Is There Any Preferential Interaction of Ions of Ionic Liquids with DMSO

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Article

Is There any Preferential Interaction of Ions of Ionic Liquids with DMSO and HO? A Comparative Study from MD Simulation 2

Yuling Zhao, Jianji Wang, Huiyong Wang, Zhiyong Li, Xiaomin Liu, and Suojiang Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b01925 • Publication Date (Web): 13 May 2015 Downloaded from http://pubs.acs.org on May 17, 2015

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The Journal of Physical Chemistry

Is There any Preferential Interaction of Ions of Ionic Liquids with DMSO and H2O? A Comparative Study from MD Simulation Yuling Zhao, † Jianji Wang, †* Huiyong Wang, † Zhiyong Li, † Xiaomin Liu, ‡ Suojiang Zhang‡

†

Collaborative Innovation Center of Henan Province for Green Manufacturing of

Fine Chemicals, School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, P. R. China. ‡

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of

Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China.

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ABSTRACT Recently, some binary ionic liquid (IL)/co-solvent systems have shown better performance than the pure ILs in the fields such as CO2 absorption, catalysis, cellulose dissolution and electrochemistry. However, interactions of ILs with co-solvents are still not well understood at the molecular level. In this work, H2O and DMSO were chosen as the representative protic and aprotic solvents to study the effect of co-solvent nature on solvation of a series of ILs by molecular dynamics simulations and quantum chemistry calculations. The conception of preferential interaction of ions was proposed to describe the interaction of co-solvent with cation and anion of the ILs. By comparing the interaction energies between IL and different co-solvents, it was found that there were significantly preferential interactions of anions of the ILs with water, but the same was not true for the interactions of cations/anions of the ILs with DMSO. Then, a detailed analysis and comparison of the interactions in IL/co-solvent systems, hydrogen bonds between cations and anions of the ILs, and the structure of the first coordination shells of the cations and the anions were performed to reveal the existing state of ions at different molar ratios of the co-solvent to a given IL. Furthermore, a systematic knowledge for the solvation of ions of the ILs in DMSO was given to understand cellulose dissolution in IL/co-solvent systems. The conclusions drawn from this study may provide new insight into the ionic solvation of ILs in co-solvents, and motivate further studies in the related applications.

Keywords: ionic liquid, H2O, DMSO, solvation, preferential interaction of ions, molecular dynamics simulation

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1. INTRODUCTION Room temperature ionic liquids (ILs) are composed of organic cations and inorganic/organic anions1. The most interesting feature of ILs is that their structures and properties can be finely tuned by a judicious structural design of cations and anions. ILs have emerged as a unique class of versatile greener solvents that hold great application promise for a diverse range of chemical synthesis, biocatalysis, electrochemistry, biomass dissolution, and analytical and separation sciences2-6. However, viscosity of ILs is usually 1-3 order of magnitude higher than conventional organic solvents, such high viscosity is a bottleneck problem for most of the IL solvents. This makes it difficult to mix and stir, and causes chemical engineering problem of mass transfer, thus weakening their application in industry7,8. In recent years, binary IL/co-solvent systems have attracting particular attention as promising solvents to avoid the impediment of the high viscosity of pure ILs. Fortunately, some IL/co-solvent systems have shown better performance than pure ILs in the fields of CO2 absorption9,10, catalysis11, cellulose dissolution12-14 and electrochemistry15,16. For instance, Rinaldi13 and our group12,14 reported a series of highly effective cellulose solvents consisted of imidazolium-based IL and a molecular solvent, such as dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and 1,3-dimethyl-2-imidazolidinone (DMI). It was found that cellulose can be readily dissolved in these IL/co-solvent systems at ambient temperature with high solubility. This not only increases the solubility of cellulose, but also greatly reduces the cost. Thus, in order to rationally extend the application of IL/co-solvent systems, it is of great importance to understand the solvation of ILs in different kinds of co-solvents. It is known that the commonly used polar co-solvents are generally divided into two categories: protic and aprotic solvents. The effect of different categories of co-solvents on the solvation of ILs is different. For the protic solvents, there are a lot of researches on the solvation of ILs in H2O. The structure of aqueous ILs and the interactions of ILs with water molecules have been studied by attenuated total reflection infrared spectroscopy (ATR-IR), NMR, molecular dynamics (MD)

