DOI: 10.1002/chem.200802742

Probing Intermolecular Interactions in Ionic Liquid–Water Mixturesby Near-Infrared Spectroscopy

Bo Wu,[a] Yang Liu,[b] Yumei Zhang,[a] and Huaping Wang*[a]

Introduction

Ionic liquids (ILs) have attracted wide attention by a grow-ing number of academics and engineers due to their wideapplication.[1] Since Seddon[2] revealed that the presence ofwater in ILs could strongly affect the physical and chemicalproperties of ILs such as viscosity, electrical conductivity,polarity, and reactivity, as well as solvation and solubilityproperties, a certain number of both experimental[3–28] andtheoretical works[29–33] have shed light on the interactions be-

tween ILs and water. These previous reports have greatlyenhanced our perspectives on the way that water moleculesinteract with ILs. However, most of them are limited to thesimulations, and/or the measurement is only limited to themixture of ILs and a small amount of water molecules. Forexample, Dupont[10] reported that pure 1,3-dialkylimidazoli-um ionic liquids are better described as hydrogen-bondedpolymeric supramolecules, and this structural pattern ismaintained to a great extent in the solvents with low dielec-tric constant. The introduction of water molecules occurswith a disruption of the hydrogen-bond network and insome cases can generate nanostructures with polar and non-polar regions.[11] As we know, hydrogen bonding is best ob-served in IR or Raman spectra as vibrational spectroscopyis a very powerful tool to explore the structural and dynamicproperties of the mixtures of ILs and water. Unfortunately,these studies are very limited,[22–28] in particular for researchdevote to the interaction of ILs–water in the water-richregion. This may be due to lack of a suitable technique thathas non-invasive and in situ capabilities without any pre-treatment of samples. Near-infrared (NIR) spectrometry canoffer a solution to this problem. Apart from its non-inva-siveness, non-destructiveness, the reason for that NIR spec-

Abstract: In this contribution, an in-sight into the interactions betweenionic liquids (ILs), 1-butyl-3-methylimi-dazolium tetrafluoroborate,[Bmim]BF4, and 1-allyl-3-methylimida-zolium chloride, [Amim]Cl, and wateris presented using near-infrared (NIR)spectroscopic measurements. Distinctdifferences were found in the NIRspectra of pure [Amim]Cl and[Bmim]BF4, whereby we propose opti-mized conformations. It was found thatthe relative position of the anion withrespect to imidazolium cation is differ-ent in these two ILs. The geometry dif-ference determined their different in-

teraction modes with water, for exam-ple, the NIR spectra in alkyl groupregion were different for these two ILs/H2O mixtures, irrespective of being ina water-rich region or IL-rich region.However, their NIR spectra for aro-matic group were similar, whereby wededuced that for both ILs, the watermolecules were favorable to form hy-drogen bonds with the proton H2 on

imidazolium ring, rather than H4 andH5. Furthermore, it was shown thatwater molecules preferred to interactwith BF4

�, but Cl� interacted more spe-cifically with aromatic C-H groupscompared with BF4

�. This was con-firmed by the fact that the supramolec-ular structure of aqueous [Bmim]BF4

solution was destroyed as mole fractionof water surpasses 0.3979, which waslower than the value of 0.5822 for[Amim]Cl/H2O. These results wouldhave important directive significancefor the study of the aggregation behav-ior and recovery of hydrophilic ILs inwater.

Keywords: aggregation behaviour ·intermolecular interactions · ionicliquids · near-infrared spectroscopy ·supramolecular chemistry

[a] Dr. B. Wu, Prof. Y. Zhang, Prof. H. WangState Key Laboratory for Modification ofChemical Fiber and Polymer Materials, Donghua UniversityNorthern Ren-ming Road 2999, Shanghai 201620 (China)Fax: (+86) 021-6779-2950E-mail : wanghp@dhu.edu.cn

[b] Dr. Y. LiuTextile Materials and Technology LaboratoryDonghua University, Northern Ren-ming Road 2999Shanghai 201620 (China)

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trometry has been used extensively in recent years forchemical analysis and characterization is that the NIRregion covers the overtone and combination transitions ofthe CH, OH, and NH groups. Since all organic and most in-organic compounds possess at least one or more of thesegroups, this technique can, in principle, be used for analysisof all organic and most inorganic compounds. Unfortunately,in spite of its potentials, to date, the NIR technique has notbeen used to probe intermolecular interactions in mixturesof ILs and water, except that Tran[34] used this techniqueonly for the determination the amount of water absorbed byILs.

