Coupled Orientational Disorder and Hydrogen-Bond Destabilization Drive Anisotropic Surface Melting in Curcumin Polymorphs

Polymorph-dependent surface stability influences the performance of molecular crystals, yet the microscopic origins of anisotropic interfacial softening remain poorly understood. We employ layer-resolved molecular dynamics simulations to investigate the temperature evolution of the free surfaces of the three curcumin polymorphs along their low-index crystallographic faces. Substantial structural rearrangements emerge below the bulk melting temperature, consistent with the onset of thermally activated interfacial softening. The onset temperature and the magnitude of the response strongly depend on which face is exposed. Early softening is associated with the loss of orientational order, π-π stacking, and population of short hydrogen-bonded configurations. Faces enriched in peripheral polar and conformationally flexible moieties tend to soften earlier, whereas surfaces exposing the planar β-ketoenol backbone retain structure to higher temperature. Conformational redistribution further modulates this response. These findings show that anisotropic interfacial softening in curcumin is governed by face-dependent coupling between molecular orientation, intermolecular packing, hydrogen bonding, and conformational flexibility.