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Molecular interactions and macroscopic effects in binary mixtures of an imidazolium ionic liquid with water, methanol, and ethanol
Journal of Molecular Structure 1018 (2012) 45–53
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Molecular interactions and macroscopic effects in binary mixtures of an imidazolium ionic liquid with water, methanol, and ethanol Kristina Noack a,b, Alfred Leipertz a,b, Johannes Kiefer b,c,⇑ a
Institute of Engineering Thermodynamics, University Erlangen-Nuremberg, Germany Erlangen Graduate School in Advanced Optical Technologies, University Erlangen-Nuremberg, Germany c School of Engineering, University of Aberdeen, Aberdeen, Scotland, United Kingdom b
a r t i c l e
i n f o
Article history: Available online 23 February 2012 Keywords: Ionic liquid Hydrogen bonding Excess properties Water Alcohol Raman spectroscopy
a b s t r a c t Intermolecular interactions in mixtures of room-temperature ionic liquids (RTILs) and co-solvents define the properties of the solution. In this work, we study the mixing behavior in the binary systems [EMIM][EtSO4]/water, [EMIM][EtSO4]/methanol and [EMIM][EtSO4]/ethanol, which is governed by a change in the balance of molecular interactions present in neat [EMIM][EtSO4]. The mixing behavior and interactions are investigated at molecular level by means of Raman spectroscopy, and at macroscopic level utilizing excess data taken from the literature. The discussion of the results aims at a distinct interpretation of the spectroscopic data and at identifying the relationships between molecular phenomena and macroscopic behavior. The Raman spectra of the binary systems indicate that the balance of intermolecular interactions in the neat RTIL is dominantly distorted by solute–solvent interactions involving hydrogen atoms (IIHAs). In concert with former studies, the spectroscopic and macroscopic data suggest, that the IIHA include a combination of conventional (red-shifting) and unconventional (blue-shifting) hydrogen bonds. With increasing co-solvent concentration, the interionic bonds become successively weaker and eventually ion-co-solvent interactions even replace those between the RTIL counter ions leading to ion pair dissociation. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Room-temperature ionic liquids (RTILs) are a group of innovative solvents increasingly gaining interest because of their unique properties. In general, RTILs are organic salts with melting points near room temperature or by convention below 100 °C [1]. The variety of possible cation–anion combinations leads to a large number of different RTILs with a wide range of thermophysical and physicochemical properties [2–8], which consequently means a strong potential for a variety of practical applications [9–15]. In fact, it is even possible to tune the properties by adjusting the ratio of Coulomb and van der Waals forces, i.e. short and long range interactions, respectively. However, as it is virtually impossible to experimentally investigate all potential cation–anion combinations, a molecular-based understanding of their macroscopic properties is crucial for a rational design [16,17]. The balance between electrostatic and dispersive interactions can be selectively changed by alteration of the strength of the respective interaction. For example, substitution of functional groups or adding a co-solvent can influence this balance [18–20]. Of course, the addition of a sec⇑ Corresponding author at: School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE, United Kingdom. Tel.: +44 1224 272495. E-mail address: [email protected] (J. Kiefer). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.02.031
ond substance is certainly not the desirable way to obtain the required properties, however mixtures of RTILs and co-solvents are of practical interest in general as process fluids are always in danger of contamination, e.g., by water or organic solvents. Interactions involving hydrogen atoms (IIHAs) have the potential to shift the balance between electrostatic and dispersive forces and with them the macroscopic properties of an RTIL [18–23]. Within this group of molecular interactions hydrogen bonds (HBs) play a crucial role. However, although extensively studied, a precise definition of HBs is yet to be found and subject of controversial discussions. In the framework of a IUPAC project on hydrogen bonding, a novel definition has been summarized in a very recent essay [24]: ‘‘The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment XAH in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.’’ Along with this primary definition, a list of HB characteristics is given [24], one of which refers to vibrational spectroscopy as a suitable tool to experimentally investigate hydrogen bonding. In vibrational spectroscopy HB formation or breakage manifests as line shifts due to changes in vibration frequencies – either towards higher values (blue-shift) or lower ones (red-shift). Typically, the XAH bond (proton donor) is lengthened upon conventional HB formation indicated by a red-shifted line
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with increased intensity and width. However, in some situations the opposite trend, i.e. bond shortening and thus a blue-shifted vibrational line upon unconventional HB formation, can be observed. The debate on the origin of unconventional HBs has recently been expanded to RTILs [25]. In this debate two questions have been discussed: (1) Whether or not blue-shifting HBs refer to ‘‘real’’ hydrogen bonding, and (2) how this interaction mechanism differs from the conventional mechanism of charge transfer to the anti-bonding orbital of the proton donor. Hobza et al. introduced a two-step model to describe the formation of blue-shifting hydrogen bonds in the generic system of ZAXAH Y [26,27]. Firstly, charge is transferred from the proton acceptor to the so called remote part of the proton donor (ZAX). As a result the bond lengthens and shows a red-shift. This leads to a structural reorganization of the proton donor in the second step, and eventually results in a bond contraction of the XAH bond and hence a blue-shift. In contrast, the second school of thought established by Scheiner and Kar discusses blue-shifting CH O HBs with respect to their spectroscopic and structural signature [28], fundamental distinctions [29] and properties [30], cooperativity [31,32], hybridization and substitution [33]. However, no fundamental distinctions in the physical origin could be found and thus CH O bonds were categorized as true HBs [29,30] which was supported by Li et al. [34]. On this basis, it was concluded, that a blue-shifted CH stretch is not an indicator for being proton donor in a HB, since CH X bonds do not always shift to the blue or even shift at all. To give a unified explanation for the shifting behavior of stretching frequencies upon HB formation, Joseph and Jemmis state that all XAH bonds face opposing contracting and lengthening forces while they are involved in HBs. Just the electron affinity of the atoms involved in the HB donor and the resulting electron density distribution predetermines whether a red- or a blue-shift will occur [35]. Besides these attempts, further experimental and theoretical studies trying to prove or generalize the origin respectively shifting behavior of HBs have been carried out using different model systems [36–51]. Concerning IIHA in mixtures of RTILs and solvents the hygroscopic nature of many RTILs is very interesting. Investigations of aqueous RTIL systems usually aim at a preferably complete structural and thermodynamic understanding. To bridge the gap between molecular phenomena and macroscopic properties in a way such that experimental studies can be carried out, the excess enthalpy has proven itself as a suitable measure [52,53]. Moreover, various properties including the diffusion coefficient, electrical conductivity and viscosity are parameters which indicate intermolecular interaction structures as well [54,55]. Leskiv et al., for instance, investigated the enthalpies of aqueous solutions and concluded that the intermolecular interactions in the RTIL 1ethyl-3-methylimidazolium ethylsulfate, [EMIM][EtSO4], are mainly of electrostatic nature rather than based on dispersion [56]. A similar idea was pursuit by Anthony et al., who utilized solution thermodynamics to find a relationship between enthalpy and entropy of absorbing water into RTILs and the affinity to do so [57]. This was found to depend on the alkyl side chain length. In this context, vibrational spectroscopy is a favorable tool to provide experimental insights. Previous studies employed IR and/ or Raman spectroscopy to analyze the HB network in RTIL/water mixtures [58–63]. Very recently, Lehmann et al. [64] found that decreasing stability or interaction energy is not correlated with the same decrease in hydrogen bonding. They focused on the role of HBs in the interionic interactions of RTILs and concluded that HBs contribute to a perturbation of the ion pairs by stabilizing conformational transition states. Eventually, this leads to higher or lower ion pair mobility in dependence on the interionic interaction strength. These results are in concert with studies of Hunt et al. regarding the viscosity change and ion mobility of imidazolium-