In our earlier paper,[35] aggregation behavior of 1-butyl-3-methylimidazolium tetrafluoroborate, ([Bmim]BF4) in waterwas demonstrated by conductometry. As a continuation, thegoal of this paper is to provide more insight into interactionsbetween [Bmim]BF4 and water on the molecular level byNIR. In addition, probing the interactions between 1-allyl-3-methylimidazolium chloride ([Amim]Cl) and water by NIRhave never been studied previously. The results on the inter-actions between ILs and water is of great importance forbetter understanding the recovery of ILs from their aqueoussolution.[36–39]

Results and Discussion

Figure 1 displays the NIR spectra of [Bmim]BF4 and[Amim]Cl obtained under ambient pressure in the 700–1850 nm region. As indicated in Figure 1, the spectrum ofpure [Bmim]BF4 exhibits four discernible peaks, that is,1) 1180 nm, second overtones of the aliphatic C-H groups;2) 1376 nm, combination of the aliphatic C-H groups;3) 1612 nm, combination of the aromatic C4,5-H groups; 4)1704 nm, first overtone of the aromatic C2-H groups. Uponswitching the counteranion from BF4

� to Cl�, the most dis-tinct is the appearance of peak at about 1452 nm owing toovertone of the allyl group, apart from the red/blueshifts ofabove four bands. These significant changes in NIR spectramay be ascribed to their difference in the optimized confor-mation simulated by semiempirical computations of Cam-bridge Software. In Figure 2, the butyl chain of [Bmim]BF4

is all anti, but the chain is gauche around C7–C8 for[Amim]Cl. Notable in Figure 2 is that the positions of coun-teranions; Cl�, planar with the imidazolium ring, lies closeto C5-H, but BF4

� is positioned on top of the imidazoliumring. In other words, the difference between C-H-BF4

� andC-H-Cl� interactions seems to play a non-negligible role fortheir distinct spectra. The issue how the relative position be-tween the anion and cation comprising the bulk liquid deter-mines the physicochemical properties of IL is still unre-solved and awaits further investigation.

Figure 3 shows the NIR spectra recorded with aqueous so-lution of [Amim]Cl at different concentrations. The dottedline was used to exhibit the shift of maximum absorption ofsolutions at IL increasing concentration. Upon addition ofwater, it can be seen a new band at about 970 nm, which

was attributed to the second overtone of the O-H groups ofwater, appeared. To better interpret the water–ILs in the O-H region from about 970–1190 nm, we amplified the spectrain the region 800–1250 nm as shown in Figure 4. Fromwhich, we noticed that the amplitude of O-H peak increasesgreatly upon addition of [Amim]Cl, and then decreases withfurther addition of [Amim]Cl. Interestingly, it undergoes aredshift with the concentration of [Amim]Cl in the wholerange. This redshift may be attributable to the fact the addi-tion of [Amim]Cl to water disrupts the flickering cluster ofwater, breaking some water H-bonds but generating morenew H-bonds. The perturbation induced by the [Amim]Cl inthe stretching and bending vibrations determines a shift inthe maximum water absorption band towards the H-bondedmolecules. The higher the perturbation, the higher the shift.With increasing the amount of [Amim]Cl, it increases thenumber of H-bonds within water, between water and solute,and within the solute molecule.

To better understand the effect of water on ILs structureand the effect of ILs on water structure, we stressed the IL-rich region and water-rich region. In [Amim]Cl-rich region,the intensity of peak at about 970 nm decreases when themole fraction of added water is less than 0.3979, after which

Figure 1. NIR spectra of [Amim]Cl and [Bmim]BF4.

Figure 2. Optimized conformation of [Amim]Cl (a,b) and [Bmim]BF4 ionpairs (c,d).