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simulations and quantum mechanical (QM) calculations17-20. The main conclusion is that water molecules are more prone to interact with the anions first, and then interact with the cations. With the addition of water, ILs undergo structural transition from huge ionic clusters, ionic clusters, ionic pairs, and even free cations and anions. At the same time, a number of investigations have been carried out for the solvation of ILs in aprotic co-solvents21-28. However, much of the researches have been focused on the non-polar and weak polar aprotic solvents, there are only a few reports on the solvation of ILs in the strong polar aprotic co-solvents, such as DMSO. In this context, a comparison study for water and DMSO addition to an IL has been made by Yu et al.29, Bardow et al. 30 and Moyna et al. 31 using experimental and simulation methods. It is shown that ILs stay in a form of isolated ions in IL/H2O mixtures, while dissociation into ions is much less observed in IL/DMSO systems. Nevertheless, the structure of the IL/co-solvent mixtures and the interactions between IL and co-solvent are still not fully understood. Thus, further studies are necessary for systematical understanding of solvation mechanism of ILs in aprotic solvents at the molecular level. In this work, H2O and DMSO were chosen as representative protic and aprotic solvents widely used in practical applications to comparatively examine the effect of co-solvent nature on the solvation of ILs. For this purpose, MD simulations were performed on the mixtures consisting of an IL ([C4mim][Tf2N], [C4mim][N(CN)2], [C4mim][OAc], [C4mim][NO3], [bpy][NO3], [N1112OH][NO3], or [P4444][NO3]) and one of the co-solvents at 298 K. The effects of the cation and anion structures of the ILs on their interactions with the co-solvents were also considered. In addition, hydrogen bonds between cations and anions of the ILs, structure of the first coordination shell of each co-solvent around the cations and anions were calculated and compared in [C4mim][NO3]/co-solvent systems at different molar ratios of the co-solvent to the IL. Based on these results, a systematic knowledge for the solvation of ions of the ILs in DMSO was understood.

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2. COMPUTATIONAL DETAILS 2.1. Molecular Dynamics Simulations Force Field Parameters. All-atom force fields were used in our simulations. For DMSO32, [C4mim][Tf2N]33, [C4mim][N(CN)2]34, [C4mim][OAc]33, [C4mim][NO3]35, [bpy][NO3]36, [N1112OH][NO3]32 and [P4444][NO3]37, the reported parameters of AMBER force field were used, and the SPC model38 was employed for water molecules. Optimization of the isolated ion structures was performed by using the Gaussian 09 D.01 version39 at the B3LYP/6-31+G* level. Atom charges were obtained by fitting the electrostatic potentials calculated at the B3LYP/6-311++G* level, and one-conformation, two-step restraint electrostatic potential method was used for this purpose40. Simulation details. All simulations were performed with MDynaMix 5.2 package41. The double time-step algorithm42 was adopted with long and short time steps of 2 and 0.5 fs, respectively. Ewald summation method43 was used to treat the long-range electrostatic interaction, in which the long-range parts were cut off at 15Å. The simulations were first performed by using mixtures of the IL ([C4mim][Tf2N], [C4mim][N(CN)2], [C4mim][OAc], [C4mim][NO3], [bpy][NO3], [N1112OH][NO3] or [P4444][NO3]) with each co-solvent (H2O or DMSO) at the same molar ratio (R) of the co-solvent to the IL (1:1). Then additional simulations were carried out on the mixtures of [C4mim][NO3] with H2O and DMSO as a function of R. All the simulations were carried out at 298 K, and the initial configuration was prepared by PACKMOL44 in a square box, typically larger than the “real” size to make the packing easier. The number of solvent molecules and molar ratio of the co-solvent to the IL for each IL/co-solvent system were given in Table S1 (See supporting information). A starting simulation was carried out at 700 K in NVE ensemble. After a relaxation for a few MD steps to reduce the possible overlapping in the initial configuration, the Nose-Hoover NPT ensemble simulation45 was performed. Descending from 700 K to the sampling temperature of 298 K, a series of NPT simulations were carried out under the standard atmospheric conditions. At the

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sampling temperature point, the system was equilibrated for at least 5 ns, and then the production phase was lasted for 4 ns. The conformations in trajectories were dumped with an interval of 20 fs for further analysis. 2.2 Quantum chemistry calculations The ion pairs of [Tf2N]-, [N(CN)2]-, [OAc]- and [NO3]- anions paired with [C4mim]+ cation and the isolated ions were optimized by Gaussian 09 D.01 program39. The structural optimizations were carried out at the B3LYP/aug-cc-pVDZ theoretical level. All the optimized geometries were recognized as local minima without any negative vibrational frequency. The initial configurations of the ion pairs were mainly based on the charge distribution and electrostatic potential on the isolated cation and anion. Thus, the favored positions of anions were distributed around the C2, C4, and C5 sites of [C4mim]+ cation. The binding energies calculated at the B3LYP/aug-cc-pVDZ theoretical levels were compared with the results obtained at the B3LYP-D3(BJ)/aug-cc-pVDZ level. In the B3LYP-D3(BJ) method, the D3 term represents

a

dispersion

correlation46.

Based

on

the

results

produced

at

B3LYP-D3(BJ)/aug-cc-pVDZ level, the electron density [ρ(r)], potential energy density V(r) and the corresponding Laplacian value [▽2ρ(r)] at the critical points were calculated by Multiwfn 3.1 program47. In the work reported by Espinosa et al.48, the authors stated that for the hydrogen bond [X-H…O (X=C,N,O)], the relationship between bond energy EHB and potential energy density V(r) at corresponding bond critical point (BCP) could be approximately described by EHB=V(rbcp)/2. Therefore, the hydrogen bond energies of these ion pairs at BCP were calculated according to this formula.