based ionic liquids [65,66]. In addition, theoretical studies have investigated the effects of alkyl side chain length and the anion structure on the aggregation behavior [67], diffusion coefficients [67] and HB formation [68]. Spickermann et al. used DFT calculations to study why RTILs mainly associate in water. They found that the structure and orientation of water as well as the HB network in the RTIL play crucial roles in determining whether dissociation or association is favored [69]. In this work, we study and compare IIHA in binary mixtures of the RTIL [EMIM][EtSO4] with water, methanol, and ethanol. While aqueous RTIL solutions are frequently studied, investigations of alcoholic mixtures are rare in the literature although they are important from practical viewpoints as well. Moreover, as the polarization of the hydroxyl group varies among the three solvents under consideration, a systematic investigation can be carried out. [EMIM][EtSO4] has been chosen because (1) it is highly hydrophilic and miscible with the three co-solvents in all proportions at ambient conditions [70,71] and (2) its vibrational structure is well understood [72,73]. In a first step, the Raman spectra of the binary systems with systematically varied composition are analyzed focusing on co-solvent induced frequency shifts. Secondly, the spectroscopic results are discussed against thermodynamic and physicochemical property data from the literature. This allows a more comprehensive interpretation in order to gain deeper insights into how the chemical/vibrational structure is related to the macroscopic properties. 2. Materials and methods 2.1. Chemicals and sample preparation 1-Ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4], methanol and ethanol with 99.9%, 99.5% and 99.5% purity, respectively, were purchased from Solvent Innovation GmbH and Merck AG, Germany and used as received. Water was taken from the water supply and additionally de-ionized using an ion exchanger system (SG 2800 SK) providing water with conductivity below 0.1 lS/cm. Binary mixtures of [EMIM][EtSO4] and co-solvent with co-solvent mass fractions of 0, 0.01, 0.05, 0.10, 0.25, 0.50, 0.65, 0.75, 0.90, 0.95, and 1 were prepared gravimetrically in glass cuvettes and grouted with film in order to avoid leakage of the liquids. In the following, the mass fractions are presented as mole fractions (water: 0.00, 0.12, 0.41, 0.60, 0.81, 0.93, 0.96, 0.97, 0.99, 1.00; methanol: 0.00, 0.07, 0.28, 0.45, 0.71, 0.88, 0.93, 0.96, 0.98, 0.99, 1.00; ethanol: 0.00, 0.05, 0.21, 0.36, 0.63, 0.84, 0.91, 0.94, 0.98, 0.99, 1.00) in order to ease an interpretation of molecular phenomena. 2.2. Raman spectroscopy The experimental Raman setup consisted of a continuous-wave grating-stabilized diode laser operating at 785 nm. The laser beam was focused into the cuvette containing the sample under investigation and the backscattered light was collected in a ‘hole in a mirror’ arrangement [74] by an achromatic lens. The signal was spectrally filtered using two long-pass filters (cut-off wavelengths 785 and 800 nm) in order to suppress elastically scattered laser light, and focused into a fiber-coupled spectrometer. Eventually, the Raman scattering signal was detected in the wavenumber range 250–2000 cm 1 with a spectral resolution of approximately 2 cm 1 on a charge-coupled-device chip. The frequency-stabilized laser source and the temperature-controlled spectrometer resulted in high reproducibility of the spectra which allowed a reliable determination of vibrational frequencies and Raman intensities.
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2.3. Excess enthalpy, viscosity, molar volume The data were taken from the following Refs. [52,53,70,71,75]. 3. Results and discussion The chemical structure of the individual ions in [EMIM][EtSO4] is depicted schematically in Fig. 1 showing the denotations of the 1-ethyl-3-methylimidazolium cation used in the following discussion and in Table 1. The electronic structure of the cationic imidazolium ring can be described as a ring uniting a delocalized 3center-4-electron configuration across the N1AC2AN3 moiety, a double bond between C4 and C5 at the opposite side, and a weak delocalization in the central region [48]. Looking at the partial charges within the cation, the peripheral hydrogen atoms at C2, C4 and C5 carry most of the positive charge, while the negative charge is distributed at N1 and N2 as well as in between the Catoms. In this context it has to be noted that the C4- and C5-atoms appear to be almost neutral while the C2-atom exceptionally possesses positive charge owing to the electron deficit in the C@N bond. Moreover, the large positive charge at the C2AH unit and the repulsive interactions due to the C@C bond explain the observed higher acidity of C2AH and the better electron-donating properties of C4, C5, respectively [66,76]. 3.1. Spectroscopic analysis In the following, we discuss the spectroscopic results referring to [72,73,77]. The Raman spectrum between 300 and 1800 cm 1 is utilized for studying molecular interactions in the binary mix-
Fig. 1. Chemical structure and denotations of the 1-ethyl-3-methylimidazolium cation and the ethylsulfate anion (see Table 1).
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tures. The relevant vibrational assignments of neat [EMIM][EtSO4] in this spectral range are summarized in Table 1. According to Grondin et al. [77], the CH vibrations of the [EMIM]+ cation can serve as probes for changes in the interionic interactions in dependence on the anion, and in case of a mixture also in dependence on the co-solvent concentration. Therefore we analyze the cationic CH vibrations in order to identify red- and blue-shifts due to bonds involving hydrogen atoms and discuss their role in the intermolecular and interionic interactions. Although sensitive probes for hydrogen bonding as well, the changes in the co-solvents’ OH vibrations are not studied in detail here, because they provide very limited information about the RTIL structure. For the analysis of Raman spectra of imidazolium-based ionic liquids either the mipNACH3 line at 696 cm 1 or the mCAC line of the ethyl group at 952.7 cm 1 can be employed as an internal reference. The latter is used here. Unlike for [EMIM][Br] (which was used by Grondin et al. because it allows the cationic vibrations to be studied without significant perturbations by the anion), in [EMIM][EtSO4] the anion does exert influence on the cation due to its higher mass [72], asymmetry (higher conformational isomerism and thus higher entropy) and its basicity (pKa(ethyl sulfuric acid) = 3.14). Therefore, in [EMIM][EtSO4] several cationic vibrations originating from interaction sites between anion and cation are shifted and/or broadened in comparison to [EMIM][Br]. An interesting example is the band at 758 cm 1 which can be assigned to overlapping bending modes of CAH at the C2 and C4,5 positions (for completeness we note that a small contribution to the band may also result from a cationic HCH wagging vibration of the ethyl moiety, which however plays only a minor role in the interionic interactions [73]). This choice is reasoned on the high sensitivity of the C2 position to the anion nature based on its function as main interaction site and, consequently, as the basicity of the anion increases the c C2AH vibration is red-shifted. In contrast, the C4,5AH bending vibrations are hardly sensitive to the anion nature and are rather constant in frequency. For these reasons, a detailed analysis of the Raman band at 758 cm 1 provides a suitable tool for studying IIHA with co-solvent molecules.
3.1.1. The [EMIM][EtSO4]/water system The unprocessed Raman spectra in the region from 700 to 820 cm 1 recorded in the aqueous solutions of [EMIM][EtSO4] are displayed in Fig. 2 and give a first impression. Unlike Grondin et al. found for [EMIM]Br, the intensity of the peak at 758 cm 1
Table 1 Observed Raman bands of 1-ethyl-3-methylimidazolium ethylsulfate and their assignments [72,73,77].
mexp (cm 1)
Refs.
