20
deep eutectic solvents’ components caused the initial increase in the conductivity
which was later decreased owing to the dilution of the electrolytes at higher water
content. The polarity linearly increased with the increasing water content for all
three solvents (Gabriele et al. 2019). A similar non-monotonic behavior of the ionic
conductivity was observed with aqueous solutions of ChCl:U and ChCl:EG via
molecular dynamics (MD) simulations. The values reached the maximum also at
60 wt% water (Celebi et al. 2019). When varying the water content from 16 to
30 wt% in citric acid:sucrose deep eutectic solvent, a linear decrease of the density
was observed, while a major impact was seen on the viscosity which decreased by
up to 99.73% at the maximum studied water content (30 wt%) (Savi et al. 2019b).
Likewise, the addition of 10 wt% water resulted in an 85.9% decrease in the viscosity of lactic acid:glucose natural deep eutectic solvent (Savi et al. 2019a).
Rublova et al. conducted an in-depth study about the effect of water on the surface tension of binary mixtures of “ChCl + water,” “ChCl + ethylene glycol,” and
“ethylene glycol + water” and a ternary mixture of “ChCl + ethylene glycol +
water.” The interpretation of the variation of the surface tension and the thermodynamic characteristics of adsorption at the interface “solution/air” led to some interesting findings. When comparing the aqueous solution of ChCl to that of EG,
stronger adsorption of choline cation was obtained owing to the choline cationwater hydrophobic interactions. The ternary mixture results revealed interactions
between deep eutectic solvents’ constituents in an adsorbed surface layer formed at
the interface air/diluted solution of ChCl:EG which explains the way higher values
of equilibrium adsorption constants for the ternary mixture compared to those
related to “ChCl + water” and “ethylene glycol + water” mixtures (Rublova et al.
2020). When studying the effect of water on the surface tension of
DL-menthol:octanoic acid deep eutectic solvent, Nunes et al. detected two consecutive behaviors: a decrease of the surface tension while increasing the water content
reaching a minimum value at around 4000 ppm of water, followed by an increase of
the surface tension (Nunes et al. 2019). The same behavior was observed by
Sanchez-Fernandez et al. when studying the surface tension of ChCl:malonic acid
as a function of water (Sanchez-Fernandez et al. 2017).
The papers dealing with the effect of water on the deep eutectic solvents’ polarity
are rather limited. The polarity of ChCl:U, ChCl:G, and ChCl:EG was investigated
through the solvatochromic behavior of different absorbance and fluorescence
probes in deep eutectic solvent-water mixtures. This approach would inform us
about the interactions dominating these mixtures. The water addition happens to
increase the dipolarity/polarizability and decrease the hydrogen bond basicity of all
three solvents. The behavior of the fluorescent probes further revealed more relevant
hydrogen bond interactions between added water and deep eutectic solvents’ constituents for ChCl:G and ChCl:EG when compared to ChCl:U. The structural differences between the adopted hydrogen bond donors as well as the interstitial
accommodation of water molecules within ChCl:U would explain the greater influence of water addition on ChCl:G and ChCl:EG rather than ChCl:U (Pandey and
Pandey 2014).
T. El Achkar et al.
deep eutectic solvents’ components caused the initial increase in the conductivity
which was later decreased owing to the dilution of the electrolytes at higher water
content. The polarity linearly increased with the increasing water content for all
three solvents (Gabriele et al. 2019). A similar non-monotonic behavior of the ionic
conductivity was observed with aqueous solutions of ChCl:U and ChCl:EG via
molecular dynamics (MD) simulations. The values reached the maximum also at
60 wt% water (Celebi et al. 2019). When varying the water content from 16 to
30 wt% in citric acid:sucrose deep eutectic solvent, a linear decrease of the density
was observed, while a major impact was seen on the viscosity which decreased by
up to 99.73% at the maximum studied water content (30 wt%) (Savi et al. 2019b).
Likewise, the addition of 10 wt% water resulted in an 85.9% decrease in the viscosity of lactic acid:glucose natural deep eutectic solvent (Savi et al. 2019a).
Rublova et al. conducted an in-depth study about the effect of water on the surface tension of binary mixtures of “ChCl + water,” “ChCl + ethylene glycol,” and
“ethylene glycol + water” and a ternary mixture of “ChCl + ethylene glycol +
water.” The interpretation of the variation of the surface tension and the thermodynamic characteristics of adsorption at the interface “solution/air” led to some interesting findings. When comparing the aqueous solution of ChCl to that of EG,
stronger adsorption of choline cation was obtained owing to the choline cationwater hydrophobic interactions. The ternary mixture results revealed interactions
between deep eutectic solvents’ constituents in an adsorbed surface layer formed at
the interface air/diluted solution of ChCl:EG which explains the way higher values
of equilibrium adsorption constants for the ternary mixture compared to those
related to “ChCl + water” and “ethylene glycol + water” mixtures (Rublova et al.
2020). When studying the effect of water on the surface tension of
DL-menthol:octanoic acid deep eutectic solvent, Nunes et al. detected two consecutive behaviors: a decrease of the surface tension while increasing the water content
reaching a minimum value at around 4000 ppm of water, followed by an increase of
the surface tension (Nunes et al. 2019). The same behavior was observed by
Sanchez-Fernandez et al. when studying the surface tension of ChCl:malonic acid
as a function of water (Sanchez-Fernandez et al. 2017).
The papers dealing with the effect of water on the deep eutectic solvents’ polarity
are rather limited. The polarity of ChCl:U, ChCl:G, and ChCl:EG was investigated
through the solvatochromic behavior of different absorbance and fluorescence
probes in deep eutectic solvent-water mixtures. This approach would inform us
about the interactions dominating these mixtures. The water addition happens to
increase the dipolarity/polarizability and decrease the hydrogen bond basicity of all
three solvents. The behavior of the fluorescent probes further revealed more relevant
hydrogen bond interactions between added water and deep eutectic solvents’ constituents for ChCl:G and ChCl:EG when compared to ChCl:U. The structural differences between the adopted hydrogen bond donors as well as the interstitial
accommodation of water molecules within ChCl:U would explain the greater influence of water addition on ChCl:G and ChCl:EG rather than ChCl:U (Pandey and
Pandey 2014).
T. El Achkar et al.