Microscale pH inhomogeneity in frozen NaCl solutions

Area of Science:

• Environmental Chemistry
• Physical Chemistry
• Solution Chemistry

Background:

• Freeze-concentrated solutions (FCS) form during ice nucleation above the eutectic point.
• Reactions in FCS can be accelerated compared to bulk solutions, with pH being a critical factor.
• Ion partitioning and surface-specific behaviors at the ice/FCS interface influence FCS properties.

Purpose of the Study:

• To investigate the pH distribution within freeze-concentrated solutions.
• To determine if pH inhomogeneity occurs in frozen aqueous solutions.
• To understand the role of the ice/FCS interface on pH.

Main Methods:

• Ratiometric fluorescence microscopy was employed to measure pH distribution.
• Frozen aqueous solutions of NaCl, buffered solutions, and glycerol were analyzed.
• Comparison of pH near the ice/FCS interface versus the bulk FCS.

Main Results:

• pH inhomogeneity was confirmed in frozen aqueous NaCl solutions.
• Buffered solutions and frozen aqueous glycerol exhibited uniform pH.
• A consistent finding was higher pH near the ice/FCS interface in frozen NaCl.

Conclusions:

• The pH of freeze-concentrated solutions is not always uniform.
• NaCl freezing leads to interfacial pH elevation, unlike buffered or glycerol solutions.
• Understanding interfacial pH is crucial for predicting reaction kinetics in frozen environments.