13
vs 527.28 mPa.s for 1:2 ChCl:U at 30 °C and 202 mPa.s vs 2142 mPa.s for 1:1
ChCl:oxalic acid at 40 °C) (García et al. 2015). These major differences can be
attributed not only to the preparation method as stated by Florindo et al. (Florindo
et al. 2014) but also to the experimental method and the impurities (García
et al. 2015).
1.4.4 Ionic Conductivity
Since the viscosity is the main controller of the conductivity, most of the deep eutectic solvents tend to have poor ionic conductivities (к < 2 mS cm
−1
at room temperature). Therefore, increasing the temperature results in a decrease in the viscosity and
an increase in the conductivity (Lapeña et al. 2019; Zhang et al. 2012). This property is also affected by the hydrogen bond acceptor/hydrogen bond donor molar
ratio (Abbott et al. 2004a), the nature of both the organic salt and the hydrogen bond
donor as well as the salt’s anion (García et al. 2015), and of course the water addition (Dai et al. 2015).
1.4.5 Surface Tension
The studies related to the surface tension of deep eutectic solvents are quite limited
compared to the studies dealing with other physicochemical properties. Yet, it is an
essential property since it is highly dependent on the intensity of the intermolecular
forces taking place between the hydrogen bond donor and the corresponding salt.
That said, highly viscous liquids present high surface tensions. The values relative
to the reported deep eutectic solvents generally vary between 35 and 75 mN m
−1
at
25 °C (García et al. 2015; Ibrahim et al. 2019). Once again remarkable high values
were recorded for sugar-based deep eutectic solvents such as ChCl:D-glucose
(Hayyan et al. 2013) and ChCl:D-fructose (Hayyan et al. 2012), reflecting their
extensive hydrogen bond network. Besides, the surface tension is influenced by the
salt mole fraction and the cation type since an additional hydroxyl group or a longer
alkyl chain in the quaternary ammonium salt leads to higher surface tensions. Also,
the surface tension linearly decreases with increasing temperature (García et al.
2015; Lapeña et al. 2019; Nunes et al. 2019).
1.4.6 Polarity
Polarity is a key property since it reflects the overall solvation capability of solvents.
Despite its significance, the polarity of the deep eutectic solvents was not considerably studied and was not addressed until recently. This property is often estimated
1 Understanding the Basics and Properties of Deep Eutectic Solvents
vs 527.28 mPa.s for 1:2 ChCl:U at 30 °C and 202 mPa.s vs 2142 mPa.s for 1:1
ChCl:oxalic acid at 40 °C) (García et al. 2015). These major differences can be
attributed not only to the preparation method as stated by Florindo et al. (Florindo
et al. 2014) but also to the experimental method and the impurities (García
et al. 2015).
1.4.4 Ionic Conductivity
Since the viscosity is the main controller of the conductivity, most of the deep eutectic solvents tend to have poor ionic conductivities (к < 2 mS cm
−1
at room temperature). Therefore, increasing the temperature results in a decrease in the viscosity and
an increase in the conductivity (Lapeña et al. 2019; Zhang et al. 2012). This property is also affected by the hydrogen bond acceptor/hydrogen bond donor molar
ratio (Abbott et al. 2004a), the nature of both the organic salt and the hydrogen bond
donor as well as the salt’s anion (García et al. 2015), and of course the water addition (Dai et al. 2015).
1.4.5 Surface Tension
The studies related to the surface tension of deep eutectic solvents are quite limited
compared to the studies dealing with other physicochemical properties. Yet, it is an
essential property since it is highly dependent on the intensity of the intermolecular
forces taking place between the hydrogen bond donor and the corresponding salt.
That said, highly viscous liquids present high surface tensions. The values relative
to the reported deep eutectic solvents generally vary between 35 and 75 mN m
−1
at
25 °C (García et al. 2015; Ibrahim et al. 2019). Once again remarkable high values
were recorded for sugar-based deep eutectic solvents such as ChCl:D-glucose
(Hayyan et al. 2013) and ChCl:D-fructose (Hayyan et al. 2012), reflecting their
extensive hydrogen bond network. Besides, the surface tension is influenced by the
salt mole fraction and the cation type since an additional hydroxyl group or a longer
alkyl chain in the quaternary ammonium salt leads to higher surface tensions. Also,
the surface tension linearly decreases with increasing temperature (García et al.
2015; Lapeña et al. 2019; Nunes et al. 2019).
1.4.6 Polarity
Polarity is a key property since it reflects the overall solvation capability of solvents.
Despite its significance, the polarity of the deep eutectic solvents was not considerably studied and was not addressed until recently. This property is often estimated
1 Understanding the Basics and Properties of Deep Eutectic Solvents