Protic Ionic Liquid-H₂O Mixtures─Structure, Interactions, and Structure-Property Relationships.

Area of Science:

• Physical Chemistry
• Materials Science

Background:

• Protic ionic liquids (PILs) are versatile solvents with tunable properties.
• Water's role in PILs can range from impurity to essential co-solvent, significantly impacting mixture behavior.
• Accurate structure-property relationship models are crucial for optimizing PIL applications.

Purpose of the Study:

• To investigate the physicochemical properties of low-toxic alkanolammonium- and carboxylate-based PIL-water mixtures.
• To elucidate the molecular-level distribution and interactions of water within PIL structures.
• To determine critical aggregation concentrations (CAC) and understand their dependence on mixture composition.

Main Methods:

• Experimental determination of density, viscosity, and electrical conductivity.
• Calculation of derived properties such as thermal expansion coefficients and excess molar volumes.
• Utilized 1D and 2D nuclear magnetic resonance (NMR) spectroscopy to probe water distribution.
• Determined CAC using physicochemical measurements and 1H longitudinal relaxation times.

Main Results:

• Water-PIL interactions are primarily governed by the PIL's anion, influencing interaction strength.
• The PIL's cation dictates the spatial arrangement and location of water molecules within the solvent network.
• Physicochemical properties and NMR data revealed distinct behaviors of PIL-water mixtures.
• Critical aggregation concentrations were successfully determined, correlating with interaction strengths.

Conclusions:

• The study provides a detailed molecular understanding of protic ionic liquid-water mixtures.
• Findings enhance the ability to predict properties and optimize the design of PIL-based systems.
• Highlights the importance of considering water's role in PIL applications and development.