The influence of nanoscale size confinement on the phase behavior of molecular organic crystals.

Abstract

This thesis details the evolution of the crystallization of molecular organic compounds under nanoconfinement. Within the confines of nanoporous matrices, crystals are limited to sizes comparable to their critical sizes, where their unfavorable surface energy outweights their favorable volume energy. The central contribution of this thesis is the crystallization of glycine within nanoporous matrices. Namely, crystallization of glycine by evaporation of aqueous solutions in nanometer-scale channels of controlled-pore glass (CPG) powders and porous polystyrene-poly(dimethyl acrylamide) (p-PS-PDMA) monoliths, the latter prepared by etching polylactide (PLA) from aligned PS-PDMA-PLA triblock copolymers, preferentially results in exclusive formation of the beta polymorph, which is not observed during crystallization in bulk form under identical conditions. X-ray diffraction (XRD) reveals that the dimensions of the embedded crystals are commensurate with the pore diameter of the matrix. Beta glycine persists for at least one year in CPG and p-PS-PDMA with pore diameters less than 24 nm, but it transforms slowly to alpha glycine over several days when confined within 55 nm CPG. Moreover, variable temperature XRD reveals that beta glycine nanocrystals embedded within CPG are stable at temperatures at which bulk beta glycine ordinarily transforms to the alpha form in the bulk. XRD and differential scanning calorimetry (DSC) reveal the melting of glycine nanocrystals within CPG below the temperature at which bulk glycine melts with concomitant decomposition. The melting point depression conforms to the Gibb-Thompson equation, with the melting points decreasing with decreasing pore size. This behavior permits an estimation of the melting temperature of bulk beta glycine, which cannot be measured directly owing to its metastable nature. Collectively, these results demonstrate size-dependent polymorphism for glycine and the ability to examine certain thermal properties under nanoscale confinement that cannot be obtained in bulk form. The observation of beta glycine at nanometer-scale dimensions suggests that glycine crystallization likely involves formation of beta nuclei followed by their transformation to the other more stable forms as crystal size increases, in accord with Ostwald's rule of stages. When embedded in p-PS-PDMA, the nanocrystals also adopt preferred orientations with their fast-growth axes aligned parallel with the pore direction. When grown from aqueous solutions alone, the nanocrystals were oriented with their [010] and [0-10] axes, the native fast growth directions of the (+) and (-) enantiomorphs of beta 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 [0-10] directions of the enantiomorphs produced beta 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 beta glycine nanocrystals form by homogeneous nucleation, with minimal influence of the pore walls on orientation. Collectively, these results suggest new routes for controlling crystallization outcomes and new studies on the exploration of crystal properties on the nanometer length scale.