Microscale pH inhomogeneity in frozen NaCl solutions

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 FC...

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Bibliographic Details
Published inPhysical chemistry chemical physics : PCCP Vol. 23; no. 34; pp. 18595 - 1861
Main Authors Kataoka, Shun, Harada, Makoto, Okada, Tetsuo
Format Journal Article
LanguageEnglish
Published Cambridge Royal Society of Chemistry 14.09.2021
Subjects
Aqueous solutions
Fluorescence
Freezing
Glycerol
Glycerol-Water
Inhomogeneity
Sodium chloride
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Summary: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. Higher pH values are confirmed at the ice interface in frozen aqueous NaCl.
Bibliography:Electronic supplementary information (ESI) available: Dependence of the fluorescence intensity from the FCS on the observation depth; freezing depression curves for NaCl/water and glycerol/water; pH distributions in the FCS on the surface of frozen 20 mM NaCl; relationship between pH and fluorescence intensity in the FCS. See DOI
10.1039/d1cp01655e
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ISSN:1463-9076
1463-9084
DOI:10.1039/d1cp01655e