The molar enthalpies of solution of a drug Albicar or 1,4-diethyl-3,6-dimethyltetrahydroimidazo[4,5-d]
imidazole-2,5(1H,3H)-dione in ordinary (H2O) and heavy (D2O) water at T = (278.15, 279.15, 288.15,
298.15, 308.15, and 313.15) K and ambient pressure were measured calorimetrically. The standard (at
infinite dilution) molar enthalpies and heat capacities of the solute dissolution along with the
(D2O–H2O) solvent isotope effects (IEs) on the specified quantities were computed. The enthalpic effects
of Albicar dissolution in H2O and D2O experience a negative-to-positive sign inversion nearby Tinv = (306
and 307) K, respectively, whereas the corresponding IEs were found to be wholly negative (by sign) and
decreasing in magnitude with increasing temperature. These facts indicate that the Albicar hydration,
being dualistic (hydrophobic–hydrophilic) in nature, is enhanced in the D2O medium. The hydration
behavior of the studied solute was discussed in comparison with those for Mebicar (1,3,4,6-tetramethylderivative)
and Bicaret (1,3,4,6-tetraethyl-derivative) using the previously obtained data.
