Alkylphosphonium carboxylate ionic liquids with tuned microscopic structures and properties

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Ionic liquids (ILs) are salts with low melting points, widely explored for their unique properties.
• Phosphonium-based ILs offer an alternative to imidazolium ILs, with potential for diverse applications.
• Understanding the structure-property relationships of ILs is crucial for designing new materials.

Purpose of the Study:

• To synthesize and characterize a series of pure alkylphosphonium carboxylate ionic liquids.
• To investigate the physico-chemical properties, structure, and thermal stability of these novel ILs.
• To explore the potential of phosphonium ILs as alternatives to conventional ionic liquids.

Main Methods:

• Synthesis of eleven alkylphosphonium carboxylate ionic liquids with varying cations and anions.
• Experimental characterization including viscosity, thermal stability, and Small-Angle X-ray Scattering (SAXS).
• Molecular dynamics (MD) simulations using polarizable force fields for structural analysis.

Main Results:

• Synthesized ILs exhibit high thermal stability (>250°C) and manageable viscosity above 50°C.
• SAXS and MD simulations revealed rich microscopic structures, including unique anion-anion correlations in tetrazolate-based ILs.
• Viscosities decrease dramatically with increasing temperature, comparable to other ILs.

Conclusions:

• Alkylphosphonium carboxylate ILs are thermally stable and possess tunable properties.
• The structural insights, particularly anion-anion correlations, explain observed physico-chemical behaviors.
• These phosphonium ILs show promise for applications requiring high-temperature stability and specific solvation properties.