Abstract
Deep eutectic solvents (DESs) are formed by a hydrogen bond donor and an acceptor. The hydrogen bond interactions between these two components significantly depress the melting temperature of the mixture. DESs have been used as an alternative for organic solvents in various branches of the chemical industry. Many DESs are very hygroscopic and water is known to change the properties of DESs, but there has neven been a systematic study performed on the deliquesence behavior of DESs. Therefore, this study investigated the thermal and deliquescent behavior of four DESs. The DES mixtures were stored in desiccators at different relative humidities (RH) to investigate the critical RH (RH0) for deliquescence. It was found that, due to the formation of a eutonic mixture, the RH0 to induce deliquescence for a given DES mixture was lower compared to the individual components comprising the DES. The results showed that, even though all investigated DESs had eutectic melting temperatures above room temperature, but due to the low RH0, they were able to appear liquid at room temperature under ambient conditions. The eutonic and eutectic compositions were identified at different compositions for the DESs. The results emphasize that great care must be taken to control the process and storage conditions for DESs.
Highlights
We investigated the thermal phase behaviour of the dry deep eutectic solvents (DESs) by thermomicroscopy, and, subsequently, investigated the RH0 by placing different molar compositions of the DES systems in desiccators with relative humidities (RH) ranging from 0% RH to 84% RH
The black dots are experimentally measured melting temperatures, and the red solid-liquid equilibrium lines visualise the phase behaviour predicted by the simplified Schröder-Van Laar equation
At humid conditions above the critical RH for deliquescence, a transition from solid to aqueous solution was observed at room temperature
Summary
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 Schröder-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
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