Article
pubs.acs.org/IECR
An Overview of Mutual Solubility of Ionic Liquids and Water
Predicted by COSMO-RS
Teng Zhou,† Long Chen,† Yinmei Ye,† Lifang Chen,† Zhiwen Qi,†,* Hannsjörg Freund,‡
and Kai Sundmacher‡,§
†
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai
200237, China
‡
Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany
§
Process Systems Engineering, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany
S Supporting Information
*
ABSTRACT: To properly screen and use ionic liquids (ILs) as environmental-friendly solvents in chemical reactors and
separation processes, the knowledge of their solubilities with water is essential. In the present work, mutual solubilities of 1500
ILs (50 cations, 30 anions) with water at 298.15 K were predicted by using the conductor-like screening model for real solvents
(COSMO-RS) as a thermodynamic model. On the basis of the COSMO-RS calculations, the influence of the types of anion and
cation, side chain modifications and substituent groups on the mutual solubility with water was extensively analyzed. The data
obtained can be used for the prescreening of ILs as solvent candidates. Moreover, to understand the intrinsic solubility behavior
in detail, different types of molecular interactions between ILs and water in solution were compared on the basis of the
determination of multiple water−IL interaction energies from COSMO-RS computation. The results confirm that hydrogen
bonding interactions between anions and water molecules have the dominant influence on the solubility. Finally, for the purpose
of fast solubility estimation and solvent selection, COSMO-RS derived molecular descriptors which indicate the strength of
anionic HB acceptors were calculated for typical anions and anion families.
1. INTRODUCTION
Ionic liquids (ILs) are molten salts that exist as liquids at or
near room temperature. The ionic nature of these liquids results
in many unique and attractive physical and chemical properties.
The characteristics and applications of ILs have been
elaborately summarized in a number of excellent reviews.1−4
With regard to potential applications in chemical processes,
there is considerable interest in replacing conventional volatile
organic solvents by ILs in liquid−liquid extraction processes.5−9
In particular, it has been found that ILs have a high capacity for
separating organics,10−13 metal ions,14 or biofuels15 from
aqueous phases. Additionally, ILs are promising solvent
candidates for intensifying chemical reactions that take place
in the aqueous phase and/or use water as a reactant.16−18 Even
a small amount of water in ILs can dramatically change their
physicochemical characteristics in mixtures.19−24 Furthermore,
the solubility of ILs in water is an essential property that has to
be determined for evaluating their impact on the environment,
health, and safety. The critical aspect here is that ILs can be
toxic because they can accumulate in organisms due to their
hydrophobicity.25−28
In conclusion, the knowledge of the mixing behavior between
ILs and water is of great practical relevance for their potential
use in future chemical processes. But up to now, experimental
phase equilibrium data and mutual solubilities of ILs with water
are reported only for a very few ion combinations.29−39 It is
expected that, due to the very high number of possible cation−
anion combinations, even on the long-term it will not be
possible to generate a complete experimental data basis for all
© 2012 American Chemical Society
binary IL/water mixtures. For this reason, one has to make use
of reliable theoretical models for predicting the solubility and
phase behavior of IL-systems such as the conductor-like
screening model for real solvents (COSMO-RS). This model
is based on the quantum chemical description of individual
molecules. It only requires universal parameters and elementspecific parameters. For this reason, COSMO-RS is a widely
accepted tool for predicting the thermodynamic properties of
ILs. For example, COSMO-RS was applied for the prediction of
liquid−liquid equilibria and for solvent prescreening.9−11,40,41
More recently, the solubility characteristics of biological
macromolecules and polymers in ILs have also been modeled
by COSMO-RS.42,43
In the present work, the solubility for IL/water binary
systems predicted by COSMO-RS is presented. The effects of
the type of anion, the type of cation, and structural variations of
the cation were studied systematically by means of extensive
computational screening within the IL molecular structure
space.
