Quantifying Acidity in Heterogeneous Systems: Biphasic p Values.

Acidities of lipophilic compounds, such as various ligands or catalysts, in systems consisting of an aqueous phase at equilibrium with a water-immiscible phase (lipid bilayers, phase transfer catalysis, sensor membranes, to name just few) are typically approximated by the aqueous p values. Our research shows that such approximations can lead to seriously biased estimations of the acidities as the bulk of solvated H ions reside in the aqueous phase, while the lipophilic species─both neutral acid and anion─predominantly reside in the organic phase. Therefore, the use of aqueous p in such situations is not justified.