Article pubs.acs.org/JPCB
Effect of the Cation on the Interactions between Alkyl Methyl Imidazolium Chloride Ionic Liquids and Water Imran Khan,† Mohamed Taha,† Paulo Ribeiro-Claro,† Simaõ P. Pinho,‡,§ and Joaõ A. P. Coutinho*,† †
Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal UNIFACS-Universidade de Salvador, Rua Dr. José Peroba 251, CEP 41770-235 Salvador, Brazil § Associate Laboratory LSRE/LCM, Departamento de Tecnologia Química e Biológica, Instituto Politécnico de Bragança, Campus de Santa Apolónia, 5301-857 Bragança, Portugal ‡
S Supporting Information *
ABSTRACT: A systematic study of the interactions between water and alkyl methyl imidazolium chloride ionic liquids at 298.2 K, based on activity coefficients estimated from water activity measurements in the entire solubility range, is presented. The results show that the activity coefficients of water in the studied ILs are controlled by the hydrophilicity of the cation and the cation−anion interaction. To achieve a deeper understanding on the interactions between water and the ILs, COSMO-RS and FTIR spectroscopy were also applied. COSMO-RS was used to predict the activity coefficient of water in the studied ionic liquids along with the excess enthalpies, suggesting the formation of complexes between three molecules of water and one IL molecule. On the basis of quantum-chemical calculations, it is found that cation−anion interaction plays an important role upon the ability of the IL anion to interact with water. The changes in the peak positions/band areas of OH vibrational modes of water as a function of IL concentration were investigated, and the impact of the cation on the hydrogen-bonding network of water is identified and discussed. water has a significant impact upon its solubility.20 Even for electronic applications, a very small amount of water may influence the electrochemical window.21 ILs−water binary systems have also recently attracted increased attention because of their potential for absorption cooling22−27 and their application in extraction processes with both hydrophobic ILs28−30 or with aqueous biphasic systems based on hydrophilic ILs, which are used in the purification of biomolecules and other added-value compounds.31−33 Mixtures of ILs and water were first investigated computationally by Hanke et al.,34 performing molecular dynamics (MD) simulations, where the coordination structure of water with ions was analyzed. The arrangement of water molecules in imidazolium-based ILs has been studied in various subsequent works by the same group and others.35,36 It has been observed that water molecules strongly associate with anions. Furthermore, water molecules in the IL were found to be isolated, while dimers and clusters of water were observed only at very high water concentrations in water miscible ILs.37−39 Mele et al.40 reported that tight ion pairs in ILs remain even after interactions with large amounts of water. Some other authors reported that water molecules interact with the IL clusters without forming a hydrogen-bond network among them.13,41,42 Other theoretical studies also shed light on how the water
1. INTRODUCTION Ionic liquids (ILs) are novel solvents with unique characteristics that make them attractive candidates for use in synthesis, catalysis, sensoristics, and electrochemistry processes and as alternative media for extractions and purifications.1−3 A detailed understanding of their behavior at molecular level is important to further explore the application of ILs.4 Comprehensive characterization of the molecular dynamics and organization of ILs as a function of cation and anion nature,5,6 concentration,7,8 and temperature9 is crucial to understand the molecular interaction in the bulk. The presence of water in ILs affects many of their properties such as polarity, viscosity, conductivity, reactivity and solvating ability.10−12 The role of water in ILs is complex and depends on the constituent ions and supramolecular structure of the ILs.13−16 Most ILs are hygroscopic and absorb water vapor from the atmosphere,8,17 and due to their high molecular weight even traces of water, on a mass per mass basis, turn out to be abundant water in these materials on a mole fraction scale, with important impact on their behavior. For that reason, Seddon and coworkers have early emphasized that efficient drying is necessary in studies of neat RTILs and for most of their applications as solvents.8 Application in capturing CO2 from flue gas using ILs has shown the presence of water in ILs to play an important role.18 Also, in membrane applications using ILs, it has been found that the presence of water significantly alters membrane performance.19 The same has been observed for cellulose processing, where the presence of © 2014 American Chemical Society
Received: June 10, 2014 Revised: August 13, 2014 Published: August 13, 2014 10503
dx.doi.org/10.1021/jp5057495 | J. Phys. Chem. B 2014, 118, 10503−10514
The Journal of Physical Chemistry B
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Figure 1. continued
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dx.doi.org/10.1021/jp5057495 | J. Phys. Chem. B 2014, 118, 10503−10514
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Figure 1. Structures of the investigated ionic liquids. The position of the anion (distances in angstroms) was obtained from DFT calculations based on the binding energy for the complexes between imidazolium cation (HC2/HN1) and Cl anion.
