eﬀect of the cation on the interactions between alkyl...

Effect of the Cation on the Interactions between Alkyl MethylImidazolium Chloride Ionic Liquids and WaterImran Khan,† Mohamed Taha,† Paulo Ribeiro-Claro,† Simao P. Pinho,‡,§ and Joao A. P. Coutinho*,†

†Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal‡UNIFACS-Universidade de Salvador, Rua Dr. Jose Peroba 251, CEP 41770-235 Salvador, Brazil§Associate Laboratory LSRE/LCM, Departamento de Tecnologia Química e Biologica, Instituto Politecnico de Braganca, Campus deSanta Apolonia, 5301-857 Braganca, Portugal

*S Supporting Information

ABSTRACT: A systematic study of the interactions between water and alkyl methylimidazolium chloride ionic liquids at 298.2 K, based on activity coefficients estimated fromwater activity measurements in the entire solubility range, is presented. The results showthat the activity coefficients of water in the studied ILs are controlled by the hydrophilicityof the cation and the cation−anion interaction. To achieve a deeper understanding on theinteractions between water and the ILs, COSMO-RS and FTIR spectroscopy were alsoapplied. COSMO-RS was used to predict the activity coefficient of water in the studiedionic liquids along with the excess enthalpies, suggesting the formation of complexesbetween three molecules of water and one IL molecule. On the basis of quantum-chemicalcalculations, it is found that cation−anion interaction plays an important role upon theability of the IL anion to interact with water. The changes in the peak positions/band areasof OH vibrational modes of water as a function of IL concentration were investigated, andthe impact of the cation on the hydrogen-bonding network of water is identified anddiscussed.

1. INTRODUCTION

Ionic liquids (ILs) are novel solvents with unique characteristicsthat make them attractive candidates for use in synthesis,catalysis, sensoristics, and electrochemistry processes and asalternative media for extractions and purifications.1−3 A detailedunderstanding of their behavior at molecular level is importantto further explore the application of ILs.4 Comprehensivecharacterization of the molecular dynamics and organization ofILs as a function of cation and anion nature,5,6 concentration,7,8

and temperature9 is crucial to understand the molecularinteraction in the bulk.The presence of water in ILs affects many of their properties

such as polarity, viscosity, conductivity, reactivity and solvatingability.10−12 The role of water in ILs is complex and depends onthe constituent ions and supramolecular structure of theILs.13−16 Most ILs are hygroscopic and absorb water vaporfrom the atmosphere,8,17 and due to their high molecularweight even traces of water, on a mass per mass basis, turn outto be abundant water in these materials on a mole fractionscale, with important impact on their behavior. For that reason,Seddon and coworkers have early emphasized that efficientdrying is necessary in studies of neat RTILs and for most oftheir applications as solvents.8 Application in capturing CO2from flue gas using ILs has shown the presence of water in ILsto play an important role.18 Also, in membrane applicationsusing ILs, it has been found that the presence of watersignificantly alters membrane performance.19 The same hasbeen observed for cellulose processing, where the presence of

water has a significant impact upon its solubility.20 Even forelectronic applications, a very small amount of water mayinfluence the electrochemical window.21 ILs−water binarysystems have also recently attracted increased attention becauseof their potential for absorption cooling22−27 and theirapplication in extraction processes with both hydrophobicILs28−30 or with aqueous biphasic systems based on hydrophilicILs, which are used in the purification of biomolecules andother added-value compounds.31−33

Mixtures of ILs and water were first investigated computa-tionally by Hanke et al.,34 performing molecular dynamics(MD) simulations, where the coordination structure of waterwith ions was analyzed. The arrangement of water molecules inimidazolium-based ILs has been studied in various subsequentworks by the same group and others.35,36 It has been observedthat water molecules strongly associate with anions. Further-more, water molecules in the IL were found to be isolated,while dimers and clusters of water were observed only at veryhigh water concentrations in water miscible ILs.37−39 Mele etal.40 reported that tight ion pairs in ILs remain even afterinteractions with large amounts of water. Some other authorsreported that water molecules interact with the IL clusterswithout forming a hydrogen-bond network among them.13,41,42