2. THEORETICAL BASIS OF COSMO-RS
COSMO-RS is a novel and fast methodology for predicting
thermo-physical and chemical properties of fluids and liquid
mixtures based on unimolecular quantum chemical calculations.
Received:
Revised:
Accepted:
Published:
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March 8, 2012
April 16, 2012
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two polar hydrogen and lone-pairs of oxygen atom,
respectively. The carbonyl oxygen of acetone results in its
peak at +0.013 e/Å2 and the polar hydrogen of chloroform
leads to the peak at −0.014 e/Å2. As for hexane, its distribution
of charge densities is narrowed in the middle region due to the
nonpolar character. Qualitatively, when the screening charge
density goes beyond ±0.008 e/Å2, the molecule is polar enough
to form a hydrogen-bond. The higher the charge density is, the
stronger the HB acceptor (+) or HB donor (−) will be. Based
on the definition of σ-profile, there are two important σmoments related to hydrogen bonds, namely HB_acc3 and
HB_don3, representing the capability of a hydrogen bond
acceptor and a hydrogen bond donor, respectively.46 Because of
the ionic characters of ILs, anions are strong HB acceptors but
weak donors, and cations can act as strong HB donors but as
weak acceptors.
Since the full description of the theory has been given
elsewhere,44 only the major features for understanding the
analysis and discussion in the present work are highlighted
here. There are two major steps in the COSMO-RS prediction
procedure. In the first step, the continuum solvation model
COSMO is applied in order to simulate a virtual conductor
environment for the molecule of interest. A screening charge
density σi on the nearby conductor is ultimately induced and
obtained through the standard quantum chemical computation
(usually DFT method). The 3D distribution of the screening
charge density on the surface of each molecule x is converted
into a surface composition function, namely the σ-profile,
Px(σ). In the second step, the statistical thermodynamics
treatment of the molecular interactions is performed in the
COSMOtherm program.45 The interaction energies of the
surface pairs are defined in terms of the screening charge
densities σ and σ′ of the respective surface segments. Finally,
the chemical potential of a surface segment, the so-called σpotential, is calculated by
μs (σ ) = −
3. COMPUTATIONAL METHOD
The solubility predictions were performed using the software
package COSMOtherm (COSMOlogic GmbH & Co KG,
Leverkusen, Germany).45 The COSMO input files for water
and most ILs were taken from the COSMO database. The
input files of ILs not stored in this database were obtained from
quantum chemical calculations performed on the Density
Functional Theory (DFT) level using the program package
Gaussian 3.0, utilizing the functional of BP (B88-VWN-P86)
and basis set of TZVP.47 To ensure an efficient computation,
the effect of different conformations was studied beforehand. In
our calculation, all the stable conformations were considered
and weighted according to the Boltzmann distribution function,
which is different from the usual treatment only considering the
lowest-energy conformation.
In COSMO-RS, the solubility is calculated from the
differences between the chemical potentials of the solute in
(P)
(S)
the solvent μ(S)
j and in pure solute μj (eq 2), where μj is the
infinite dilution chemical potential of the solute j in the solvent
S. The solubility thus computed is a zeroth order approximation.
a
RT ⎡
ln⎢ ps (σ ′) exp eff [μS (σ ′)
RT
aeff ⎣
∫
{
⎤
− Emisfit(σ , σ ′) − E HB(σ , σ ′)] dσ ′⎥
⎦
}
(1)
where aeff represents the effective contact area and ps(σ) stands
for the surface screening charge distribution of the whole
system. The chemical potential of a compound is available from
the integration of the σ-potential over the surface of the
molecule. The capability of COSMO-RS to calculate the
chemical potential of an arbitrary solute x in any pure or mixed
solvent S at variable temperature enables the prediction of
thermodynamic properties such as the solubility.
