Comment on “Surface Acidity of Water Probed by Free Energy Calculation for Trimethylamine Protonation

Abstract

Tabe et al. recently argued (on the basis of calculations) that the protonation of trimethylamine (TMA) on the surface of pH ≤ 4 water^2 reflects that TMA is a much weaker base at the surface than in bulk water (pK_(a,TMA) = 9.8) and conjectured that “if another indicator species (X) were used which is less stabilized at the surface than its protonated form (XH+), we would expect an apparently opposite shift of pK_a”. It appears that Tabe et al. have overlooked the experiments we had performed using indicator species covering a wide range of acidity constants, which clearly fail to support their prediction.