Standard volumetric characteristics of N-dimethyl-substituted bicyclic bisureas (glycolurils) in water at T = (278.15 to 318.15) K and p ∼0.1 MPa

Densities of aqueous solutions of achiral cis-dimethylglycoluril (2,8-DMGU) and chiral (in the racemic form) trans-dimethylglycoluril (2,6-DMGU) were measured using a hermetically sealed vibrating-tube densimeter, with an uncertainty of similar to 1 x 10(-5) g.cm(-3), at T = (278.15, 288.15, 298.15, 308.15, and 318.15) K and p = (99.6 +/- 0.4) kPa. The solution molality was ranged from (0.037 to 0.396) mol.kg(-1) for aqueous 2,6-DMGU and from (0.046 to 0.202) mol.kg(-1) for aqueous 2,8-DEGU. The standard (at infinite dilution) molar volumes and isobaric molar expansibilities for the specified N-dimethyl-substituted heterocyclic compounds in water were computed and discussed in comparison with previously derived results for the equimolecular 2,4-dimethylglycoluril (2,4-DMGU) whose isomeric molecules contain simultaneously both the unsubstituted and fully methylated five-membered rings. It was established that the compared glycoluril-derivatives are the prevailingly structure-breaking solutes in water despite of presence of two methyl groups in each of heterocyclic molecules. In other words, the hydrophobic hydration effect is being less pronounced than the hydrophilic hydration or H-bonding effect (via amide and carbonyl groups). The temperature-dependent behavior of packing related hydration effects was described taking into account the structural (stereochemical) features of a solute molecule. (C) 2016 Elsevier B.V. All rights reserved.