Ionic liquids as solvents of polar and non-polar solutes: affinity and coordination.

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Cite this: DOI: 10.1039/c5cp01774b

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Ionic liquids as solvents of polar and non-polar solutes: affinity and coordination† Elixabete Rezabal*a and Thomas Scha¨ferbc The use of ionic liquids (ILs) as highly tuneable solvents requires a deep understanding of the intermolecular interactions they can establish with the solutes. In the present work, we study the solvation patterns of two small but highly important molecules in the framework of IL properties and applications, namely H2O and

Received 26th March 2015, Accepted 6th May 2015

CO2. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) techniques are used for a

DOI: 10.1039/c5cp01774b

of the non specific and specific interactions on the affinity of the IL for the solute, and how these

systematic study of the interactions established between the solute and the solvent, identifying the influence interactions change with the surrounding environment. The nature, spatial distribution and strength of these

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interactions are described by means of topological analysis of the electronic density of the system.

1 Introduction Ionic liquids (ILs) are salts whose melting temperature is around or below ambient temperature. Since the late 1990s, ILs have attracted remarkable attention from chemists due to their great potential for diverse applications and the easy tuneability of their physicochemical properties. Typical ILs are composed of, but not limited to, an organic cation (most often an alkyl-substituted imidazolium, a pyridinium or a quaternary ammonium ion) and an inorganic anion.1 In particular, solvent properties of methylimidazolium-based ILs have been widely studied, as media for chemical reactions, catalysis,2,3 gas separation4 and for the solvation of bigger systems such as DNA,5–10 nanotubes,2 or enzymes.11–17 Apart from the wealth of experimental research conducted on the properties of methylimidazolium based ILs,18–29 a great effort has been made by theoreticians to provide insight into their physicochemical properties,30–48 and, to a lesser extent, into their solvation properties.49–63 The variety and strength of the intermolecular solute–IL interactions requires large model systems, what in turn constitutes a great challenge for the quantum theoretical chemistry methods. Quantum chemical calculations provide a rigorous representation of the interactions at an electronic level, but can only deal with relatively small systems (some ion pairs at most, employing the so called ab initio a

Laboratoire de Chimie Moleculaire, Department of Chemistry, Ecole Polytechnique and CNRS, 91128 Palaiseau Cedex, France. E-mail: [email protected] b POLYMAT, University of the Basque Country UPV/EHU 20018 Donostia-San ń, Spain. E-mail: [email protected] Sebastia c Ikerbasque, Basque Foundation for Science, Bilbao, Spain † Electronic supplementary information (ESI) available: Additional RDFs, aNCI analysis and methodological details. See DOI: 10.1039/c5cp01774b

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molecular dynamics (AIMD)).30,39,61 Instead, rather large systems can be described by classical molecular dynamics (MD), but at the cost of a less reliable, parameterized approximation and molecular level description.40,64–66 As a first step towards the comprehensive approach to IL solvation processes, systematic ab initio computational studies of single, isolated ion pairs were previously carried out.49 A direct relationship between the gas phase electronic structure and the experimentally observed behavior was found and the nature of the non-covalent interactions (charge–dipole, charge– induced dipole and dispersion interactions) between the methylimidazolium based ILs and a set of different solutes was described. In the present work, we extend our model to several ion pairs, with the aim of understanding the microsolvation of the solute and the role of the bulk on the solvation process. As a compromise between the real system and the affordable size for quantum chemical calculations, we have chosen a small, prototypical IL, the 1,3-methylimidazolium chloride ([MIM][Cl]), and small solutes, namely H2O and CO2, for their intrinsic interest and fundamentally distinct solvation patterns observed previously. On one hand, H2O presents significant affinity for ILs, and furthermore, the properties of ILs are dependant on the amount of absorbed water.67 In fact, water is the most important solute in ILs since in many applications water is ubiquitous, rendering it the most relevant impurity of the ILs.68,69 On the other hand, the ILs show very promising features for capture and recycling of CO2.3 Besides, these two solutes can be considered representative of two types of solutes dissolved in IL: those which have a strong dipole moment and establish an specific interaction with the IL, and those with no dipole moment, which establish weaker, non-specific interactions with the IL.49