Other theoretical studies also shed light on how the water

Received: June 10, 2014Revised: August 13, 2014Published: August 13, 2014

Article

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Figure 1. continued

The Journal of Physical Chemistry B Article

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molecules interact with the IL when the binary mixture iscreated, which shows the structural organization of ILs in diluteregime and percolating network of water molecules in the IL-rich region.37 Jiang et al.43 analyzed the effect of varying waterconcentrations on the nanostructural organization of IL−watermixtures using MD simulation. Feng and Voth44 investigatedthe effects of cation tail length and anion on dynamics andstructure in imidazolium-based IL/water mixtures. Raabe andKohler45 performed MD simulations and found that tailaggregation depends on alkyl side-chain length and temperaturefor imidazolium-based ILs.Interactions of water dissolved in various ILs have been

extensively studied using spectroscopy.16,46−48 The vibrationalmodes of water that result in the OH-stretching region (3000−3800 cm−1) of the bulk water represent the inhomogeneousenvironment of water molecules due to hydrogen bonding,which are relevant to the intermolecular interactions, to studythe structural change of water due to anion and cation ofILs.49,50 The most widely used spectroscopic technique forprobing the interaction of ILs−water is infrared(IR).16,46,48,51−58 The water content of various ILs is stronglyinfluenced by the nature of the anion and, to a lesser extent, thecation.48,59−61 Most of these studies are related to theinteraction of water and the anion, while interactions of theproton on the cation with water are much less evaluated.62−64

Jeon et al.58 studied binary mixtures of [C4mim][BF4] withwater to explain the relative position of the anion with respectto the imidazolium cation, and the same authors investigatedthe binary mixtures of [C4mim][BF4] with water, over thewhole composition range, using attenuated total reflectanceinfrared (ATR-IR) and studied the effect of the IL on the waterstructure by analyzing the OH stretching vibrations of liquidwater.52 Cammarata et al.62 studied the interaction strength ofvarious anions with water and additionally showed the minor

cation influences on the molecular states of water in the OHstretching region. Recently, Singh and Kumar48 studied thestructure of imidazolium-based ILs in terms of conformationalchange in alkyl chain and positioning of anions with respect tocations as a function of water concentration using Fouriertransform infrared (FTIR) spectroscopy.The activity coefficients measure the degree of nonideality of

a compound in solution and provide relevant information onthe thermodynamic behavior of a solvent. In the present work,11 hydrophilic ILs with methylimidazolium-based cations andchloride as common anion were chosen to investigate the effectof the number and size of the alkyl side chains on theimidazolium cation upon the interaction between water and thechloride anion at 298.2 K in the entire solubility range of ILswith water. The activity coefficients were assessed by measuringthe water activities. Contributing to deepening of theunderstanding of these important binary mixtures, furtherinformation about the water−IL interactions was obtainedusing FTIR spectroscopy, and density functional theory (DFT)calculations were used to evaluate the strength of the cation−anion interaction. Finally, COSMO-RS is applied to supportthe information obtained from experimental data and gatherfurther knowledge on the water−IL interactions.65−69

2. EXPERIMENTAL SECTION

2.1. Materials. The studied ILs 1-methylimidazoliumchloride, [C1im]Cl(98 wt %); 1,3-dimethyl-imidazoliumchloride, [C1mim]Cl (99 wt %); 1-ethyl-3-methylimidazoliumchloride, [C2mim]Cl (98 wt %); 1-butyl-3-methylimidazoliumchloride, [C4mim]Cl; 1-butyl-2,3-dimethylimidazolium chlor-ide, [C4C1mim]Cl (99 wt %); 1-pentyl-3-methylimidazoliumchloride, [C5mim]Cl (98 wt %); 1-hexyl-3-methylimidazoliumchloride, [C6mim]Cl (98 wt %); 1-heptyl-3-methylimidazoliumchloride, [C7mim]Cl (98 wt %); 1-methyl-3-octylimidazolium

Figure 1. Structures of the investigated ionic liquids. The position of the anion (distances in angstroms) was obtained from DFT calculations basedon the binding energy for the complexes between imidazolium cation (HC2/HN1) and Cl anion.



chloride, [C8mim]Cl (99 wt %); 1-decyl-3-methylimidazoliumchloride, [C10mim]Cl (98 wt %); and 1-allyl-3-methylimidazo-lium chloride, [a1C1mim]Cl (98 wt %) were obtained fromIoLiTec (Germany). For calculation of mole fraction, actualweight mass of ILs has been considered, that is, treated as ionpairs. Figure 1 depicts the chemical structures of the studiedmethyl imidazolium ILs. Prior to the measurement of theactivity coefficient, the individual samples of each IL were driedat moderate temperature (∼323 K) and at high vacuum (∼10−3Pa), under constant stirring, and for a minimum period of 48 hto remove traces of water and volatile compounds. The puritiesof these ILs were further checked by 1H and13C NMR and wereshown to be ≥99 wt %. The water content of each IL wasdetermined by Karl Fischer titration (Mettler Toledo DL32Karl Fischer coulometer using the Hydranal-Coulomat E fromRiedel-de Haen as analyte) and found to be <30 × 10−6 massfraction. Double-distilled water, passed through a reverse-osmosis system and further treated with a Milli-Q plus 185water purification equipment, was used in all experiments.2.2. Measurement of Water Activities and Water

