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Physicochemical and Structural Properties of a Hydrophobicity/Hydrophilicity Switchable Ionic Liquid
Kota Ohkubo1, Keiya Yanagisawa1, Akio Kamimura1
1Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan.
The Journal of Physical Chemistry. B
|April 16, 2020
Summary
We developed new solubility-switchable ionic liquids (ILs) with weaker ion interactions and poor conductivity. These novel ILs show tunable water miscibility based on cation structure, enabling phase separation or aqueous solutions.
Area of Science:
- Materials Science
- Physical Chemistry
- Chemical Engineering
Background:
- Ionic liquids (ILs) are tunable solvents with unique properties.
- Controlling IL hydrophilicity is crucial for applications like separation and catalysis.
- Understanding structure-property relationships in ILs is key to designing new materials.
Purpose of the Study:
- To synthesize and characterize novel solubility-switchable ionic liquids (ILs).
- To investigate the impact of cation structure on IL physicochemical and structural properties.
- To evaluate the ion-ion interactions, conductivity, and water miscibility of the synthesized ILs.
Main Methods:
- Synthesis of glycerammonium (GA) and protected GA (PGA) cations with bis(trifluoromethanesulfonyl)amide (TFSA) anion.
- Measurement of solution density to determine interionic volumes (Vinter).
- Evaluation of ionic conductivity (σ) and self-diffusion coefficients (D).
- High-energy X-ray total scattering experiments and molecular dynamics (MD) simulations for structural analysis (radial distribution functions, G(r)).
- Analysis of atom-atom pair correlation functions from MD simulations to understand local interactions.
Main Results:
- Synthesized hydrophobic [PGA][TFSA] and hydrophilic [GA][TFSA] ILs.
- Observed weaker ion-ion interactions and significantly lower ionic conductivity and self-diffusion coefficients compared to conventional ILs.
- Structural analysis revealed comparable nearest-neighbor interactions but weaker long-range order in [PGA][TFSA] and [GA][TFSA] compared to imidazolium-based ILs.
- Identified electrostatic interactions in [PGA][TFSA] and hydrogen bonding between GA cations and TFSA anions in [GA][TFSA].
- Demonstrated tunable water miscibility: [PGA][TFSA] phase-separates from water due to weak ion-hydration, while [GA][TFSA] is miscible due to hydrogen bonding.
Conclusions:
- The designed ILs exhibit switchable solubility in water, controlled by the cation's hydrophilic/hydrophobic nature.
- The structural differences, particularly in long-range ordering and specific interactions, influence IL properties like conductivity and water miscibility.
- These findings provide insights for designing ILs with tailored properties for separation processes and other applications.
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