Solvent fluctuations in the solvation shell determine the activation barrier for crystal growth rates
Solvent fluctuations in the solvation shell determine the activation barrier for crystal growth rates
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DOI：
10.1073/pnas.1910691116
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发表时间：
2019-11-26
影响因子：
11.1
通讯作者：
Singh, Meenesh R.
中科院分区：
文献类型：
--
作者：
Dighe, Anish V.;Singh, Meenesh R.
溶液结晶是生长先进功能晶体材料的常用技术。已知过饱和度、温度和溶剂组合物会影响晶体材料的生长速率，从而影响晶体材料的性质;然而，这些因素如何影响生长速率的活化势垒尚未得到令人满意的解释。我们在这里报告说，这些影响可以归因于一个以前未被认识到的后果，在溶剂化壳的溶质分子附着到晶体表面的溶剂波动。随着过饱和度的增加，谷氨酸分子的平均水合数降低，并可达到对应于分子上吸附位点数目的渐近极限。谷氨酸分子的水合数也波动，由于溶剂化壳层中溶剂的快速交换和局部过饱和度的变化。这些快速的波动允许分子的完全溶剂化和部分去溶剂化状态之间的准平衡，这可以用于构建双阱势，从而识别过渡态和所需的激活势垒。部分去溶剂化的分子是不稳定的，并且可以自发地附着到晶体表面。活化能垒与水合数遵循埃文斯-波兰尼关系。预测的α-谷氨酸晶体在较低的过饱和度的绝对生长速率是在合理的协议与实验观察。
Solution crystallization is a common technique to grow advanced, functional crystalline materials. Supersaturation, temperature, and solvent composition are known to influence the growth rates and thereby properties of crystalline materials; however, a satisfactory explanation of how these factors affect the activation barrier for growth rates has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of solvent fluctuations in the solvation shell of solute molecules attaching to the crystal surface. With increasing supersaturation, the average hydration number of the glutamic acid molecule decreases and can reach an asymptotic limit corresponding to the number of adsorption sites on the molecule. The hydration number of the glutamic acid molecule also fluctuates due to the rapid exchange of solvent in the solvation shell and local variation in the supersaturation. These rapid fluctuations allow quasi-equilibrium between fully solvated and partially desolvated states of molecules, which can be used to construct a double-well potential and thereby to identify the transition state and the required activation barrier. The partially desolvated molecules are not stable and can attach spontaneously to the crystal surface. The activation barrier versus hydration number follows the Evans-Polanyi relation. The predicted absolute growth rates of the alpha-glutamic acid crystal at lower super-saturations are in reasonable agreement with the experimental observations.