Activity Coefficients. The measurements of water activities(aw) were performed using a Novasina hygrometer LabMaster-aw (Switzerland). The measuring principle of the instrument isbased on the resistive-electrolytic method. The accuracy of theinstrument is 0.001aw, enabling measurements under controlledchamber temperature conditions (±0.15 K), and was calibratedwith six saturated pure salt standard solutions (water activitiesranging from 0.113 to 0.973), which were included in theinstrument. However, to achieve the given accuracy, acalibration curve was built using either KCl or CaCl2 aqueoussolutions at different salt molalities, depending on themagnitude of the water activity values to be measured. Theobtained values were compared with those recommended inthe extensive reviews by Archer70 for KCl or Rard and Clegg71

for CaCl2. For each measurement, samples of approximately 2to 3 cm3 were prepared by weighing the appropriate amounts ofeach compound, presenting a water mole fraction uncertaintyof ±0.0001, after being charged in the measuring dishes andfinally placed in the airtight equilibrium chamber. The exchangeof free water takes place until the partial pressure of water vaporreaches the equilibrium, which is confirmed following the awvariation with time. When a constant value is reached, the wateractivity is recorded. Diluted solutions reach equilibrium in <1 h,but solutions with high concentration of ILs could take up to 8h. The water activity coefficient (γw) is then calculated from thewater activities as

γ =axw

w

w (1)

where aw is the water activity and xw is the mole fraction ofwater.2.3. FT-IR Measurements. ATR and FTIR spectra of

water/ILs/water−ILs solutions were measured using ABBMB3000 FTIR spectrometer equipped with PIKE MIRacleand single-reflection diamond/ZnSe crystal plate. The spectralregion was 400−4000 cm−1 with resolution 4 cm−1 and 100scans. At least five repeated measurements were performed foreach sample. Second-derivative spectra of the hydroxyl (∼3250cm−1) region were used as peak position guides for theGaussian curve-fitting analysis. The second-derivative and thecurve fitting were done using PeakFit v4.0 (AISN Software).2.4. COSMO-RS Modeling. COSMO-RS (conductor-like

screening model for real solvents) is the extension of COSMO

model to statistical thermodynamics used for predictingthermo-physico-chemico properties of fluids in its pure andmixture state. The detailed theory on COSMO-RS can befound at the original work of Klamt.72 The detailed calculationand procedure of estimating activity coefficient using COSMO-RS can be found elsewhere,73,74 and the procedure ofestimating excess enthalpy can be found in the literature.75

The first step in the COSMO-RS prediction procedure isapplying the continuum solvation model COSMO to simulate avirtual conductor environment for the molecule of interest. It isthen followed by a screening charge density, σi, on the nearbyconductor, obtained through the standard quantum-chemicalcalculation. The 3D distribution of the screening charge densityon the surface of each molecule is converted into a surfacecomposition function, called by the sigma profile (σ profile),p(σ). In the second step, the statistical thermodynamicstreatment of the molecular interactions is performed in theCOSMOtherm software using the parameter fi leBP_TZVP_C30_1301 (COSMOlogic, Leverkusen, Ger-many).76

2.5. Binding Energy. The cation, anion, and ion-pairstructures of the studied ILs were geometry-optimized at thedensity functional theory (DFT) and hybrid Becke 3-Lee−Yang−Parr (B3LYP) exchange−correlation functional level.The DGDZVP basis set and the integral equation formalism-polarizable continuum model (IEF-PCM) were employed. AllDFT calculations were carried out using Gaussian 09program,77 and the calculation results were visualized withChemcraft 1.6 program [G. A. Zhurko, ChemCraft 1.6, (http://www.chemcraftprog.com)]. The ion-pair structures wereconstructed by combining the Cl anion in the proximity ofthe H atom attached to the 2-C atom of the imidazolium ring ofthe lowest energy conformation of the corresponding cations of[Cnmim]Cl. In the case of [C1im]Cl, the Cl anion is also placednear the H atom attached to 1-N atom, and it is placed at thefront of the methyl group attached to the 2-C atom of[C4C1mim]Cl. Vibrational frequencies were carried out toverify that the optimized structures were at the minimumenergy structure, as no imaginary frequencies were found. Thestable ion-pair structures and the H----Cl distances inangstroms are shown in Figure 1. The binding energies forthe ion-pair formations were calculated as the differencesbetween the total energies of the ion pairs and the separatedions.

