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首页> 外文期刊>Acta crystallographica.Section D. Biological crystallography >An accurate numerical model for calculating the equilibration rate of a hanging-drop experiment.
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An accurate numerical model for calculating the equilibration rate of a hanging-drop experiment.

机译：一个精确的数值计算模型平衡悬滴实验。

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A numerical model of the equilibration of a hanging-drop experiment has been developed and tested. To obtain accurate calculations with a given precipitant, the vapor pressure of water over water/precipitant solutions must be known for various concentrations of the precipitant. The calculations of the model are in excellent agreement with all available experimental data on hanging-drop equilibration when the necessary vapor pressures are known (ammonium sulfate and sodium chloride). By varying each of the relevant rate constants in the model, the rate-limiting step in the equilibration of a hanging drop is determined. This analysis clearly shows that the rate-limiting step is the diffusion of water vapor from the drop to the reservoir, which agrees with experimental findings. Since the diffusion of water vapor is the rate-limiting step, there is virtually no precipitant concentration gradient in the drop during equilibration. As a result, there is no gravity-induced convection owing to the equilibration. Thus, whereas gravity might have an effect during crystal growth, gravity does not affect the equilibration rate of a hanging-drop experiment to a significant extent, and the diffusion of water vapor will remain the rate-limiting step in the absence of gravity. Finally, the effects of several of the parameters, such as initial drop volume, drop-to-reservoir distance and temperature, are considered quantitatively. The equilibration rate was found to vary nearly linearly with drop volume. The equilibration rate decreases roughly by a factor of three as the temperature decreases from 293 to 276 K. This decrease in the equilibration rate is greater than would be expected when just considering the change in the diffusion coefficient of water vapor in air. This large dependence can, however, be attributed to the change in water-vapor pressure. Most surprisingly, a linear dependence on drop-to-reservoir distance is found, a result that agrees very well with experiment.
机译：一个数值模型的平衡悬滴已经开发和实验测试。鉴于沉淀剂,水的蒸汽压在水/沉淀剂必须是已知的解决方案对于不同的沉淀剂的浓度。计算模型的优秀协议和所有可用的实验数据悬滴平衡时必要的是已知的(硫酸铵和蒸汽压力氯化钠)。速率常数模型中,病原一步的平衡悬滴确定。病原反应步骤是水的扩散蒸汽从下水库,同意实验结果。水蒸气的扩散是病原一步,几乎没有沉淀剂浓度梯度在下降平衡。gravity-induced对流由于平衡。不影响在晶体生长过程中,重力影响悬滴的平衡率实验在很大程度上,水蒸气的扩散将保持病原反应一步失重。最后,几位的影响参数,比如初始体积下降,drop-to-reservoir距离和温度认为定量。发现不同的近线性下降体积。随着温度的下降是由三个因素造成的从293年到276 K。平衡率大于时只是考虑的变化在空气中水蒸气的扩散系数。然而,大的依赖可以归因于水蒸气压力的变化。令人惊讶的是,一个线性依赖找到drop-to-reservoir距离,结果很同意与实验。

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