Manipulating crystal orientation in nanoscale cylindrical pores by stereochemical inhibition

Glycine nanocrystals, grown in aligned nanometer-scale cylindrical pores of nanoporous polystyrene-poly(dimethyl acrylamide) monoliths by evaporation of imbibed aqueous solutions, adopt preferred orientations with their fast-growth axes aligned parallel with the pore direction. X-ray diffraction analysis revealed the exclusive formation of the metastable β-polymorph, with crystal size comparable with the 22 nm pore diameter, in contrast to the formation of a-glycine in the absence of nanoscale confinement. When grown from aqueous solutions alone, the nanocrystals were oriented with their [010] and [010] axes, the native fast growth directions of the (+) and (-) enantiomorphs of β-glycine, respectively, aligned parallel with the pore direction. In contrast, crystallization in the presence of racemic mixtures of chiral auxiliaries known to inhibit growth along the [010] and [010] directions of the enantiomorphs produced β-glycine nanocrystals with their [001] axes nearly parallel to the pore direction. Enantiopure auxiliaries that inhibit crystallization along the native fast growth direction of only one of the enantiomorphs allow the other enantiomorph to grow with the [010] axis parallel to the cylinder. Collectively, these results demonstrate that crystal growth occurs such that the fast-growing direction, which can be altered by adding chiral auxiliaries, is parallel to the pore direction. This behavior can be attributed to a competition between differently aligned crystals due to critical size effects, the minimization of the surface energy of specific crystal planes, and a more effective reduction of the excess free energy associated with supersaturated conditions when the crystal grows with its fast-growth axis unimpeded by pore walls. These observations suggest that the β-glycine nanocrystals form by homogeneous nucleation, with minimal influence of the pore walls on orientation.