225
microextraction techniques because most of the separation processes are based on
the density differences between the two phases. Deep eutectic solvent’s density
decreases linearly when the temperature is increased as it was shown by Florindo
et al. (2014) and as presented in Fig. 6.16. Also, it is highly influenced by the deep
eutectic solvent components molar ratio. For example, it was seen by Wahaibi et al.
(2019) that the densities of the deep eutectic solvent choline chloride/malonic acid
with 1:0.5 molar ratio are slightly higher than the same solvent with 1:1 molar ratio
at all temperatures. As it was explained by the authors, that this is due to the presence of high amount of choline chloride in the first solvent because, as a general
rule, the bulkier the cation is, the lower the density is (Wahaibi et al. 2019). Another
reference proved also a similar result that is presented in Fig. 6.17 (Abbott
et al. 2011).
6.3.3 Viscosity
The range of deep eutectic solvent’s viscosity values are around 20–1000 times
higher than that of water at room temperature (Dai et al. 2013a, b). Most of the deep
eutectic solvents possess high viscosity values (>100 Cp) at room temperature
(Zhang et al. 2012). The choline chloride/ethylene glycol (1:4 molar ratio) deep
eutectic solvent possesses the lowest viscosity (Zhang et al. 2012). Mainly, the viscosity depends on the ion size, the void volume, the temperature, and the water
content (Smith et al. 2014; Tang et al. 2014). The possible high viscosity of deep
eutectic solvents is attributed to the presence of an extensive hydrogen-bonding
network, Van der Waals, and/or electrostatic interactions between the compounds
that restrict the mobility of the free species inside deep eutectic solvent and restrict
the dispersion of deep eutectic solvent in the extraction medium during the extraction process (Zhang et al. 2012; Habibollahi et al. 2018). The high viscosity is good
for the prevention of entrapped agents from vaporization, but it hampers the mass
transfer and thus produces lower extraction efficiency. Therefore, this problem can
be solved by increasing the temperature, thus leading to a better penetration of the
solvents in the sample; this is known as Arrhenius-like behavior (Bubalo et al.
2018). Also, adding a certain percentage of water to a deep eutectic solvent, at
which the deep eutectic solvent’s network is still maintained, is another way to
decrease deep eutectic solvent’s viscosity (Qi et al. 2015; Fernández et al. 2018).
However, Dai et al. (2015) showed that the addition of water above 50% can dissolve the deep eutectic solvent components in water (Dai et al. 2015). Also, the
addition of water can alter the hydrogen bonds between deep eutectic solvent and
the target compound (Bajkacz and Adamek 2017). Water-added deep eutectic solvents are more suitable for the extraction of polar compounds, whereas deep eutectic solvents with low water content are better for the extraction of nonpolar
compounds (Dai et al. 2013a, b). Other parameters that may influence the viscosity
of the deep eutectic solvent are the type of its components and the molar ratio. Cao
et al. (2018) showed that the extraction yield of proanthocyanidin was decreased
6 Methods for Extraction of Bioactive Compounds from Plant and Animal Matter…
microextraction techniques because most of the separation processes are based on
the density differences between the two phases. Deep eutectic solvent’s density
decreases linearly when the temperature is increased as it was shown by Florindo
et al. (2014) and as presented in Fig. 6.16. Also, it is highly influenced by the deep
eutectic solvent components molar ratio. For example, it was seen by Wahaibi et al.
(2019) that the densities of the deep eutectic solvent choline chloride/malonic acid
with 1:0.5 molar ratio are slightly higher than the same solvent with 1:1 molar ratio
at all temperatures. As it was explained by the authors, that this is due to the presence of high amount of choline chloride in the first solvent because, as a general
rule, the bulkier the cation is, the lower the density is (Wahaibi et al. 2019). Another
reference proved also a similar result that is presented in Fig. 6.17 (Abbott
et al. 2011).
6.3.3 Viscosity
The range of deep eutectic solvent’s viscosity values are around 20–1000 times
higher than that of water at room temperature (Dai et al. 2013a, b). Most of the deep
eutectic solvents possess high viscosity values (>100 Cp) at room temperature
(Zhang et al. 2012). The choline chloride/ethylene glycol (1:4 molar ratio) deep
eutectic solvent possesses the lowest viscosity (Zhang et al. 2012). Mainly, the viscosity depends on the ion size, the void volume, the temperature, and the water
content (Smith et al. 2014; Tang et al. 2014). The possible high viscosity of deep
eutectic solvents is attributed to the presence of an extensive hydrogen-bonding
network, Van der Waals, and/or electrostatic interactions between the compounds
that restrict the mobility of the free species inside deep eutectic solvent and restrict
the dispersion of deep eutectic solvent in the extraction medium during the extraction process (Zhang et al. 2012; Habibollahi et al. 2018). The high viscosity is good
for the prevention of entrapped agents from vaporization, but it hampers the mass
transfer and thus produces lower extraction efficiency. Therefore, this problem can
be solved by increasing the temperature, thus leading to a better penetration of the
solvents in the sample; this is known as Arrhenius-like behavior (Bubalo et al.
2018). Also, adding a certain percentage of water to a deep eutectic solvent, at
which the deep eutectic solvent’s network is still maintained, is another way to
decrease deep eutectic solvent’s viscosity (Qi et al. 2015; Fernández et al. 2018).
However, Dai et al. (2015) showed that the addition of water above 50% can dissolve the deep eutectic solvent components in water (Dai et al. 2015). Also, the
addition of water can alter the hydrogen bonds between deep eutectic solvent and
the target compound (Bajkacz and Adamek 2017). Water-added deep eutectic solvents are more suitable for the extraction of polar compounds, whereas deep eutectic solvents with low water content are better for the extraction of nonpolar
compounds (Dai et al. 2013a, b). Other parameters that may influence the viscosity
of the deep eutectic solvent are the type of its components and the molar ratio. Cao
et al. (2018) showed that the extraction yield of proanthocyanidin was decreased
6 Methods for Extraction of Bioactive Compounds from Plant and Animal Matter…