3. RESULTS AND DISCUSSIONTable 1 presents the experimental results for the activitycoefficient of water in the studied substituted methylimidazolium chloride ILs at 298.2 K. To the best of ourknowledge, no systematic study on the activity coefficient ofwater in such a large series of compounds was previouslyreported. However, a few sets were possible to find in the openliterature used here for comparison purposes. In general, a goodagreement was found between the experimental valuesmeasured in this work and scattered literature data usingvarious techniques for dilute solutions of alkyl methylimidazolium chloride ([C2mim]Cl,78−80 [C4mim]Cl,66,80,81

[C5mim]Cl,82 [C6mim]Cl

83) in aqueous solution in the highrange of water mole fractions at 298.2 K as shown in Figure S1in the Supporting Information. Our choice of severalsubstituted alkyl-methyl-imidazolium-based ILs as cation andchloride as common anion is aimed at studying the effect of thenumber and length of the cation alkyl chain in the interaction



http://www.chemcraftprog.com

http://www.chemcraftprog.com

with water at 298.2 K. All of the previously reported water−ILssystems were studied in the region of complete miscibility.Figure 2 represents the behavior of all studied systems of ILswith water and shows the significant effect of water in the ILs.In general, the studied systems show activity coefficients lower

than unity, indicating a favorable interaction between water andthe studied imidazolium-based ILs, following the rankingpresented here

≅ ≅ <

< < < <

≅ < <

[C mim]Cl [C mim]Cl [C mim]Cl [C mim]Cl

[C im]Cl [C mim]Cl [C mim]Cl [C C mim]Cl

[alC mim]Cl [C mim]Cl [C mim]Cl

10 8 7 6

1 5 4 4 1

1 2 1

It is known that the anion is dominant in the interactionsthat control the water miscibility,62 but the results herereported highlight the cation influence on the hydrophobic-ity84−86 and the hydrogen-bonding ability of the IL with water.The experimental activity coefficients are reported in Figure 2along with the COSMO-RS predictions that are shown toprovide a correct description of the trend observed.Presently, predictive models for activity coefficients like those

based on functional groups such as UNIFAC,87 are not simpleto apply to ILs containing systems because an extensive andreliable parameter table is still missing.88 A more recentalternative is the COSMO-RS model for which only a small setof universal parameters must be calibrated. COSMO-RS is anefficient model alternative to structure-interpolating groupcontribution methods with the advantage that it providesinformation on the molecular interactions that can be obtainedfrom the σ profiles, depicted in Figure 3. The σ profile of waterdisplays a very broad peak on its negative side, water shows abroad peak at about −1.6 e·nm−2 resulting from the two polarhydrogen atoms, while on the positive side the same broad peakat 1.8 e·nm−2 results from the lone pairs of electrons of theoxygen atom. Both peaks lie beyond the threshold (σhb = ±0.79 e·nm−2), which means large parts of the surface of watermolecules are able to hydrogen bond. Both H-bond acceptorsand donors can undergo efficient H bonding with water if theyovercome that threshold. One of the other remarkable featuresof the water σ profile is the symmetry with respect to σ; that is,it shows the same amount of strongly positive and equallystrong negative surface area, which energetically enables veryfavorable pairing of positive and negative surfaces and theformation of strong hydrogen bonding. The σ profiles of thepure substituted methyl imidazolium chloride ILs are shown inFigure 3 with the cationic imidazolium group lying somewherein negative region and mostly in the nonpolar part (between−1 and 1). The σ potentials are depicted in SupportingInformation (Figure S2).As previously discussed by us,66 the COSMO-RS is not able

to provide a quantitative description of the activity coefficientsfor the water-chloride-based IL systems, but as previouslydescribed in this work, the qualitative trend of the water activitycoefficients is correctly predicted. Additionally, COSMO-RShas previously been shown to be a good tool for the predictionof excess enthalpies for water−IL systems,75 and thus themodel is here used to foster the understanding of these systems.The Hm

E is the change in enthalpy upon mixture of the twocomponents, and its estimation in the COSMO-RS method isthe sum of three specific molecular interactions, that is,electrostatic/misfit, Hm,MF