3. RESULTS AND DISCUSSION 3.1. The interaction energies In this work, a serious of MD simulations was performed on the mixtures formed by different ILs with H2O or DMSO at 298 K. The effect of anionic and cationic structures on the interactions of ILs with H2O and DMSO was considered by comparing the following two groups of typical ILs, respectively: (i) [C4mim][Tf2N], 6


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[C4mim][N(CN)2],

[C4mim][OAc],

and

[C4mim][NO3];

(ii)

[C4mim][NO3],

[bpy][NO3], [N1112OH][NO3] and [P4444][NO3]. After such simulations, the interaction energies of cation-anion, cation-cosolvent and anion-cosolvent in these ILs/co-solvent systems were calculated by sum of the electrostatic and van de Waals energies. At first, the cation-anion interaction in pure [C4mim][NO3], [C4mim][NO3]/H2O and [C4mim][NO3]/DMSO systems (Figure 1) were compared to examine the effect of co-solvent on the dissociation of the ILs. It is clear from Figure 1 that the interaction energy between cation and anion in pure [C4mim][NO3] was remarkably higher than that in the two [C4mim][NO3]/co-solvent systems at the molar ratio of 1:1. This suggests that the extended three-dimensional hydrogen bond network between cations and anions of the pure IL was destroyed with addition of the co-solvent, and then the interaction between cation and anion was weakened at the given IL concentration. As the molar volume of DMSO molecules is larger than that of H2O molecules, the degree of co-solvent effect in the cation-anion interaction energy decreased in the order: DMSO > H2O. The interaction energies between ILs and co-solvent were calculated and the results were shown in Figure 2, and the detailed data for the electrostatic and van de Waals energies of these interactions were included in Tables S2 and S3. From the comparison of the interaction energies of the both co-solvents with cation and anion of these ILs, it is evident that the interaction between H2O and anion was significantly stronger than that between H2O and cation. However, DMSO - anion interaction did not show any significant difference from DMSO - cation interaction. In order to describe the significant difference in the interaction of the cation and anion with the same co-solvent, a conception of “preferential interaction of ions” is proposed here. If the interaction of the co-solvent with cation (or anion) was significantly stronger than that of the co-solvent with anion (or cation), preferential interaction of cation (or anion) with the solvent will be defined. It should be pointed out that this conception is different from the preferential solvation frequently reported in literatures49, where preferential solvation was usually used to indicate that an ion was more strongly solvated by which solvent in a binary solvent mixture. By using this definition, it is 7



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clear that there was a significantly preferential interaction between anion of the ILs and water, but neither the ILs anions nor their cations was preferentially interacted with DMSO. In a recent work, Budtova et al.50 studied the interactions of [C2mim][OAc] with DMSO by experimental measurements of density, viscosity, NMR spectroscopy, relaxometry and diffusion coefficients. They suggested that DMSO molecules could disrupt the ionic clusters formed within the IL to a certain extent, and confirmed that the interactions between [C2mim][OAc] and DMSO were significantly weaker than those between [C2mim][OAc] and water, which is in agreement with our simulation results. To examine the effect of anionic structure on the solvation of the ILs, the interaction energies of anions of [C4mim][Tf2N], [C4mim][N(CN)2], [C4mim][OAc] and [C4mim][NO3] with the co-solvents were calculated and compared at the molar ratio of 1:1. It can be seen from Figure 2a that for the anions of these ILs, the change of anion-H2O interaction energies in ILs/H2O systems was very significant. The strength of anion-H2O interaction decreased in the order: [OAc]

-

> [NO3]- >

[N(CN)2]- > [Tf2N]-, which is consistent with hydrophilicity of the ILs51. In ILs/DMSO systems, only a little change was observed for the anion-DMSO interaction energies of these ILs, indicating the small effect of anionic structure on the solvation of the ILs in DMSO. Based on the data shown in Table S2, it is evident that the anions – H2O interactions were electrostatic (including hydrogen bonding) in nature, while electrostatic and van der Waals interactions were both important for the anions - DMSO interactions. By comparing the interaction energies of these anions with different co-solvents, it was found that the interaction energies of anions with H2O were significantly higher than those with DMSO except for [Tf2N]- which is strongly hydrophobic. The remarkable difference in the interactions observed between the anions - water and the anions - DMSO could be ascribed to the much stronger electrostatic interactions (including hydrogen bonding) of the anions with H2O (Table S2). The ILs with the same anion [NO3]- but different cations [C4mim]+, [bpy]+, [N1112OH]+ and [P4444]+ were chosen to investigate the effect of cationic structure on 8


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the solvation of the ILs in different co-solvents. It is evident from Figure 2b that the change of cation-H2O and anion-H2O interaction energies in these ILs/H2O systems was relatively small, and the effect of cationic structure on the solvation of the ILs in DMSO was obvious in ILs/DMSO systems. For these ILs/DMSO systems, the interaction between cation and DMSO in [P4444][NO3]/DMSO system was the strongest, while the interaction between anion and DMSO in this system was the weakest owing to the effect of the cation size. Based on the data shown in Table S3, it was easy to find that the cations – DMSO interactions were predominated by van der Waals interactions, while the anions - DMSO interactions were largely ascribed to electrostatic interactions. At this stage, it is appropriate to suggest that hydration of the ILs is dependent on the chemical structures of their anions, whereas solvation of the ILs in DMSO is affected by the size of cations.

