Nucleation of urea from aqueous solution: Structure, critical size, and rate.

Using fully atomistic simulations, we find that the structure of the critical urea crystal nucleus (monoclinic, four molecules per unit cell) in an aqueous solution differs from the known crystal structure of bulk urea (orthorhombic, two molecules per unit cell). Following a frequently used "seeding technique" combined with the classical nucleation theory, we also find that at room temperature the critical nucleus is very large (containing ∼530 molecules) and the nucleation rate is very slow (∼5×10-24cm-3s-1), suggesting that the homogeneous nucleation of urea is improbable at room temperature.