A Kirkwood−Buff Derived Force Field for Mixtures of Urea and Water

A new nonpolarizable force field for mixtures of urea and water is described. The model was parametrized to reproduce the experimental Kirkwood−Buff integrals between urea−urea, urea−water, and water−water pairs, as defined by the Kirkwood−Buff theory of solution mixtures. It was observed that the integrals were sensitive to the charge distribution used, and that none of the literature charge distributions investigated produced the correct degree of urea association. However, a hybrid charge distribution was found which accurately reproduced the integrals over a range of concentrations. Correspondingly, the solution thermodynamics, including the activity of urea, were well described. In addition, other physical properties (density, diffusion constants, compressibility) were also well reproduced. The model displayed little or no urea self-aggregation, in agreement with the experimental data. The ideal nature of urea mixtures (molar activity scale) appeared to result from a balance between water−water and urea-water interactions, with a smaller urea−urea interaction. Although developed for use with SPC/E water, the new model performed equally well with the SPC and TIP3P water models.