333.8, 409.2 591.3 696.6 758.4 909.4, 952.7 1014.4 1054.3 1085.5 1247.0 1330.6 1382.1 1416.0 1447.6 1560.3
[73] [73] [72,73] [72,73,77] [72,73] [73] [72,73] [72,73] [72,73] [72,73] [72,73] [72,73] [72,73]
Vibrational assignment [72,73,77] dO21S20O22, CCring
mC(7)N(1), wO21S20O23 wC(4,5)H(11) dC(4,5)H(11),cC(2)H, wC(7)H2, S20O24 mC25C26,msO21S20O22 mC(2)N(1)C(5)masCOSO3 mC25C26, mO24C25 mC25O24S20,dipC(4,5)H, sC(6)H2 dC(4,5)H, mCH3(2)N rC(7)H2, rC(7)H(16)N(1) rC(7)H2, rC(8)H2, C(26)H2, C(25)H2 rC(7)N(1)H(15), rC(6)N(3)H(13) sC(8)H3, sC(7)H2, sC(6)H2, dasCH3(Me), dasCH3(Me) rC(2)N(1), mip, asring, rC(4)N(3)(10), mC(6)H3N(3), mC(7)H2N(1)
d: in plane bending, c: out of plane bending, s: twisting, r: rocking, w: wagging, s: scissoring, m: stretching.
Fig. 2. Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/H2O mixtures. Increasing water mole fraction from top to bottom: 0.00, 0.15, 0.40, 0.61, 0.82, 0.93, 0.96, 0.98, 0.99, 1.00.
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is not very weak, but exhibits medium intensity and hence provides sufficient signal-to-noise ratio for a detailed analysis. In this region, a number of modes are in close vicinity with each other and overlap enhancing the overall peak intensity. This band centered at 758 cm 1 is remarkably affected by the addition of co-solvent observable as an increasing blue-shift with increasing water content, as well as it undergoes a change in the overall spectral shape. In order to extract more quantitative information the band has been fitted to a sum of pseudo-Voigt functions using a Matlab code. Pseudo-Voigt profiles for fitting vibrational spectra are the result of an inverse Fourier transformation of the convolution of a Lorentz and Gauss profile. A reasonably good fit could be achieved by employing six individual profiles as shown in Fig. 3. Based on the fact that fitting of the spectra was mainly influenced by alterations of the two most intense individual modes (at 758.1 cm 1 and 770 cm 1 in the neat RTIL), we conclude that these vibrations refer to the cationic C4,5AH and C2AH bending vibrations, respectively. The other profiles are also blue-shifted, but they have very low intensity and hence their contribution to the overall band position and shape is small. For the sake of completeness, we note that the blue-shifted bending modes are in concert with blue-shifts of the corresponding stretching vibrations for both the C2AH (3105 cm 1) and C4,5AH (3160 cm 1) stretches observed in the IR spectra (not shown). Interestingly, when we compare the IR absorption intensity ratio of these two stretching modes, we find that approximately at a water mole fraction of about 0.67 this ratio starts to reverse. In the pure ionic liquid, the C2AH peak shows the strongest absorption which is based on its higher positive charge compared to C4,5 whereas at high water mole fractions this is reverse. This can be an indicator for a shifted force balance. Concerning the C4,5AH bond, which appears to be almost neutral and exhibits more a repulsive than attractive nature in neat [EMIM][EtSO4] [66,76], this change is related to an increase of the C4,5 position’s attractive nature accompanied by a blue-shifting vibration frequency. The strengthening of the covalent CAH bonds at the C2 and C4,5 positions of the cation are due to IIHA. These interactions can have two origins. On the one hand, the blue shifts due to IIHA are based on the formation of HBs between the co-solvent and the RTIL cation which weakens the strong interionic interactions. The replacement of strong interionic interactions by weak long range HB interactions would lead to a charge transfer back to the CH bonds and thus to a strengthening of these covalent bonds. Furthermore, the much lower mass of the co-solvent molecules supports a bond strengthening also. On the other hand, there can be interactions between the water oxygen and the p-electrons of the aromatic imidazolium ring. Such configurations were proposed by Kempter and
Kirchner [25]and have very recently been investigated by Lehmann et al. [64] and Kiefer et al. [78]. Due to an arrangement of the cosolvent molecules at the top or bottom of the cationic ring, the water–oxygen is able to transfer charge to the ring. The result is an increase in the electron densities and hence a bond strengthening observable as blue shifts in the respective vibrational modes. As a consequence, the interionic IIHA are weakened. Both interaction types do influence the overall force balance. However, in the existing literature on mixtures of RTILs with hydroxyl containing solvents (see introduction), the formation of HBs between the co-solvent and the C2 and C4,5 position is usually considered the dominating mechanism. In concert with the published work, we therefore deduce that water addition strengthens dispersion and not electrostatic forces. The electrostatic forces are successively decreased upon water addition, which is one reason for the blue-shift observed. Another reason is the increasing formation of strongly bound cation-water and anion-water pairs. The formation of anion-water pairs manifests as red-shifted sulfate vibrational mode, because the OAH bond in water is more polarized than the C2AH moiety of the imidazolium cation. On the other hand, when the cation is hydrated at the C2 proton, the interactions with water are weaker than those with the anion and, as a consequence, the C2AH vibration is blue-shifted with increasing water content. By contrast, when we look at the C4,5 moiety a blue-shift can be observed, but less pronounced than the one for C2AH. This can be attributed to the C4,5 protons probably forming HBs with water molecules, because in the neat IL the C4,5 position is almost neutral and thus only possesses very weak interactions with the anion compared to the C2 position. If now water molecules are added, they start to interact with the anion and the cation at the C2 position. Thus, the already very low tendency of the anion to interact with the cation by means of the C4,5 position successively decreases further upon dilution with water. Therefore, the observed blue-shift is more likely due to the interactions with water. This explains also, why the strength of the C4,5AH bond (indicated by a less blue shifting vibration) is not as much affected as it is at the C2 position. To sum up, on the basis of our spectroscopic results from the [EMIM][EtSO4]/H2O system we conclude for the blue-shifts of the C2AH vibrations, that they are primarily based on the successive off-switching of the anion-cation interactions at the C2 position being the main interaction site in the neat RTIL. The main interaction between cation and anion at the C2 position gets successively replaced by water-ion interactions. On the other hand, the blueshift of the C4,5AH bending and stretching vibrations is to a lesser extend a result of successive off-switching of anion-cation interactions at the C2 position going along with an increase of its attractive force, but to a greater proportion due to the formation of
Fig. 3. Fitted spectra between 700 and 800 cm 1 using pseudo-Voigt functions calculating the sum curve to approximate the experimental curve; water mole fractions: 0.00 (A), 0.60 (B), 0.82 (C). Bold solid lines: experimental spectra; solid lines: pseudo-Voigt profiles; dashed lines: pseudo-Voigt sum curve.
