 308.15, and 318.15) K and p = 0.1 MPa. The standard molar enthalpies and heat capacities of TMDETA dissolution/hydration, along with D2O–H2O solvent isotope effects (IEs) in these quantities, were computed. Solution enthalpies and corresponding IEs were found to be negative, decreasing in magnitude as the temperature is rising. It was concluded that hydrophobic effects are the predominant part in the TMDETA hydration and they are enhanced in D2O and weakened going from TMDETA to its lower-molecular aminal derivative, hexamethylenetetramine (HMTA) being the known drug urotropine. Unlike the aqueous HMTA, the “dualistic” character of TMDETA hydration is less dependent on the initial structure state of a solvent isotopologue. The results of analyzing the contributions to the solvation enthalpy within the scope of Scaled Particle Theory showed that IEs in the enthalpy of TMDETA – water interaction and the solvent cavity formation is almost cancelling each other at T > 288.15 K.