Kataoka Shun, Harada Makoto, Okada Tetsuo
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan.
Phys Chem Chem Phys. 2021 Sep 14;23(34):18595-18601. doi: 10.1039/d1cp01655e. Epub 2021 Aug 17.
When an aqueous solution freezes at temperatures above the eutectic point, a freeze concentrated solution (FCS) is separated from the ice phase. Reactions of environmental importance often occur in the FCS and, in some cases, are accelerated compared to those in solution conditions. The pH of the FCS is an essential factor governing the thermodynamics and kinetics of the reactions occurring therein. It is known that freezing of aqueous NaCl causes an increase in the FCS pH, which arises from the difference in the partition to the ice phase between Na and Cl. It has also been shown that H and other ions show surface-specific behaviors on ice. Although the details are not known, the ice/FCS interface can also affect the behaviors of ions. In this study, the pH distribution in the FCS is evaluated using ratiometric fluorescence microscopy, and the pH inhomogeneity is confirmed for frozen aqueous NaCl. However, interestingly, buffered solutions and frozen aqueous glycerol result in a uniform pH value. The pH in frozen NaCl is always higher near the ice/FCS interface than in the middle of the FCS vein.
当水溶液在高于共晶点的温度下冻结时，会从冰相中分离出冷冻浓缩溶液（FCS）。具有环境重要性的反应通常在FCS中发生，并且在某些情况下，与溶液条件下的反应相比会加速。FCS的pH值是控制其中发生的反应的热力学和动力学的关键因素。已知NaCl水溶液的冻结会导致FCS的pH值升高，这是由于Na和Cl在冰相中的分配差异所致。研究还表明，H和其他离子在冰上表现出表面特异性行为。尽管细节尚不清楚，但冰/FCS界面也会影响离子的行为。在本研究中，使用比率荧光显微镜评估FCS中的pH分布，并确认冷冻NaCl水溶液存在pH不均匀性。然而，有趣的是，缓冲溶液和冷冻甘油水溶液的pH值是均匀的。冷冻NaCl中靠近冰/FCS界面处的pH值总是高于FCS脉管中部的pH值。