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unconventional HBs at the C4,5 moiety itself. This is because the tendency of the anion to interact with the cation by means of the C4,5 position is still very low based on the successive solvation of the anion by water molecules. For completeness sake we note that interactions between water molecules and the cation through the imidazolium ring p-electrons do in principle explain the observed phenomena as well. However, we believe they are less likely in systems with hydroxyl containing solvents and also have less influence on the shift of the force balance in the studied binary system. 3.1.2. The [EMIM][EtSO4]/alcohol systems The spectroscopic data of the [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH systems indicate similar but less pronounced phenomena compared to the aqueous solutions. Their Raman spectra in the region from 700 to 800 cm 1 are shown in Fig. 4A and B. In order to obtain a more quantitative picture, the position of the 758 cm 1 band from the neat IL is plotted as a function of co-solvent mole fraction in Fig. 5. The maximum blue-shift of the C4,5AH bending frequency decreases from water to ethanol: Dmmax,[EMIM][EtSO4]/H2O = 22 cm 1, Dmmax,[EMIM][EtSO4]/MeOH = 17 cm 1, Dmmax,[EMIM][EtSO4]/EtOH = 11 cm 1. In general, in the high concentration regime the frequency shifts displayed in Fig. 5 reveal the steepest slope for water and the least steep slope for ethanol. The methanol curve lies in between the two. These observations indicate that the strength of the molecular interactions between the RTIL and the co-solvent decrease from water to ethanol. This is in concert with what can be expected intuitively, because the co-solvents under investigation exhibit dipole moments in the sequence H2O > MeOH > EtOH reflecting their individual abilities to form IIHA. As discussed for the [EMIM][EtSO4]/water system, IIHA can in principle be formed either as HBs or as interactions between the co-solvent oxygen and the cationic ring p-electrons. In addition, the increasing molecular weight of the co-solvent further supports the smaller blue-shifts of the C2AH and C4,5AH bends in the alcoholic systems. Furthermore, ethanol is slightly hydrophobic due to the aliphatic ethyl group and thus repulsive forces counteract hydrophilic dispersion and electrostatic forces resulting in weaker interactions between the solvents. Interestingly, when we look at the frequency shifts in the regime of low co-solvent concentration it appears that each binary system has an individual composition at which the co-solvent induced frequency shifting starts to develop. In contrast to the intuitively expected trend, i.e. that water molecules interact with the RTIL at very low concentration due to the strongest dipole mo-
Fig. 5. C2-, C4,5AH bending vibration of the imidazolium ring of the cation of [EMIM][EtSO4] in binary mixtures with water, methanol and ethanol in dependence of the mole fraction of the co-solvent.
ment, the [EMIM][EtSO4]/MeOH system shows a frequency shift already at the lowest mole fraction xMethanol = 0.10 studied. Water and ethanol addition leads to first observable frequency changes around 0.25–0.3 co-solvent mole fraction. This indicates that for water and ethanol the solvation of the ions requires a certain minimum amount of co-solvent in the system in order to effectively replace interionic interactions by ion-co-solvent interactions, which goes along with a dissociation of ion pairs. However, in the case of methanol our results indicate that adding a small amount of co-solvent already results in significant interactions with the ions. In contrast, ethanol as co-solvent does not affect the [EMIM][EtSO4] vibration frequency at ethanol mole fractions below 0.3, and the maximum blue-shift of the 758 cm 1 band is 11 cm 1. Another, but less frequently utilized indicator for intermolecular interactions and non-idealities due to the formation of a solvation shell is the Raman signal intensity, especially when it is normalized with respect to concentration. In an ideal system, the Raman intensity of a vibrational mode would scale proportionally with concentration, hence the concentration-normalized intensity would be constant. Fig. 6 displays the Raman intensities and concentration-normalized Raman intensities as functions of co-solvent mole fraction. The Raman intensities in the two RTIL/alcohol systems show a very similar and linear behavior until mole fractions as high as 0.83 and 0.88 for ethanol and methanol, respectively.
Fig. 4. (A) Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/MeOH Increasing methanol mole fractions from top to bottom:0.00, 0.10, 0.29, 0.45, 0.71, 0.88, 0.93, 0.96, 0.98, 0.99, 1.00. (B) Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/EtOH (B) mixtures. Increasing ethanol mole fractions from top to bottom: 0.00, 0.04, 0.21, 0.36, 0.63, 0.84, 0.90, 0.94, 0.98, 0.99, 1.00.
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Fig. 6. Raman intensities and normalized Raman intensities in dependence on co-solvent mole fraction.
At these compositions the Raman intensities suggest an additional transition point for the individual binary systems. However, looking at the normalized intensities no significant deviations from an ideal curve progression are observed below mole fractions as high as 0.97. Especially in the high concentration range the normalized intensities represent a better relation between ideal and non-ideal concentration-dependent changes based on solvation and dissociation effects. The strong change in the curve progression in the highly diluted RTIL/co-solvent systems indicates a very strong non-ideality, which can be due to complete dissociation of the ions. For the alcoholic systems this would be around a mole fraction of 0.98 and for water around 0.96. In the aqueous system, the Raman intensity is relatively constant at water mole fractions below 0.4. Then a linear decrease is found with a transition point suggested at 0.93. In this case, also the normalized Raman intensities reveal deviations from the ideal behavior at this point and beyond. This indicates strong non-idealities in the mixtures at high water content, suggested as a result of ion pair dissociation due to solvation effects. Especially, when only a small amount of RTIL is present in the RTIL/water system, the formation of a complete hydration shell is possible indicating strong non-ideality. Concluding from our spectroscopic data, we must note that there is no simple and intuitive rule of thumb that a lower dipole moment associated with a lower proton donor ability compared to water does necessarily mean weaker effects on the intermolecular interaction structure as can be seen in the case of the co-solvent methanol. 3.2. Physicochemical and thermophysical properties In order to obtain a better understanding of the intermolecular interactions evident in the Raman data and influencing the macroscopic behavior of the investigated binary [EMIM][EtSO4] systems, we discuss in this section our spectroscopic findings against the physicochemical properties, i.e. excess enthalpy HE, excess molar volume VE, and excess viscosity Dg taken from the literature. Note that for the alcoholic mixtures no excess enthalpy HE data were available in the literature, and their determination would go beyond the scope of this study. 3.2.1. Excess enthalpy, HE The excess enthalpy is a measure for the difference in interaction strength between like species, i.e. in our case RTIL–RTIL and water–water, and unlike species, i.e. RTIL-water [52,53]. Fig. 7 presents the excess enthalpy HE of the aqueous [EMIM][EtSO4] solutions as function of water mole fraction. The experimental data were taken from Ficke et al. [53] and fitted based on the Red-
Fig. 7. Excess Enthalpy HE in dependence of the water mole fraction of the binary system [EMIM][EtSO4]/H2O. The diamonds represent experimental data taken from [53], the solid line is a fitted Redlich–Kister function.
