Concentrated aqueous urea solutions: A molecular dynamics study of different models

Molecular dynamics studies of concentrated aqueous urea solutions, with urea mole fraction ranging from 0.04 to 0.25, have been performed, essentially focussed on the study of the tendency of urea to self-aggregate. Four of the existing urea models have been studied herein, with number of particles of $\text{N=256}$ and $\text{N=864.}$ It was generally found that unusually long MD runs were required in order to reach proper thermodynamic equilibrium, and particularly so for the $\text{N=256}$ systems. This slow convergence of the trajectories is related here to the urea self-aggregation tendency that appears in our calculation not to be limited to pairing processes only. Moreover, no evidence for urea dimerization has been found in the present calculations. Evidence for urea self-aggregation is the most visible in the structural properties, particularly in the way density–density correlations decay to their unit asymptotic limit. Densities, energies, diffusion constants, and structural data have been calculated for all models, and compared with corresponding experimental data. This comparison allows a classification of the different models in terms of their relative agreements with experimental data. Urea aggregation appears to be quite different for different models.