Effect of Solvent Topography and Steric Hindrance on Crystal Morphology

Effect of steric hindrance resulting from solvent topography on the resultant crystal morphology was examined via cooling crystallization of various carboxylic acids in isomeric butyl and pentyl alcohols. Our experiments show that the magnitude of hindrance is related to the degree of branching at the substituted carbon, with hindrance increasing in the order of 1° < 2° < 3° alcohols. The resulting crystals displayed a trend of low, intermediate, and high aspect ratios, corresponding to 1°, 2°, and 3° alcohols, respectively. In particular, 3° alcohols have a tendency to yield significantly different crystal morphologies, compared to 1° and 2° alcohols. Hence, the position of the hydroxyl functional group plays a major role in enhancing or limiting solute–solvent hydrogen bonding interactions and thereby influencing the resultant crystal morphology. A simple molecular model, with succinic acid as test case, was used to demonstrate the extended hydrogen bonding network and surface chemistry binding at the dominant {100} face. This molecular-level exploration of solvent–carboxyl hydrogen bonding interaction at the crystal interface helped explain observed macroscopic morphological trends.