The densities of aqueous solutions of 4-(3,7-dioxo-2,4,6,8-tetraazabicyclo[3.3.0]oct-2-yl)butanoic acid or glycoluril-butyric acid (GUBA) and its N,N-dimethyl-substituted analogue (DMGUBA), with molality ranged from m = (0.004–0.06) mol·kg−1, were measured at T = (278.15, 288.15, 298.15, 308.15, and 318.15) K and p ≈ 0.1 MPa. The uncertainty in density measurements was estimated to be 3 × 10−5 g cm−3. The apparent and standard (at infinite dilution) molar volumes,V0,2 and Vo2 , as we as the standard molar expansibilities,Eop,2 , of GUBA and DMGUBA were calculated. The volume-hydration effects,►tr Vo2, induced by the conversion of glycoluril (GU) and N,N-dimethylglycoluril (DMGU) into the derivatives of γ-aminobutyric acid (GUBA and DMGUBA) point to squeezing of the structure of the non-methylated “hydration complex” and its loosening at N-methylation of one of glycolurilic rings of the solute under the influence of increasing temperature. It is inferred that the decisive role in the process of hydration of both GUBA and DMGUBA belongs to hydrophilic effects associated with the interactions of proton-donor/acceptor moieties of solution components as well as protons and carboxyl ions formed during the solute partial dissociation under the influence of an aqueous medium (electrostriction effect).
