Defying Decomposition: The Curious Case of Choline Chloride

21 September 2023, Version 3
This content is a preprint and has not undergone peer review at the time of posting.


Chemists aim to meet modern sustainability, health, and safety requirements by replacing conventional solvents with deep eutectic solvents (DESs). Through large melting point depressions, DESs may incorporate renewable solids in task-specific liquids. Yet, DES design is complicated by complex molecular interactions and a lack of comprehensive property databases. Even measuring pure component melting properties can be challenging, due to decomposition before melting. Here we overcame the decomposition of the quintessential DES constituent, choline chloride (ChCl). We measured its enthalpy of fusion (13.8±3.0 kJ·mol−1) and melting point (687±9 K) by fast scanning calorimetry combined with micro-XRD and high-speed optical microscopy. Our thermodynamically coherent fusion properties identify ChCl as an ionic plastic crystal and demonstrate negative deviations from ideal mixing for ChCl—contradicting previous assumptions. We hypothesise that the plastic crystal nature of ammonium salts governs their resilience to melting; pure or mixed. We show that DESs based on ionic plastic crystals can profit from (1) a low enthalpy of fusion and (2) favourable mixing. Both depress the melting point and can be altered through ion selection. Ionic plastic crystalbased DESs thus offer a platform for task-specific liquids at a broad range of temperatures and compositions.


deep eutectic solvents
fusion properties
choline chloride
fast scanning calorimetry
thermal decomposition
synchrotron XRD

Supplementary materials

Supplementary Information
A detailed description of sample preparation and handling, the experimental methodology, supporting data, literature data, and a thermodynamic analysis of the impact of the new accurate fusion properties for choline chloride.
Supporting videos
Video S1–S8: The obtained μ-XRD results were animated to obtain an interactive overview of the Bragg-peaks with the temperature program. Video S9–S12: The images obtained from the high-speed camera were combined with the temperature response of the FSC to obtain an interactive overview of the particle morphology with time and temperature.

Supplementary weblinks


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