Densities of aqueous 1,1-dimethylurea (1,1-DMU) solutions, with a molality ranged up to 0.2 mol·kg−1, were measured using an Anton Paar DMA 5000 M vibrating-tube densimeter. All the experiments were performed with a density uncertainty did not exceeding 0.03 kg·m−3·in the temperature range between (278.15 and 318.15) K at pressure to be 99.6 ± 0.8 kPa. The standard (partial at infinite dilution) molar volumes and expansibilities of 1,1-DMU in water were derived from the density data. The influence of nature of N-dimethyl-substitution in urea and achiral glycoluril (bisurea) molecules on structure-packing effects at forming the hydration complex was discussed. A nontrivial fact of the similarity in changing both volume (δV2o) and enthalpy (δΔsolH2o) effects of a solute dissolution in water during the isomeric replacements of 1,1-DMU → 1,3-dimethylurea and 2,8-dimethylglycoluril → 2,4-dimethyglycoluril was established. It was found that δV2o ~7 × 10−6 m3·mol−1 and δΔsolH2o ~11 kJ·mol−1 at T = 298.15 K in both transfer cases considered. The comparability in differently overlapping thermodynamic effects during the dissolution process at the specified replacements can be the main cause for the phenomenon discovered.