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While several AIMD studies on [MIM][Cl] are available in the literature,41,42,44 scarce AIMD studies of CO256,63 and H2O70 solvation in methylimidazolium based ILs are found, and in related works the issue is mainly addressed by experimental and classical MD methods.22,58,60,62,71–86 Despite some discrepancies, it is generally accepted that in methylimidazolium based ILs both water22,72,74,76,87 and CO256,73,84 interact mainly with two anions of the IL, this interaction being the major factor to both solutes’ solubility. In addition, both solutes can ćzki establish interactions with the cation.22,56,63,73,74,88,89 Hollo et al. observed interactions between CO2 and the p cloud of the cation in 1-ethyl-3-methylimidazolium acetate ([C2C1IM][OAc]) by performing AIMD simulations.63 With the same method, Bhargava and coworkers56 found weak hydrogen bonds of CO2 with the Hc hydrogen of the cation, in 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), while for this IL and 1-n-butyl-3methylimidazolium tetrafluoroborate ([BMIM][BF4]) high pressure NMR spectroscopy finds the CO2 atoms close to the methyl groups of the cations.88 Similarly, a diffuse arrangement of imidazolium cation has been observed around H2O in [MIM][Cl] and [MIM][BF4] by MD calculations.74,89 Other works, based on classical MD calculations75 and NMR spectroscopy studies67 on [MIM][BF4] found CH–O interactions between the O atom of H2O and the Hr and Hmet atoms of the cation. On the contrary, no experimental evidence of solute–cation interaction in methylimidazolium based ILs (combined with various anions, namely BF4 , PF6 , ClO4 , CF3SO3 , NO3 , CF3CO2 , SbF6 ) was found using ATR and transmission IR spectroscopy.22 In the present work, AIMD calculations will be carried out for studying the solvation of H2O and CO2 in [MIM][Cl]. These simulations will be combined with Density Functional Theory (DFT) calculations in small systems and topological study of the electronic density distribution to analyse the non-covalent interactions taking place between the solute and the ions. Special attention is paid to the nature and strength of the interactions established between the solute and the surrounding environment, performing a qualitative and quantitative approximation to the microsolvation of the selected solutes in [MIM][Cl]. Our aim is to analyse these interactions and elucidate how they vary throughout the solvation process.

2 Methods The interaction of the solutes (Sol) with the IL is studied on Sol–([MIM][Cl])n models at different theory levels. Throughout this work, Hr denotes the hydrogen attached to the C2 carbon, Hc refers to those in positions 4 and 5 of the ring, and finally the hydrogens of the methyl groups are named Hmet (see Fig. 1). DFT calculations of n = 1, 2 systems are carried out in gas phase and in IL phase. The latter was simulated implicitly by means of the SMD method;90 the numerical values of the atomic and molecular parameters were adjusted to our model and are available in the ESI.† BLYP91–93 geometry optimisations, with Grimme’s dispersion correction94 are performed, combined with TZVP95,96 basis set as implemented in Gaussian09.97 This functional


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Fig. 1 Schematic representation of the (a) IL and (b) solutes considered in this work.

was chosen in order to be consistent between the different theoretical approaches used in this work. Frequency calculations are carried out in order to ensure the absence of imaginary modes. The structures generated in a previous work49 are used as a starting point to obtain the optimised geometries. However, in the n = 1, 2 complexes we do not aim at a complete exploration of the potential energy surface, but rather an overview of the preferred interaction sites for the solute and the influence of the solvent on the coordination preferences and the overall affinity. Furthermore, it must be considered that, in general, ionic liquids are not ion pairs,98 and therefore, this model is a rather crude approximation to real systems. The affinity of the solute is defined as the energy balance of the hypothetical reaction Sol + ([MIM][Cl])n - Sol–([MIM][Cl])n

