As a result of a recent development in our group, it is now, for the first time, possible to rigorously measure acidity of acids in biphasic systems (aqueous phase at equilibrium with a water-immiscible phase) by using biphasic pKa values (pKaow values). This work has now been published in Analytical Chemistry 2022, 94, 4059–4064.
In this work, the octanol-water biphasic pKaow values have been determined for 35 acids of various structures and chemical properties (carboxylic acids, phenols, diphenylamines, imides, different CH acids) using UV-Vis and NMR (1H, 13C, or 31P) spectrometry.
Biphasic pKa values enable quantifying acid dissociation in biphasic systems in a more realistic and rigorous way than the conventional “mono-phasic” pKa values. The latter completely ignores a large part of the picture – partitioning of the neutrals and the ions between the two phases and ion-pairing in the low-polarity phase. In contrast, pKaow values account for these effects. The ratio of the acid and its conjugate base is measured in the 1-octanol phase, using UV-Vis and/or NMR spectrometric method. The activity of H+ is measured in the aqueous phase with a conventional pH-meter. The pKaow values are obtained at different concentrations and extrapolated to zero concentration.
Biphasic systems are present in many biological and technological systems and processes: cell membranes, solvent extraction, phase-transfer catalysis, sensor membranes, etc. In all such systems, acid-base properties of the participating compounds would be best described using biphasic rather than “mono-phasic” pKa values.