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Understanding ionic mesophase stabilization by hydration: a solid-state NMR study
Debashis Majhi1, Jing Dai1, Andrei V Komolkin2
1Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. sergeid@kth.se.
Physical Chemistry Chemical Physics : PCCP
|June 9, 2020
Summary
Water enhances ionic liquid crystal stability by strengthening hydrogen bonds, despite reducing molecular order. This hydration effect increases ion diffusion anisotropy, crucial for layered structures.
Area of Science:
- Materials Science
- Physical Chemistry
- Supramolecular Chemistry
Background:
- Ionic liquid crystals (ILCs) exhibit unique mesophases influenced by ion interactions and molecular ordering.
- Understanding the role of hydration in ILCs is crucial for tailoring their properties and applications.
- Previous studies have explored ILC structure-property relationships, but the specific impact of water on hydrogen bonding and dynamics remains less understood.
Purpose of the Study:
- To investigate the correlation between water's hydrogen bonding contribution, mesophase order, and ion diffusion in layered ILCs.
- To elucidate the effects of hydration on hydrogen bonding, molecular dynamics, and orientational order in imidazolium-based ILCs.
- To determine the influence of water on the stability and properties of the smectic mesophase.
Main Methods:
- Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy to monitor changes in hydrogen bonding, conformational dynamics, and translational diffusion.
- Density Functional Theory (DFT) analysis to complement experimental findings and provide insights into molecular interactions.
- Investigation of monohydrated and anhydrous imidazolium-based ionic liquids across varying temperatures.
Main Results:
- Monohydrated ILCs exhibited enhanced smectic mesophase stability over a wider temperature range compared to anhydrous counterparts.
- Hydration counterintuitively decreased the orientational order of organic cations, reducing the contribution of cation alignment and dispersion forces to mesophase stability.
- Increased anisotropy in translational diffusion was observed in hydrated samples, supporting a layer-stabilizing effect of water, outweighing the decrease in molecular order.
Conclusions:
- Water plays a critical role in stabilizing layered ionic liquid crystalline phases through hydrogen bonding, even with reduced molecular order.
- The increased interaction energy from water's hydrogen bonding within the ionic sublayer is the dominant factor in mesophase stabilization.
- Hydration significantly influences the delicate balance between molecular order, hydrogen bonding, and ion dynamics in ILCs.
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