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Updated: Jul 3, 2026
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Published on: May 27, 2018
Structural Transformation of Hydrated Excess Protons at the Water Surface.
Mohammed Ahmed1,2, Tatsuya Ishiyama3, Satoshi Nihonyanagi1,2
1Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Journal of the American Chemical Society
|July 1, 2026
Summary
The hydration structure of excess protons at water surfaces changes with acidity. At low acidity, protons form localized structures, while high acidity leads to more delocalized proton hydration structures at the interface.
Area of Science:
- Physical Chemistry
- Surface Science
- Spectroscopy
Background:
- The microscopic hydration structure of excess protons at the air/water interface is not well understood.
- Hydrated protons in bulk water show a vibrational 'proton continuum' due to delocalized structures.
- Proton hydration at aqueous interfaces has not been previously resolved.
Purpose of the Study:
- To investigate the hydration structure of excess protons at aqueous HCl surfaces using advanced spectroscopic and simulation techniques.
- To understand how proton hydration structures change with varying HCl concentrations at the air/water interface.
Main Methods:
- Interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy.
- Ab initio molecular dynamics (AIMD) simulations.
- Studied aqueous HCl surfaces across a concentration range of 0-9 M.
Main Results:
- HD-VSFG revealed concentration-dependent changes in interfacial vibrational spectra.
- Free OH band amplitude decreased, indicating depletion of normal water molecules.
- Hydrogen-bonded OH band amplitude increased due to water molecule orientation in the electric double layer.
- A low-frequency feature evolved into a proton continuum at high HCl concentrations (>5 M).
- AIMD simulations showed proton location asymmetry decreased with increasing HCl concentration.
Conclusions:
- Interfacial proton hydration structure is dependent on HCl concentration.
- At low concentrations, protons adopt localized Eigen-like structures.
- At high concentrations, more delocalized Eigen-Zundel-Eigen-like structures become accessible.
- Provides crucial insights into proton transfer mechanisms at aqueous interfaces.
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