The enthalpies of dilution of solutions of hexamethylenetetramine (HMTA), the well-known pharmaceutical urotropine, in water (H2O) and heavy water (D2O) were determined with a new heat conduction calorimeter at 298.15 K. The enthalpy-related homotactic coefficients of pairwise,h22 , and triplet,h22 , interactions between hydrated solute molecules were computed using the excess thermodynamic function concept (based on the McMillan-Mayer theory formalism). Theh22 and h22 values were found to be large and positive in both H2O and D2O due to a partial overlapping of solute hydrophobic hydration shells and appearance of solute – solute correlations at rather large distances. This phenomenon is stronger pronounced in heavy water both for pairwise and for triplet HMTA – HMTA interactions indicating a highly ordered hydration structure in D2O. The established correlation between the h22 values and the corresponding solvent isotope effects, ðh22(H2O → D2O), for HMTA and tetramethylurea mono- and bicyclic derivatives as solute species confirms the hypothesis of the differentiating effect of a solvent isotopic substitution on both the energetics of solvation and solute – solute interactions in aqueous solutions of proton-accepting non-electrolytes. In other words, the more negative or positive the h22 value due to stronger homo- and heterocomponent D-bonds, the more negative or positive the corresponding isotopic effect is.
