Molecular-dynamics simulations of interfaces between water and crystalline urea

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and ( 111) interfaces reveal strong adsorption of water while the ( 110) and (iii) interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only ( 111) on the urea growth form is predicted. We argue that on the other hand the dominance of ( 110) over (001) cannot be explained using a simple layer growth model.