# Deliquescence Behavior of Deep Eutectic Solvents
## 3 Discussion  
The experimental melting temperatures were significantly lower than those predicted by the simplified Schroder-Van Laar equation and confirmed the presence of a DES system for ChCl/succinic acid and betaine/xylitol. The phase behaviour of ChCl/xylitol and betaine/urea almost followed the predicted phase behaviour. However, the measured melting temperatures were slightly lower than predicted by the simplified Schroder-Van Laar equation. Thus, these two systems can also be classified as DESs. Often in the literature, DESs are referred to as liquid at room temperature without further information about their thermal phase behaviour or their water sorption behavior. However, under dry conditions, a distinction between a DES and a simple eutectic can only be made based on the phase behaviour rather than its physical appearance at room temperature. In addition, when preparing a physical mixture of two DES components under uncontrolled humidity, the physical appearance may be liquid as a result of deliquesence rather than a melting point depression below room temperature. All DESs characterised in this study had a eutectic melting temperature above room temperature for the dry physical mixtures. However, the DESs became liquid at ambient conditions as the humidity in our lab exceeded the RH\({}_{0}\), as captured by microscope in Figure 2. In the literature, betaine/urea has previously been reported to be liquid in a molar range from 50:50 to 25:75 [18]. In addition, ChCl/xylitol has been reported to be liquid at room temperature [19] even in a wide composition range from 33:66 to 66:33 [20]. Our data suggest that the liquid appearance at room temperature of these DESs is due to deliquescence and not alone due to the eutectic behaviour that these systems also exhibit. The preparation of DESs are often performed over a timespan of hours [11,19], possibly allowing sorption of a significant amount of water if proper humidity control was not in place. The results showed that the RH\({}_{0}\) was lowered for all the studied systems, making humidity control even more critical than for the individual hygroscopic DES-components. The eutonic compositions were not identical to the eutectic compositions. The eutonic compositions were observed at a larger mole fraction of the hydrogen bond acceptors when compared with the eutectic compositions. This means that sorption of water might also lead to unintentionally falsely reported DES compositions if preparation of the mixtures was done in an uncontrolled, humid environment. For example, the two DESs, known as ChCl/Glucose [21] and ChCl/acetylsalicylic acid [22], have been reported to be liquid at room temperature. However, other studies have reported the two DESs to have eutectic melting temperatures of 57.4 \({}^{\circ}\)C [23] and 63 \({}^{\circ}\)C [24], respectively. The difference in melting temperatures has also been previously explained by sorbed moisture [23]. For DESs liquid at room temperature at the eutectic composition, sorption of water can also broaden the composition range at which the DES appears liquid at room temperature. There was no correlation between the RH\({}_{0}\) and the amount of sorbed moisture, and it was found that it depends alone on the affinity of the individual components towards water rather than the mixture. This is in line with the mechanism that water interrupts the DES nanostructure by formation of domains with water-in-DES and DES-in-water. Moisture sorption has also been investigated for other DESs with a melting temperature below room temperature (at dry conditions). It was found that these systems sorbed moisture until the liquid-vapor equilibrium was established. In addition, here, the amount of sorbed moisture varied significantly even between DESs with a similar hydrogen bond acceptor [25]. A common strategy to prevent water within the DES is to dry the DES-components before preparation or subsequent drying of the prepared DES using vacuum [9; 25].