145
measured isobarically, gas solubility is directly calculated by the difference of volume of the vapor phase before and after the gas absorption.
The mass of the solution in equilibrium with a given partial pressure of gas can
also be used to determine the solubility in gravimetric methods. To measure the
solubility of gases in deep eutectic solvents, two different gravimetric methods have
been reported. One, somewhat less accurate, consists on the low pressure bubbling
of gas in the liquid mixture that is weighed after equilibrium is reached. Other methodologies involve the use of gravimetric microbalances where a few milliliters of
liquid are put in contact with the gas and the change in mass of the liquid phase is
monitored as a function of the gas pressure at different temperatures. The amount of
required sample is, in this case, much lower and the measurements are precise with
relatively short equilibrium times. The disadvantages are linked with the necessity
of having a very precise balance in order to be able to measure light or scarcely
soluble gases. Furthermore, the accuracy of the gas absorption data obtained using
a microbalance is considerably lower when the liquid samples have a non-negligible
vapor pressure at the temperature of the measurements.
4.3 Gas Solubilities
Although the gas solubility data are reported in different units – mass ratio, molar
ratio, molality, or mole fraction – here it was decided to represent only the gas solubility data reported as Henry’s law constant, K H , which is defined as:
K
f x
i
x
i
i
i
H
,
,
lim
/
{ o
0
where f i is the fugacity of the gas and x i its molar fraction concentration in the solution at a given pressure and temperature. Henry’s law coefficients allow an easier
comparison between different gases and absorbents, making it easier to evaluate the
published data on gas absorption by different deep eutectic solvents in this chapter. For comparison purposes we considered the solubility data from Duan et al.
(2019) although Henry’s law constants in this case are based on molality instead of
molar fraction.
In Fig. 4.2, Henry’s law constants are presented for CO 2 , H 2 S, SO 2 , and NO – the
gases with larger solubilities in deep eutectic solvents. Solvents with different compositions but based on the same salts and hydrogen bond donors were studied by
different authors. Figure 4.3 represents Henry’s law constants for CO 2 in mixtures
of choline chloride with urea at different compositions and as a function of temperature. The differences in K H are significant (from ca. 18 bar to 30 bar in [Ch]Cl:U
(1:2.5) and [Ch]Cl:U (1:1.5), respectively) but correspond to mole fraction concentrations of CO 2 of the same order of magnitude – 5.5 × 10
−2
and 3.3 × 10
−2
at a
partial pressure of 1 bar of gas in [Ch]Cl:U (1:2.5) and [Ch]Cl:U (1:1.5), respectively. We have represented in Fig. 4.2 only one of the compositions studied for each
deep eutectic solvent.
4 Solubility of Gases in Deep Eutectic Solvents
measured isobarically, gas solubility is directly calculated by the difference of volume of the vapor phase before and after the gas absorption.
The mass of the solution in equilibrium with a given partial pressure of gas can
also be used to determine the solubility in gravimetric methods. To measure the
solubility of gases in deep eutectic solvents, two different gravimetric methods have
been reported. One, somewhat less accurate, consists on the low pressure bubbling
of gas in the liquid mixture that is weighed after equilibrium is reached. Other methodologies involve the use of gravimetric microbalances where a few milliliters of
liquid are put in contact with the gas and the change in mass of the liquid phase is
monitored as a function of the gas pressure at different temperatures. The amount of
required sample is, in this case, much lower and the measurements are precise with
relatively short equilibrium times. The disadvantages are linked with the necessity
of having a very precise balance in order to be able to measure light or scarcely
soluble gases. Furthermore, the accuracy of the gas absorption data obtained using
a microbalance is considerably lower when the liquid samples have a non-negligible
vapor pressure at the temperature of the measurements.
4.3 Gas Solubilities
Although the gas solubility data are reported in different units – mass ratio, molar
ratio, molality, or mole fraction – here it was decided to represent only the gas solubility data reported as Henry’s law constant, K H , which is defined as:
K
f x
i
x
i
i
i
H
,
,
lim
/
{ o
0
where f i is the fugacity of the gas and x i its molar fraction concentration in the solution at a given pressure and temperature. Henry’s law coefficients allow an easier
comparison between different gases and absorbents, making it easier to evaluate the
published data on gas absorption by different deep eutectic solvents in this chapter. For comparison purposes we considered the solubility data from Duan et al.
(2019) although Henry’s law constants in this case are based on molality instead of
molar fraction.
In Fig. 4.2, Henry’s law constants are presented for CO 2 , H 2 S, SO 2 , and NO – the
gases with larger solubilities in deep eutectic solvents. Solvents with different compositions but based on the same salts and hydrogen bond donors were studied by
different authors. Figure 4.3 represents Henry’s law constants for CO 2 in mixtures
of choline chloride with urea at different compositions and as a function of temperature. The differences in K H are significant (from ca. 18 bar to 30 bar in [Ch]Cl:U
(1:2.5) and [Ch]Cl:U (1:1.5), respectively) but correspond to mole fraction concentrations of CO 2 of the same order of magnitude – 5.5 × 10
−2
and 3.3 × 10
−2
at a
partial pressure of 1 bar of gas in [Ch]Cl:U (1:2.5) and [Ch]Cl:U (1:1.5), respectively. We have represented in Fig. 4.2 only one of the compositions studied for each
deep eutectic solvent.
4 Solubility of Gases in Deep Eutectic Solvents