M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

6 8 H I G H L I G H T S 2 0 2 3 I

Melting the unmeltable

Rational design of deep eutectic solvents (DESs) is hindered because fundamental DES components like choline chloride (ChCl) decompose before melting. A combination of fast scanning calorimetry and X-ray diffraction was used to overcome ChCl decomposition and measure its melting properties. Plastic ionic crystals are proposed as a platform for renewable DESs, based on the identification of ChCl as a plastic crystal.

Deep eutectic solvents (DESs) are characterised by substantial melting point depressions upon mixing, allowing high-melting salts to be liquefied at ambient temperatures. DESs are explored as alternative liquid electrolyte media for versatile applications to meet modern sustainability, health and safety requirements. However, a thermodynamic description of the conditions at which DESs remain liquid requires accurate melting properties for all constituents. Choline chloride (ChCl) is an abundant biodegradable salt and a common DES constituent. However, it decomposes before melting because its crystal lattice is more stable than its covalent bonds. This obstructed the direct measurement of ChCl melting properties, and only indirect estimations could be made. In this work, these limitations were overcome through fast scanning calorimetry (FSC).

An FSC is a chip calorimeter that can heat small ChCl particles (5 100 ng) at extreme heating rates (100 5000 K s-1). Differential FSC (DFSC) was used to analyse the phase transitions of ChCl thermally. However, it is not possible to identify structural changes based on enthalpic data only. At beamline ID13, FSC was combined with micro-focused synchrotron X-ray diffraction (XRD) (ø ≈ 15 μm) to follow the crystal structure evolution of small ChCl particles. An X-ray diffraction pattern was collected every 2 ms, ensuring an excellent temperature resolution (Figure 49b). ChCl is highly hygroscopic, but a dry atmosphere was guaranteed by a unique hermetic FSC cell designed at ID13. The sample morphology was evaluated using the same cell under an optical microscope equipped with a high-speed camera.

First, ChCl was heated to its melting point faster than the kinetics of decomposition (> 1000 K s−1). The original crystal was kept intact well above its decomposition temperature (Td), as is demonstrated by the monotonic DFSC baseline (Figure 49a), the shift in XRD peaks to slightly lower q owing to thermal expansion (Figure 49c), and the unchanged visual aspect (Figure 49d).

ChCl melts at 687 ± 9 K with a melting enthalpy of 13.8 ± 3.0 kJ mol-1, as measured from the onset and integral of the first DFSC peak (n = 26), respectively.

Fig. 49: a) Heat flow signal of ChCl from differential FSC at 1000 K s−1 and 5000 K s−1. The decomposition temperature (Td) is highlighted in grey. b) Temperature (T) vs. time (t) profile of ChCl using an FSC at a heating rate of 1000 K s−1. The ChCl solid solid transition temperature (Ttrs) is highlighted in grey. c) Corresponding selected XRD patterns that increase with t from bottom to top (black arrow). The q of β-ChCl are highlighted in grey. d) Selection of high-speed

microscopy images of a ChCl particle on silicon grease heated at 5000 K s−1 and the corresponding T t profile.