Defying Decomposition: The Curious Case of Choline Chloride

Abstract

Chemists are exploring deep eutectic solvents (DESs) as alternatives to conventional solvents to meet modern sustainability, health, and safety requirements.[1–3] Through large melting point depressions, DESs allow for the incorporation of renewable solids in taskspecific liquids.[3] Yet, DES design is complicated by complex molecular interactions and a lack of comprehensive and reliable property databases.[3–6] Even measuring the melting properties of pure DES components can be challenging, as they often decompose before melting.[7–9] 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 (FSC) combined with micro-XRD and high-speed optical microscopy. Our thermodynamically coherent fusion properties identify ChCl as an ionic plastic crystal[ 10,11] and demonstrate negative deviations from ideal mixing for ChCl—contradicting the conclusions based on previously assumed fusion properties.[5,7,12] We hypothesise that the plastic crystal nature of ammonium salts is at the basis of their resilience to melting when pure or mixed with other components. We show that DESs based on ionic plastic crystals can profit from (1) a low enthalpy of fusion and (2) favourable mixing of the ionic and molecular compounds. Both lower the mixture’s melting point and can be altered through the nature of the ions. Ionic plastic crystal-based DESs thus offer a platform for task-specific liquids at accessible temperatures and over a broad range of compositions.