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首页> 外文期刊>Crystal growth & design >Does the gamma Polymorph of Glycine Nucleate Faster? A Quantitative Study of Nucleation from Aqueous Solution
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Does the gamma Polymorph of Glycine Nucleate Faster? A Quantitative Study of Nucleation from Aqueous Solution

机译：甘氨酸的γ多晶型物能更快地成核吗？水溶液中成核的定量研究

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We quantitatively study the crystallization of glycine from solution by following the crystallization of a plate with 96 wells, each with 0.1 mL of supersaturated solution. Our first aim is to address the difficult problem of obtaining nucleation data that is reproducible. This problem is difficult because of the extreme sensitivity of nucleation times. Nucleation is sensitive to factors that include how the crystallizing system is prepared and even small (1%) variations in the supersaturation. We discuss the appropriate statistical tests needed to show reproducibility. Our second aim is to study the competition between the nucleation of the alpha and gamma polymorphs of glycine. We find that nucleation appears to be heterogeneous: Some samples crystallize in minutes, whereas others do not crystallize after days, which indicates that there is no well-defined nucleation rate for the set of samples. Homogeneous nucleation gives a well-defined rate. Those samples that crystallize in minutes mostly yield the metastable alpha polymorph. We speculate that these crystals may be the result of seed crystals formed by transient local increases in supersaturation in pipette tips during sample preparation. However, those that crystallize in hours are largely the equilibrium gamma polymorph. This is perhaps surprising because typically the alpha polymorph is obtained from crystallization from aqueous solutions near neutral pH. We speculate that the nucleation rate of the gamma polymorph may be higher than that of the alpha form but that in earlier work with larger solution volumes (increasing the probability of seeding) and with stirring the alpha polymorph dominates because of its much faster growth rate.
机译：我们通过跟随96孔板的结晶来定量研究甘氨酸从溶液中的结晶，每个板有0.1 mL过饱和溶液。我们的首要目标是解决获得可再现的成核数据的难题。由于成核时间的极度敏感性，这个问题很难解决。成核对各种因素敏感，这些因素包括如何制备结晶系统，以及过饱和中的微小变化（1％）。我们讨论了显示重现性所需的适当统计检验。我们的第二个目标是研究甘氨酸的α和γ多晶型的成核之间的竞争。我们发现成核似乎是异质的：一些样品在数分钟内结晶，而另一些样品在数天后不结晶，这表明该组样品没有明确的成核速率。均匀成核的速率很明确。在几分钟内结晶的那些样品大多数产生亚稳态的α多晶型物。我们推测这些晶体可能是在样品制备过程中移液器吸头中过饱和的瞬时局部增加所形成的晶种的结果。但是，在几小时内结晶的那些大部分是平衡的γ多晶型物。这可能令人惊讶，因为通常α多晶型物是从接近中性pH的水溶液中结晶而获得的。我们推测，γ多晶型物的成核速率可能高于α晶型，但在较早的工作中，溶液体积较大（增加了晶种的可能性），并且在搅拌下，由于α多晶型物的生长速度快，因此它占主导地位。

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