Hamilton Benjamin D, Weissbuch Isabelle, Lahav Meir, Hillmyer Marc A, Ward Michael D
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Am Chem Soc. 2009 Feb 25;131(7):2588-96. doi: 10.1021/ja807193s.
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 beta-polymorph, with crystal size comparable with the 22 nm pore diameter, in contrast to the formation of alpha-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 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 [010] 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.
通过吸涨水溶液的蒸发在纳米多孔聚苯乙烯 - 聚（二甲基丙烯酰胺）整体材料的排列纳米级圆柱孔中生长的甘氨酸纳米晶体，其快速生长轴与孔方向平行排列，呈现出择优取向。X射线衍射分析表明，与在没有纳米尺度限制时形成α - 甘氨酸相反，在这种情况下排他性地形成了亚稳的β - 多晶型，晶体尺寸与22纳米的孔径相当。当仅从水溶液中生长时，纳米晶体的[010]和[010]轴（分别为β - 甘氨酸的（ + ）和（ - ）对映体的天然快速生长方向）与孔方向平行排列。相比之下，在手性助剂的外消旋混合物存在下结晶，已知这些助剂会抑制沿对映体的[010]和[010]方向生长，从而产生β - 甘氨酸纳米晶体，其[001]轴几乎与孔方向平行。仅抑制其中一种对映体沿天然快速生长方向结晶的对映体纯助剂，会使另一种对映体以[010]轴平行于圆柱体的方式生长。总体而言，这些结果表明晶体生长的方式是，可通过添加手性助剂改变的快速生长方向与孔方向平行。这种行为可归因于由于临界尺寸效应，不同排列的晶体之间的竞争、特定晶面表面能的最小化，以及当晶体以其快速生长轴不受孔壁阻碍的方式生长时，与过饱和条件相关的过量自由能更有效地降低。这些观察结果表明β - 甘氨酸纳米晶体通过均匀成核形成，孔壁对取向的影响最小。
