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Orientation‐Induced Adsorption of Hydrated Protons at the Air–Water Interface
Author(s) -
Mamatkulov Shavkat I.,
Allolio Christoph,
Netz Roland R.,
Bonthuis Douwe Jan
Publication year - 2017
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201707391
Subject(s) - adsorption , surface tension , ionic bonding , electrokinetic phenomena , chemical physics , dipole , chemistry , ion , molecular dynamics , surface charge , computational chemistry , thermodynamics , organic chemistry , physics
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 H 3 O + adsorbs to the air–water interface. The adsorption is explained by the deep potential well caused by the orientation of the H 3 O + dipole in the interfacial electric field, which is confirmed by ab initio simulations.