Chemical Phenomena at the Critical Point of Solution

The principle of critical point universality is known to govern critical effects in physical phenomena such as superconductivity, superfluidity, and the liquid – vapor transition. Using a binary liquid mixture with a critical point of solution, we have shown that critical effects can also be observed in chemical phenomena as diverse as solubility, adsorption, and ion exchange. At fixed temperature and pressure, chemical critical effects occur when no more than one composition variable is held fixed. The assertion that only three variables are fixed, namely temperature, pressure, and one composition variable, is affirmed by application of the phase rule. In the case of three fixed variables, critical effects have been observed in more than 20 different experiments involving a wide variety of solids and solvent pairs.¹ Recently, we have added to this tally by observing a critical effect in the solubility of the optically active R-enantiomer of zinc tartrate dissolving in a critical mixture of isobutyric acid + water.
¹J. K. Baird, J. R. Lang, X. Wang, A. Mukherjee, and P. Norris, J. Phys. Chem. B 123, 5545 – 5554 (2019).