3.2. The microstructures As the strength of hydrogen bond between [C4mim]+ and [NO3]- is moderate and the shape of [NO3]- anion can be regarded approximately as a simple sphere, [C4mim][NO3] was used, here, as a model IL to study microstructure of the ILs in different co-solvent systems. To this end, spatial distribution functions (SDFs) of the co-solvents around [C4mim]+ were calculated and the results were shown in Figure 3. The coordinate lengths in the x, y and z directions were chosen to be 20 Å in the calculations. The green contour surface was drawn at 10 times of the average density, and the red contour surface was drawn at 3 times of the average density. As depicted in Figure 3, in the equimolar mixture, H2O molecules were mainly distributed in the between of cation and anion, while DMSO molecules were mainly distributed around the cation and anion. This indicates that H2O molecules could break the interaction between cation and anion, but the ability of DMSO to break ion pairs of the IL was very weak at this concentration. Moreover, the structures of [C4mim][NO3]/co-solvent systems were also investigated at different molar ratios of the co-solvent to the IL. Figure 4 showed the snapshots of [C4mim][NO3] in different solvent systems at R= 0.0 and 20.0, respectively. It can be seen that free cations, free anions and a few ion 9



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pairs in [C4mim][NO3]/H2O system were distributed evenly throughout the box, but there were a lot of ion pairs, small ion clusters and small amount of free ions in [C4mim][NO3]/DMSO system. In addition, the snapshot of [C4mim]+ and [NO3]- in [C4mim][NO3]/co-solvent systems with the same IL volume fraction was also shown in Figure S1 to compare the distribution of the IL in different co-solvents more clearly. It was suggested that with the addition of water, solvation of the IL in H2O was accompanied by the transformation from extended three-dimensional hydrogen bond network, small ion clusters, and ion pairs to free cations and anions. However, solvation of the IL in DMSO tended to become solvent-surrounded ion pairs and small amounts of free cations and anions within the simulated concentration range of the IL. Similar conclusions were obtained for the other ILs in water and DMSO co-solvents.

3.3. The hydrogen bonds Besides Coulomb force, hydrogen bond (HB) is another important non-covalent interaction in ILs and is closely related to some important properties, such as melting point, viscosity, and vaporization enthalpy. Therefore, [C4mim][OAc], [C4mim][NO3], [C4mim][N(CN)2] and [C4mim][Tf2N] were chosen, in this part, to investigate the effect of co-solvents on the formation of hydrogen bonds between cation and anion of the ILs. First, the strength of hydrogen bonding in these ILs was compared by QM calculations. The lowest-energy complexes for the ion pairs of [C4mim]+ with different anions were obtained and the conformers of these ion pairs were shown in Figure 5. Table 1 showed the binding energies (∆E) for these four conformers at the B3LYP/aug-cc-pVDZ and B3LYP-D3(BJ)/aug-cc-pVDZ theoretical levels. The binding energy52 for a dimer (AB) complex was calculated by ΔE=EAB−EA−EB

(1)

It was found from Table 1 that the binding energies decreased in the order: [C4mim][OAc] > [C4mim][NO3] > [C4mim][Tf2N] > [C4mim][N(CN)2]. The differences between the energies at B3LYP and B3LYP-D3(BJ) levels were the dispersion correction. For a given complex shown in Table 1, the B3LYP-D3(BJ) 10


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energy was larger than the B3LYP energy, which indicated a better description of the long-range interaction at the B3LYP-D3(BJ) theoretical level. From the hydrogen bonds in these complexes indicated by the dashed lines in Figure 5, it is clear that the most stable interaction site with O atom of [OAc] -, [NO3]- and [Tf2N]- or N atom of [N(CN)2]- appeared to be the C2-H of [C4mim]+. The lengths and energies (EHB) for these hydrogen bonds were listed in Table 2. It can be seen that the strength of hydrogen

bonds

in

the

complexes

decreased

in

the

order:

