Nanodomains and Their Temperature Dependence in a Phosphonium-Based Ionic Liquid: A Single-Molecule Tracking Study

Area of Science:

• Materials Science
• Physical Chemistry
• Nanotechnology

Background:

• Ionic liquids (ILs) possess unique nanoscale structures known as nanodomains.
• Understanding the formation and dynamics of these nanodomains is crucial for their application.
• Previous studies suggest complex organizational principles within ILs.

Purpose of the Study:

• To provide evidence for the existence of nanodomains in trihexyl(tetradecyl)phosphonium chloride ([P66614][Cl]).
• To investigate the mechanism of nanodomain formation in [P66614][Cl] using varying temperatures and probe molecules.
• To elucidate the role of temperature and viscosity on the nanostructure of ionic liquids.

Main Methods:

• Single-molecule tracking (SMT) was employed to monitor the diffusion of fluorescent probes.
• The maximum entropy method (MEM) was utilized to analyze single-molecule trajectories and determine diffusion coefficients.
• Experiments were conducted on [P66614][Cl] at different temperatures (20 °C, 35 °C, 45 °C, 50 °C) and viscosities, using multiple fluorophores (ATTO 647N, DiD, Nile Blue chloride).

Main Results:

• At 20 °C, [P66614][Cl] exhibited a two-population diffusion distribution, indicating the presence of nanodomains (16% slow, 84% fast).
• At 50 °C, the slow diffusion population disappeared, resulting in a single fast diffusion population.
• Similar results were observed for a different ionic liquid, [P66614][NTf2], at comparable viscosity but lower temperature, and multiple probes confirmed the physical nature of nanodomain formation.

Conclusions:

• The temperature-dependent elimination of the slow diffusion population in [P66614][Cl] supports liquid-liquid phase separation (LLPS) as the mechanism for nanodomain formation.
• The observation of two diffusing populations across different chemical probes suggests a physical, rather than chemical, origin for these nanodomains.
• This study provides key insights into the dynamic nanoscale organization of ionic liquids and their temperature-dependent behavior.