148
Sulfur dioxide, SO 2 , is the most soluble of the four gases with reported K H at
293 K lower than 0.18 bar. Many of the deep eutectic solvents studied also dissolve
large quantities of NO, several of these mixtures, containing tetraalkylphosphonium
and tetraalkylammonium salts, having K H values at 303 K lower than 0.20 bar. H 2 S
and CO 2 are less soluble gases in the deep eutectic solvents studied so far with K H
values at 298 K and 303 K lower than 6 bar and 40 bar, respectively.
The mixtures that dissolve the largest quantities of SO 2 (K H < 0.12 bar) are based
on imidazolium or choline chloride salts with imidazole, methylimidazole, or glycerol as H-bond donors. For NO, these deep eutectic solvents were not studied; therefore, amongst the reported data, the eutectic mixtures based on tetraalkylphosphonium
salts and ethylene glycol or 1,3-dimethylthiourea are the ones that absorb larger
quantities of gas with reported K H lower than 0.15 bar. Eutectic mixtures of tetraalkylphosphonium salts are also the ones that absorb larger quantities of H 2 S with
Henry’s law constants at 298 K lower than 1 bar. For CO 2 , tetrabutylammonium
salts mixed with diethanolamine and allyltriphenylphosphonium salts mixed with
triethylene glycol are the deep eutectic solvents with larger capacities with K H values at 303 K lower than 5 bar, probably ought to a chemical reaction between the
gas and one of the components of the liquid mixture. Even if no evident trends could
be found, tetraalkylphosphonium or tetraalkylammonium salts seem to be the constituents of the deep eutectic solvents that are capable of absorbing larger quantities of gas.
Polarity seems to be at first sight a good descriptor for the gas absorption in deep
eutectic solvents as polar gases, like SO 2 or H 2 S, seem to be more soluble in deep
eutectic solvents. Nevertheless, non-polar gases like NO are also very soluble in
deep eutectic solvents (although a relatively small number of solvents have been
studied) and quadrupolar gases as CO 2 have a much lower solubility.
The analysis of the behavior of the gas absorption with temperature is important
as it allows to ascertain the relative importance of the gas-solvent interactions and
of the structural organization of the solution to the solvation process. The Gibbs
energy of solvation, defined as the difference in chemical potential when the solute
is transferred from an ideal gas standard pressure into the reference state in the solution at infinite dilution, is related with Henry’s law coefficient through
' solv
H
G RT
K
p
i
i
§
©
¨
·
¹
¸
ln
,
0
its variation with temperature being directly related with the enthalpy of solvation
'
'
solv
solv
H
T T
G
T
i
i
p
w
w
§
©
¨
·
¹
¸
2
Figure 4.4 represents Henry’s law constants as a function of temperature for seven
different gases in the deep eutectic solvent [Ch]Cl:U (1:2). Three groups of solubility data can be identified with the lowest values of K H corresponding to SO 2 , followed by a second group of gases including H 2 S and CO 2 and a third group of less
L. Moura et al.
Sulfur dioxide, SO 2 , is the most soluble of the four gases with reported K H at
293 K lower than 0.18 bar. Many of the deep eutectic solvents studied also dissolve
large quantities of NO, several of these mixtures, containing tetraalkylphosphonium
and tetraalkylammonium salts, having K H values at 303 K lower than 0.20 bar. H 2 S
and CO 2 are less soluble gases in the deep eutectic solvents studied so far with K H
values at 298 K and 303 K lower than 6 bar and 40 bar, respectively.
The mixtures that dissolve the largest quantities of SO 2 (K H < 0.12 bar) are based
on imidazolium or choline chloride salts with imidazole, methylimidazole, or glycerol as H-bond donors. For NO, these deep eutectic solvents were not studied; therefore, amongst the reported data, the eutectic mixtures based on tetraalkylphosphonium
salts and ethylene glycol or 1,3-dimethylthiourea are the ones that absorb larger
quantities of gas with reported K H lower than 0.15 bar. Eutectic mixtures of tetraalkylphosphonium salts are also the ones that absorb larger quantities of H 2 S with
Henry’s law constants at 298 K lower than 1 bar. For CO 2 , tetrabutylammonium
salts mixed with diethanolamine and allyltriphenylphosphonium salts mixed with
triethylene glycol are the deep eutectic solvents with larger capacities with K H values at 303 K lower than 5 bar, probably ought to a chemical reaction between the
gas and one of the components of the liquid mixture. Even if no evident trends could
be found, tetraalkylphosphonium or tetraalkylammonium salts seem to be the constituents of the deep eutectic solvents that are capable of absorbing larger quantities of gas.
Polarity seems to be at first sight a good descriptor for the gas absorption in deep
eutectic solvents as polar gases, like SO 2 or H 2 S, seem to be more soluble in deep
eutectic solvents. Nevertheless, non-polar gases like NO are also very soluble in
deep eutectic solvents (although a relatively small number of solvents have been
studied) and quadrupolar gases as CO 2 have a much lower solubility.
The analysis of the behavior of the gas absorption with temperature is important
as it allows to ascertain the relative importance of the gas-solvent interactions and
of the structural organization of the solution to the solvation process. The Gibbs
energy of solvation, defined as the difference in chemical potential when the solute
is transferred from an ideal gas standard pressure into the reference state in the solution at infinite dilution, is related with Henry’s law coefficient through
' solv
H
G RT
K
p
i
i
§
©
¨
·
¹
¸
ln
,
0
its variation with temperature being directly related with the enthalpy of solvation
'
'
solv
solv
H
T T
G
T
i
i
p
w
w
§
©
¨
·
¹
¸
2
Figure 4.4 represents Henry’s law constants as a function of temperature for seven
different gases in the deep eutectic solvent [Ch]Cl:U (1:2). Three groups of solubility data can be identified with the lowest values of K H corresponding to SO 2 , followed by a second group of gases including H 2 S and CO 2 and a third group of less
L. Moura et al.