21
1.5.2 Effect on Deep Eutectic Solvents’ Network
As discussed above, water clearly impacts the physicochemical properties of the
deep eutectic solvents, whether present in low or large amounts. Furthermore, investigating deep eutectic solvent-water interactions is crucial especially that binary
mixtures of deep eutectic solvents and water have been commonly adopted in many
applications already. In fact, the presence of water allows the circumvention of
some of the shortcomings of deep eutectic solvents like their relative high viscosity
while maintaining their unique and appealing properties, which explains the rising
interest in deep eutectic solvent-water mixtures over the past few years. However,
the high polarity of water and its propensity to interact with the hygroscopic components of the eutectic system makes it of paramount importance to check out if and
how water affects the intra- and intermolecular bonds lying behind the supramolecular network of deep eutectic solvents. Despite their relevance, the investigations
of the effect of water on deep eutectic solvent’s system are rather restricted and
mostly cover ChCl-based eutectics. Table 1.2 provides an overview of the reported
studies dealing with the effect of water on various deep eutectic solvents’ systems.
This effect was examined via multiple techniques mainly NMR, Brillouin spectroscopies, and neutron total scattering, not to mention the MD simulations. Some studies proposed a passage from deep eutectic solvent to an aqueous solution of its
individual components while adding water, and others suggested that a transition
from “water-in-DES” to a “DES-in-water” system occurs at a certain hydration
level. In the former system, water is seen as another hydrogen bond donor (Hammond
et al. 2017b; López-Salas et al. 2019; Zhekenov et al. 2017), thus integrating into
the deep eutectic solvent’s network and subsequently strengthening the hydrogen
bonds taking place between the hydrogen bond acceptor and the hydrogen bond
donor at a low water content (Hammond et al. 2017a; Weng and Toner 2018).
However, further dilution results in the weakening of the interactions that usually
dominate in a deep eutectic solvent supramolecular structure owing to the tendency
of water to interact with the deep eutectic solvent’s forming compounds. The preferential hydration of chloride anions was reported in numerous papers dealing with
different ChCl-based deep eutectic solvents like ChCl:U, ChCl:G, ChCl:EG, and
ChCl:LA (Alcalde et al. 2019; Fetisov et al. 2018; Kaur et al. 2020; Kumari et al.
2018; Weng and Toner 2018). Yet, when it comes to the hydration level at which the
transition happens, the values are not always consistent for the same deep eutectic
solvent. For instance, the transition point varied between 15 and 51 wt% for ChCl:U
(Hammond et al. 2017a; Kumari et al. 2018; Posada et al. 2017; Shah and Mjalli
2014). There are not enough studies to compare between the transition points of
other deep eutectic solvents. Few studies also proved that temperature does not
affect the structure of deep eutectic solvent-water mixtures (Celebi et al. 2019;
Weng and Toner 2018). Further studies must be conducted on other deep eutectic
solvents because although this transition is likely to occur in all the aqueous mixtures of deep eutectic solvents, the changeover water content obviously depends on
the hydrogen bond acceptor and hydrogen bond donor types as well as their molar
1 Understanding the Basics and Properties of Deep Eutectic Solvents
1.5.2 Effect on Deep Eutectic Solvents’ Network
As discussed above, water clearly impacts the physicochemical properties of the
deep eutectic solvents, whether present in low or large amounts. Furthermore, investigating deep eutectic solvent-water interactions is crucial especially that binary
mixtures of deep eutectic solvents and water have been commonly adopted in many
applications already. In fact, the presence of water allows the circumvention of
some of the shortcomings of deep eutectic solvents like their relative high viscosity
while maintaining their unique and appealing properties, which explains the rising
interest in deep eutectic solvent-water mixtures over the past few years. However,
the high polarity of water and its propensity to interact with the hygroscopic components of the eutectic system makes it of paramount importance to check out if and
how water affects the intra- and intermolecular bonds lying behind the supramolecular network of deep eutectic solvents. Despite their relevance, the investigations
of the effect of water on deep eutectic solvent’s system are rather restricted and
mostly cover ChCl-based eutectics. Table 1.2 provides an overview of the reported
studies dealing with the effect of water on various deep eutectic solvents’ systems.
This effect was examined via multiple techniques mainly NMR, Brillouin spectroscopies, and neutron total scattering, not to mention the MD simulations. Some studies proposed a passage from deep eutectic solvent to an aqueous solution of its
individual components while adding water, and others suggested that a transition
from “water-in-DES” to a “DES-in-water” system occurs at a certain hydration
level. In the former system, water is seen as another hydrogen bond donor (Hammond
et al. 2017b; López-Salas et al. 2019; Zhekenov et al. 2017), thus integrating into
the deep eutectic solvent’s network and subsequently strengthening the hydrogen
bonds taking place between the hydrogen bond acceptor and the hydrogen bond
donor at a low water content (Hammond et al. 2017a; Weng and Toner 2018).
However, further dilution results in the weakening of the interactions that usually
dominate in a deep eutectic solvent supramolecular structure owing to the tendency
of water to interact with the deep eutectic solvent’s forming compounds. The preferential hydration of chloride anions was reported in numerous papers dealing with
different ChCl-based deep eutectic solvents like ChCl:U, ChCl:G, ChCl:EG, and
ChCl:LA (Alcalde et al. 2019; Fetisov et al. 2018; Kaur et al. 2020; Kumari et al.
2018; Weng and Toner 2018). Yet, when it comes to the hydration level at which the
transition happens, the values are not always consistent for the same deep eutectic
solvent. For instance, the transition point varied between 15 and 51 wt% for ChCl:U
(Hammond et al. 2017a; Kumari et al. 2018; Posada et al. 2017; Shah and Mjalli
2014). There are not enough studies to compare between the transition points of
other deep eutectic solvents. Few studies also proved that temperature does not
affect the structure of deep eutectic solvent-water mixtures (Celebi et al. 2019;
Weng and Toner 2018). Further studies must be conducted on other deep eutectic
solvents because although this transition is likely to occur in all the aqueous mixtures of deep eutectic solvents, the changeover water content obviously depends on
the hydrogen bond acceptor and hydrogen bond donor types as well as their molar
1 Understanding the Basics and Properties of Deep Eutectic Solvents