E ; hydrogen bonds, Hm,HBE ; and van

derWaals forces, Hm,VdWE described here

= + +H H H HmE

m,MFE

m,HBE

m,VdWE

(2)

The mixing of water and the methyl-imidazolium-based ILsstudied shows the negative excess enthalpy as predicted byusing COSMO-RS at 298.2 K, which indicates favorable

Table 1. Experimental Values of Activity Coefficient ofWater in Imidazolium-Based Ionic Liquids at 298.2 K

H2O + [C1im]Cl H2O + [C1mim]Cl H2O + [C2mim]Cl

xH2O γH2O xH2O γH2O xH2O γH2O

0.9836 0.981 0.9852 0.984 0.9876 0.9890.9633 0.974 0.9673 0.969 0.9643 0.9640.9365 0.945 0.9455 0.947 0.9397 0.9310.9075 0.916 0.9137 0.907 0.9086 0.8750.8682 0.855 0.8802 0.8080.8142 0.750 0.8357 0.7070.7316 0.592 0.7741 0.5710.6233 0.483 0.7212 0.4550.4461 0.318 0.6353 0.349

0.5822 0.273H2O + [C4mim]Cl

66 H2O + [C4C1mim]Cl H2O + [C5mim]Cl


0.9980 1.002 0.9891 0.993 0.9890 0.9980.9894 1.005 0.9888 0.993 0.9757 0.9860.9815 0.990 0.9766 0.982 0.9624 0.9760.9677 0.976 0.9620 0.973 0.9403 0.9440.9537 0.961 0.9618 0.968 0.9082 0.8970.9403 0.941 0.9404 0.934 0.8785 0.8480.9190 0.913 0.9152 0.894 0.8157 0.7250.9037 0.890 0.9136 0.892 0.7064 0.5170.8872 0.858 0.8691 0.7890.8661 0.808 0.8169 0.6680.8207 0.713 0.7258 0.4700.7923 0.653 0.5584 0.2850.8301 0.7380.7607 0.5850.6908 0.4430.6498 0.376H2O + [C6mim]Cl H2O + [C7mim]Cl H2O + [C8mim]Cl


0.9902 1.002 0.9909 0.996 0.9915 0.9970.9781 0.991 0.9798 1.001 0.9806 1.0040.9631 0.981 0.9643 1.012 0.9675 1.0130.9439 0.952 0.9498 1.013 0.9504 1.0150.9174 0.916 0.9231 1.002 0.9286 1.0130.8831 0.865 0.8982 0.959 0.8949 0.9820.8269 0.747 0.8410 0.872 0.8489 0.9070.7374 0.597 0.7488 0.6650.5548 0.424 0.4941 0.467

H2O + [C10mim]Cl H2O + [alC1mim]Cl

xH2O γH2O xH2O γH2O

0.9918 0.998 0.9877 0.9920.9831 1.005 0.9785 0.9830.9709 1.015 0.9595 0.9650.9555 1.018 0.9303 0.9180.9344 1.018 0.8987 0.861

0.8614 0.7600.7832 0.6090.7012 0.4410.5044 0.295



interactions. COSMO-RS is even able to predict thecontribution of each specific intermolecular interactionindividually for the studied systems throughout the wholecomposition, which is depicted in Figure S3 in the SupportingInformation. From Figure 4, it can be clearly seen that in allstudied binary mixtures, the dominant interaction is thehydrogen bonding, largely contributing to the exothermicityof the mixtures, which is in agreement with the literature,confirming that hydrogen bonding is the dominant interactionin water−IL system.37,89

The electrostatic interactions are attractive, as indicated by itsnegative values, which slightly contribute to the total excessenthalpy of the mixture, whereas van der Waals interactionsmake a little contribution to the excess enthalpy values of thesemixtures. Contribution of HMF

E is negative, as depicted in FigureS3 (see Supporting Information), which indicates that waterreduces the electrostatics interaction between the cation andthe anion of the ILs, which happens first by breaking hydrogenbond between H2O−H2O molecule and [Cation]−[Anion] ofthe ILs, where the latter is followed by the formation of newhydrogen bonds between H2O−[Anion] and for higher waterconcentrations between H2O−[Cation] also.90 It has beenreported that the hydrogen bonds between [Anion]−H2O−[Cation] are stronger than [Cation]−[Anion].90,91 Figure S4(see Supporting Information) for the binary system of waterand ILs shows a minima at xH2O ≈ 0.75 that seems to indicatethe formation of the complex between three water moleculesand one molecule of methyl-imidazolium-based ILs, which isalso in accordance with the literature for methyl-imidazolium-based ILs−water systems.37,92 This value is considered thewater percolation limit. As the water concentration changes inprogressively more concentrated water-in-IL solutions, thewater state changes from isolated water molecules, to string-likewater clusters, to a percolated water continuous subphase. Thepercolation limit, first posited by Lynden-Bell et al.,34,37 wasafter confirmed by MD simulations at around xw = 0.75,43

