Nanoscopic structures and molecular interactions leading to a dystectic and two eutectic points in [EMIm][Cl]/urea...

Area of Science:

• Materials Science
• Physical Chemistry
• Supramolecular Chemistry

Background:

• Deep eutectic solvents (DES) are emerging non-classical solvents with tunable properties, often derived from ionic liquids (ILs) and hydrogen bond acceptors.
• Understanding the nanoscale structure and dynamics of DES is crucial for predicting and controlling their macroscopic behavior.
• The interplay between IL components and hydrogen bond acceptors significantly influences DES properties.

Purpose of the Study:

• To investigate the structure, dynamics, and molecular processes in mixtures of 1-ethyl-3-methylimidazolium chloride ([EMIm][Cl]) and urea at various molar ratios.
• To elucidate the nanoscale origins of macroscopic properties in DES, particularly focusing on dynamic heterogeneities.
• To correlate macroscopic phase behavior (eutectic/dystectic points) with nanoscale structural changes.

Main Methods:

• Electron paramagnetic resonance (EPR) spectroscopy using nitroxide spin probes to monitor local environment and dynamics.
• Differential scanning calorimetry (DSC) to determine phase behavior and melting point extrema.
• Molecular dynamics (MD) simulations, Raman spectroscopy, and pulsed-field gradient (PFG) NMR spectroscopy to complement EPR and DSC findings.

Main Results:

• Specific molar ratios corresponding to melting point extrema showed altered EPR spectra, indicating changes in spin probe reorientational dynamics and polarity.
• Analysis revealed distinct nanoscale regions within the DES, comprising IL-rich and urea-enriched domains.
• The chloride anion's hydrogen bonding with urea was identified as a key factor driving structural changes and influencing the nanointerface.

Conclusions:

• Macroscopic DES properties are dictated by the nanoscale interface between IL-based and urea-enriched regions.
• The hydrogen-bonding capability of the chloride anion plays a critical role in forming the DES nanostructure.
• This study provides detailed insights into the structure-dynamics-property relationships in [EMIm][Cl]-urea DES.