lich–Kister-type equation given therein. According to Ficke et al. [52,53], a negative or exothermic excess enthalpy can initially be interpreted as an increase in the interaction strength upon mixing. This means that the interactions between [EMIM][EtSO4] and water are stronger than those in pure water respectively the pure RTIL. The excess enthalpy exhibits a minimum at water mole fraction 0.8. Interestingly, the CAH bending mode frequencies in the aqueous system (see Fig. 5) show the steepest slope in this region indicating strong intermolecular interactions and thus are in concert with the findings of Ficke et al. [52,53]. As an additional measure for strong interactions between unlike species, the acid dissociation constant of [EtSO4] (pKa = 3.14) can be utilized to support the hypothesis of strong interactions between water and the IL anion, whereas the glass transition temperature of [EMIM][EtSO4] at 192.85 K can be used as indicator for strong like-like interactions in the neat RTIL [79]. If the IIHA are dominated by HBs, increasing interactions upon water addition would support either the theory of Spickermann et al. or the theory of unconventional blue-shifting hydrogen bonding [26,27,29]. Spickermann et al. investigated the phenomenon of associated RTIL ions in aqueous solution. They found the water molecules being orientated tangential. This supports hydrogen bonding and ion pairing, instead of ion pair dissociation. However, in this case, the increase in vibration frequency and decrease in excess enthalpy would be only explained by increasing like-like interactions. This would be contrary to the basicity of [EtSO4]expecting strong anion-water interactions as well as the finding
K. Noack et al. / Journal of Molecular Structure 1018 (2012) 45–53
of Ficke et al., that [EMIM][EtSO4] decomposes in aqueous solutions to [EMIM][HSO4] and EtOH supporting the idea of dissociation, at least for the anion. Based on that, the presence of blue shifting HBs seems more likely and in concert with our spectroscopic findings discussed in the preceding section. 3.2.2. Excess volume, VE The excess molar volume can provide a measure of how the packing in the system rearranges upon mixing. Negative values express a decreased overall volume compared to an ideal mixture, for example due to shorter distances between atoms and molecules based on stronger interactions or geometrical rearrangements. On the other hand, positive values indicate a volume increase owing to increasing repulsive interactions or the replacement of short range by long range intermolecular interactions (e.g. van der Waals forces) leading to longer mean free paths, for instance. In Fig. 8, the excess volumes based on density measurements as well as fitted curves (Redlich–Kister-type equation) are displayed. The experimental data were taken from Gomez et al. [70] and Gonzales et al. [71]. Generally speaking, all binary systems under investigation exhibit negative excess molar volumes over the entire composition range [75]. Water shows the smallest negative excess volume while the largest is found for methanol. The spectroscopic data discussed above have already suggested a kind of exceptional behavior of the [EMIM][EtSO4]/MeOH system as even small amounts of co-solvent resulted in significant intermolecular interactions. This is in concert with the macroscopic observations regarding the excess volume. The enhanced packing of methanol molecules in between the interstitials of [EMIM][EtSO4] provides an explanation of this phenomenon, although the interactions between the methanol molecules are not as strong as they are between water molecules. Moreover, water has two protons which can form IIHA and consequently it is possible that rather bulky trimer structures form in which one water molecule interacts with two ions. The same explanation can be used for ethanol in comparison to water. The differences between the two alcoholic systems can be explained by the slightly increasing hydrophobic forces, which influence the balance between polar and non-polar interactions, along with the even lower dipole moment and ability of donating protons in the case of ethanol. Assuming the presence of blue-shifting hydrogen bonding with the background of Hobza’s theory of an accompanying geometric rearrangement and subsequent contraction of bonds or molecular
Fig. 8. Excess molar volume VE of the binary systems plotted against the co-solvent mole fraction. Experimental points for [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH, [EMIM][EtSO4]/EtOH taken from [70,71]; solid lines represent fitted Redlich–Kister functions.
51
parts, the packing respectively excess molar volume is also strongly influenced by the strength of such weak and blue-shifting HBs. Furthermore, it appears that strong HBs, like those in the [EMIM][EtSO4]/H2O system, no matter whether of conventional or unconventional nature, oppose a decreasing excess molar volume and therefore a less non-ideal mixing behavior is observed. Water possesses the strongest IIHA and the greatest dipole moment and thus supports ion association, which leads to a rather bulky packing. In case of methanol and ethanol, which are less polarized, the molecules fit better into the interstitial space of the ionic liquid and therefore show a more negative excess molar volume. The excess molar volume of ethanol solutions is less negative than that of methanol solutions due to the presence of slightly hydrophobic forces.
3.2.3. Excess viscosity, Dg Viscosity can also be interpreted with respect to the strength of intermolecular interactions. It depends on attractive forces and cohesion as well as ‘internal friction’. Basically, high viscosity values can be explained by strong interactions and hence high internal friction. A strong viscosity decrease upon co-solvent addition can be observed for all binary systems studied. The non-ideal behavior becomes obvious in Fig. 9 showing the excess viscosity, i.e. the viscosity deviation Dg from the ideal case, in dependence on the co-solvent mole fraction. The experimental data displayed were again taken from Gomez et al. [70] and Gonzales et al. [71]. As for the excess molar volume and enthalpy, the aqueous mixtures show the lowest deviation from the ideal mixing behavior followed by ethanol and, again, methanol exhibits the strongest deviations. However, the differences between methanol and ethanol (Fig. 9) are relatively small in contrast to the observations concerning the excess volumes discussed before (Fig. 8). Interestingly, only a small difference between the alcoholic systems was also found for the normalized Raman intensities (see Fig. 6), while the aqueous solutions revealed a stronger deviation, especially in the high concentration regime. This indicates that the co-solvent polarization plays a more important role for the viscosity than for the excess volume as can be seen from the relative permittivity values: water (78.5), methanol (32.6), ethanol (24.6). By considering the mass of the co-solvent molecules, which successively hydrate the ions, and the fact that the strong, short-range electrostatic interactions get partially replaced by weaker longrange dispersion forces as well as the increase in mean free path,
Fig. 9. Viscosity deviation Dg of the binary systems [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH. Symbols represent experimental data taken from [70,71]. The solid lines are fitted Redlich–Kister functions.
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the shear stress in the mixture decreases leading to reduced viscosity values and increased ion mobility. Furthermore, the conductivity increases, because more free charge carriers are present since the neutral ion pairs bonded by Coulombic forces are now replaced by charged ion pairs bonded by van der Waals interactions [54]. Moreover, owing to the weakness and the unsteady nature of HBs, the conformational isomerism increases and thus the entropy of the system supporting higher ion mobility and thus a much lower viscosity, even so the molar volume, decreases. This is in concert with our previous work on interionic interactions in neat ionic liquids: Based on the theories of Fumino et al. [80] and Hunt [65], we have recently discussed the effects of a decrease in Coloumbic interactions and the replacement of electrostatic interactions by dispersion forces [18].
As water, methanol and ethanol molecules arrange themselves inside the interstitials of the ionic liquid, even the negative excess values for the molar excess volume as well as for the viscosity can be explained. The negative excess enthalpies for the aqueous [EMIM][EtSO4] solutions indicate stronger interactions between unlike species and hence would suggest that the interactions between water and the individual ions are stronger than the interionic interactions in [EMIM][EtSO4]. However, this is not inevitably true, if we consider the tendency of water to induce stronger ion pair associations in the low concentration range, the effect of the low size and mass of water molecules as well as the presence of blue-shifting hydrogen bonding, which lead to shortened bonds.
4. Conclusion
Parts of the work have been supported by the German Research Foundation (DFG) which funds the Erlangen Graduate School of Advanced Optical Technologies (SAOT) and the Cluster ‘‘Engineering of Advanced Materials’’ (EAM) in the framework of the German Excellence Initiative as well as the Priority Program DFG SPP-1191. In addition, J.K. acknowledges support from the British Council (ARC 1378).