C2–H···O1

([C4mim][OAc]) > C2–H···O1 ([C4mim][NO3]) > C2–H···O1 ([C4mim][Tf2N]) > C2–H···N1 ([C4mim][N(CN)2]). The electron density (ρ(r)) and the corresponding Laplacian value [▽2ρ(r)] at the critical points are also important measurements for hydrogen bonds with the expected values of ρ(r) ≈ 0.002−0.035 and▽2ρ(r) ≈ 0.024−0.13953,54. It was also found from Table 2 that most of the electron density values lied in the range from 0.002 to 0.035. However, the ρ(r) values of C2–H···O1 in [C4mim][OAc] and [C4mim][NO3] were larger than 0.0035, suggesting that these HB interactions were very strong. On the other hand, the ρ(r) values of C2–H···O1 and C2–H···N1 were 0.0257 and 0.021, respectively, in the complexes of [C4mim][Tf2N] and [C4mim][N(CN)2]. This indicated a very weak hydrogen bond interaction. Moreover, all of the Laplacian values of the electron densities at the critical points of hydrogen bonds were in the range from 0.024 to 0.139, the same trend in electron densities was found in these complexes (see Table 2). Analysis of ∆E, EHB, ρ(r) and ▽2ρ(r) indicated that the strength of hydrogen bonds in the investigated ILs decreased in the order: [C4mim][OAc] > [C4mim][NO3] > [C4mim][Tf2N] > [C4mim][N(CN)2]. To understand the effect of co-solvent on the formation of hydrogen bonds between cation and anion, average hydrogen bond number in the pure ILs, ILs/H2O and ILs/DMSO systems was calculated by MD simulations (Table 3) at 1:1 molar ratio of the co-solvent to the IL. The criterion for the formation of hydrogen bonds was defined as55: the distance from a donor hydrogen to an acceptor is less than 2.5 Å and the angle of acceptor-hydrogen-donor is larger than 120°. From the comparison of the hydrogen bond number in the pure ILs listed in Table 3, it is clear that the average 11



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hydrogen bond number in the ILs decreased in the order: [C4mim][OAc] > [C4mim][NO3] > [C4mim][Tf2N] > [C4mim][N(CN)2], which is in agreement with the above QM results. Furthermore, when H2O or DMSO co-solvent was added into the ILs, hydrogen bond number in both IL/co-solvent systems was found to decrease in different extent, and the percentage of decline was also shown in Table 3. It was noted that by the addition of water, the percentage of decline of hydrogen bond number in these ILs decreased in the order: [C4mim][OAc] > [C4mim][NO3] > [C4mim][Tf2N] > [C4mim][N(CN)2]. However, the reversed order was observed in DMSO. This can be rationalized from the fact that ion pairs of the ILs could be broken by H2O molecules, but could not be broken by DMSO molecules. Therefore, the hydrogen bond number of the ILs with stronger hydrogen bonding interaction may be decreased more significantly in H2O than in DMSO. In addition, the number of hydrogen bond between cation and anion in [C4mim][NO3]/H2O and [C4mim][NO3]/DMSO systems was calculated at different molar ratios, and the results were shown in Figure 6. It was noted that with the increase of the molar ratio of the co-solvent to [C4mim][NO3], the number of hydrogen bond in [C4mim][NO3]/H2O system decreased significantly, while that in [C4mim][NO3]/DMSO system only showed a small decline. This further suggests that the protic solvent such as water is able to break the hydrogen bonds of ion pairs in the ILs, but the aprotic solvent such as DMSO is not.

3.4. The first coordination shell The radial distribution function, g(r), is a function that describes the spherically averaged local organization around any given atom. By integrating g(r) out to the location of system, coordination number in the coordination shell, N, can be calculated from the following equation33: r

N = 4π ∫ ρ N g ( r ) r 2 dr 0

(2)

where ρN is number density, if r refers to the first minimum (rmin1) in g(r), N is the coordination number in the first coordination shell. In this work, g(r) and coordination number N(r) of the co-solvents around the cation or anion of the IL were calculated in

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[C4mim][NO3]/co-solvent systems (R=1:1), and the results were shown in Figures 7 and 8. It can be seen from Figure 7a that the first peak height for H2O molecules around the anion was much higher than that around the cation, which is in agreement with the result of [C4mim]Br]/water mixture reported by Migliorati et al.56 This indicated that [NO3]- had a strong interaction with H2O molecules at 3.8 Å, and the preferential hydration of [NO3]- was quite evident. By contrast, the first peak height for DMSO molecules around the anion was close to that around the cation (Figure 7b). This confirms that cation and anion of the IL were not preferentially interacted with DMSO. The first-minimum positions for H2O molecules around cation and anion in Figure 7a were found to be about 4.3 Å and 6.2 Å, while the first-minimum positions for DMSO molecules around cation and anion in Figure 7b were about 7.0 Å and 6.7 Å, respectively. It was suggested that the radius of the first coordination shell of ions in [C4mim][NO3]/DMSO systems were longer than that in [C4mim][NO3]/H2O system. The coordination number N(r) of the co-solvent around the IL cation or anion at R=1:1 was also calculated. As is shown in Figure 8, the N(r) values at the given molar ratio increased abruptly for [C4mim][NO3]/H2O system, which was different from those for [C4mim][NO3]/DMSO system. The main reason is that the IL anion was preferentially interacted with H2O molecules. In order to have a better understanding for the coordination of the co-solvents in the first coordination shells of [C4mim]+ and [NO3]-, Figure 9a shows the coordination number of DMSO in the first coordination shell of [C4mim]+ cation as a function of molar ratio of the co-solvent to the IL. Since the size of [C4mim]+ was relatively large and the interaction of [C4mim]+ with H2O molecules was weak, the boundary between the first and second coordination shells cannot be distinguished clearly in integrating g(r). Thus it was difficult to obtain quantitative data for the coordination number of H2O around [C4mim]+ from MD simulations. In such a case, the simulation results can only be understood in a qualitative way. Therefore, the hydration number data for [C4mim]+ cation were not given in Figure 9a. The coordination numbers of H2O and DMSO in the first coordination shell of [NO3]- anion were also shown in Figure 9b at different molar ratios. It was clearly indicated that the coordination numbers of both 13



