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Probing intermolecular interactions in water/ionic liquid mixtures by far-infrared spectroscopy
Ana Dominguez-Vidal1, Nina Kaun, Maria Jose Ayora-Cañada
1Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164AC, A-1060 Wien, Austria.
The Journal of Physical Chemistry. B
|April 6, 2007
Summary
Far-infrared spectroscopy reveals how water interacts with ionic liquids. Hydrophilic ionic liquids show stronger water-anion interactions, while hydrophobic ones exhibit weaker interactions and no water dimer formation.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Ionic Liquids
Background:
- Ionic liquids (ILs) are salts that are liquid below 100°C, composed entirely of ions.
- Water interactions with ILs are crucial for understanding their properties and applications.
- Far-infrared (FIR) spectroscopy is a powerful tool for probing molecular vibrations and interactions.
Purpose of the Study:
- To investigate the interactions between water and different types of ionic liquids using FIR spectroscopy.
- To analyze the effect of anion type on water molecule behavior in ILs.
- To identify and quantify different species formed in IL-water mixtures.
Main Methods:
- Far-infrared (FIR) spectroscopy was employed to study hydrophilic (1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate) and hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate) ionic liquids.
- Mixtures of ILs with water at various concentrations were analyzed.
- Multivariate curve resolution (MCR) was applied to spectral data, particularly in the 50–350 cm-1 range, to resolve overlapping bands and identify distinct spectral contributions.
Main Results:
- Shifts in the librational water bands were observed, correlating with anion type and interaction strength.
- Hydrophilic ILs showed a librational band around 460 cm-1, while the hydrophobic IL exhibited a band at 388 cm-1, indicating less restricted water rotation.
- MCR resolved a band at 153 cm-1 into contributions from water dimers (202 cm-1) and hindered translation due to water-BF4- interactions (148 cm-1).
- Water dimer formation was absent in the hydrophobic IL, and water-PF6- interactions were extremely weak.
Conclusions:
- The nature of the anion significantly influences water molecule behavior and interactions within ionic liquids.
- Hydrophilic ILs (e.g., with BF4-) exhibit stronger hydrogen bonding between water and anions, leading to hindered translation of water.
- Hydrophobic ILs (e.g., with PF6-) show weaker interactions with water, allowing for more free rotation and preventing water dimer formation.
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