Bowler Michael G, Bowler David R, Bowler Matthew W
Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK.
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
J Appl Crystallogr. 2017 Mar 29;50(Pt 2):631-638. doi: 10.1107/S1600576717003636. eCollection 2017 Apr 1.
The humidity surrounding a sample is an important variable in scientific experiments. Biological samples in particular require not just a humid atmosphere but often a relative humidity (RH) that is in equilibrium with a stabilizing solution required to maintain the sample in the same state during measurements. The controlled dehydration of macromolecular crystals can lead to significant increases in crystal order, leading to higher diffraction quality. Devices that can accurately control the humidity surrounding crystals while monitoring diffraction have led to this technique being increasingly adopted, as the experiments become easier and more reproducible. Matching the RH to the mother liquor is the first step in allowing the stable mounting of a crystal. In previous work [Wheeler, Russi, Bowler & Bowler (2012). F, 111-114], the equilibrium RHs were measured for a range of concentrations of the most commonly used precipitants in macromolecular crystallography and it was shown how these related to Raoult's law for the equilibrium vapour pressure of water above a solution. However, a discrepancy between the measured values and those predicted by theory could not be explained. Here, a more precise humidity control device has been used to determine equilibrium RH points. The new results are in agreement with Raoult's law. A simple argument in statistical mechanics is also presented, demonstrating that the equilibrium vapour pressure of a solvent is proportional to its mole fraction in an ideal solution: Raoult's law. The same argument can be extended to the case where the solvent and solute molecules are of different sizes, as is the case with polymers. The results provide a framework for the correct maintenance of the RH surrounding a sample.
样品周围的湿度是科学实验中的一个重要变量。特别是生物样品，不仅需要潮湿的环境，而且通常需要相对湿度（RH）与一种稳定溶液达到平衡，这种稳定溶液是在测量过程中保持样品处于相同状态所必需的。大分子晶体的可控脱水可导致晶体有序度显著增加，从而提高衍射质量。随着实验变得更加简便且可重复，能够在监测衍射的同时精确控制晶体周围湿度的设备使得这项技术越来越受到采用。使相对湿度与母液相匹配是实现晶体稳定安装的第一步。在之前的工作中[惠勒、鲁西、鲍勒和鲍勒（2012年）。F，111 - 114页]，测量了大分子晶体学中一系列最常用沉淀剂浓度下的平衡相对湿度，并展示了这些湿度如何与溶液上方水的平衡蒸气压的拉乌尔定律相关。然而，测量值与理论预测值之间的差异无法得到解释。在此，使用了一种更精确的湿度控制设备来确定平衡相对湿度点。新的结果与拉乌尔定律相符。还给出了一个统计力学中的简单论证，证明了溶剂的平衡蒸气压与其在理想溶液中的摩尔分数成正比：即拉乌尔定律。同样的论证可以扩展到溶剂和溶质分子大小不同的情况，就像聚合物的情况一样。这些结果为正确维持样品周围的相对湿度提供了一个框架。
