NMR self-diffusion study of a phosphonium bis(mandelato)borate ionic liquid

Area of Science:

• Materials Science
• Physical Chemistry
• Tribology

Background:

• Halogen-free boron based ionic liquids (hf-BILs) offer hydrolytic stability and low melting points.
• These ionic liquids possess excellent wear and friction-reducing properties, making them promising for lubrication.
• Understanding their self-diffusion behavior is crucial for optimizing performance.

Purpose of the Study:

• To investigate the self-diffusion peculiarities of a representative hf-BIL, trihexyltetradecylphosphonium bis(mandelato)borate ([P6,6,6,14][BMB]).
• To explore the influence of temperature on the diffusion behavior and phase coexistence within the ionic liquid.

Main Methods:

• Nuclear Magnetic Resonance (NMR) techniques were employed to study self-diffusion.
• Diffusion coefficients of cations ([P6,6,6,14]) and anions ([BMB]) were measured.
• Experiments were conducted across a temperature range of 20-100 °C.

Main Results:

• At lower temperatures (20-50 °C), the ionic liquid exists in two phases with distinct diffusion coefficients for cations and anions.
• Cation diffusion is significantly slower (factor of 20) than anion diffusion due to aggregation.
• Above 60 °C, the two phases merge into a single phase where cations and anions diffuse as a pair with equal coefficients.

Conclusions:

• The self-diffusion behavior of [P6,6,6,14][BMB] is temperature-dependent, exhibiting complex phase separation at lower temperatures.
• Cation aggregation significantly impacts ion mobility, but thermal motion disrupts these aggregates at higher temperatures.
• This temperature-induced transition in diffusion behavior is key to understanding and utilizing these ionic liquids in tribological applications.