This paper reports an analysis using molecular dynamics simulations of the effect of urea on the structure of water. Two definitions of the tetrahedral distributions are used to quantify this effect. The first one is sensitive to the mutual orientation between a reference water molecule and the water molecules forming the tetrahedron, and the second is sensitive to their radial distribution. The analysis shows that increasing urea mole fraction results in a reduction of the structured tetrahedral arrangement contribution in favor of an unstructured one. In order to understand this behavior, we used the nearest neighbor approach which allows us to get unambiguous information on the radial and orientation distributions of the water molecules around a probe one. The results indicate that the decrease of the tetrahedral arrangement of the nearest neighbors around a probe water molecule is associated with both the increase of the fluctuation in their radial distances as well as with the loss of their mutual orientations with respect to those observed in pure water. The tetrahedral distribution of water in the hydration shell of urea as well as that around its carbonyl and amine groups is also discussed.

Molecular dynamics simulation of the aqueous solutions of urea of seven different concentrations (including neat water as a reference system) has been performed on the isothermal-isobaric (N,p,T) ensemble. The ability of the urea molecules of self-association is investigated by means of the method of Voronoi polyhedra. For this purpose, all the analyses are repeated by removing one of the two components from the sample configurations and considering only the other one. In this way, the analysis of self-aggregation is reduced to the analysis of voids, a problem that can routinely be addressed by means of Voronoi analysis. The obtained results show that the urea molecules exhibit self-association behavior, which is found to be the strongest at the urea mole fraction of 0.23. However, the size of these urea aggregates is found to be rather limited; on average, they are built up by 3-4 molecules, and never exceed the size of 20-25 molecules.

The universal critical-scattering function ͑CSF͒ has been measured on a CO 2 sample at critical density by small-angle neutron scattering. The experiment was designed and conducted in such a way as to reach an accuracy of 0.3% in the determination of the CSF in the large-Q range. Several approximate equations proposed in the literature are used to model the results, and their validity is discussed, but the main purpose of this work was to produce accurate data in the region of the crossover between the Ornstein-Zernike and critical regimes. ͓S0163-1829͑98͒06042-1͔

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