The riddle of resorcinol crystal growth revisited:: Molecular dynamics simulations of α-resorcinol crystal-water interface

The mechanism by which solvent exerts its effect during the process of crystallization is poorly understood. An important and ongoing problem is the uneven growth of the faces {011} and {01 (1) over bar} of a-resorcinol in water. Growth occurs mainly at the{0 (1) over bar (1) over bar} surface. In an attempt to determine the mechanism, molecular-dynamics simulations have been carried out of the two surfaces in contact with water. The dynamical properties of the water close to the surface as well as the overall interaction energies of the water with the respective faces have been calculated. The strongest water-binding sites have also been determined and energetically characterized. The data indicate that the adsorption of water molecules is stronger at the slower growing {011} face, with the strongest binding occurring at specific sites on this face. The motion of the water molecules in the surface layer at this face is also more localized and restricted compared with that at the faster growing {0 (1) over bar (1) over bar} face. The binding sites at the {011} surface are not within the grooves that are present at this surface but are located above the outermost part of the crystal surface. The water molecules form strong hydrogen bonds with the limited number of hydroxyl oxygens of the resorcinol molecules protruding from the surface. The overall inference is that the stronger binding of the water molecules at the {011} surface serves to retard crystal growth, rather than enhancing it as predicted by the surface roughening theory.