Angewandte Chemie - International Edition
Angewandte Chemie - International Edition
4 years ago

Orientation-Induced Adsorption of Hydrated Protons at the Air–Water Interface

Orientation-Induced Adsorption of Hydrated Protons at the Air–Water Interface
Shavkat I. Mamatkulov, Douwe Jan Bonthuis, Roland R. Netz, Christoph Allolio
The surface tension of the air—water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air–water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid-19th century. Herein, we calculate surface tensions and ionic surface propensities at air–water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H3O+ adsorbs to the air–water interface. The adsorption is explained by the deep potential well caused by the orientation of the H3O+ dipole in the interfacial electric field, which is confirmed by ab initio simulations. Surface tensions and ionic surface propensities at air–water interfaces were calculated by molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions agree remarkably well with experiment, and of the studied ions, only H3O+ adsorbs to the air–water interface. The adsorption is explained by the deep potential well caused by the orientation of the H3O+ dipole in the interfacial electric field.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201707391

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