Small-angle neutron scattering and volumetric studies of dilute solutions of N,N′-dimethylethyleneurea in heavy water

Small-angle neutron scattering experiments and density measurements were carried out on dilute solutions of N,N′-dimethylethyleneurea (DMEU) in heavy water in the aquamolality ranges 0.28-2.92 and 0.28-4.17, respectively, at 298.15 and 313.15 K. The results obtained for DMEU, the cyclic analogue of tetramethylurea (TMU), were compared with those reported for TMU solutions in the literature. Whereas in the aqueous solution of TMU the hydrophobic interaction increases with increasing temperature, the second osmotic virial coefficient obtained from the concentration dependence of the forward scattering intensities in the DMEU solution is temperature independent. In contrast with the TMU solution, the radius of gyration in the DMEU solution appears to be constant below the aquamolality of about 2. The molal volume second virial coefficient (Vxx), calculated from the densities of the solutions, is negative, which indicates that DMEU has a structure-making effect on water. In the TMU solution the value of Vxx was found to be more negative, which suggests that the water molecules are more structured in the hydration sphere of TMU than in that of DMEU molecule. The results of both the SANS and the volumetric studies on dilute solutions of DMEU lead to the conclusion that the pair-wise solute-solute interactions are weaker in DMEU than in TMU solutions.