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co-solvents around [NO3]- increased rapidly with the molar ratio, and the change of coordination number slowed down at R=6.0. This suggested that the coordination of H2O and DMSO in the first coordination shell was near to saturation. The saturated coordination numbers of H2O and DMSO around [NO3]- in the first coordination shell were found to be about 8 and 7, respectively, which were in good agreement with the results reported in literature57,58. To further investigate the local structure of the cation and anion of the IL in the co-solvents, representative examples for the first coordination shells of [C4mim]+ and [NO3]- in different co-solvents were extracted from the trajectory files. Figure 10 shows the snapshots for [C4mim][NO3] /co-solvent systems at R= 20.0, it can be seen from that in the first coordination shell of [C4mim]+ (Figure 10 (a) and (b) ), O atoms of H2O and DMSO molecules orient towards the acidic protons of the imidazole ring to form weak C-H···O hydrogen bonds. In the first coordination shell of [NO3](Figure 10 (c) and (d)), strong O-H···O hydrogen bonds are formed by hydroxyl proton of H2O molecules with [NO3]-, while the CH3 group of DMSO molecules was close to the O atom of [NO3]- which may results in weak C-H···O hydrogen bonds. Compared with the first coordination shells of cation and anion in [C4mim][NO3] /H2O system (Figure 10 (a) and (c)), it is noted that in [C4mim][NO3] / DMSO system (Figure 10 (b) and (d)), there exist some cations or anions of the IL in the first coordination shell of the ions. This result confirms that water molecules were able to break the hydrogen bonds between cations and anions of the ILs, but DMSO molecules were not, even in the solution with IL mole fraction of about 0.05 (R= 20.0).

3.5. Comparison with the solvation of common electrolytes in DMSO and application to cellulose dissolution in IL/co-solvent systems It is known that the polar group of DMSO is electronegative and involved in solvating the cation of electrolytes59. For this reason, most of the solvation studies concluded that preferential interaction of cation of the electrolytes (such as LiNO3, NaBF4) arisen in DMSO59,60. However, different conclusion was drawn from the 14


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present study. It seems that the conventional solvation viewpoint for the common electrolytes was not applicable to imidazolium ILs. The reason is that the size of imidazolium cation is big and its positive charge density is small compared with the cation of common inorganic salts. Therefore, it should be careful to discuss ILs solvation by using the conventional concepts. In addition, Lynden-Bell et al.61 studied the solvation of some small molecules in dimethylimidazolium chloride by molecular simulation. For this purpose, water, methanol, dimethyl ether and propane were chosen which have a range of properties from polar and hydrogen-bonding to non-polar. It was shown that the strongest interactions are hydrogen bonding to the chloride ion for water and methanol, while dimethyl ether and propane interact more strongly with the cation. In other words, anion of the IL preferentially interacted with water and methanol, while cation of the IL preferentially interacted with dimethyl ether and propane. This is true, but can not be misunderstood by the statement that protic co-solvents were preferentially interacted with anion, whereas aprotic co-solvents were preferentially interacted with cations of ILs, because DMSO is an aprotic solvent but no such preferential interaction was observed. Actually, although DMSO, dimethyl ether and propane are all aprotic solvents, the polarity of DMSO is strongest. Thus preferential interaction between ions of ILs and aprotic solvent was also dependent on the polarity of the solvent. Qualitatively speaking, for non-polar and weak polar aprotic solvents such as propane and dimethyl ether, they do have preferential interactions with cations of ILs. However, with the increase of polarity of the aprotic solvents, such as DMSO, no obvious preferential interactions can be observed. If a solvent contains potential proton, its polarity is strong enough such as methanol, it is possible for us to see preferential interactions between anions of ILs and the solvent. It is known from the recent studies that in the dissolution process of cellulose in ILs, the addition of DMSO into an IL can increase the solubility of cellulose, but water or ethanol was always used to precipitate cellulose from IL/cellulose mixture12,14,62. Actually, this phenomenon can be reasonably interpreted from the results of this work. Indeed, it was reported that the dissolution of cellulose in ILs was 15



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mainly determined by the hydrogen bond interactions between anion of the IL and cellulose. When DMSO was added into an IL, the extended three-dimensional structure and ionic clusters among cation and anion of the IL were broken to generate a lot of solvent-surrounded ion pairs and some free ions in the co-solvent system. As the interactions of ions of the IL with DMSO were weaker than their interactions with cellulose, these ion pairs and free anions were readily available to interact with cellulose, leading to the increase of cellulose solubility. By contrast, when H2O was added into an IL, anion of the IL was strongly preferentially interacted with H2O molecules, which hindered the interaction of anion with cellulose due to the stronger interaction between anion and H2O. Under these circumstances, the IL was not able to dissolve cellulose, resulting in the remarkable decrease of cellulose solubility and even precipitation of cellulose. Liu et al.63 performed molecular dynamics simulations for the interfaces between an Iβ cellulose crystal and different mixed solvent systems of [C4mim]Cl/water and [C4mim]Cl/DMSO. It was found that in the presence of DMSO, an increase in the proportion of the 3-HB pattern of Cl- with cellulose led to a shift of the pair energy distribution to a stronger interaction energy, but the proportion of the 1-HB pattern increased upon the addition of water, resulting in the decrease of the anion-cellulose interaction. This suggests that in the presence of water, the Clanions have to adjust their positions and tend to form hydrogen bonds with water. This structure change further supports the solvation mechanism of preferential interaction of ions of ILs in molecular solvents proposed by us. On the other hand, when DMSO was added into IL, system viscosity was greatly reduced. This would increase mobility of the ionic pairs and free ions as well as the possibility of their interactions with cellulose, which also led to the increase of cellulose solubility.

