Effect of differential surface anisotropy on performance of two plate shaped crystals of aspirin form I.

Differential surface anisotropy of different crystals of the same API can have a significant impact on their pharmaceutical performance. The present work investigated the impact of differential surface anisotropy of two plate-shaped crystals of aspirin (form I) on their hygroscopicity, stability and compaction behavior. These crystals differed in their predominant facets (100) and (001) and were coded as AE-100 & E-001. (100) facets exposed polar carbonyl groups which provided hydrophilicity to the facets. In contrast, (001) facets possessed hydrophobicity as they exposed non-polar aryl and methyl groups. Both the samples showed different degradation behavior, at various stability conditions (i.e. 40°C/75%RH, 30°C/90%RH and 30°C/60%RH) and different time intervals. Polar groups of aspirin have been reported to be prone to hydrolysis due to which AE-100 was less stable than E-001. Dynamic vapor sorption (DVS) analysis at different simulated stability conditions also supported this observation, wherein AE-100 showed higher moisture sorption than E-001. Both the samples having similar particle size, shape, surface area and hardness value, showed differences in their compactibility. However, milling narrowed down the predominance of facets and both the milled samples showed similar stability and compaction behavior. This study was also supported by surface free energy determination, molecular modeling and face indexation of unmilled and milled samples.