The Structure of a Supersaturated Solution: A Neutron Scattering Study of Aqueous Urea

In this study a combination of neutron diffraction measurements and empirical potential structure refinement (EPSR) was used to investigate the time-averaged structure of a 0.30 mol fraction solution of urea in water at both 25 and 35 °C. These two temperatures were chosen in order to compare the structure of an undersaturated solution at 35 °C with that of a supersaturated solution at 25 °C. The results indicate that in moving into the supersaturated state the solutions show a significant and unexpected change in the self-aggregation of urea molecules. Urea−urea interactions present in the undersaturated solution become suppressed in the supersaturated region, in favor of interactions between urea and water so that overall the urea becomes more hydrated. Such a change may be viewed as a response of the system to the imposed supersaturation and a factor playing an important role in the observed metastability of the solutions.