# Deliquescence Behavior of Deep Eutectic Solvents
## 1 Introduction  
Deep eutectic solvents (DESs) are a promising class of solvents suggested as alternatives for organic solvents in the chemical industry [1; 2; 3; 4]. DESs are formed by intimate mixing of components with a hydrogen bond donor and an acceptor. The large melting point depression found in DESs is caused by the strong hydrogen bond interactions between the two components [3]. Due to the hydrogen bonds between the two DES components, the system does not follow the phase behaviour of simple eutectic systems that can be predicted by the simplified Schroder-Van Laar equation [5]. For melting temperature depressions larger than predicted, an interaction stronger than in the molten phase of the individual components is expected. For DESs, this interaction is a hydrogen bond between the two components. This means that only systems with negative deviations from an ideal phase behaviour can be defined as DESs. Systems that follow the predicted phase behaviour are classified as simple eutectics.  
By combining different hydrogen bond donors and acceptors, it is possible to prepare DESs with specific eutectic melting points and/or solvation properties suitable for various technological applications. However, DESs are often hygroscopic, and the presence of sorbed moisture within DESs can change the DES properties and nanostructure [6; 7]. After sorption of low amounts of water, a heterogeneous mixture is formed with domains of _water-in-DES_ and _DES-in-water_ because water acts as a second hydrogen bond donor/acceptor interacting with the DES-components. The relative prevalence of these domains changes with the concentration of water until the solution becomes an aqueous electrolyte-like mixture when the sorbed amount of moisture is able to fully hydrate both DES components [6; 7; 8]. The presence of water has also been reported to decrease the melting temperature, as illustrated by a DES consisting of choline chloride (ChCl) and urea.  
The eutectic melting temperature of this DES decreased from 23.5 \({}^{\circ}\)C at 0 wt% water to 2 \({}^{\circ}\)C at 10 wt% water [9]. Other studies have reported that the presence of water within DESs dramatically decreases the viscosity and changes the polarity, conductivity, density, and solvation properties of organic compounds such as drugs [10; 11; 12]. The components used to form DESs are typically hygroscopic, able to form hydrates, or exhibit deliquescence at a low level of relative humidity (RH). Despite the impact of water on the properties of a DES, water sorption of DES during deliquescence has not been thoroughly studied.  
Deliquescence of a solid crystalline material is a physical phenomenon, where a water-soluble material dissolves in the sorbed moisture at and above a critical relative humidity (RH\({}_{0}\)) [13]. If the RH decreases below the RH\({}_{0}\), the material will lose water and recrystallize. It has been reported that the RH\({}_{0}\) for a given deliquescing material can decrease if said material is in contact with a second material [14]. The lowest RH\({}_{0}\) of a binary mixture is called the eutonic point and is the binary composition at which both components reach their solubility limit in the solution [13; 14; 15].  
In our preliminary experiments, we experienced that many DESs reported to be liquid at room temperature were solid in our laboratory at dry conditions. However, at high absolute humidity conditions, the DESs become liquid. We believe that the liquid appearance of these mixtures was due to deliquescence and deliquescence lowering and not solely due to the eutectic behavior that these mixtures exhibit. Therefore, this study investigates the phase behaviour of four hygroscopic DESs (ChCl/xylitol, ChCl/succinic acid, betaine/xylitol, and betaine/urea) including two of which (ChCl/xylitol and betaine/urea) that have previously been reported to be liquid at room temperature but remained solid in our laboratory at dry conditions with eutectic melting temperatures above room temperature.