Nanodomain Formation and Temperature-Dependent Diffusion in Deep Eutectic Solvents Revealed by Single-Molecule...

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Deep eutectic solvents (DESs) are typically considered homogeneous but exhibit nanoscale heterogeneity.
• Current research often attributes DES nanoscale features to short-range chemical interactions.
• The role of long-range physical mechanisms in DES nanostructure formation remains unclear.

Purpose of the Study:

• To investigate the origin of nanoscale structural heterogeneity in hydrophobic deep eutectic solvents.
• To differentiate between chemical and physical mechanisms driving nanostructure formation in DESs.
• To analyze the temperature-dependent behavior of nanostructures in specific DES formulations.

Main Methods:

• Utilized single-molecule tracking (SMT) to monitor fluorescent probe diffusion.
• Applied maximum entropy method (MEM) analysis to identify diffusion populations.
• Examined two hydrophobic DESs: tetrabutylammonium bromide:l-menthol (DES-butyl) and tetraoctylammonium bromide:l-menthol (DES-octyl).

Main Results:

• DES-butyl showed a narrow diffusion distribution, indicating a relatively homogeneous structure.
• DES-octyl exhibited two distinct diffusion populations at 20°C (0.12 μm²/s and 0.53 μm²/s).
• The slow-diffusing population in DES-octyl diminished with heating, disappearing above ~30°C, suggesting nanodomain collapse.

Conclusions:

• The temperature-dependent homogenization of DES-octyl supports a physical mechanism, such as liquid-liquid phase separation (LLPS), for nanostructure formation.
• Long-range physical interactions, rather than solely short-range chemical interactions, are the most plausible origin of the observed nanoscale structure in DES-octyl.
• These findings challenge the traditional view of DES homogeneity and highlight the importance of physical mechanisms in their structural organization.