(1)

where the reactants keep their geometry in the product. The negative sign of the energy denotes the exothermicity of the reaction. n = 18 models are treated with the Car–Parrinello MD (CPMD) method (see ESI† for details). The structure of the systems throughout the CPMD trajectories is analyzed by radial distribution function (RDF), which gives the probability of finding an atom at a distance r away from a given reference atom. A topological method, namely the recently developed noncovalent index (NCI) is used for analysing and visualising the non-covalent interactions between the solute and IL. NCI plots are computed with the NCIPLOT program99 starting from (1) the BLYP-D wave functions of the optimized geometries, and (2) the promolecular densities calculated at the optimised geometries. Both approximations are compared for n = 1 complexes, and found to give similar results (see ESI†). Therefore, only promolecular densities are used to perform the NCI calculations on the n = 18 system. The non-covalent interactions throughout the simulation are analysed by means of the averaged NCI (aNCI),100 which characterises the interactions for an ensemble of structures, and is a very useful tool for characterizing noncovalent interaction patterns in fluctuating environments.

3 Results 3.1

H2O solvation

3.1.1 H2O–([MIM][Cl])n=1,2 systems. Three low lying thermoneutral isomers are found for the H2O–[MIM][Cl] system


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Fig. 2

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H2O–([MIM][Cl]) isomers optimized in (a) gas phase and (b) IL phase.

Table 1 Relative stability (DE), interaction energy (DEint) and dispersion interaction (DEdisp) contribution in H2O–([MIM][Cl])n complexes, in kcal mol 1 (BLYP-D/TZVP)

Gas phase

IL phase DEdisp

DE

n=1

1 2 3

0.0 0.9 0.9

19.9 19.0 18.9

5.3 3.7 3.2

1.1 0.7 0.0

9.9 7.8 8.4

3.0 2.7 2.5

n=2

1 2 3 4 5

0.0 0.0 0.6 0.7 1.1

26.0 26.0 27.5 26.1 24.8

8.4 8.4 8.3 8.4 8.8

0.0 0.2 0.4 0.4 0.4

13.4 9.5 13.6 13.6 13.8

7.7 5.4 7.7 7.7 7.3

DE

DEint

DEint

DEdisp

(see Fig. 2 and Table 1), where the water molecule locates itself in front (isomers 1 and 3) or on top of the ring (isomer 2) and interacts via hydrogen bond with Cl .49 The interaction established with the anion is very similar in length for all the three isomers, between 2.113 Å for 1 and 2.167 Å for 3. Additionally, H2O interacts with the methyl group and the Hr atom of the cation (isomer 1), or the p system of the cation, in particular, the C–N bond (isomers 2 and 3) (see ESI† for geometrical parameters). In gas phase, water molecule shows an interaction energy of around 19 kcal mol 1 with the ion pair (see Table 1),

Fig. 3

mainly due to the Hw–Cl dipole–charge interaction. The dispersion interactions, which take place with the cation (see NCI data in ESI†) contribute weakly to the affinity, specially in isomers 2 and 3. Instead, in IL phase, the interaction with the p cloud of the ring is disfavoured (see Fig. 2 and Table 1); the most stable isomer is 3, where both the solute and anion are in the ring plane, followed by 2, which bears the H2O molecule on top of the ring. As compared to gas phase, larger separation between anion and cation is observed. The Hr–Cl bond is elongated from 2.167–2.818 Å in gas phase to 2.440–3.262 Å in IL phase (see ESI†). Hr–O and Hw–Cl distances increase only slightly, while HMet–O distances are enlarged. Nevertheless, a slight shortening of Hw–O shows that even if the H2O–anion distance is kept, the ion pair–solute interaction is weaker. This is also confirmed by the interaction energies which range between 9.9 and 7.8 kcal mol 1 (see Table 1). Similarly, dispersion interactions become slightly weaker, due to the larger H2O– MIM+ distances. The insertion of a second ion pair permits the simultaneous interaction of the solute with both ion pairs (see Fig. 3 and Table 1). The low lying isomers of the H2O–([MIM][Cl])2 system show, in the gas phase, the water molecule between both cation rings and interacting via hydrogen bonds with two Cl anions. The simultaneous interaction with both ion pairs results in less efficient Hw–Cl bonds; Hw–Cl and Hw–O bond lengths increase and decrease (see ESI†), respectively, as compared to H2O–[MIM][Cl] isomers. On the other hand, in this system the solute interacts with the p cloud and the methyl of the cation, while the Hr–O distance increases. In particular, the p interaction takes place between the H2O and the C–N bond of the imidazolium, except in isomer 5, where it takes place with the N atom, and results in slightly higher relative energy. The strong interaction of the water with the second ion pair, particularly with the Cl anion, results in stronger affinity (see Table 1), around 26 kcal mol 1 in gas phase. The dispersion interactions taking place between the solute and the cations ( 8 kcal mol 1) are twofold when the second ion pair is included.

