The Effect of Urea on the Structure of Water: A Molecular Dynamics Simulation

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.