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further addition of water leads to an increase in absorptionstrength. This indicates most of water molecules in[Amim]Cl-rich region tend to associate to the anions ratherthan self-aggregate, thereby probably forming symmetricallyH-bonded complexes, anion···H-O-H···anion. That is, nowater clusters are formed and water molecules are isolatedfrom each other, being involved in H-bonding with the Cl�

ions. However, on increasing the water content abovexwater =0.3979, water–anion interactions enter into competi-tion with water–water interactions, and water forms clustersand self-aggregates. As a consequence, a strong and narrowpeak due to the O-H stretching vibration (centered at1460 nm) appears at xwater =0.5822, after which it splits intotwo peaks. This may indicate a complete destruction ofsupramolecular structure of [Amim]Cl, which is somewhatlower than the value 0.5–0.6 reported by Katayanagi.[20]

More importantly, looking into more detail in Figure 4, weobserved no drastic change in the concentration dependenceof the alkyl C-H band wavelength (at ca. 1150 nm) at highconcentrations of [Amim]Cl (x[Amim]Cl>0.6). This behavior

may indicate a clustering of ionic liquid in non-polar regionsand a slight perturbation by the presence of H2O at highconcentration of [Amim]Cl. The aggregation of ionic liquidin solution is apparently a general trend. However, in water-rich region, a redshift for peaks at about 1150 nm occurredwith addition more water, and the change trend of intensitywith the concentration of [Amim]Cl shown in Figure 4, issimilar to that of peaks at about 970 nm. This observationsuggests the formation of a certain water structure aroundalkyl C-H groups of ionic liquids in water-rich mixtures.

Additionally, from Figure 3, no appreciable changes in theband wavelength of the imidazolium C4,5-H at about1628 nm occurred at the beginning of dilution of ionic liquidwith H2O, but a monotonic blueshift in wavelength was ob-served when further diluted in water-rich region. This is as aresult of dilution effect, and is remarkably contrary to whatwas revealed for the alkyl groups. This blueshift also indi-cates there is no C4,5-H···O interactions between imidazoli-um C4,5-H groups and water molecules, if there is, a redshiftshould occur, like the wavelength of imidazolium C2-Hgroup at about 1688 nm, which implies the imidazolium C2-H group seems to be more favorable sites for hydrogen-bond-like C-H···O than the imidazolium C4,5-H groups.Therefore, this behavior may be explained as follows. As theCl� ion should be strongly bound to the imidazolium cationfor pure [Amim]Cl, one can suggest that they are still at-tached in the presence of few water, while excess water fa-cilitates the dissociation of anion and cation. In other words,water can be added to change the structural organization of[Amim]Cl by introducing C2-H···O interactions between C2-H group and Cl�, as well as Cl�-H···O interactions betweenCl� and water molecules. This is consistent with the recentinvestigations which suggest that spatial heterogeneity existsin ionic liquids/H2O mixture.[3,4] Our results in Figures 3 and4 indicated that the presence of water significantly perturbsthe ionic liquid–ionic liquid associations in the polar region.

In order to investigate the effects of cation and anion onthe interactions between ionic liquids and water, we addedwater to [Bmim]BF4. It can be seen from Figure 5, a newband at about 1412 nm owing to O-H generated by waterappears as the mole fraction of added water is 0.1126, lowerthan the value (xwater =0.5822) for [Amim]Cl/H2O system.Another interesting difference is this peak does not splituntil the water mole fraction is 0.3979, which means theaqueous solution [Bmim]BF4 remains its supramolecularstructure as its mole fraction surpasses 0.6021. This value islarger than the value 0.4178 for [Amim]Cl/H2O, but agreeswith the value of Katayanagi (0.5–0.6).[20] These remarkabledifferences may be due to the fact it is more easy for watermolecules to break the equilibrium of [Bmim]+ ACHTUNGTRENNUNG[BF4]

� ionpairs or higher aggregates by introducing water–BF4

� inter-actions. This may be explained by the variation in the posi-tion of the anions, where Cl� is closed to the C5-H of theimidazolium cation, but BF4

� lies on top of the imidazoliumring, Consequently, Cl� interacts more specifically with aro-matic C-H groups as compared to BF4

� which prefers to in-teract with water molecules.

Figure 3. NIR spectra of [Amim]Cl/H2O mixture with different amountsof [Amim]Cl.

Figure 4. Partial amplification spectra of [Amim]Cl/H2O mixtures in 800–1250 nm range.