Top view of H2O–([MIM][Cl])2 isomers optimised in (a) gas phase and (b) IL phase.



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In the IL phase, most of the systems (except 2), show the cation rings perpendicular to each other, and a shift of the water molecule towards the edge of the ring is observed. In isomers 1, 2 and 5 the solute does not interact with the p system of the rings, but rather with the Hr atom; in isomers 3 and 4 the interaction with the C–N bond is maintained. As observed for H2O–[MIM][Cl] isomers, the interaction between ions is weakened when passing from the gas phase to the IL phase (see Hr–Cl bonds in ESI†); the distance of the water from the Cl molecule becomes slightly shorter, resulting in the cations being separated from the H2O molecule. However, the bond lengths within the water molecule become slightly shorter, indicating a weaker Hw–Cl interaction. Even if the Cl –H2O–Cl motif observed in gas phase is also highly preferred in IL, the weakening of the Hw–Cl bond results in the presence of isomer 2, which presents a single H2O–Cl interaction, among the most stable isomers. H2O presents in isomer 2 an affinity and bond parameters very similar to H2O– ([MIM][Cl]), with stronger contribution from dispersion interactions. Overall, also in the n = 2 system the interaction energy is weaker in the IL phase (around 13.5 kcal mol 1) as compared to the gas phase, however, with a very important contribution of the dispersion interactions (around 7 kcal mol 1). 3.1.2 H2O–([MIM][Cl])18 system. Explicitly solvated H2O shows very similar geometrical parameters as those obtained for H2O–([MIM][Cl])2 complex. The solute shows an Hw–O–Hw angle of 103.7 degrees in average, with a standard deviation of 5.8 degrees, while the bond lengths are in average of 1.018 Å and 1.010 Å, with a standard deviation of 0.03 Å. Regarding the interaction of H2O with the surrounding environment, the RDFs show that the first coordination shell of the water in the IL is well defined (see Fig. 4); each Hw atom interacts with one Cl . The Hw–Cl RDF shows a first peak at 2.15 Å, and a second at 3.60 Å, corresponding to the Cl attached to the other Hw, while the rest of the anions are found quite far away from the water molecules, more precisely at around 7 Å. On the other hand, analysing the distribution of the cation around H2O, a single peak is observed on the O–Hc RDF, pointing to two Hc atoms specifically interacting within 2.15 Å of the O atom. Besides, there is a diffuse distribution of Hmet

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Fig. 5 Reduced density gradient isosurfaces (s = 0.5 a.u.) of the H2O– [MIM][Cl]18 system.

