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 (pKa) of the indicator species at the water surface. We calculated the pKa 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 pKa 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films