4. CONCLUSION In this paper, a series of binary mixtures of ILs with H2O or DMSO were performed at 298 K by MD simulations. The interaction energies between the ILs and different co-solvents, the microstructure of the ILs/co-solvent mixtures, the hydrogen

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bonds between cations and anions of the ILs and the first coordination shell of cations and anions of a given IL were systematically investigated to demonstrate the interactions and the existing state of ions in ILs/co-solvent systems. In agreement with the conclusions reported in literatures, anion of the ILs was found to have strong interaction preferentially with H2O molecules, leading to the destruction of hydrogen bonding structure among cations and anions of the ILs and the generation of more free cations and anions with the increase of the molar ratio of the co-solvent to the IL. However, obvious preferential interactions of cations and anions of the ILs with DMSO were not observed, and the ILs in DMSO tended to become solvent-surrounded ion pairs and a small amount of free ions with the increase of the molar ratio because it was difficult for DMSO to break the ion pairs into free ions. The above solvation viewpoint of ILs in co-solvents has been applied to explain the dissolution process of cellulose in IL/co-solvent systems. Moreover, it was found that the number of hydrogen bond between cations and anions of the ILs was decreased more significantly in H2O than in DMSO. Also, it was noted that in [C4mim][NO3] / DMSO system, there exist some cations or anions of the IL in the first coordination shells of ions. These results confirmed that water molecules were able to break the hydrogen bonds between cations and anions of the ILs, but the DMSO molecules were not. The simulation results reported here may be helpful for us to understand the ionic solvation of ILs in polar aprotic solvents, and to promote further exploration of new applications of ILs/co-solvent systems.

AUTHOR INFORMATION Corresponding Author *Tel: +86-373-3325805. E-mail: [email protected]; Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant 17



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No. 21403060, 21133009) and scientific research key project fund of education of Henan province (No.14A150039).

ASSOCIATED CONTENT Supporting Information Further details including the molecule number and molar ratio of the co-solvents to the ionic liquids for each simulated system, the interaction energy between ionic liquids and co-solvent in different IL/co-solvent systems and the long shots of [C4mim]+ and [NO3]- in [C4mim][NO3]/co-solvent systems which have the same volume fraction occupied by the IL in each solution by molecular dynamics simulations are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1. The binding energies (∆E) of the conformers at 298 K calculated at the B3LYP/aug-cc-pVDZ and B3LYP-D3(BJ)/aug-cc-pVDZ theoretical levels △E (KJ/mol)

Ion pair

DFT

DFT-D

[C4mim][OAc]

-424.488

-441.421

[C4mim][NO3]

-380.264

-403.388

[C4mim][Tf2N]

-362.866

-392.735

[C4mim][N(CN)2]

-348.507

-371.500

Table 2. The bond lengths, bond energies EHB, electron densities ρ(r) and Laplacian values▽2ρ(r) for the hydrogen bonds of the cation - anion complex of the ILs Type of Length EHB hydrogen ρ(r) IL V(r) ▽2ρ(r) (Å) (KJ/mol) bond -75.6 0.0636 0.158 C2-H-O1 1.60 -0.0576 [C4mim][OAc] C7-H-O2 2.18 -0.0132 -17.3 0.0184 0.055 -46.3 0.0447 0.135 C2-H-O1 1.75 -0.0353 [C4mim][NO3] C2-H-O2 2.25 -0.0092 -12.1 0.0130 0.043 C2-H-O1 1.94 -0.0187 -24.5 0.0257 0.093 [C4mim][Tf2N] C2-H-O2 2.24 -0.0117 -15.3 0.0160 0.055 C2-H-N1 2.11 -0.0129 -16.9 0.0210 0.061 [C4mim][N(CN)2] C6-H-N1 2.37 -0.0082 -10.8 0.0146 0.407 C7-H-N2 2.29 -0.0095 -12.5 0.0154 0.045

Table 3. The average hydrogen bond number in the pure ILs, ILs/H2O and ILs/DMSO systems calculated from MD simulations at 298 K Pure IL IL [C4mim][OAc] [C4mim][NO3] [C4mim][Tf2N] [C4mim][N(CN)2]

HB number 2.22 2.08 1.50 0.24

IL/H2O (R=1:1) HB number 1.54 1.62 1.22 0.21

Percentage of decline (%) 30 22 19 13

22


IL/DMSO (R=1:1) HB number 2.18 1.85 1.30 0.20

Percentage of decline (%) 2 11 13 17

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-3.2

-2.8

3

E (10 kJ/mol)

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-2.4

-2.0

[C4mim][NO3]

[C4mim][NO3]/H2O [C4mim][NO3]/DMSO

Figure 1. The interaction energies between cation and anion in the pure [C4mim][NO3] and [C4mim][NO3]/co-solvent (R=1:1) systems obtained from MD simulations at 298 K.