(see ESI†) around the oxygen atom which starts at 2.5 Å; even if the lack of a defined distribution denotes the absence of specific interactions, the short distance reveals an interaction between the O and the methyl substituents. In summary, the analysis of the CPMD trajectory indicates a tetracoordination of H2O, establishing two Hw–Cl and two O–Hc interactions, all of them with bond lengths around 2.15 Å. Accordingly, the NCI analysis of a single structure of the trajectory (see Fig. 5(a)) shows two strong Hw–Cl bonds (sign(l2)r = 0.028), other two weaker O–Hc interactions (sign(l2)r = 0.014 and 0.016), and several areas of dispersion interactions around the oxygen with the methyl groups of the cations (r between 0.011 and 0.001). Extending the analysis to the whole CPMD trajectory, aNCI analysis (see Fig. 5(b) and (c)) permits to observe that the bonds formed between the Hw atoms and the Cl anions are stable during the simulation (sign(l2)r = 0.025 and 0.03, blue surfaces), while more flexible structuration around the O atom takes place (green surface, r between 0.012 and 0.001). Even though the tetracoordination pattern can still be observed, oxygen establishes very flexible interactions; the two large green areas around the O atom correspond to very weak interactions of the solute with the surrounding IL molecules. An estimation of the interaction energy between H2O and the surrounding environment is obtained by performing DFT calculations on 14 structures extracted throughout the trajectory (one structure is extracted from the trajectory each 2.5 ps). The average interaction energy of the H2O with the IL bulk has

Fig. 4 (a) Ow–Cl (solid line) and Hw–Cl (dashed line) and (b) Ow–Hc (solid) and Hw–Hc (dashed line) RDFs (black lines) and the corresponding integral (red lines) in the H2O–[MIM][Cl]18 system. The distance between the atoms, r, (x axis) is in Å.



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been found to be 12.5 kcal mol 1 (see ESI†), with a contribution of the dispersion interactions of around 8.0 kcal mol 1, which is very similar to the DEint found for H2O–([MIM][Cl])2 isomers, considering the different models used. In line with aNCI results, this suggests that the additional Hc–O interactions found in the n = 18 system do not contribute significantly to the affinity of water towards the IL, which is mainly driven by Hw–Cl interactions.


3.2

CO2 solvation

3.2.1 CO2–([MIM][Cl])n=1,2 systems. As was observed for H2O[MIM][Cl], the low lying isomers of CO2[MIM][Cl] also show the solute on top or in front of the cation (see Fig. 6 and Table 2),49 interacting with both ions. The most stable isomer (isomer 1) shows CO2 in front of the imidazolium cation, interacting with Hr, Hmet and Cl , which is on top of the ring. In the other two isomers, instead, CO2 locates itself on top of the imidazolium plane, interacting with Cl and the p system and methyl group of the cation. In isomer 2, the CO2 locates parallel to the C–N bond, and on isomer 3, along the N–N axis, allowing a better interaction with the p system. The distances between CO2 and Cl are similar for all the isomers (see ESI† for geometrical parameters). The interaction of CO2 with the anion provokes a distorsion from linearity in the former, giving rise to a charge–induced dipole interaction that is kept in the different conformations. On the other hand, the interaction with the imidazolium ring, based on dispersion interactions, changes remarkably. Even though isomers 2 and 3, bearing CO2 on top of the ring, are less stable than 1, they present slightly better affinities (see Table 2). The long distances between CO2 and the ion pair suggest weak interactions between the solute and the IL, in agreement with the weak interaction energy calculated in Table 2, around 8.0 kcal mol 1. Dispersion interactions, which take place with the cation ring, have an important role in this system, as almost half of the affinity is due to these interactions; in particular, isomer 3 shows the strongest contribution from dispersion interactions. Geometry optimisations in IL phase result in very similar isomers, albeit with a different stability order (see Table 2). As the separation between the ions increases, (about 0.2 Å longer in IL), the interaction of the solute with the cation is maintained (with slightly

Fig. 6

CO2[MIM][Cl] isomers optimized in (a) gas phase and (b) IL phase.


