19
eutectic solvent, respectively, at only 6 wt% water content (Du et al. 2016). Likewise,
Shah et al. found that within 10 wt% of water, the viscosity of ChCl:U was reduced
by more than 80% and the conductivity was 3 times higher compared to the dried
deep eutectic solvent. In contrast, the presence of water only gradually decreased
the melting point and the density of the mixture (Shah and Mjalli 2014). According
to Silva et al., the addition of 3 to 9 wt% water to ChCl:sugar deep eutectic solvents
decreased their melting point, making them liquid at room temperature. The presence of water also contributes to a slight density reduction of 0.82 to 2.22%, a substantial drop in viscosity values, and an increase in the liquid’s polarizability (Silva
et al. 2018). For ChCl:carboxylic acids, a 5% difference in density was detected
between a dried and water-saturated deep eutectic solvent (the water content of the
hydrated ones varied between 9.88 and 19.40 wt% depending on the deep eutectic
solvent following their exposition to air) at the same temperature which proves once
again that this property is not very sensitive to the amount of water present in the
sample. On the other hand, viscosity was strongly decreased in the hydrated deep
eutectic solvents with 10 to 200 times lower values than the dried ones (Florindo
et al. 2014). The presence of water (up to 5 wt%) also significantly decreased the
viscosity and increased the conductivity of natural deep eutectic solvents based on
ChCl and betaine as hydrogen bond acceptors and sugars and carboxylic acids as
hydrogen bond donors (Aroso et al. 2017). Increasing the water content from 2 to
22 wt% in ChCl:lactic acid decreased the viscosity by two orders of magnitude and
increased the electrical conductivity in a similar trend (Alcalde et al. 2019). Another
study noticed a threefold and a tenfold decrease in the viscosity of glucose:ChCl:water
natural deep eutectic solvent after the addition of 5 and 10% water (v/v), respectively (Dai et al. 2013). A significant decrease in the viscosity and a linear decrease
in the density of several natural deep eutectic solvents were also observed in the
presence of water, while the conductivity of five selected ternary natural deep eutectic solvents made of choline chloride, organic acids, sugars, and water firstly
increased with the increasing water content and then decreased after reaching a peak
value of around 10–100 times higher than that of pure natural deep eutectic solvent
at 60–80 wt% water (Dai et al. 2015). Similar water effects on the density and viscosity of some organic acid-based natural deep eutectic solvents were obtained by
Mitar et al. However, though clearly higher conductivities were seen in ChCl:organic
acid-based natural deep eutectic solvents compared to sugar:organic acid in the
studies of both Dai et al. and Mitar et al., no rise and fall was obtained by Mitar’s
group when studying the natural deep eutectic solvent’s conductivity as a function
of water content. The ionic conductivities simply increased with the water content,
and this difference can be attributed to the narrower water range that was considered
by Mitar et al. (10–50 wt%) (Mitar et al. 2019). The viscosity and conductivity of
ChCl-based deep eutectic solvents with different glycols as hydrogen bond donor
were also massively affected by the water content. In fact, the viscosity value was
halved after the addition of about 7–10 wt% water. On the other hand, the conductivity of deep eutectic solvents firstly increased with the increasing water content,
reached a maximum at 60 wt% water, at which values were 6–15 times higher than
that of pure deep eutectic solvents, and then decreased. The ionic dissociation of
1 Understanding the Basics and Properties of Deep Eutectic Solvents
eutectic solvent, respectively, at only 6 wt% water content (Du et al. 2016). Likewise,
Shah et al. found that within 10 wt% of water, the viscosity of ChCl:U was reduced
by more than 80% and the conductivity was 3 times higher compared to the dried
deep eutectic solvent. In contrast, the presence of water only gradually decreased
the melting point and the density of the mixture (Shah and Mjalli 2014). According
to Silva et al., the addition of 3 to 9 wt% water to ChCl:sugar deep eutectic solvents
decreased their melting point, making them liquid at room temperature. The presence of water also contributes to a slight density reduction of 0.82 to 2.22%, a substantial drop in viscosity values, and an increase in the liquid’s polarizability (Silva
et al. 2018). For ChCl:carboxylic acids, a 5% difference in density was detected
between a dried and water-saturated deep eutectic solvent (the water content of the
hydrated ones varied between 9.88 and 19.40 wt% depending on the deep eutectic
solvent following their exposition to air) at the same temperature which proves once
again that this property is not very sensitive to the amount of water present in the
sample. On the other hand, viscosity was strongly decreased in the hydrated deep
eutectic solvents with 10 to 200 times lower values than the dried ones (Florindo
et al. 2014). The presence of water (up to 5 wt%) also significantly decreased the
viscosity and increased the conductivity of natural deep eutectic solvents based on
ChCl and betaine as hydrogen bond acceptors and sugars and carboxylic acids as
hydrogen bond donors (Aroso et al. 2017). Increasing the water content from 2 to
22 wt% in ChCl:lactic acid decreased the viscosity by two orders of magnitude and
increased the electrical conductivity in a similar trend (Alcalde et al. 2019). Another
study noticed a threefold and a tenfold decrease in the viscosity of glucose:ChCl:water
natural deep eutectic solvent after the addition of 5 and 10% water (v/v), respectively (Dai et al. 2013). A significant decrease in the viscosity and a linear decrease
in the density of several natural deep eutectic solvents were also observed in the
presence of water, while the conductivity of five selected ternary natural deep eutectic solvents made of choline chloride, organic acids, sugars, and water firstly
increased with the increasing water content and then decreased after reaching a peak
value of around 10–100 times higher than that of pure natural deep eutectic solvent
at 60–80 wt% water (Dai et al. 2015). Similar water effects on the density and viscosity of some organic acid-based natural deep eutectic solvents were obtained by
Mitar et al. However, though clearly higher conductivities were seen in ChCl:organic
acid-based natural deep eutectic solvents compared to sugar:organic acid in the
studies of both Dai et al. and Mitar et al., no rise and fall was obtained by Mitar’s
group when studying the natural deep eutectic solvent’s conductivity as a function
of water content. The ionic conductivities simply increased with the water content,
and this difference can be attributed to the narrower water range that was considered
by Mitar et al. (10–50 wt%) (Mitar et al. 2019). The viscosity and conductivity of
ChCl-based deep eutectic solvents with different glycols as hydrogen bond donor
were also massively affected by the water content. In fact, the viscosity value was
halved after the addition of about 7–10 wt% water. On the other hand, the conductivity of deep eutectic solvents firstly increased with the increasing water content,
reached a maximum at 60 wt% water, at which values were 6–15 times higher than
that of pure deep eutectic solvents, and then decreased. The ionic dissociation of
1 Understanding the Basics and Properties of Deep Eutectic Solvents