14
via the solvatochromic parameters which consider the hypsochromic (blue) shift or
bathochromic (red) shift of UV-vis bands for the negatively solvatochromic dyes
(e.g., Reichardt’s betaine dye) or the positively solvatochromic dyes (e.g., Nile red),
respectively, as a function of the solvent’s polarity (Reichardt 1994). The most frequently used scales are the polarity scales of Dimroth and Reichardt (E T and E T N)
(Reichardt 1994) and the multiparameter scale of Kamlet and Taft (the hydrogen
bond donating ability α, the hydrogen bond accepting ability β, and dipolarity/
polarizability π*) (Kamlet et al. 1977; Kamlet and Taft 1976). The common probes
adopted for the establishment of Dimroth and Reichardt’s scale include Reichardt’s
betaine dyes and Nile red, while molecules like 4-nitroaniline and N,N-diethyl-4nitroaniline are used to determine the parameters following the Kamlet and Taft
multiparameter scale. However, it is worthy to mention that the polarity scales are
not universal and are probe dependent, which means that we cannot compare polarity parameters obtained by different solvatochromic probes (Valvi et al. 2017). A
general overview on the studies conducted on deep eutectic solvents’ polarity is
presented in Table 1.1 following a chronological order.
1.5 Effect of Water
Given the omnipresence of water and the hygroscopic character of some deep eutectic solvents and their forming compounds, the water uptake by the eutectic solvents
is inevitable (Du et al. 2016; Florindo et al. 2014). While traces of water in deep
eutectic solvents are usually considered as impurities, a plethora of papers intentionally added water to their solvents in order to fine-tune their properties so they
can respond to the requirements of some desired applications and water allowed, in
many cases, to improve the performance of deep eutectic solvents. On the other
hand, the presence of water not only affects the physicochemical properties but may
also jeopardize the integrity of deep eutectic solvents (El Achkar et al. 2019), which
explains the inconsistency in the literature given that deep eutectic solvents are prepared in different operating conditions. Therefore, studying the effect of water on
the eutectic systems is of utmost importance. This section highlights the impact of
water on the physicochemical properties of deep eutectic solvents and the characteristics of their supramolecular organizations.
1.5.1 Effect on Deep Eutectic Solvents’
Physicochemical Properties
Herein, the effect of water on the main physicochemical properties (melting point,
density, viscosity, conductivity, surface tension, and polarity) will be discussed
according to the reported studies so far. Some investigated the effect of low water
T. El Achkar et al.
via the solvatochromic parameters which consider the hypsochromic (blue) shift or
bathochromic (red) shift of UV-vis bands for the negatively solvatochromic dyes
(e.g., Reichardt’s betaine dye) or the positively solvatochromic dyes (e.g., Nile red),
respectively, as a function of the solvent’s polarity (Reichardt 1994). The most frequently used scales are the polarity scales of Dimroth and Reichardt (E T and E T N)
(Reichardt 1994) and the multiparameter scale of Kamlet and Taft (the hydrogen
bond donating ability α, the hydrogen bond accepting ability β, and dipolarity/
polarizability π*) (Kamlet et al. 1977; Kamlet and Taft 1976). The common probes
adopted for the establishment of Dimroth and Reichardt’s scale include Reichardt’s
betaine dyes and Nile red, while molecules like 4-nitroaniline and N,N-diethyl-4nitroaniline are used to determine the parameters following the Kamlet and Taft
multiparameter scale. However, it is worthy to mention that the polarity scales are
not universal and are probe dependent, which means that we cannot compare polarity parameters obtained by different solvatochromic probes (Valvi et al. 2017). A
general overview on the studies conducted on deep eutectic solvents’ polarity is
presented in Table 1.1 following a chronological order.
1.5 Effect of Water
Given the omnipresence of water and the hygroscopic character of some deep eutectic solvents and their forming compounds, the water uptake by the eutectic solvents
is inevitable (Du et al. 2016; Florindo et al. 2014). While traces of water in deep
eutectic solvents are usually considered as impurities, a plethora of papers intentionally added water to their solvents in order to fine-tune their properties so they
can respond to the requirements of some desired applications and water allowed, in
many cases, to improve the performance of deep eutectic solvents. On the other
hand, the presence of water not only affects the physicochemical properties but may
also jeopardize the integrity of deep eutectic solvents (El Achkar et al. 2019), which
explains the inconsistency in the literature given that deep eutectic solvents are prepared in different operating conditions. Therefore, studying the effect of water on
the eutectic systems is of utmost importance. This section highlights the impact of
water on the physicochemical properties of deep eutectic solvents and the characteristics of their supramolecular organizations.
1.5.1 Effect on Deep Eutectic Solvents’
Physicochemical Properties
Herein, the effect of water on the main physicochemical properties (melting point,
density, viscosity, conductivity, surface tension, and polarity) will be discussed
according to the reported studies so far. Some investigated the effect of low water
T. El Achkar et al.