cation influences on the molecular states of water in the OH stretching region. Recently, Singh and Kumar48 studied the structure of imidazolium-based ILs in terms of conformational change in alkyl chain and positioning of anions with respect to cations as a function of water concentration using Fourier transform infrared (FTIR) spectroscopy. The activity coefficients measure the degree of nonideality of a compound in solution and provide relevant information on the thermodynamic behavior of a solvent. In the present work, 11 hydrophilic ILs with methylimidazolium-based cations and chloride as common anion were chosen to investigate the effect of the number and size of the alkyl side chains on the imidazolium cation upon the interaction between water and the chloride anion at 298.2 K in the entire solubility range of ILs with water. The activity coefficients were assessed by measuring the water activities. Contributing to deepening of the understanding of these important binary mixtures, further information about the water−IL interactions was obtained using 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 support the information obtained from experimental data and gather further knowledge on the water−IL interactions.65−69
molecules interact with the IL when the binary mixture is created, which shows the structural organization of ILs in dilute regime and percolating network of water molecules in the ILrich region.37 Jiang et al.43 analyzed the effect of varying water concentrations on the nanostructural organization of IL−water mixtures using MD simulation. Feng and Voth44 investigated the effects of cation tail length and anion on dynamics and structure in imidazolium-based IL/water mixtures. Raabe and Köhler45 performed MD simulations and found that tail aggregation depends on alkyl side-chain length and temperature for imidazolium-based ILs. Interactions of water dissolved in various ILs have been extensively studied using spectroscopy.16,46−48 The vibrational modes of water that result in the OH-stretching region (3000− 3800 cm−1) of the bulk water represent the inhomogeneous environment of water molecules due to hydrogen bonding, which are relevant to the intermolecular interactions, to study the structural change of water due to anion and cation of ILs.49,50 The most widely used spectroscopic technique for probing the interaction of ILs−water is infrared (IR).16,46,48,51−58 The water content of various ILs is strongly influenced by the nature of the anion and, to a lesser extent, the cation.48,59−61 Most of these studies are related to the interaction of water and the anion, while interactions of the proton on the cation with water are much less evaluated.62−64 Jeon et al.58 studied binary mixtures of [C4mim][BF4] with water to explain the relative position of the anion with respect to the imidazolium cation, and the same authors investigated the binary mixtures of [C4mim][BF4] with water, over the whole composition range, using attenuated total reflectance infrared (ATR-IR) and studied the effect of the IL on the water structure by analyzing the OH stretching vibrations of liquid water.52 Cammarata et al.62 studied the interaction strength of various anions with water and additionally showed the minor
2. EXPERIMENTAL SECTION 2.1. Materials. The studied ILs 1-methylimidazolium chloride, [C1im]Cl(98 wt %); 1,3-dimethyl-imidazolium chloride, [C1mim]Cl (99 wt %); 1-ethyl-3-methylimidazolium chloride, [C2mim]Cl (98 wt %); 1-butyl-3-methylimidazolium chloride, [C4mim]Cl; 1-butyl-2,3-dimethylimidazolium chloride, [C4C1mim]Cl (99 wt %); 1-pentyl-3-methylimidazolium chloride, [C5mim]Cl (98 wt %); 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl (98 wt %); 1-heptyl-3-methylimidazolium chloride, [C7mim]Cl (98 wt %); 1-methyl-3-octylimidazolium 10505
dx.doi.org/10.1021/jp5057495 | J. Phys. Chem. B 2014, 118, 10503−10514
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model to statistical thermodynamics used for predicting thermo-physico-chemico properties of fluids in its pure and mixture state. The detailed theory on COSMO-RS can be found at the original work of Klamt.72 The detailed calculation and procedure of estimating activity coefficient using COSMORS can be found elsewhere,73,74 and the procedure of estimating excess enthalpy can be found in the literature.75 The first step in the COSMO-RS prediction procedure is applying the continuum solvation model COSMO to simulate a virtual conductor environment for the molecule of interest. It is then followed by a screening charge density, σi, on the nearby conductor, obtained through the standard quantum-chemical calculation. The 3D distribution of the screening charge density on the surface of each molecule is converted into a surface composition function, called by the sigma profile (σ profile), p(σ). In the second step, the statistical thermodynamics treatment of the molecular interactions is performed in the COSMOtherm software using the parameter file BP_TZVP_C30_1301 (COSMOlogic, Leverkusen, Germany).76 2.5. Binding Energy. The cation, anion, and ion-pair structures of the studied ILs were geometry-optimized at the density functional theory (DFT) and hybrid Becke 3-Lee− Yang−Parr (B3LYP) exchange−correlation functional level. The DGDZVP basis set and the integral equation formalismpolarizable continuum model (IEF-PCM) were employed. All DFT calculations were carried out using Gaussian 09 program,77 and the calculation results were visualized with Chemcraft 1.6 program [G. A. Zhurko, ChemCraft 1.6, (http:// www.chemcraftprog.com)]. The ion-pair structures were constructed by combining the Cl anion in the proximity of the H atom attached to the 2-C atom of the imidazolium ring of the lowest energy conformation of the corresponding cations of [Cnmim]Cl. In the case of [C1im]Cl, the Cl anion is also placed near the H atom attached to 1-N atom, and it is placed at the front of the methyl group attached to the 2-C atom of [C4C1mim]Cl. Vibrational frequencies were carried out to verify that the optimized structures were at the minimum energy structure, as no imaginary frequencies were found. The stable ion-pair structures and the H----Cl distances in angstroms are shown in Figure 1. The binding energies for the ion-pair formations were calculated as the differences between the total energies of the ion pairs and the separated ions.
chloride, [C8mim]Cl (99 wt %); 1-decyl-3-methylimidazolium chloride, [C10mim]Cl (98 wt %); and 1-allyl-3-methylimidazolium chloride, [a1C1mim]Cl (98 wt %) were obtained from IoLiTec (Germany). For calculation of mole fraction, actual weight mass of ILs has been considered, that is, treated as ion pairs. Figure 1 depicts the chemical structures of the studied methyl imidazolium ILs. Prior to the measurement of the activity coefficient, the individual samples of each IL were dried at moderate temperature (∼323 K) and at high vacuum (∼10−3 Pa), under constant stirring, and for a minimum period of 48 h to remove traces of water and volatile compounds. The purities of these ILs were further checked by 1H and13C NMR and were shown to be ≥99 wt %. The water content of each IL was determined by Karl Fischer titration (Mettler Toledo DL32 Karl Fischer coulometer using the Hydranal-Coulomat E from Riedel-de Haen as analyte) and found to be