Figure 1 shows σ-profiles and COSMO-cavities of four
representative compounds: water, acetone, chloroform, and
hexane. Water has a very broad σ-profile with two pronounced
peaks around −0.016 e/Å2 and +0.018 e/Å2 resulting from the
log10(xjSOL) = [μj(P) − μj(S) − max(0, ΔGfus)]
/(RT ln(10))
(2)
To improve the accuracy of prediction, the solubility can be
refined iteratively, as expressed in eq 3
log10(xjSOL(n + 1)) = [μj(P) − μj(S)(xjSOL(n)) − max(0, ΔGfus)]
/(RT ln(10))
(3)
In this work, all the solubility data were obtained using the
iterative algorithm. But for better comparability of the effects of
different molecular interactions on solubilities, a noniterative
mode was utilized. It should also be pointed out that, in the
COSMO-RS calculation, ILs are treated as electro-neutral
mixtures of separated cations and anions with the molar ratio
reflecting the stoichiometry of the IL. The mole fraction used in
the calculations differs from the mole fraction normally defined
in experiments, where the IL is treated as one compound.
Therefore, COSMO-RS derived properties that depend on the
mole fraction definition, such as solubilities, activity coefficients,
and Henry constants, have to be converted to ensure the same
reference standard as used for the experimental value. In this
Figure 1. σ-Profiles and COSMO-cavities of representative compounds.46 The blue color indicates a negative surface screening charge
resulting from positive partial charges within the molecule. The red
color indicates a positive surface charge and green symbolizes almost
neutral charges.
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molecular structure variations on mutual solubilities with the
aid of COSMO-RS σ-profile theory.
4.2.1. Influence of Anion Identity. As can be seen from
Figure 3, mutual solubilities between water and ILs with the
same anion vary in a relatively limited range. For instance, the
mole fraction of water dissolved in [PF6]−-based ILs is found to
be on the order of 10−1 (Figure 3A) and the corresponding ILin-water solubilities are mainly around 10−2.5 (Figure 3B). For
[Tf2N]−, the logarithmic mole fraction of water solubility and
IL solubility generally falls into the region of [−0.5, −0.8] and
[−3.0, −5.0], respectively. Interestingly, when combined with
suitable cations, some very hydrophilic anions (e.g., acetate,
methylsulfate, dihydrogenphosphate) are completely miscible
with water according to COSMO-RS predictions. However,
mutual solubilities between water and ILs with the same cation
usually vary significantly, generally from completely miscible to
almost insoluble with the alteration of anions. On the basis of
these trends it can be concluded that the mutual solubilities of
ILs with water are mainly dependent on the nature of the
anion, expect for some special cations like guanidinium and
diethanolammonium which lead to almost complete miscibility
of the corresponding ILs with water, according to our
COSMO-RS calculations.
Mean values of mutual solubilities of typical anion families
were evaluated based on more extensive COSMO-RS
calculations considering 62 anions. As can be seen from
Table 1, the anions were classified into three families according
to their mutual solubilities. Group 1 comprises decanoate,
acetate, halogen, alkyl-phosphinate/phosphate/sulfonate, and
-sulfate, which usually have a large miscibility with water (some
are theoretically completely miscible according to COSMORS). Group 2 has intermediate solubility of ILs with water. It
includes fluoroalkyl-phosphinate, fluoroalkyl-sulfonate, and
polar borates. It is worth mentioning that the phosphinate-,
phosphate-, and sulfonate-based ILs are miscible with water
when combined with an alkyl group. However, solubilities
decrease sharply when the alkyl hydrogen is replaced by
fluorine due to a more decentralized charge density. Group 3
contains the most hydrophobic anions, namely the less polar
borates, PF6, AsF6, SbF6, (fluoroalkylsulfonyl)-amide, -imide,
-methane, -methide, and fluoroalkyl-phosphate. Correspondingly, the IL-water solubilities of this group are much smaller,
some ILs can even be regarded as completely immiscible with
water. Figure 4 shows σ-profiles and COSMO-cavities of four
representative anions ([Tf2N]−, [MeS]−, [TfS]−, [FEP]−) from
different anion groups (see Supporting Information, Table S1).
