Mechanism of Urea Crystal Dissolution in Water from Molecular Dynamics Simulation

Molecular dynamics simulations are used to determine the mechanism of urea crystal dissolution in water under sink conditions. Crystals of cubic and tablet shapes are considered, and results are reported for four commonly used water models. The dissolution rates for different water models can differ considerably, but the overall dissolution mechanism remains the same. Urea dissolution occurs in three stages: a relatively fast initial stage, a slower intermediate stage, and a final stage. We show that the long intermediate stage is well described by classical rate laws, which assume that the dissolution rate is proportional to the active surface area. By carrying out simulations at different temperatures, we show that urea dissolution is an activated process, with an activation energy of ∼32 kJ mol^-1. Our simulations give no indication of a significant diffusion layer, and we conclude that the detachment of molecules from the crystal is the rate-determining step for dissolution. The results we report for urea are consistent with earlier observations for the dissolution of NaCl crystals. This suggests that the three-stage mechanism and classical rate laws might apply to the dissolution of other ionic and molecular crystals.