23



Cation-solvent Anion-solvent

-12

0

-60 -30 ] Ac [O ] im ] O3 4m [N [C ] im ] 4m 2N [C [Tf ] im )2] 4m [C CN ( [N m] mi 4 [C

H2O

0

The interact ion

-90

energy / KJ. mol -1

- 15 0

DMSO

(a)

Cation-solvent Anion-solvent

-80 -60 -40 ] O3 ][N 4 ] 44 O3 [P4 ][N y ] [bp O3 ][N m i ] 4m O3 [C ][N H 2O 111 [N

-20 0

H2O

ion energy / KJ.mol -1

-10 0

The interact

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Page 24 of 33

DMSO

(b) Figure 2. Comparison of interaction energies of different ILs with co-solvent at the molar ratio of 1:1 and 298 K: (a), the effect of anion structure on the interaction energies; (b), the effect of cation structure on the interaction energies.

24


Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[C4mim][NO3]/H2O

[C4mim][NO3]/DMSO

Figure 3. Spatial distribution function (SDF) of the co-solvents and [NO3]- around [C4mim]+ in [C4mim][NO3]/co-solvent systems (R=1:1): red, co-solvent; green, [NO3]-; blue, [C4mim]+.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[C4mim][NO3]

[C4mim][NO3] / H2O

Page 26 of 33

[C4mim][NO3] / DMSO

Long shot

Close-up

Figure 4. The long shots and close-ups of [C4mim]+ and [NO3]- in different co-solvent systems at R= 0.0 and 20.0.

26


Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) [C4mim][OAc]

(b) [C4mim][NO3]

(c) [C4mim][Tf2N]

(d) [C4mim][N(CN)2]

Figure 5. Optimized geometries for the ion pairs of [C4mim]+ with different anions: (a), [OAc]-; (b), [NO3]-; (c), [Tf2N]-; (d), [N(CN)2]-. The dashed lines note the hydrogen bonds in the ion pairs, and C atom (gray), H atom (white), O atom (red), S atom (yellow) and N atom (blue) are shown in different colors for clarity.

27



2.5 The number of hydrogen bond

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Page 28 of 33

DMSO H2O

2.0

1.5

1.0

0.5

0.0 0

4

8

12

16

20

R

Figure 6. The number of hydrogen bonds between cation and anion in [C4mim][NO3]/H2O and [C4mim][NO3]/DMSO systems at different molar ratios of the co-solvents to the IL.

28


Page 29 of 33

7 6 Cation-H2O

5

Anion-H2O

g(r)

4 3 2 1 0 4

6

8

10

r / 0.1 nm

(a)

2.0

Cation-DMSO Anion-DMSO

1.5

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

0.5

0.0 4

6

8

10

r / 0.1 nm

(b) Figure 7. Radial distribution function of the co-solvents around cation or anion of the IL in [C4mim][NO3]/co-solvent systems (R=1:1): (a), H2O; (b), DMSO.

29



10

8

Cation-H2O Anion-H2O

N(r)

6

4

2

0 2

4

6

8

10

r / 0.1nm

(a)

10

8

Cation-DMSO Anion-DMSO

6

N(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

4

2

0 4

6

8

10

r / 0.1nm

(b) Figure 8. The coordination number of the co-solvent around the cation or anion of the IL in [C4mim][NO3]/co-solvent systems (R=1:1): (a), H2O; (b), DMSO. 30


Page 31 of 33

Coordination number

10

DMSO

5

0 0

4

8

12

16

20

R

(a)

8 Coordination number

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6

4 DMSO H2O

2

0 0

4

8

12

16

20

R

(b) Figure 9. The coordination number of H2O and DMSO in the first coordination shells of cation and anion of the IL as a function of the molar ratio: (a), [C4mim]+; (b), [NO3]-.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

[C4mim][NO3] / H2O

[C4mim][NO3] / DMSO

(a)

(b)

(c)

(d)

[C4mim]+

[NO3]-

Figure 10. Simulation snapshots showing H2O and DMSO molecules in the first coordination shells of [C4mim]+ and [NO3]- in different [C4mim][NO3] /co-solvent systems at R= 20.0. Red, [NO3]-; blue, [C4mim]+; green, co-solvent.

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Page 33 of 33

Table of Contents Graphic

+ -

+

-

+

+

-+

+

Anion

-

-

+ -

-

Ionic Liquid Cation

-

+

-

-

+ -

+

DMSO

-

33


+

+

+ +

-

+

H2O

+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Is There Any Preferential Interaction of Ions of Ionic Liquids with DMSO

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