Their HB accepting properties can be ranked as [MeS]− >
[TfS]− > [Tf2N]− > [FEP]− which is consistent with their
solubility trend and suggests that HB interactions between
water and the anions play an important role on the solubility
behavior. Related quantitative studies, from the molecular
interaction point of view, will be presented in section 4.3.
As indicated by the presented COSMO-RS data, mutual
solubilities between water and ILs are dominated by anions.
Therefore, the mixing behavior of ILs and water can be
manipulated simply by changing the anion. On the basis of this
idea, Table 1 directly provides data for the fast preselection of
ILs. However, as a matter of fact, the cation structure such as
the alkyl chain length, side chain number, and substituent
groups has a non-negligible influence on the thermodynamic
properties of ILs. In fact, these cationic factors of influence can
be utilized for fine-tuning of the solubility properties. This is
discussed in the subsequent sections.
work, the transition calculations were ultimately performed for
all the predicted solubility data.
4. RESULTS AND DISCUSSION
4.1. Validation of COSMO-RS Predictions. It is quite
difficult to measure ILs solubilities in water and there are
usually large differences between the results of different
researchers.29−31 Nevertheless, the published experimentally
determined water-in-IL solubilities were used to validate our
COSMO-RS predictions. The cation set covers typical cationic
categories, like imidazolium, pyridinium, pyrrolidinium, and
phosphonium. The anions include [PF6]‑, [BF4]‑, [Tf3C]‑,
[C(CN)3]‑, and [Tf2N]‑ (see Supporting Information, Table
S1). As can be seen from the parity plot in Figure 2, water
Figure 2. Comparison of COSMO-RS predictions with experimental
data31−36 of water solubilities in 19 different ILs at 298.15 K.
solubilities predicted by COSMO-RS are in good agreement
with the experimental results despite some reasonable
deviations. Besides, this advanced predictive model has also
been evaluated and validated in literatures, with the conclusion
that COSMO-RS can yield reliable qualitative and satisfactory
quantitative predictions for water/IL systems.31,33,34,48 Nevertheless, COSMO-RS is a suitable tool for qualitatively ranking
ILs with regard to their solubility in water.
4.2. Mutual Solubilities. Mutual solubilities between 1500
ILs (50 cations and 30 anions) and water at 298.15 K were
predicted by COSMO-RS (see Figure 3). The set of cations
used for these calculations covers the most representative types,
such as imidazolium, pyridinium, pyrrolidinium, piperidinium,
sulfonium, ammonium, and phosphonium. The anions
examined include the widely used ones (e.g., [BF4]−, [PF6]−,
[Tf2N]−) and other typical categories, such as halogen,
-phosphate, -sulfate, and -sulfonate (see Supporting Information, Table S1). As a general trend found in the simulated
results, water-in-IL solubilities (Figure 3A) and IL-in-water
solubilities (Figure 3B) show a similar dependency on IL
molecular structure, that is, on the cation−anion combination.
But one should note that the absolute solubility values are quite
different. While the water-in-IL solubility varies from Xw =
10−1.8 up to 1, the IL-in-water solubility XIL ranges from 10−12
to almost completely miscible. It is worth mentioning that the
main objective of this study is not only to predict the spectrum
of solubility values, but also to understand the effect of IL
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Figure 3. Predicted mutual solubilities of water with 1500 ILs at 298.15 K. (A) water in ILs; (B) ILs in water.