Table 2 Relative stability (DE), interaction energy (D Eint) and dispersion interaction (DEdisp) contribution in CO2–([MIM][Cl])n isomers, in kcal mol 1 (BLYP-D/TZVP)

Gas phase DE

IL phase

DEint

DEdisp

DE

DEint

DEdisp

n=1

1 2 3

0.0 1.3 1.8

7.1 8.1 8.0

3.1 3.7 6.1

1.9 0.3 0.0

4.6 3.7 5.0

3.6 3.2 5.3

n=2

1 2 3 4 5

0.0 0.7 0.7 0.7 1.3

6.3 6.3 9.3 9.9 10.1

4.7 4.7 5.9 6.8 5.8

0.0 0.4 0.8 2.1 2.3

4.0 5.5 4.6 6.1 5.6

3.8 5.7 5.1 6.6 6.6

shorter bonds), while the distance with the anion is enlarged around 0.1 Å. Consequently, the isomers where the CO2 is placed on top of the ring are favoured (isomers 2 and 3), stabilised by the better interaction between CO2 and the p cloud of the ring. Nevertheless, due to the higher dielectric of the environment, the overall interaction of the solute with the IL is weaker, as pointed out by the smaller distortion of the O–C–O linearity and the interaction energy (around 4.5 kcal mol 1). The dispersion interactions, on the contrary, are maintained also in IL, owing to better CO2–cation interaction. Upon the insertion of another ion pair in the system, CO2 interacts in gas phase with both ion pairs, establishing interactions with the anion and the methyl groups of the cation (see Fig. 7). In the most stable isomer, isomer 1, the CO2 interacts with both cations and anions. In isomer 2, it interacts only with one Cl and one cation, what enables a shorter bond length, and therefore the interaction energy is maintained. Observe that the interaction energy is weaker than with only one ion pair (6.3 vs. 7.1–8.1 kcal mol 1). In complexes 3 and 5, it interacts strongly with one Cl (as reflected in angles, 173.6 and 171.6, and C–Cl bond lengths, 3.134 and 3.045, respectively), and the methyl groups of both cations. In isomer 4, instead, the solute interacts with one Cl and the methyl groups of one cation. Affinity relies mainly on cation interactions, as reflected in the important contribution of dispersion interactions. In IL phase, the interaction between anion and cation is relaxed and tends to elongate, what permits the cations to reaccommodate and maximize the p–p interactions in isomers 1, 2 and 3. On the contrary, isomers 4 and 5 present the cations perpendicular to each other, in T-shape conformation. The CO2 interacts now with only one ion pair. The contribution of dispersion interactions is maintained, while CO2–Cl is weakened, as reflected in the O–C–O angles, even though the bond lengths remain similar. Overall, affinity is lost. 3.2.2 CO2–([MIM][Cl])18 system. The geometrical parameters of CO2 in the CO2–([MIM][Cl])18 system are in good agreement with the static calculations, evidencing a very weakly interacting solute molecule. The otherwise linear solute molecule is slightly distorted to an angle of 172.7 (with a standard deviation of 4.8 degrees) and C–O bonds are in average of 1.192 and 1.190 Å, with a standard deviation of 0.02 Å.


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Fig. 7 Top view of CO2([MIM][Cl])2 isomers optimized in (a) gas phase and (b) IL phase.

Fig. 8 (a) O–Cl (solid line) and C–Cl (dashed line) and (b) O–Hc (solid) and C–Hc (dashed line) RDFs (black lines) and the respective integral (red lines) in the CO2–[MIM][Cl]18 system. The distance between the atoms, r, (x axis) is in Å.

The RDFs show a much less structured distribution of IL around CO2, as compared to the first coordination shell of H2O. A coordination of 1.5 Cl atoms around the C atom can be found at 3.3 Å (see Fig. 8(a)). This distance correlates with the lengthening of the C–Cl bond observed in the CO2–([MIM][Cl]) and CO2–([MIM][Cl])2 models. As already seen for n = 1, 2 models, the cations are found closer to the solute than anions. Even though no specific interaction is found, Hc and Hmet atoms can be found at 3.0 and 2.8 Å from the O atom, respectively (see Fig. 8(b) and ESI†). The first minimum of the C–Hc RDF, at 5 Å, shows eight Hc atoms around the molecules, namely, about four cations around the solute. NCI analysis of a single structure of the trajectory (see Fig. 9(a)) shows the interactions described above; CO2 establishes weak C–Cl (sign(l2)r = 0.02), O–Hc (sign(l2)r = 0.019) and O–Hmet (sign(l2)r = 0.014) interactions with the ions. If the whole trajectory is considered (see aNCI results in Fig. 9(b) and (c)), we observe that the interactions fluctuate significantly and are in average uniformly distributed around the CO2, suggesting that no specific interactions are formed in this system. aNCI shows purely dispersion interactions with the