In this work, we have carried out a study on the molecular interactions involving hydrogen atoms (IIHA) and the mixing behavior of binary [EMIM][EtSO4] systems. For this purpose, we have analyzed the Raman spectra of the binary [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH systems in detail and compared these results to the macroscopic behavior of these mixtures in terms of excess data of the mixture enthalpy, molar volume and viscosity. The excess data reflect the macroscopic non-idealities of the mixtures which have been investigated at a molecular level by means of concentration-normalized Raman intensities. Eventually, we can conclude, that blue-shifting CH vibrations, a decrease in viscosity, negative excess enthalpies, and excess molar volumes all indicate the same: Upon co-solvent addition the strong interactions between the anion and the C2 position of the imidazolium cation are weakened, and at high dilution they are replaced by cation-co-solvent interactions. In other words, the ion pairs dissociate. The normalized Raman intensities indicate the start of the ion dissociation leading to non-ideal mixing behavior at mole fractions exceeding 0.4 and a complete dissociation above a mole fraction of 0.96. The blue-shifts of the C2AH bend are based both on the weakening of interactions between the RTIL counter ions and on the lower molar mass of the co-solvent molecules compared to the RTIL anion. The molecular interactions are dominated by IIHA including blue- and red-shifting HBs, but may also involve interactions between the co-solvent oxygen and the cationic ring p-electrons. The latter is possible when the co-solvent molecules lie above or below the imidazolium ring. Both interaction types lead to an increase in electron density at the C2AH bond. In contrast, the blue-shifting C4,5AH bending vibrations are dominated by IIHA which mainly result from unconventional hydrogen bonding. This is supported by an increase in IR absorption of the C4,5AH vibration relative to the decrease upon dilution. In turn, this is an indicator for an increased change in the dipole moment respectively the polarization as compared to the neat RTIL. This would lead to stronger interactions between the different solvent molecules through this position, which is further supported by the fact that the excess volume decreases (although the mean free path should increase, which should be reflected by positive excess volumes when the force balance is shifted into the direction of long range dispersion forces). Our findings are partially in concert with Hobza’s theory of bond contractions as a result of improper hydrogen bonding. However, it should be noted that the molecular interactions of co-solvent molecules above/below the imidazolium ring may play a role in shifting the force balance, too. Therefore theories that only include IIHA are not sufficient to explain all aspects of the observations made.
Acknowledgment
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Molecular interactions and macroscopic effects in binary mixtures of an imidazolium ionic liquid with water, methanol, and ethanol Kristina Noack a,b, Alfred Leipertz a,b, Johannes Kiefer b,c,⇑ a
Institute of Engineering Thermodynamics, University Erlangen-Nuremberg, Germany Erlangen Graduate School in Advanced Optical Technologies, University Erlangen-Nuremberg, Germany c School of Engineering, University of Aberdeen, Aberdeen, Scotland, United Kingdom b
a r t i c l e
i n f o
Article history: Available online 23 February 2012 Keywords: Ionic liquid Hydrogen bonding Excess properties Water Alcohol Raman spectroscopy
a b s t r a c t Intermolecular interactions in mixtures of room-temperature ionic liquids (RTILs) and co-solvents define the properties of the solution. In this work, we study the mixing behavior in the binary systems [EMIM][EtSO4]/water, [EMIM][EtSO4]/methanol and [EMIM][EtSO4]/ethanol, which is governed by a change in the balance of molecular interactions present in neat [EMIM][EtSO4]. The mixing behavior and interactions are investigated at molecular level by means of Raman spectroscopy, and at macroscopic level utilizing excess data taken from the literature. The discussion of the results aims at a distinct interpretation of the spectroscopic data and at identifying the relationships between molecular phenomena and macroscopic behavior. The Raman spectra of the binary systems indicate that the balance of intermolecular interactions in the neat RTIL is dominantly distorted by solute–solvent interactions involving hydrogen atoms (IIHAs). In concert with former studies, the spectroscopic and macroscopic data suggest, that the IIHA include a combination of conventional (red-shifting) and unconventional (blue-shifting) hydrogen bonds. With increasing co-solvent concentration, the interionic bonds become successively weaker and eventually ion-co-solvent interactions even replace those between the RTIL counter ions leading to ion pair dissociation. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Room-temperature ionic liquids (RTILs) are a group of innovative solvents increasingly gaining interest because of their unique properties. In general, RTILs are organic salts with melting points near room temperature or by convention below 100 °C [1]. The variety of possible cation–anion combinations leads to a large number of different RTILs with a wide range of thermophysical and physicochemical properties [2–8], which consequently means a strong potential for a variety of practical applications [9–15]. In fact, it is even possible to tune the properties by adjusting the ratio of Coulomb and van der Waals forces, i.e. short and long range interactions, respectively. However, as it is virtually impossible to experimentally investigate all potential cation–anion combinations, a molecular-based understanding of their macroscopic properties is crucial for a rational design [16,17]. The balance between electrostatic and dispersive interactions can be selectively changed by alteration of the strength of the respective interaction. For example, substitution of functional groups or adding a co-solvent can influence this balance [18–20]. Of course, the addition of a sec⇑ Corresponding author at: School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE, United Kingdom. Tel.: +44 1224 272495. E-mail address: [email protected] (J. Kiefer). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.02.031
ond substance is certainly not the desirable way to obtain the required properties, however mixtures of RTILs and co-solvents are of practical interest in general as process fluids are always in danger of contamination, e.g., by water or organic solvents. Interactions involving hydrogen atoms (IIHAs) have the potential to shift the balance between electrostatic and dispersive forces and with them the macroscopic properties of an RTIL [18–23]. Within this group of molecular interactions hydrogen bonds (HBs) play a crucial role. However, although extensively studied, a precise definition of HBs is yet to be found and subject of controversial discussions. In the framework of a IUPAC project on hydrogen bonding, a novel definition has been summarized in a very recent essay [24]: ‘‘The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment XAH in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.’’ Along with this primary definition, a list of HB characteristics is given [24], one of which refers to vibrational spectroscopy as a suitable tool to experimentally investigate hydrogen bonding. In vibrational spectroscopy HB formation or breakage manifests as line shifts due to changes in vibration frequencies – either towards higher values (blue-shift) or lower ones (red-shift). Typically, the XAH bond (proton donor) is lengthened upon conventional HB formation indicated by a red-shifted line
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with increased intensity and width. However, in some situations the opposite trend, i.e. bond shortening and thus a blue-shifted vibrational line upon unconventional HB formation, can be observed. The debate on the origin of unconventional HBs has recently been expanded to RTILs [25]. In this debate two questions have been discussed: (1) Whether or not blue-shifting HBs refer to ‘‘real’’ hydrogen bonding, and (2) how this interaction mechanism differs from the conventional mechanism of charge transfer to the anti-bonding orbital of the proton donor. Hobza et al. introduced a two-step model to describe the formation of blue-shifting hydrogen bonds in the generic system of ZAXAH Y [26,27]. Firstly, charge is transferred from the proton acceptor to the so called remote part of the proton donor (ZAX). As a result the bond lengthens and shows a red-shift. This leads to a structural reorganization of the proton donor in the second step, and eventually results in a bond contraction of the XAH bond and hence a blue-shift. In contrast, the second school of thought established by Scheiner and Kar discusses blue-shifting CH O HBs with respect to their spectroscopic and structural signature [28], fundamental distinctions [29] and properties [30], cooperativity [31,32], hybridization and substitution [33]. However, no fundamental distinctions in the physical origin could be found and thus CH O bonds were categorized as true HBs [29,30] which was supported by Li et al. [34]. On this basis, it was concluded, that a blue-shifted CH stretch is not an indicator for being proton donor in a HB, since CH X bonds do not always shift to the blue or even shift at all. To give a unified explanation for the shifting behavior of stretching frequencies upon HB formation, Joseph and Jemmis state that all XAH bonds face opposing contracting and lengthening forces while they are involved in HBs. Just the electron affinity of the atoms involved in the HB donor and the resulting