Surface acidity of water probed by free energy calculation for trimethylamine protonation

Surface acidity/basicity of water has been elusive, due to contradicting observations from various aspects. One effective definition has been proposed by investigating the protonation equilibrium (pK_a) of the indicator species at the water surface. We calculated the pK_a of trimethylamine at the surface by using quantum mechanics/molecular mechanics (QM/MM) and thermodynamic integration calculations and elucidated the observed pK _a shift at the surface. It was revealed that the observed pK _a shift reflects specific properties of the indicator species, particularly relative solvation free energy at the surface. The pK_a shift appears within ∼5 Å thickness. One has to properly take account of the specific indicators toward deriving a general picture of surface acidity/basicity from protonation reactions.