Defying decomposition: the curious case of choline chloride

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Conventional solvents pose sustainability, health, and safety challenges.
• Deep eutectic solvents (DESs) are promising alternatives, but their design is hindered by complex interactions and limited property data.
• Measuring pure component properties like melting point can be difficult due to thermal decomposition.

Purpose of the Study:

• To overcome the decomposition issue of choline chloride (ChCl), a key DES component.
• To accurately measure the fusion properties of ChCl.
• To investigate the mixing behavior of ChCl in DES formulations.

Main Methods:

• Fast scanning calorimetry (FSC) was employed to measure thermal properties.
• Micro-X-ray diffraction (μ-XRD) and high-speed optical microscopy were used for structural and morphological analysis.
• Thermodynamic analysis was performed to understand mixing behavior.

Main Results:

• The enthalpy of fusion and melting point of ChCl were accurately determined (13.8 ± 3.0 kJ·mol⁻¹ and 687 ± 9 K, respectively).
• ChCl was identified as an ionic plastic crystal.
• Negative deviations from ideal mixing were observed for ChCl, contradicting prior assumptions.

Conclusions:

• The plastic crystalline nature of ammonium salts influences their melting behavior.
• DESs utilizing ionic plastic crystals benefit from low enthalpy of fusion and favorable mixing, enabling lower melting points.
• Ionic plastic crystal-based DESs provide a versatile platform for creating task-specific liquids across various temperatures and compositions.