electron density distribution predetermines whether a red- or a blue-shift will occur [35]. Besides these attempts, further experimental and theoretical studies trying to prove or generalize the origin respectively shifting behavior of HBs have been carried out using different model systems [36–51]. Concerning IIHA in mixtures of RTILs and solvents the hygroscopic nature of many RTILs is very interesting. Investigations of aqueous RTIL systems usually aim at a preferably complete structural and thermodynamic understanding. To bridge the gap between molecular phenomena and macroscopic properties in a way such that experimental studies can be carried out, the excess enthalpy has proven itself as a suitable measure [52,53]. Moreover, various properties including the diffusion coefficient, electrical conductivity and viscosity are parameters which indicate intermolecular interaction structures as well [54,55]. Leskiv et al., for instance, investigated the enthalpies of aqueous solutions and concluded that the intermolecular interactions in the RTIL 1ethyl-3-methylimidazolium ethylsulfate, [EMIM][EtSO4], are mainly of electrostatic nature rather than based on dispersion [56]. A similar idea was pursuit by Anthony et al., who utilized solution thermodynamics to find a relationship between enthalpy and entropy of absorbing water into RTILs and the affinity to do so [57]. This was found to depend on the alkyl side chain length. In this context, vibrational spectroscopy is a favorable tool to provide experimental insights. Previous studies employed IR and/ or Raman spectroscopy to analyze the HB network in RTIL/water mixtures [58–63]. Very recently, Lehmann et al. [64] found that decreasing stability or interaction energy is not correlated with the same decrease in hydrogen bonding. They focused on the role of HBs in the interionic interactions of RTILs and concluded that HBs contribute to a perturbation of the ion pairs by stabilizing conformational transition states. Eventually, this leads to higher or lower ion pair mobility in dependence on the interionic interaction strength. These results are in concert with studies of Hunt et al. regarding the viscosity change and ion mobility of imidazolium-
based ionic liquids [65,66]. In addition, theoretical studies have investigated the effects of alkyl side chain length and the anion structure on the aggregation behavior [67], diffusion coefficients [67] and HB formation [68]. Spickermann et al. used DFT calculations to study why RTILs mainly associate in water. They found that the structure and orientation of water as well as the HB network in the RTIL play crucial roles in determining whether dissociation or association is favored [69]. In this work, we study and compare IIHA in binary mixtures of the RTIL [EMIM][EtSO4] with water, methanol, and ethanol. While aqueous RTIL solutions are frequently studied, investigations of alcoholic mixtures are rare in the literature although they are important from practical viewpoints as well. Moreover, as the polarization of the hydroxyl group varies among the three solvents under consideration, a systematic investigation can be carried out. [EMIM][EtSO4] has been chosen because (1) it is highly hydrophilic and miscible with the three co-solvents in all proportions at ambient conditions [70,71] and (2) its vibrational structure is well understood [72,73]. In a first step, the Raman spectra of the binary systems with systematically varied composition are analyzed focusing on co-solvent induced frequency shifts. Secondly, the spectroscopic results are discussed against thermodynamic and physicochemical property data from the literature. This allows a more comprehensive interpretation in order to gain deeper insights into how the chemical/vibrational structure is related to the macroscopic properties. 2. Materials and methods 2.1. Chemicals and sample preparation 1-Ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4], methanol and ethanol with 99.9%, 99.5% and 99.5% purity, respectively, were purchased from Solvent Innovation GmbH and Merck AG, Germany and used as received. Water was taken from the water supply and additionally de-ionized using an ion exchanger system (SG 2800 SK) providing water with conductivity below 0.1 lS/cm. Binary mixtures of [EMIM][EtSO4] and co-solvent with co-solvent mass fractions of 0, 0.01, 0.05, 0.10, 0.25, 0.50, 0.65, 0.75, 0.90, 0.95, and 1 were prepared gravimetrically in glass cuvettes and grouted with film in order to avoid leakage of the liquids. In the following, the mass fractions are presented as mole fractions (water: 0.00, 0.12, 0.41, 0.60, 0.81, 0.93, 0.96, 0.97, 0.99, 1.00; methanol: 0.00, 0.07, 0.28, 0.45, 0.71, 0.88, 0.93, 0.96, 0.98, 0.99, 1.00; ethanol: 0.00, 0.05, 0.21, 0.36, 0.63, 0.84, 0.91, 0.94, 0.98, 0.99, 1.00) in order to ease an interpretation of molecular phenomena. 2.2. Raman spectroscopy The experimental Raman setup consisted of a continuous-wave grating-stabilized diode laser operating at 785 nm. The laser beam was focused into the cuvette containing the sample under investigation and the backscattered light was collected in a ‘hole in a mirror’ arrangement [74] by an achromatic lens. The signal was spectrally filtered using two long-pass filters (cut-off wavelengths 785 and 800 nm) in order to suppress elastically scattered laser light, and focused into a fiber-coupled spectrometer. Eventually, the Raman scattering signal was detected in the wavenumber range 250–2000 cm 1 with a spectral resolution of approximately 2 cm 1 on a charge-coupled-device chip. The frequency-stabilized laser source and the temperature-controlled spectrometer resulted in high reproducibility of the spectra which allowed a reliable determination of vibrational frequencies and Raman intensities.
K. Noack et al. / Journal of Molecular Structure 1018 (2012) 45–53
2.3. Excess enthalpy, viscosity, molar volume The data were taken from the following Refs. [52,53,70,71,75]. 3. Results and discussion The chemical structure of the individual ions in [EMIM][EtSO4] is depicted schematically in Fig. 1 showing the denotations of the 1-ethyl-3-methylimidazolium cation used in the following discussion and in Table 1. The electronic structure of the cationic imidazolium ring can be described as a ring uniting a delocalized 3center-4-electron configuration across the N1AC2AN3 moiety, a double bond between C4 and C5 at the opposite side, and a weak delocalization in the central region [48]. Looking at the partial charges within the cation, the peripheral hydrogen atoms at C2, C4 and C5 carry most of the positive charge, while the negative charge is distributed at N1 and N2 as well as in between the Catoms. In this context it has to be noted that the C4- and C5-atoms appear to be almost neutral while the C2-atom exceptionally possesses positive charge owing to the electron deficit in the C@N bond. Moreover, the large positive charge at the C2AH unit and the repulsive interactions due to the C@C bond explain the observed higher acidity of C2AH and the better electron-donating properties of C4, C5, respectively [66,76]. 3.1. Spectroscopic analysis In the following, we discuss the spectroscopic results referring to [72,73,77]. The Raman spectrum between 300 and 1800 cm 1 is utilized for studying molecular interactions in the binary mix-
Fig. 1. Chemical structure and denotations of the 1-ethyl-3-methylimidazolium cation and the ethylsulfate anion (see Table 1).
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tures. The relevant vibrational assignments of neat [EMIM][EtSO4] in this spectral range are summarized in Table 1. According to Grondin et al. [77], the CH vibrations of the [EMIM]+ cation can serve as probes for changes in the interionic interactions in dependence on the anion, and in case of a mixture also in dependence on the co-solvent concentration. Therefore we analyze the cationic CH vibrations in order to identify red- and blue-shifts due to bonds involving hydrogen atoms and discuss their role in the intermolecular and interionic interactions. Although sensitive probes for hydrogen bonding as well, the changes in the co-solvents’ OH vibrations are not studied in detail here, because they provide very limited information about the RTIL structure. For the analysis of Raman spectra of imidazolium-based ionic liquids either the mipNACH3 line at 696 cm 1 or the mCAC line of the ethyl group at 952.7 cm 1 can be employed as an internal reference. The latter is used here. Unlike for [EMIM][Br] (which was used by Grondin et al. because it allows the cationic vibrations to be studied without significant perturbations by the anion), in [EMIM][EtSO4] the anion does exert influence on the cation due to its higher mass [72], asymmetry (higher conformational isomerism and thus higher entropy) and its basicity (pKa(ethyl sulfuric acid) = 3.14). Therefore, in [EMIM][EtSO4] several cationic vibrations originating from interaction sites between anion and cation are shifted and/or broadened in comparison to [EMIM][Br]. An interesting example is the band at 758 cm 1 which can be assigned to overlapping bending modes of CAH at the C2 and C4,5 positions (for completeness we note that a small contribution to the band may also result from a cationic HCH wagging vibration of the ethyl moiety, which however plays only a minor role in the interionic interactions [73]). This choice is reasoned on the high sensitivity of the C2 position to the anion nature based on its function as main interaction site and, consequently, as the basicity of the anion increases the c C2AH vibration is red-shifted. In contrast, the C4,5AH bending vibrations are hardly sensitive to the anion nature and are rather constant in frequency. For these reasons, a detailed analysis of the Raman band at 758 cm 1 provides a suitable tool for studying IIHA with co-solvent molecules.