which corresponds to the minimum observed in the predictedHm

E , by COSMO-RS.The results depicted in Figure 2 show that the hydro-

phobicity increases with the cation alkyl chain length, inagreement with previous literature results.6,93 However, a fewcompounds escape this general trend. Because [C1im]Cl hasjust one alkyl group attached to the imidazolium ring, it couldbe expected to present the strongest water interaction, yet,surprisingly, activity coefficients similar to those presented by

Figure 2. Experimental activity coefficient of water, γH2O, as a function of concentration of water at 298.2 K for the [C1im]Cl (black ▲, ---),[C1mim]Cl (light blue △, ···), [C2mim]Cl (purple ■, −··−), [C4mim]Cl

66 (maroon ×, ), [C4C1mim]Cl (pink □, ---), [C5mim]Cl (light brown -,−□−□−), [C6mim]Cl (blue ●, −·−), [C7mim]Cl (green +, −+−+−), [C8mim]Cl (light green ○, − −), [C10mim]Cl (red ◆, −○−○−) and[a1C1mim]Cl (orange ◇, −·−). The symbols and lines represent the experimental and COSMO-RS predictions, respectively.

Figure 3. σ Profile of H2O (blue ), [C1im]Cl (black ---), [C1mim]Cl (light blue ···), [C2mim]Cl (purple − ·· −), [C4mim]Cl (maroon−), [C4C1mim]Cl (pink ---), [C5mim]Cl (light brown −□−□−),[C6mim]Cl (blue −·−), [C7mim]Cl (green −+−+−), [C8mim]Cl(light green − −), [C10mim]Cl(red −○−○−) and [a1C1mim]Cl(orange −·−).



[C6mim]Cl were observed, suggesting an unexpectedly poorinteraction with water. The COSMO-RS validation of thepeculiar behavior of the activity coefficient of water in [C1im]Clprompted us to further attempt to explain this unexpectedbehavior. The predicted excess enthalpies for the system[C1im]Cl−water are significantly lower than those for the other[Cnmim]Cl systems, as shown in Figure 4, further supportingthe vision of a poor interaction between [C1im]Cl and water.Binding energies between the Cl− anion and the cation wereestimated by DFT. It was observed that the chloride anion has amuch higher binding energy with the hydrogen atom attachedto nitro group (−9.38 kcal/mol) than with the available freehydrogen atom attached to the C2 (−4.18 kcal/mol), whichindicates that the Cl− anion prefers to bind with the hydrogenatom of nitro group rather than with that at the C2 position,which is the preferred interaction observed for the other ILs.This stronger structure decreases the dissociation of the IL andreduces the ability of the water to solvate the IL ions, thusincreasing the hydrophobicity of the IL.Another unexpected behavior was observed in the activity

coefficients of [C5mim]Cl and [C4mim]Cl when comparedwith [C4C1mim]Cl. This is also validated by the COSMO-RSwater activity coefficient predictions showing that the latterpresents lower activity coefficients, indicating a slightly higherinteraction with water when compared with its isomer [C5mim]

Cl and even with [C4mim]Cl. This was unexpected because theacidic proton at C2 on the imidazolium ring confers the cationwith a hydrogen-bonding donor ability, and its replacement bya methyl group should make it more hydrophobic, yet a morehygroscopic nature for methyl substituted imidazolium ILs hasbeen previously reported.62,64 Again, this behavior could beattributed to ion-pair formation and could be explained on thebasis of the excess enthalpies estimated from COSMO-RS andon the binding energies between cation and anion estimated byDFT. For [C4C1mim]Cl, the chloride anion has a lowerbinding energy (−1.94 kcal/mol) for the hydrogen atom of themethyl group attached at C2 position in comparison with[C4mim]Cl (−3.98 kcal/mol) and [C5mim]Cl (−3.98 kcal/mol), which in turn enhances the possibility of [C4C1mim]Clto dissociate and interact with water, which is confirmed by theexcess enthalpies that are clearly more negative than for theother compounds here compared. (See Figure 4.)The results concerning these two ILs (i.e., [C1im]Cl,

