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Ion Hydration and Ion Pairing as Probed by THz Spectroscopy
Gerhard Schwaab1, Federico Sebastiani1, Martina Havenith1
1Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780, Bochum, Germany.
Angewandte Chemie (International Ed. in English)
|July 20, 2018
Summary
Terahertz-Fourier Transform Infrared (THz-FTIR) spectroscopy reveals ion hydration dynamics. This method quantifies ion-cation cooperativity and hydration shell size, challenging the Hofmeister model.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Solution Chemistry
Background:
- Ion hydration is crucial for numerous chemical and biological processes.
- Understanding ion-water interactions requires studying hydration dynamics.
- Existing spectroscopic methods face limitations in characterizing these dynamics.
Purpose of the Study:
- To introduce and validate Terahertz-Fourier Transform Infrared (THz-FTIR) spectroscopy for studying ion hydration.
- To elucidate the dynamics of ions within their hydration shells.
- To investigate anion-cation interactions and their influence on hydration.
Main Methods:
- Utilized THz-FTIR spectroscopy to analyze the vibrational modes of ions in solution.
- Dissected THz spectra to identify characteristic absorption features.
- Quantified parameters such as hydration shell size, ion-water mode lifetimes, and anion-cation cooperativity.
Main Results:
- Identified distinct spectral features corresponding to the rattling modes of strongly hydrating ions and charge fluctuations of weakly hydrating ions.
- Determined the size of the dynamic hydration shell and the lifetimes of collective ion-hydration water modes.
- Provided evidence for non-additive ion behavior, questioning the simplified Hofmeister model.
- Observed and quantified ion pairing at high salt concentrations.
Conclusions:
- THz-FTIR spectroscopy is a powerful tool for probing ion hydration dynamics at a molecular level.
- The study reveals complex ion-water interactions and collective dynamics.
- Findings challenge established models like the Hofmeister series and highlight the importance of considering non-additive effects.
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