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

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• 作者：Yusuke Tabe ; Nobuaki Kikkawa ; Hideaki Takahashi ; Akihiro Morita
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：January 16, 2014
• 年：2014
• 卷：118
• 期：2
• 页码：977-988
• 全文大小：454K
• ISSN：1932-7455

文摘

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 脜 thickness. One has to properly take account of the specific indicators toward deriving a general picture of surface acidity/basicity from protonation reactions.