8
room temperature and crushing them in a mortar with a pestle, until a clear liquid is
formed (Florindo et al. 2014). Another method based on the freeze-drying of the
aqueous solutions of the components of deep eutectic solvents was also revealed by
Gutierrez et al. (Gutiérrez et al. 2009). Indeed, separate aqueous solutions of ChCl
and urea (or thiourea) were mixed to form an aqueous solution of 1:2 ChCl:U (or
ChCl:thiourea), having 5 wt% solute contents. The obtained solutions were then
frozen and freeze-dried, resulting in the formation of clear and viscous liquids.
However, water was detected in the freeze-dried mixture because it can interact with
deep eutectic solvent’s components and be part of the deep eutectic solvent’s network (Choi et al. 2011; Dai et al. 2013). That said, different deep eutectic solvents
are obtained when using different methods of preparation. An evaporation method
was also reported by Dai et al., consisting on dissolving the components of deep
eutectic solvents in water, followed by an evaporation at 50 °C. The resulting liquid
is then placed in a desiccator in the presence of silica gel (Dai et al. 2013).
Considering the optimization of time and energy consumption, a greener microwaveassisted approach was proposed for the preparation of natural deep eutectic solvents
within seconds (Gomez et al. 2018). Lastly, an ultrasound-assisted synthesis of
natural deep eutectic solvents was recently introduced (Santana et al. 2019).
1.4 Physicochemical Properties
The physicochemical properties of deep eutectic solvents are one of the main reasons behind the rising researchers’ interest in these solvents. Besides having a low
volatility, nonflammability, low vapor pressure, and chemical and thermal stability,
deep eutectic solvents are chemically tunable, meaning they can be designed for
specific applications given the wide variety of the possible deep eutectic solvents’
forming compounds. All these properties encouraged the scientists to explore deep
eutectic solvents and apply them as a good alternative to conventional solvents.
Herein, the main physicochemical properties of deep eutectic solvents, namely,
their phase behavior, density, viscosity, ionic conductivity, surface tension, and
polarity, are presented and discussed.
1.4.1 Phase Behavior
As mentioned above, deep eutectic solvents are not pure compounds but mixtures of
two or more pure compounds. This system is represented by a solid-liquid phase
diagram, which shows the melting temperature in function of the mixture composition. Therefore, if we consider a binary mixture of compounds A and B, the eutectic
point represents the composition and the minimum melting temperature at which
the melting curves of both compounds meet (Fig. 1.4).
T. El Achkar et al.
room temperature and crushing them in a mortar with a pestle, until a clear liquid is
formed (Florindo et al. 2014). Another method based on the freeze-drying of the
aqueous solutions of the components of deep eutectic solvents was also revealed by
Gutierrez et al. (Gutiérrez et al. 2009). Indeed, separate aqueous solutions of ChCl
and urea (or thiourea) were mixed to form an aqueous solution of 1:2 ChCl:U (or
ChCl:thiourea), having 5 wt% solute contents. The obtained solutions were then
frozen and freeze-dried, resulting in the formation of clear and viscous liquids.
However, water was detected in the freeze-dried mixture because it can interact with
deep eutectic solvent’s components and be part of the deep eutectic solvent’s network (Choi et al. 2011; Dai et al. 2013). That said, different deep eutectic solvents
are obtained when using different methods of preparation. An evaporation method
was also reported by Dai et al., consisting on dissolving the components of deep
eutectic solvents in water, followed by an evaporation at 50 °C. The resulting liquid
is then placed in a desiccator in the presence of silica gel (Dai et al. 2013).
Considering the optimization of time and energy consumption, a greener microwaveassisted approach was proposed for the preparation of natural deep eutectic solvents
within seconds (Gomez et al. 2018). Lastly, an ultrasound-assisted synthesis of
natural deep eutectic solvents was recently introduced (Santana et al. 2019).
1.4 Physicochemical Properties
The physicochemical properties of deep eutectic solvents are one of the main reasons behind the rising researchers’ interest in these solvents. Besides having a low
volatility, nonflammability, low vapor pressure, and chemical and thermal stability,
deep eutectic solvents are chemically tunable, meaning they can be designed for
specific applications given the wide variety of the possible deep eutectic solvents’
forming compounds. All these properties encouraged the scientists to explore deep
eutectic solvents and apply them as a good alternative to conventional solvents.
Herein, the main physicochemical properties of deep eutectic solvents, namely,
their phase behavior, density, viscosity, ionic conductivity, surface tension, and
polarity, are presented and discussed.
1.4.1 Phase Behavior
As mentioned above, deep eutectic solvents are not pure compounds but mixtures of
two or more pure compounds. This system is represented by a solid-liquid phase
diagram, which shows the melting temperature in function of the mixture composition. Therefore, if we consider a binary mixture of compounds A and B, the eutectic
point represents the composition and the minimum melting temperature at which
the melting curves of both compounds meet (Fig. 1.4).
T. El Achkar et al.