Surface characterization of aspirin crystal planes using molecular dynamics simulations

Abstract

Molecular dynamics simulations have been implemented to study the effect of polar and non-polar solvents on the different faces of the aspirin crystal and the possible mechanisms of surface growth. We focus on the role of the H-bond in stabilizing the crystal faces and do a systematic study on the effect of the solvents at intermediate stages of the process of growth. Our results show that for both polar and non-polar solvents, the (0 1 0) surface of aspirin maintains its robustness and lattice order after equilibration, which allows for the fast deposition of additional aspirin molecules onto the existing surface. We find that the growth on this face can be mostly accomplished in a molecule-by-molecule manner, which is favored both thermodynamically and kinetically. The (0 0 1) surface shows that the molecules lose their lattice structure completely if they are not paired with other aspirin molecules before they are in contact with any solvent. For this reason, growth in this direction can only be in a layer-by-layer manner if there is any at all. Our results also show that as the unpaired (1 0 0) surface of aspirin is populated with–OH groups, their hydrophilic nature favors the interaction with a polar solvent and thus growth along this face is enhanced when contacting with ethanol. The methyl and carboxyl groups covering the unpaired (0 0 1) plane cause unfavorable interaction with the polar solvent. On the (0 1 0) plane, the interlaced arrangement of hydrophilic hydroxyl groups and hydrophobic phenyl groups stabilizes the surface structure in both polar and non-polar solvents.