[C 4mim]+, [C4 mpy]+, [C 4mpyr]+, and [C 4mpip]+ (see
Supporting Information, Table S1). To avoid cases of complete
miscibility and to keep a relatively wide range of solubilities,
four hydrophobic anions ([FMC], [Tf2N], [FEN], [Tf3C])
were chosen to elucidate the effect of cation families. From
4.2.2. Influence of Cation Family. The most widely used
cations, such as imidazolium, pyridinium, pyrrolidinium, and
piperidinium, contain aromatic or aliphatic rings. Therefore,
our studies were based on the analysis of mutual solubilities of
cations with the same alkyl substituent group, that is,
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Table 1. Mutual IL/Water Solubilities at 298.15 K Predicted
with COSMO-RS
IL typea
group 1
group 2
group 3
decanoate, acetate
alkyl-phosphinate
alkyl-phosphate
alkyl-sulfonate
Cl, Br, I
-sulfate
fluoroalkyl-phosphinate
fluoroalkyl-sulfonate
-borate1
-borate2
PF6, AsF6, SbF6
(fluoroalkylsulfonyl) -amide, -imide,
-methane, -methide
fluoroalkyl-phosphate
log10(xw)
log10(xIL)
0
0
0
0
−0.001
−0.003
−0.034
−0.063
−0.068
−0.449
−0.592
−0.641
0
−0.030
0
−0.026
−0.066
−0.171
−0.710
−1.000
−1.786
−2.935
−2.653
−3.789
−0.969
−5.030
Notes: “-borate1” represents polar borates such as bismalonatoborate
(C6H4BO8), bissalicylatoborate (C14H8BO6), and bisbiphenyldiolatoborate (C24H16BO4); “-borate2” represents less polar borates without
the very polar hydrogen atom, such as bisoxalatoborate (C4BO8) and
tetracyanoborate (C4BN4).
a
Figure 5. Predicted mutual solubilities of water with ILs based on
cations with the same alkyl substituent at 298.15 K. (A) water in ILs;
(B) ILs in water.
Figure 4. σ-Profiles and COSMO-cavities of four representative
anions.
Figure 5A, water-in-IL solubilities follow the cation ranking
[C4mim]+ > [C4mpy]+ > [C4mpyr]+ > [C4mpip]+ for all anions
studied. According to the COSMO-RS approach, a broader
distribution of the screening charge density usually indicates a
higher electronic acidity (i.e., stronger cationic HB donor
capability, see Figure 6) when interacting with water, which
finally results in a higher water solubility. On the other hand,
IL-in-water solubilities can be generally ordered as [C4mim]+ >
[C4mpyr]+ > [C4mpy]+ > [C4mpip]+ (Figure 5B). The
corresponding COSMO volumes are 199.86 Å3, 215.62 Å3,
217.09 Å3, and 232.28 Å3, respectively. It seems that the
solubilities of ILs in water are more dependent on the size than
on the HB acidity of the cations.32,38 The different trends of
water-in-IL solubilities and IL-in-water solubilities are believed
to originate from the different thermodynamic solution
behaviors between IL-rich phase and water-rich phase.32
4.2.3. Effect of Cation Side Alkyl Chain and Length.
Among all IL cations, imidazolium ions are well-known to have
tunable properties such as miscibility, melting point and
viscosity. Thus property tuning can be achieved by structural
modifications, especially on the 1- and 3- positions of the
Figure 6. σ-Profiles of four typical cations with the same alkyl
substituent.
imidazolium ring. Figure 7 shows predicted mutual solubilities
between water and imidazolium-based ILs, for different side
chain length and number. In this study, four representative
anions were employed to check for possible cation−anion
interferences during the cation structural variations. As can be
seen from Figure 7A, the influence of the cation side chain
variations on ILs solubilities follows the same trend for all
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Figure 8. σ-Profiles and COSMO-cavities of three imidazolium-based
cations: [C4mim]+, [C4C1mim]+, [C6mim]+.
