18
who also followed the variation of the melting point of the same deep eutectic solvent but with a full range of water content. Though a similar linear trend was
obtained up until 10 wt% of water by the two studies, further increase in the water
content yields a minimum melting point of −48 ± 2 °C at 0.67 mole fraction of
water. Above this point, the melting temperature linearly increased as shown in
Fig. 1.7. Owing to the behavior of the studied mixture, the authors proposed that
1:2:6 ChCl:U:water makes a ternary deep eutectic solvent (P. J. Smith et al. 2019).
Nevertheless, this behavior of ChCl:U was not observed by the study of Shah et al.
in which the melting point only decreased as a function of water content studied in
full range (Shah and Mjalli 2014). Contrarily, the addition of up to 10 wt% water
slightly increased the melting point of 1:1 ChCl:boric acid which was explained by
a possible reaction between water and boric acid (Häkkinen et al. 2019).
On the other hand, all the studies have agreed that unlike the density, both viscosity and conductivity are highly sensitive to the presence of water in deep eutectic
solvents. Agieienko et al. noticed a slight decrease of 0.14% in the density of
ChCl:U at around 0.008 mass fraction of water, while 0.005 water mass fraction
decreased its viscosity by around 22%. The authors stated that different water contents along with the chosen experimental method and associated instrument calibration may be the reasons behind the poor agreement between the reported viscosity
values of ChCl:U (Agieienko and Buchner 2019). Du et al. showed that both viscosity and conductivity of ChCl:U are highly sensitive to water. In fact, the viscosity
and the conductivity were 13 times lower and 10 times higher in the hydrated deep
Fig. 1.7 Variation of the freezing point of 1:2 choline chloride:urea deep eutectic solvent with the
added mole fraction of water. (Reprinted with permission from (Smith et al. 2019). Copyright
(2019) American Chemical Society)
T. El Achkar et al.
who also followed the variation of the melting point of the same deep eutectic solvent but with a full range of water content. Though a similar linear trend was
obtained up until 10 wt% of water by the two studies, further increase in the water
content yields a minimum melting point of −48 ± 2 °C at 0.67 mole fraction of
water. Above this point, the melting temperature linearly increased as shown in
Fig. 1.7. Owing to the behavior of the studied mixture, the authors proposed that
1:2:6 ChCl:U:water makes a ternary deep eutectic solvent (P. J. Smith et al. 2019).
Nevertheless, this behavior of ChCl:U was not observed by the study of Shah et al.
in which the melting point only decreased as a function of water content studied in
full range (Shah and Mjalli 2014). Contrarily, the addition of up to 10 wt% water
slightly increased the melting point of 1:1 ChCl:boric acid which was explained by
a possible reaction between water and boric acid (Häkkinen et al. 2019).
On the other hand, all the studies have agreed that unlike the density, both viscosity and conductivity are highly sensitive to the presence of water in deep eutectic
solvents. Agieienko et al. noticed a slight decrease of 0.14% in the density of
ChCl:U at around 0.008 mass fraction of water, while 0.005 water mass fraction
decreased its viscosity by around 22%. The authors stated that different water contents along with the chosen experimental method and associated instrument calibration may be the reasons behind the poor agreement between the reported viscosity
values of ChCl:U (Agieienko and Buchner 2019). Du et al. showed that both viscosity and conductivity of ChCl:U are highly sensitive to water. In fact, the viscosity
and the conductivity were 13 times lower and 10 times higher in the hydrated deep
Fig. 1.7 Variation of the freezing point of 1:2 choline chloride:urea deep eutectic solvent with the
added mole fraction of water. (Reprinted with permission from (Smith et al. 2019). Copyright
(2019) American Chemical Society)
T. El Achkar et al.