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6 9 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Defying decomposition: the curious case of choline chloride, A. van den Bruinhorst (a), J. Avila (a), M. Rosenthal (b), A. Pellegrino (a), M. Burghammer (c), M. Costa Gomes (a), Nat. Commun. 14, 6684 (2023); https:/doi.org/10.1038/s41467-023-42267-6 (a) École Normale Supérieure de Lyon, Lyon (France) (b) KU Leuven, Leuven (Belgium) (c) ESRF

Around 730 K, the XRD pattern is free of crystallographic reflections (Figure 49c) and a droplet is formed (Figure 49d), indicating complete ChCl liquefaction. The first FDSC peak ends at the same temperature (Figure 49a), reinforcing that it corresponds to melting. Upon melting, a portion of ChCl decomposes until it cools below Td. The mass loss was observed visually (Figure 49d) and quantified by the solid solid transition energetics (Figure 49a). The XRD patterns show that the remaining ChCl recrystallises in its original form, which constitutes the first proof of reversible ChCl melting.

Accurate ChCl melting properties are imperative for a correct thermodynamic interpretation of ChCl- based DESs. Figure 50 shows that current inaccurate estimations (orange) indicate asymmetric deviations from ideal mixing for the genesis urea + ChCl DES. Instead, the accurate melting properties from this work demonstrate favourable (negative) and symmetric mixing energies for urea and ChCl. This agrees with the current molecular view on DESs: both compounds contribute to a delicate balance between intermolecular interactions and increased disorder.

These results unveil that ChCl is a plastic crystal with a low ChCl melting entropy (20.2 J mol−1 K−1) and high entropy of solid solid transition (46.3 J mol−1 K−1). It is thus proposed that the development of DESs should be centred around ionic plastic crystals that simultaneously profit from a low melting enthalpy and a favourable mixing. Both aspects lower the mixture s melting point and can be tuned by selecting the appropriate ions and molecular compounds. Ionic plastic crystal-based DESs thus provide a platform to include solids in renewable task-specific solvents at accessible temperatures and over a broad range of compositions.

Fig. 50: a) Liquidus temperatures (black circles) for urea + ChCl with the ideal liquidus temperature (T idliq, solid lines) and eutectic

depth (De) calculated from the ChCl melting enthalpy (∆fusHm) measured (blue) or the indirectly estimated value (orange).

Dashed lines indicate 95% confidence intervals. b) Partial molar excess Gibbs energy (GEm,i, circles) derived from

the experimental liquidus temperatures and T idliq shown in A. Solid lines highlight the general trends.