Cavity formation and preferential interactions in urea solutions: Dependence on urea aggregation

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Abstract
A series of models for 8 M urea solutions was investigated using molecular dynamics simulations. The models differed only in their charge distributions and displayed various degrees of urea aggregation. The relationship between urea aggregation and the thermodynamics of the solution was established using Kirkwood–Buff theory. It was observed that high urea aggregation resulted in lower predicted values for the solution activity, and that Kirkwood–Buff theory provided a sensitive test for the properties of a particular force field. The free energy for formation of repulsive cavities in the different solutions was also investigated. The free energy was more unfavorable than in pure water, but independent of the extent of urea aggregation. However, the preferential exclusion of urea from the cavities was very sensitive to the degree of urea aggregation and varied by more than an order of magnitude in response to changes in the activity derivatives. A simple explanation for these observations is presented.

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