[C4C1mim]Cl) suggest that an enhanced interaction of thecation with the water is not a guarantee of an enhancedinteraction of the IL with water because this interaction resultsfrom a balance between the [Cation]−[Anion], [Cation]−H2O, and [Anion]−H2O interactions. An increase in the[Cation]−H2O interaction is often accompanied by asimultaneous increase in the [Cation]−[Anion] interaction

Figure 4. Total predicted excess free Gibbs energy, GmE (green bars) and enthalpy (orange bars) of (water + methyl imidazolium-based ionic liquids)

binary mixture at 298.2 K using COSMO-RS at xH2O = 0.75 in terms of contribution of Hm,MFE (blue bars), Hm,HB

E (purple bars), and Hm,vdWE (yellow

bars) to the total excess enthalpy, HmE .

Figure 5. FTIR-normalized absorbance spectra of water in the OH stretch regions in the range 2800−3800 cm−1 at two fixed concentrations: xIL =0.05 and xIL = 0.10 for the various studied water−ILs mixtures.



that may actually reduce the ability of the final IL to interactwith water and then lead to an increase in the ILhydrophobicity. A counterintuitive strategy for the design ofILs with a stronger interaction with water may actually be thedecrease in the cation polarity or any other strategy that maycontribute to decrease the [Cation]−[Anion] interaction, asobserved for the [C4C1mim]Cl, where the introduction of amethyl group actually enhances the IL hydrophilicity, while theremoval of one of the methyl groups, as in [C1im]Cl, willactually enhance the hydrophobicity of the compound.As previously mentioned, IR spectroscopy has been shown to

be a tool for investigating molecular interactions between waterand ILs.46,48,94 The normalized spectra in the OH stretchingregions for the studied alkyl methyl imidazolium chloride−water systems are depicted in Figure 5 for xIL = 0.05 and 0.10.(For some other concentrations, see Supporting InformationFigure S5.) FTIR-normalized absorbance spectra of water inthe OH stretching for various studied water−ILs mixtures atdifferent water (xH2O = 0.02, 0.05, 0.10, 0.25 and 0.4) molefractions are depicted in the Supporting Information (FigureS6). As part of this study to understand the molecularinteraction of water and ILs, we monitored the changes in thewater spectrum in the presence of ILs species, that is, cationand anion. The mid-IR spectrum of liquid water has an intenseand broad band envelope at ∼3300 cm−1 that is assigned to theOH stretching modes.95−97 Curve fitting of water vibrationalspectrum is often used to describe the structure of liquid waterby the continuum model proposed by Walrafen98 and lateradvanced by Green et al.99 and Hare and Sorensen.100 A morethorough description of the fitting of the neat water spectrumcan be found in the literature.101−107 Among the OH stretchband of water, the most prominent stretching is around 3250and 3420 cm−1, which represent the strongly H-bonded patchesof molecules with tetrahedral structure (ice like) and weakly H-bonded molecules (liquid like), respectively. The othercomponent bands at 3550 and 3620 cm−1 are attributed tothe coupling of OH antisymmetric stretching vibrationalmodes,96,97 and the dangling OH stretching modes of watermolecules, respectively. Although it is not possible to assign adefined frequency to the band resulting from the Cl−waterhydrogen bonds, it is also within this broad band envelope. Thenormalized absorbance spectra of water in the OH stretchingregion clearly show a blue shift in all studied water−[Cnmim]Cl, as depicted in Figure 5.Figure 6 depicts the variation in peak position of the OH

stretch band for the various mixtures as a function of ILconcentration. A marked blue shift is observed, which increaseswith the increase in the alkyl chain length on the cation. Thesechanges in the blue shift indicate a disruption in the waterhydrogen-bonding network that results from a combined effectof the molar volume of the ions (the larger the disruption, themore important) and their ability to hydrogen bond with water(the larger the smaller is the disruption), as can be observedfrom the results reported by Singh and Kumar.48 Their resultsalso show that small anions with a strong hydrogen-bondability, such as chlorides, present a much reduced blue shiftwhen compared with other larger or more hydrophobic anions,as observed in this work (Figure 6). The blue shift hereobserved seems to follow the increase in the cation hydro-phobicity. The cation hydrophobicity is related to the alkylchain length and thus the molar volume of the cation, followingthe order