4.2.4. Effect of the Substituent Group. In Table 2 predicted
mutual solubilities between water and ILs consisting of the
Table 2. Mutual IL/Water Solubilities for Different
Substituent Groups at 298.15 K Predicted with COSMO-RS
ionic liquids
xw
[HOC2mim][Tf2N]
[C2mim][Tf2N]
[C1OC2mim][Tf2N]
[HSO3C3py][Tf2N]
[HOC3py][Tf2N]
[C2C1mim][Tf2N]
[C2C1phmim][Tf2N]
[C4py][Tf2N]
[C4-4-mpy][Tf2N]
[C4DMApy][Tf2N]
0.6637
0.3275
0.3134
0.7833
0.6173
0.2473
0.1980
0.2687
0.2268
0.1751
xIL
1.43
2.18
1.30
8.62
7.76
1.28
2.36
8.52
4.56
1.91
×
×
×
×
×
×
×
×
×
×
10−2
10−3
10−3
10−3
10−3
10−3
10−4
10−4
10−4
10−4
anion [Tf2N]‑ and cations with typical substituent groups are
given. From these data it can be concluded that the mutual
solubilities significantly increase when a hydroxyl group is
added at the end of the alkyl chain. The introduction of a
methoxy group increases the electronic capacity of the IL to
interact with water. But a methyl inclusion can also contribute
to a larger nonpolar surface which leads to a larger dissimilarity
of IL and water molecules. This is the reason why
[C1OC2mim][Tf2N] shows a similar mutual solubility with
water like [C2mim][Tf2N]. It is also interesting that cationic
sulfo groups lead to a much higher hydrophilicity than hydroxyl
groups, as can be concluded from the solubilities predicted for
the ILs [HSO3C3py][Tf2N] and [HOC3py][Tf2N]. Finally,
adding a phenyl group at the end of the alkyl chain or
introducing a dimethylamino group in para-position of the
cationic aromatic ring was found to decrease the solubilities.
4.3. Solvation Characteristics: Multiple Molecular
Interactions. As a well-founded solvation model, COSMORS is able to yield important information on molecular solute−
solvent interactions. On the basis of this model, three
interaction parameters can be derived, namely the misfit
energy, the HB energy, and the vdW energy which quantify the
electrostatic interactions, the hydrogen bonding interactions,
and the van der Waals interactions, respectively. Thereby one
can attribute macroscopic thermodynamic properties to the
most important molecular interaction forces through the
determination and comparison of different types of interaction
energies based on COSMO-RS computation. Considering the
high complexity of solute−solvent interactions in water-rich
Figure 7. Mutual solubilities of water with ILs incorporating
imidazolium cations and four typical anions at 298.15 K: (A) ILs in
water; (B) water in ILs. COSMO-RS predictions (solid line),
experimental data (dashed line).
anions examined. The IL solubility decreases significantly as the
length or number of the alkyl chain increases, which has also
been demonstrated by experimental measurements.31,33 ILs
with a larger molecular size obviously have a lower charge
density and polarity, which contributes to a lower IL solubility.
According to COSMO-RS, a larger nonpolar surface of the IL
molecule should cause more dissimilarity or mismatching when
interacting with water, thus leading to a more consumption of
misfit energy and finally to a lower IL solubility.
As illustrated in Figure 7B, a similar trend of the water-in-IL
solubility can be observed except for the cation [C4C1mim]+.
This finding can be interpreted with the help of the σ-profiles
depicted in Figure 8. Because of the introduction of the methyl
substituent, [C4C1mim]+ has a much smaller HB donor
capability. When the acidic hydrogen in the C2 position of
the imidazolium ring is replaced by an alkyl group, the
interaction between IL and water is dramatically reduced, thus
resulting in much lower water solubility.
A direct comparison of COSMO-RS predictions with
experimental solubility data for [Tf2N]-based ILs is given in
Figure 7A and Figure 7B. It is clear that COSMO-RS can yield
reliable qualitative and acceptable quantitative predictions for
both water-in-IL solubilities and IL-in-water solubilities.
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HB acceptor strength (basicity), and the σ-moment HB_don3
which indicates the HB donor strength (acidity). While water
can act as a donor and also as an acceptor for hydrogen
bonding, anions are strong HB acceptors but weak donors. In
Table 3, the HB_acc3 values of representative anions and anion
families are listed. As a general rule, the HB interactions
between water and ILs become stronger when incorporating an
anion with higher HB_acc3 value. Apart from a few exceptions
which may arise from the inconsistent effects of molecular sizes,
shapes, and general polarities, the HB_acc3 values can be
regarded as adequate descriptors for IL/water solubilities
(compare with Table 1).
phases, and because of similar tendencies of mutual solubilities
in water/IL mixtures, only the case of water dissolving in the
IL-rich phase was investigated in this work.
