Proton Availability at the Air/Water Interface

The acidity of the water surface sensed by a colliding gas is determined in experiments in which the protonation of gaseous trimethylamine (TMA) on aqueous microjets is monitored by online electrospray mass spectrometry as a function of the pH of the bulk liquid (pH(BLK)). TMAH(+) signal intensities describe a titration curve whose equivalence point at pH(BLK) 3.8 is dramatically smaller than the acidity constant of trimethylammonium in bulk solution, pK(A)(TMAH(+)) = 9.8. Notably the degree of TMA protonation above pH(BLK) 4 is enhanced hundred fold by submillimolar LiCl or NaCl and weakly inhibited at larger concentrations. Protonation enhancements are associated with the onset of significant direct kinetic solvent hydrogen isotope effects. Since TMA(g) can be protonated by H2O itself only upon extensive solvent participation, we infer that H3O+ emerges at the surface of neat water below ph(BLK) 4.