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.

Validation and extension of the gas-phase superacidity scale.

Rationale: In recent years it has become increasingly evident that the previously reported experimental gas-phase acidity (GA) values of several strong acids differ markedly from the corresponding high-level computational values. In this work, the superacidic part of the current gas-phase acidity scale was validated and extended.

Methods: For that, the strongly acidic section of the gas-phase acidity scale was remeasured using the equilibrium Fourier transform ion cyclotron resonance (FTICR-MS) method, adding new compounds and introducing methodological changes.