Baird James K, Lang Joshua R, Wang Xingjian, Mukherjee Anusree, Norris Pauline
Advanced Materials Institute , Western Kentucky University , 2413 Nashville Road , Bowling Green , Kentucky 42101 , United States.
J Phys Chem B. 2019 Jul 5;123(26):5545-5554. doi: 10.1021/acs.jpcb.9b02978. Epub 2019 Jun 19.
Critical effects have been reported in the case of chemical equilibria, in which the solvent is a binary liquid mixture having a critical point of solution. At atmospheric pressure and for temperatures near the critical point, the critical effect manifests itself as a divergence in the temperature derivative of the extent of reaction. For a critical mixture of isobutyric acid + water (IBA/HO) serving as the solvent, we report experimental results for three complex equilibria involving (i) parallel dissolution of aluminum oxide and manganese dioxide (involves 8 species); (ii) parallel dissolution of aluminum oxide and copper(I) oxide (involves 10 species); and (iii) dissolution of barium chromate (involves 9 species). In each case, we observe a divergence in the slope of the van't Hoff plot of the extent of reaction in the critical region. By phase rule analysis of these and all other existing data, we find that the chemical equilibrium critical effect occurs in coincidence with three thermodynamic intensive variables being fixed, where two of these are the temperature and the pressure. The slope of the van't Hoff plot in the critical region is observed to diverge toward negative infinity when the reaction is endothermic and toward positive infinity when it is exothermic. These two features are a characteristic of both homogeneous and heterogeneous equilibria and have been observed at both upper and lower critical solution temperatures. Taken together, these observations support the applicability of the universality concept to chemical equilibrium critical phenomena in binary liquid mixtures.
在化学平衡的情况下已报道了临界效应，其中溶剂是具有溶液临界点的二元液体混合物。在大气压下且温度接近临界点时，临界效应表现为反应程度的温度导数出现发散。对于用作溶剂的异丁酸 + 水（IBA/H₂O）临界混合物，我们报告了三个复杂平衡的实验结果，这三个平衡分别涉及：（i）氧化铝和二氧化锰的平行溶解（涉及8种物质）；（ii）氧化铝和氧化亚铜的平行溶解（涉及10种物质）；以及（iii）铬酸钡的溶解（涉及9种物质）。在每种情况下，我们都观察到临界区域内反应程度的范特霍夫图斜率出现发散。通过对这些以及所有其他现有数据进行相律分析，我们发现化学平衡临界效应与三个热力学强度变量被固定同时出现，其中两个是温度和压力。当反应为吸热时，临界区域内范特霍夫图的斜率向负无穷大发散；当反应为放热时，则向正无穷大发散。这两个特征是均相和非均相平衡的共同特点，并且在上下临界溶液温度下均已观察到。综合这些观察结果，支持了普遍性概念适用于二元液体混合物中的化学平衡临界现象。