Since water solubilities were found to depend mainly on the
anion type, the above-mentioned water−IL interaction energies
were determined based on 30 different anions, using the same
cation, namely the imidazolium [C8mim]+ (Figure 9). It can be
5. CONCLUSIONS
Mutual solubilities of 1500 different ILs and water were
predicted and systematically analyzed based on extensive
COSMO-RS calculations, which are very helpful for fast
prescreening and for guiding the molecular structure design
of ILs. On the basis of the obtained simulation data, important
molecular structure parameters that dominate the macroscopic
solubility behavior of IL/water mixtures were identified and the
strong influence of the anion type on solubilities was analyzed.
With regard to cations, short and monobranched alkyl groups
are recommended for increasing the miscibility of ILs with
water. A higher electronic acidity of the cation is preferable for
achieving higher water solubility.
COSMO-RS is also very useful in describing the solute−
solvent molecular interactions in terms of the σ-moments. The
most important moment of this type is the descriptor HB_acc3
which clearly indicates that IL/water solubilities are primarily
governed by the HB acceptor strength of the anions.
Figure 9. Predicted molecular interaction energies of water with ILs
based on the cation of [C8mim]+ and 30 anions at 298.15 K.
■
observed that among the three interaction energies, vdW and
misfit energies are around −1.1 and 0.5 kcal/mol, respectively.
Both of them show very small variations when changing the
anion type. On the contrary, the HB energy varies substantially
from −0.8 to −8.5 kcal/mol for all the ILs. Therefore, it can be
concluded that the HB interactions between water and anions
are the determinant for water solubility in ILs. In general, the
stronger HB interactions between the anion and water
molecule are, the higher HB energies are gained, and
consequently the higher water solubility values are attained.
As mentioned before, the so-called σ-moments which are
very important molecular descriptors can be determined from
COSMO-RS computation. In our cases, the most useful
descriptors are the σ-moment HB_acc3 which quantifies the
ASSOCIATED CONTENT
S Supporting Information
*
Details on cations and anions studied in this work are given in
Table S1. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: +86 21-64253528.
Notes
The authors declare no competing financial interest.
Table 3. COSMO Descriptor HB_acc3 for Typical Anions or Anion Families
anion
HB_acc3
anion
HB_acc3
alkyl-phosphinate
acetate
decanoate
Cl
alkyl-phosphate
alkyl-sulfonate
Br
salicylate
trifluoroacetate
NO3
I
-sulfate
37.43−43.59
38.928
38.072
36.63
34.45−35.88
25.65−30.08
29.64
21.686
20.208
19.041
18.84
18.34−20.46
dicyanamide
thiocyanate
fluoroalkyl-phosphinate
-borate1
fluoroalkyl-sulfonate
tricyanomethane
ClO4
-borate2
(fluoroalkylsulfonyl) -amide, -imide, -methane, -methide
fluoroalkyl-phosphate
PF6, AsF6, SbF6
I3
15.666
12.291
11.80−12.56
10.95−12.59
7.16−10.68
7.593
3.885
2.49−3.45
0.91−3.36
0.01−0.08
0
0
6262
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Industrial & Engineering Chemistry Research
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ACKNOWLEDGMENTS
■
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The authors gratefully acknowledge the financial support of this
research work by the Max Planck Partner Group program of
the Max Planck Society in Germany, the National Natural
Science Foundation of China (NSFC 21076074, 21006029),
the Shanghai Pujiang Talents Program (10PJ1402400), the
Program of Introducing Talents of Discipline to Universities
(111 Project: B08021), and the Fundamental Research Funds
for Central Universities of China.
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