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FULL PAPERIonic Liquid–Water Mixtures

Comparing the NIR spectra of [Amim]Cl/H2O and[Bmim]BF4/H2O system, the most distinct difference shownin Figure 6 is that the band at about 970 nm generated byO-H group of water suffers from a monotonic decrease inwavelength with increasing [Bmim]BF4, as well as the inten-sity. This implies that the interactions between Cl� and H2O,and between BF4

� and H2O are different, and the reasonwas given above. Another possible reason is that[Bmim]BF4 is more chaotropic than [Amim]Cl,[36] leading tobond breakage in the periphery and inside the cluster, fol-lowed by an increase in the distance among molecules andthe number of free molecules. Besides, another unique char-acter in NIR spectra of [Bmim]BF4/H2O is that the alkyl C-H band wavelength at about 1180 nm firstly undergoes ablueshift prior to x=0.6021, in the [Bmim]BF4-rich region,and then a redshift in water-rich region. But the absorptionstrength monotonically rises in the whole range, like the O-H band at about 970 nm. This confirms our previous conclu-sion that when the mole fraction of [Bmim]BF4 surpasses0.6021, the structure of its aqueous solution remains as thesupramolecular structure in solid state.

Apart from these differences, the similarity between NIRspectra of [Amim]Cl/H2O and [Bmim]BF4/H2O systems isthe change tendency of band for imidazolium ring. This indi-cates that irrespective of anions and length of alkyl chain,the water molecules are favorable to form hydrogen bondswith the cation involving imidazolium ring proton H2,rather than H4 and H5. This is consistent with the simula-tion results by Mele.[40]

Conclusions

In summary, the NIR technique could be sensitively usedfor probing the interactions between ILs and water. First,we investigated the structures of [Bmim]BF4, [Amim]Cl andtheir water mixtures using NIR spectroscopy. The NIR spec-tra were very distinct for [Bmim]BF4 and [Amim]Cl, andtheir changes with the increase in the water concentrationwere also different. These differences suggested that the rel-ative position of anion with respect to imidazolium cation isdifferent in these two ILs. The geometry difference deter-mined their different interaction modes with water. Foranions, water molecules prefer to interact with BF4

�, but Cl�

interacts more specifically with aromatic C-H groups ascompared to BF4

�. This preference suggested minor tracesof water could destroy the supramolecular structure of ILs.For [Bmim]BF4, the supramolecular structure of aqueous so-lution only remained as mole fraction surpasses 0.6021,which is larger than the value 0.4178 for [Amim]Cl/H2O. Al-though their optimized conformers are different, it wasshown for both ILs, the water molecules are favorable toform hydrogen bonds with the proton H2 on imidazoliumring, rather than H4 and H5. As a preliminary investigationon IL–water interaction by NIR, this work will do a contri-bution to the study of aggregation behavior of ILs in waterand recycling.

Experimental Section

Materials : Chlorobutane, allyl chloride, 1-methylimidazole, ethyl acetate,acetone and NaBF4 were all purchased from Shanghai Chemical Re-agents Company. They are of analytic grade and used as received.Doubly distilled water was used in all experiments. [Amim]Cl and[Bmim]BF4 were prepared based on the reported procedures.[41, 42] Thepurity of ILs was verified in terms of NMR analysis (>99 %), and theCl�1 content is smaller than 130 ppm.[43] The water content of all ILs (<0.5%) was determined by the Karl Fischer titration (ZSD-2 KF with aprecision of 0.05 %, Cany Precision Instruments Co., Ltd.).

Measurements : NIR spectra were collected by a computer-aided doublebeam spectrometer (U4100 UV/Vis/NIR, Hitachi High-Technologies Cor-poration) in the 700–1850 nm wavelength range at 6 nm intervals. An un-certainty of 1% was given by the manufacturer for the measurements ob-tained by this spectrophotometer. Data analysis was limited to 700–1850 nm to avoid the 1900 nm water band, whose high absorbance valueswere greatly distorted by stray light. All spectra were referenced to anempty cuvette.

Figure 5. NIR spectra of [Bmim]BF4/H2O mixture with different amountsof [Bmim]BF4.

Figure 6. Partial amplification spectra of [Bmim]BF4/H2O mixtures in800–1250 nm range.

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H. Wang et al.

Acknowledgements

We thank Financial support from the Cultivation Fund of the Key Scien-tific and Technical Innovation Project, Ministry of Education of China(707026), and Shanghai Science and Technology Commission(08XD14005).

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Received: December 29, 2008Published online: June 2, 2009

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FULL PAPERIonic Liquid–Water Mixtures