< < <

< < ≈ <

< < <

[C mim]Cl [C mim]Cl [C mim]Cl [C mim]Cl

[C mim]Cl [C C mim]Cl [C mim]Cl [alC mim]Cl

[C mim]Cl [C mim]Cl [C im]Cl

10 8 7 6

5 4 1 4 1

2 1 1

The only relevant exception observed is for [C4C1mim]Cl.Although its molar volume is identical to [C5mim]Cl, its IRspectra display a shift identical to [C4mim]Cl. In this case, for asimilar molar volume of the cation, the effect of the higheranion solvability, that is, a higher capacity to establish hydrogenbonding with water, appears as a reduced disruption of thehydrogen bonding network of the water by this IL. Theopposite effect observed for [C1im]Cl in the activitycoefficients is not observed here because the lower ability ofthe chloride anion to hydrogen bond with water in this IL is notcompensated by the low molar volume effect upon the waterhydrogen-bonding disruption.For all water−ILs systems, the blue shifting observed is most

prominent in the lower concentration of IL proofing to have amaximum impact upon the water hydrogen boding network forconcentrations below xIL = 0.1, as the effect is much lessimportant above this concentration. This is in agreement withthe results by Singh and Kumar,48 and it seems to be related toa region of high water concentration where the water−ionclustering is very high, as described by Zhong et al.108 Abovethat value, the water network seems to be completely disrupted,and, as shown by different authors,109−111 other regions wheresmaller clusters of water, down to the limit of solitary watermolecules, will be also present. The actual mole fraction for thistransition seems to vary with the anion.43,48,108,109

4. CONCLUSIONSThe prospect of fine-tuning properties, through the combina-tion of cations and anions, allows the design of task-specific ILsintended for a specific application. It is important to understandthe behavior of water−ILs interaction quantitatively to designthe process. The prediction based on COSMO-RS method wasof great value to design applications based on ILs because theypermit rationalization of the selection of the cation-ion pair forobtaining the type of interactions required to achieve a givenbehavior as a solvent. On the basis of new experimental wateractivity coefficient data, it was found that the interaction studyof water−ILs is affected by increasing alkyl chain length on theimidazolium ring. COSMO-RS is found to be a reliable

Figure 6. Peak shift for OH stretching band of water for variousstudied water−ILs mixtures with increasing ILs mole fraction.



predictive method to estimate at least qualitatively the excessenthalpies and Gibbs free energy of binary mixtures of waterand ILs, indicating that hydrogen bonding is the dominantinteraction in the studied binary system. COSMO-RS alsosuggests the formation of complexes in the binary mixture ofwater and studied methyl imidazolium-based ILs. Quantum-chemical calculation shows that cation−anion interaction playsan important role upon the ability of the IL to interact withwater. IR spectroscopy shows a blue shift, which increases withthe increase in the alkyl chain length on the cation, indicating adisruption in the water hydrogen-bonding network that resultsfrom a combined effect of the molar volume of the ions andtheir ability to hydrogen bond with water.

■ ASSOCIATED CONTENT*S Supporting InformationComparison of experimental water activity (aw) and activitycoefficient (γ) of some alkyl methyl imidazolium chloride([C2mim]Cl, [C4mim]Cl, [C5mim]Cl, and [C6mim]Cl) inaqueous solution at 298.2 K with reported values in literature; σpotential of H2O and studied alkyl methyl imidazoliumchloride; contribution of specific interaction of each moleculeto the excess enthalpy of binary mixture of H2O and ILs at298.2 K estimated by COSMO-RS; estimated total excessenthalpies and estimated excess Gibb’s Free energy of binarymixture of ILs and water at 298.2 K predicted by COSMO-RS;FTIR-normalized absorbance spectra of water in the OHstretch regions in the range 2800−3800 cm−1 at fixedconcentrations of ILs for the various studied water−ILsmixtures; and FTIR normalized absorbance spectra of waterin the OH stretch region in the range 2800−3800 cm−1 forvarious studied water−ILs mixtures at different water (xH2O =0.02, 0.05, 0.10, 0.25, 0.40) mole fractions. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +351 234 401 507. Fax: + 351 234 370 084. E-mail:[email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financed by national funding from Fundacaopara a Ciencia e a Tecnologia (FCT, Portugal), EuropeanUnion, QREN, FEDER and COMPETE for funding theCICECO (project PEst-C/CTM/LA0011/2013) and LSRE/LCM (project PEst-C/EQB/LA0020/2013). I.K. acknowledgesFCT exploratory project EXPL/QEQ-PRS/0224/2013, andI.K. and M.T. acknowledge Fundacao para a Ciencia eTecnologia for the postdoctoral grants SFRH/BPD/76850/2011 and SFRH/BPD/78441/2011, respectively.

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