Fig. 9 Reduced density gradient isosurfaces (s = 0.5 a.u.) of the CO2– ([MIM][Cl])18 system.

surrounding (r between 0.007 and 0.001), which are weaker than those found for H2O. DFT calculations in 14 structures extracted from the trajectory (one structure extracted each 2.5 ps, approximately) show that the average interaction of the solute with the environment is of 6.9 kcal mol 1, similar to the affinity observed for the smaller complexes. Nevertheless, in the n = 18 system, the contribution of the dispersion interactions to this energy increases to around 11 kcal mol 1 (see ESI†), which means


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that the solute presents favorable interactions with the media only thanks to the dispersion interactions whereas electrostatic interactions are repulsive.


4 Discussion As expected, fundamentally different solvation processes are found for H2O and CO2 in [MIM][Cl]. The water molecule, with its dipolar moment, results in significant electrostatic interaction between the partially positively charged Hw atoms and the Cl anions. Similarly, the partial negative charge on the oxygen atom permits its interaction with MIM+. CO2, due to its lack of dipolar moment, shows instead very weak electrostatic interactions with the ions, as reflected in the significantly weaker affinity for the IL. In gas phase, interaction of the solutes with the anion is preferred, even though interaction with the cation is also established in both systems. As the dielectric constant of the environment increases, the charges are stabilised. As a consequence, the charge–charge/charge–solute interactions are weakened, and so is the affinity. However, the separation between the ions as compared to the gas phase permits a closer interaction with the solutes, such that a competition between the ions for the solute takes place. H2O optimises the Hw–Cl interaction, while CO2 tends to favour the dispersion interactions with the cation. H2O–([MIM][Cl])n complexes show that the solute can interact with two Cl atoms at the same time, and, as dielectric and n increase, the preferred interaction site of the cation changes from Hr to the p cloud of the cation and finally to Hc. The explicit solvation of H2O permits to observe, besides the Cl –H2O–Cl moiety, other specific interactions of the IL with the solute. Besides the two Hw–Cl bonds where the H2O acts as a hydrogen bond donor, it is known that O can act as a hydrogen bond acceptor, even though these interactions fluctuate more.100 The peak in the Hc–O RDF of the system denotes the presence of a stable interaction between the O atom and the Hc atom of two cations, reaching a tetracoordination previously described for other ILs.101 Additionally, a poorly defined Hmet atom distribution is observed around O. aNCI analysis suggests that O–Hc and O–Hmet interactions are significantly weaker than Hw–Cl interactions, due to the poor acidity of the Hc and Hmet ions. In agreement with the literature, we can see the fluctuation and the decreased density value reflected on the large area on which O–cation interactions take place. Even though interactions between O and methylimidazolium cations have previously been observed in [MIM][BF4],67,75,89 no specific interaction between O and Hc is mentioned, and other experimental studies did not report any interaction between the solute and the cations.22 Although the discrepancies could be due to the methodology and system chosen, the different nature of the anions considered has also to be taken into account. It is known that the anion plays an important role in H2O miscibility, which establishes weaker interactions with BF4 and PF6 than


