660
4.1.2 Density
Density is an important property for the chemical processing. DESs have density
more than water but less than ILs. Density usually changes with type of HBD and
HBA and their molar ratio. In general, density is significantly affected by the HBA/
HBD molar ratio and decreases with increase in temperature. Like ionic liquids, the
density effects in DESs are explained based on the hole theory or free volume [120].
The density (ρ) of DESs can be modelled using a quadratic equation as given below:
ρ ρ
= +
+
0
2
aT bT
(1)
where “a,” “b,” and ρ 0 are constant and temperature T is in K.
4.1.3 Viscosity
The viscosity represents the resistance to flow which indirectly indicates the strength
of intermolecular interaction among the DESs’ precursor compounds. Most of the
DESs show high viscosity at room temperature which is due to extensive hydrogen
bond network between the DES precursors [113]. Viscosity of DESs depends on the
nature of precursor compounds, water content, and temperature. The viscosity
behavior of DESs can be also explained through hole theory. The viscosity of DES
is high if the empty vacancies are less due to higher structuring effect. At higher
temperature, the empty vacancies in DESs increase, and therefore viscosity
decreases. The effect of temperature (T) on the viscosity (μ) of DESs can be
described using Arrhenius equation as given below:
ln /
µ µ
µ
∞
(
)=
E
RT
(2)
where μ ∞ is the empirical constant, E μ is the activation energy for viscous flow, and
R is the gas constant.
4.1.4 Volatility
In the synthesis of DESs, one of the precursors is a salt, and other is volatile compounds. This makes DESs partially volatile as compared to non-volatile ILs.
Moreover, volatility of the DESs depends on the individual precursor. For example,
DESs formed by mixing two solid have low volatility compared to DESs formed by
mixing one volatile and one solid compound [112]. However, all the DESs have
sufficiently low volatility to categorize them under green solvent.
M. B. Haider et al.
4.1.2 Density
Density is an important property for the chemical processing. DESs have density
more than water but less than ILs. Density usually changes with type of HBD and
HBA and their molar ratio. In general, density is significantly affected by the HBA/
HBD molar ratio and decreases with increase in temperature. Like ionic liquids, the
density effects in DESs are explained based on the hole theory or free volume [120].
The density (ρ) of DESs can be modelled using a quadratic equation as given below:
ρ ρ
= +
+
0
2
aT bT
(1)
where “a,” “b,” and ρ 0 are constant and temperature T is in K.
4.1.3 Viscosity
The viscosity represents the resistance to flow which indirectly indicates the strength
of intermolecular interaction among the DESs’ precursor compounds. Most of the
DESs show high viscosity at room temperature which is due to extensive hydrogen
bond network between the DES precursors [113]. Viscosity of DESs depends on the
nature of precursor compounds, water content, and temperature. The viscosity
behavior of DESs can be also explained through hole theory. The viscosity of DES
is high if the empty vacancies are less due to higher structuring effect. At higher
temperature, the empty vacancies in DESs increase, and therefore viscosity
decreases. The effect of temperature (T) on the viscosity (μ) of DESs can be
described using Arrhenius equation as given below:
ln /
µ µ
µ
∞
(
)=
E
RT
(2)
where μ ∞ is the empirical constant, E μ is the activation energy for viscous flow, and
R is the gas constant.
4.1.4 Volatility
In the synthesis of DESs, one of the precursors is a salt, and other is volatile compounds. This makes DESs partially volatile as compared to non-volatile ILs.
Moreover, volatility of the DESs depends on the individual precursor. For example,
DESs formed by mixing two solid have low volatility compared to DESs formed by
mixing one volatile and one solid compound [112]. However, all the DESs have
sufficiently low volatility to categorize them under green solvent.
M. B. Haider et al.