Protonation and Deprotonation on Water's Surface

How the acidity of bulk water (pHbulk) regulates the degree of protonation of Brönsted acids and bases on water surfaces facing hydrophobic media is a key unresolved issue in chemistry and biology. We addressed experimentally the important case of the air/water interface and report the strikingly dissimilar pHbulk-dependences of the protonation/deprotonation of aqueous versus gaseous n-hexanoic acid (HxOH) determined on the surface of aqueous microjets by online electrospray mass spectrometry. We confirm that HxOH(aq) is deprotonated at pHbulk > pKa(HxOH) = 4.8, but find that the deprotonation of HxOH(g) into interfacial HxO-(s) displays two equivalence points at pHbulk ~ 2.5 and ~ 10.0. The weak base HxOH(aq) (pKa(HxOH2+) < - 4) is barely protonated at pHbulk > 1, whereas HxOH(g) is significantly protonated to HxOH2+(s) on pHbulk < 4 water, as expected from the proton affinities PA(HxOH) > PA(H2O) of gas-phase species. The exceptionally large kinetic isotope effect for the protonation of HxOH(g) on D2O/H2O: KIE = HxOH2+/HxODH+ ~ 100, is ascribed to a desolvated transition state. Since ion creation at the interface via proton transfer between H2O itself and neutral species is thermodynamically disallowed i.e., HxOH(g) is actually deprotonated by interfacial OH-(s), whereas Me3N(g) is hardly protonated by H3O+(s) on pHbulk ~ 4 - 8 water (Enami et al., J. Phys. Chem. Lett. 2010, 1, 1599) we conclude that [OH-(s)] > [H3O+(s)] above pHbulk ~ 4, at variance with inferences drawn from spectroscopic signatures or model calculations of water’s surface.