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with Cl .102 A weaker interaction of the solute with the anion would result in less polarisation of H2O and therefore, weaker interaction of O with cations. Considering the low energy of solute–cation interactions established, a change in the Hw–anion coordination would explain the fact that other authors observed no specific interaction of the water with the cation. This hints at a cooperativity effect between the ions during the solvation of H2O, rather than a competition between anion and cation for the solute.102 In line with the weak H2O–cation interactions suggested by aNCI analysis, we observe that the interaction energies do not significantly change from n = 2 to n = 18 complexes (around 13 kcal mol 1 and 12 kcal mol 1 respectively), suggesting that the water coordination shell is saturated with two ion pairs and does not profit significantly from the O–Hc and O–Hmet interactions with the rest of the surrounding ion pairs. Therefore, even if these interactions between H2O and the cation are not captured in n = 1, 2 complexes, we observe that H2O– ([MIM][Cl])2 is a good model for low water concentration IL solutions. On the contrary, in CO2–([MIM][Cl])n complexes, even if the overall affinity of the system does not vary remarkably, the role of dispersion interactions with the cation gains importance as dielectric increases and more ion pairs are included. The interactions of CO2 evolve with the complexity of the model by gradually losing charge–induced dipole interaction with Cl and compensating with dispersion interactions with the cation. CO2 shows a much weaker interaction and less defined solvation than H2O with the IL. A coordination of 1.5 Cl atoms are found around the C atom, close to the 1.8 PF6 molecules observed on other imidazolium based ILs,73 while around two cation molecules interact with each O atom, as seen in similar systems.63 As opposed to the anion, cations are more numerous around and closer to the solute which is why we can assume that the cations contribute more than anions to the solvation of CO2 in this IL. Very low affinity of the CO2 for the IL is observed,49 in fact, it shows a stronger affinity in gas phase (around 9 kcal mol 1) with a moderate contribution from dispersion interactions. On the contrary, when explicitly solvated in IL, it shows an affinity of around 7 kcal mol 1. The principal contributor to this affinity are dispersion interactions, without which the affinity would be positive, as was previously observed in similar ILs.103 In fact, aNCI analysis shows a uniform surface of dispersion interactions around the molecule, suggesting that in bulk IL the solute establishes dispersion interactions with both anions and cations. This is in line with the poor structuration observed around the solute. Furthermore it is in agreement with the well known fact that other anions as PF6 and BF4 both of which show stronger vdW interactions in solution,103 contribute to the solubility of CO249 even though they establish weaker acid–base type interactions with CO2.50 Nevertheless, when comparing IL–solute systems with different anions, it must be taken into account that not only the anion–solute interaction is altered. The nature of the counterion influences significantly the chemical properties of the cation,104


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and consequently, the interactions it establishes with the solute. Therefore, when comparing the coordination and affinity of a given solute in different ILs, the whole complexity of the system must be carefully considered.


5 Concluding remarks In this work, the microsolvation of H2O and CO2 by [MIM][Cl] has been studied by means of different theoretical approximations. DFT calculations on n = 1, 2 complexes have permitted to analyse the preferred interaction sites of the solutes in gas phase, and the influence of the dielectric environment on this coordination. In fact, it is observed that both solutes interact preferentially with the anion in gas phase, forming dipole/ induced dipole–charge interactions. The increase of the dielectric results in a loss of DEint for both solutes and favours the anion–H2O (dipole–charge) and the cation–CO2 (dispersion) interactions. CPMD calculations provide a complete explicit solvation of the solute and a proper exploration of the potential surface, and therefore, the influence of the specific interactions established between the solute and the IL has been studied. We observe that the coordination of water in [MIM][Cl] is mainly based on the dipole–charge interactions with the anions. Even if two specific O–Hc interactions are also established, these interactions are very weak and do not significantly contribute to the affinity. On the contrary, in IL, the affinity for CO2 is exclusively based on dispersion interactions with both ions. A distribution of around two anions and four cations are found in the first solvation shell. Consequently, cations interact more efficiently with the CO2 and are the main source of the affinity in this IL, as opposed to what is generally assumed in the literature. An enhancement of dispersion interactions with the anions would remarkably increase the solubility of CO2 and the relevance of the anions on the solvation process.

Acknowledgements This research was funded by the ERC Starting Grant 209842¨fer). Technical and human support provided MATRIX (T. Scha by IZO-SGI, SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) and Barcelona Supercomputing Center-Centro Nacional de Supercomputacion are gratefully acknowledged for assistance and generous allocation of computational resources. E. R. thanks Dr E. C. Beret, Dr R. Castillo and Dr G. Hantal for fruitful discussion.

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