3.1.1. The [EMIM][EtSO4]/water system The unprocessed Raman spectra in the region from 700 to 820 cm 1 recorded in the aqueous solutions of [EMIM][EtSO4] are displayed in Fig. 2 and give a first impression. Unlike Grondin et al. found for [EMIM]Br, the intensity of the peak at 758 cm 1
Table 1 Observed Raman bands of 1-ethyl-3-methylimidazolium ethylsulfate and their assignments [72,73,77].
mexp (cm 1)
Refs.
333.8, 409.2 591.3 696.6 758.4 909.4, 952.7 1014.4 1054.3 1085.5 1247.0 1330.6 1382.1 1416.0 1447.6 1560.3
[73] [73] [72,73] [72,73,77] [72,73] [73] [72,73] [72,73] [72,73] [72,73] [72,73] [72,73] [72,73]
Vibrational assignment [72,73,77] dO21S20O22, CCring
mC(7)N(1), wO21S20O23 wC(4,5)H(11) dC(4,5)H(11),cC(2)H, wC(7)H2, S20O24 mC25C26,msO21S20O22 mC(2)N(1)C(5)masCOSO3 mC25C26, mO24C25 mC25O24S20,dipC(4,5)H, sC(6)H2 dC(4,5)H, mCH3(2)N rC(7)H2, rC(7)H(16)N(1) rC(7)H2, rC(8)H2, C(26)H2, C(25)H2 rC(7)N(1)H(15), rC(6)N(3)H(13) sC(8)H3, sC(7)H2, sC(6)H2, dasCH3(Me), dasCH3(Me) rC(2)N(1), mip, asring, rC(4)N(3)(10), mC(6)H3N(3), mC(7)H2N(1)
d: in plane bending, c: out of plane bending, s: twisting, r: rocking, w: wagging, s: scissoring, m: stretching.
Fig. 2. Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/H2O mixtures. Increasing water mole fraction from top to bottom: 0.00, 0.15, 0.40, 0.61, 0.82, 0.93, 0.96, 0.98, 0.99, 1.00.
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is not very weak, but exhibits medium intensity and hence provides sufficient signal-to-noise ratio for a detailed analysis. In this region, a number of modes are in close vicinity with each other and overlap enhancing the overall peak intensity. This band centered at 758 cm 1 is remarkably affected by the addition of co-solvent observable as an increasing blue-shift with increasing water content, as well as it undergoes a change in the overall spectral shape. In order to extract more quantitative information the band has been fitted to a sum of pseudo-Voigt functions using a Matlab code. Pseudo-Voigt profiles for fitting vibrational spectra are the result of an inverse Fourier transformation of the convolution of a Lorentz and Gauss profile. A reasonably good fit could be achieved by employing six individual profiles as shown in Fig. 3. Based on the fact that fitting of the spectra was mainly influenced by alterations of the two most intense individual modes (at 758.1 cm 1 and 770 cm 1 in the neat RTIL), we conclude that these vibrations refer to the cationic C4,5AH and C2AH bending vibrations, respectively. The other profiles are also blue-shifted, but they have very low intensity and hence their contribution to the overall band position and shape is small. For the sake of completeness, we note that the blue-shifted bending modes are in concert with blue-shifts of the corresponding stretching vibrations for both the C2AH (3105 cm 1) and C4,5AH (3160 cm 1) stretches observed in the IR spectra (not shown). Interestingly, when we compare the IR absorption intensity ratio of these two stretching modes, we find that approximately at a water mole fraction of about 0.67 this ratio starts to reverse. In the pure ionic liquid, the C2AH peak shows the strongest absorption which is based on its higher positive charge compared to C4,5 whereas at high water mole fractions this is reverse. This can be an indicator for a shifted force balance. Concerning the C4,5AH bond, which appears to be almost neutral and exhibits more a repulsive than attractive nature in neat [EMIM][EtSO4] [66,76], this change is related to an increase of the C4,5 position’s attractive nature accompanied by a blue-shifting vibration frequency. The strengthening of the covalent CAH bonds at the C2 and C4,5 positions of the cation are due to IIHA. These interactions can have two origins. On the one hand, the blue shifts due to IIHA are based on the formation of HBs between the co-solvent and the RTIL cation which weakens the strong interionic interactions. The replacement of strong interionic interactions by weak long range HB interactions would lead to a charge transfer back to the CH bonds and thus to a strengthening of these covalent bonds. Furthermore, the much lower mass of the co-solvent molecules supports a bond strengthening also. On the other hand, there can be interactions between the water oxygen and the p-electrons of the aromatic imidazolium ring. Such configurations were proposed by Kempter and
Kirchner [25]and have very recently been investigated by Lehmann et al. [64] and Kiefer et al. [78]. Due to an arrangement of the cosolvent molecules at the top or bottom of the cationic ring, the water–oxygen is able to transfer charge to the ring. The result is an increase in the electron densities and hence a bond strengthening observable as blue shifts in the respective vibrational modes. As a consequence, the interionic IIHA are weakened. Both interaction types do influence the overall force balance. However, in the existing literature on mixtures of RTILs with hydroxyl containing solvents (see introduction), the formation of HBs between the co-solvent and the C2 and C4,5 position is usually considered the dominating mechanism. In concert with the published work, we therefore deduce that water addition strengthens dispersion and not electrostatic forces. The electrostatic forces are successively decreased upon water addition, which is one reason for the blue-shift observed. Another reason is the increasing formation of strongly bound cation-water and anion-water pairs. The formation of anion-water pairs manifests as red-shifted sulfate vibrational mode, because the OAH bond in water is more polarized than the C2AH moiety of the imidazolium cation. On the other hand, when the cation is hydrated at the C2 proton, the interactions with water are weaker than those with the anion and, as a consequence, the C2AH vibration is blue-shifted with increasing water content. By contrast, when we look at the C4,5 moiety a blue-shift can be observed, but less pronounced than the one for C2AH. This can be attributed to the C4,5 protons probably forming HBs with water molecules, because in the neat IL the C4,5 position is almost neutral and thus only possesses very weak interactions with the anion compared to the C2 position. If now water molecules are added, they start to interact with the anion and the cation at the C2 position. Thus, the already very low tendency of the anion to interact with the cation by means of the C4,5 position successively decreases further upon dilution with water. Therefore, the observed blue-shift is more likely due to the interactions with water. This explains also, why the strength of the C4,5AH bond (indicated by a less blue shifting vibration) is not as much affected as it is at the C2 position. To sum up, on the basis of our spectroscopic results from the [EMIM][EtSO4]/H2O system we conclude for the blue-shifts of the C2AH vibrations, that they are primarily based on the successive off-switching of the anion-cation interactions at the C2 position being the main interaction site in the neat RTIL. The main interaction between cation and anion at the C2 position gets successively replaced by water-ion interactions. On the other hand, the blueshift of the C4,5AH bending and stretching vibrations is to a lesser extend a result of successive off-switching of anion-cation interactions at the C2 position going along with an increase of its attractive force, but to a greater proportion due to the formation of
Fig. 3. Fitted spectra between 700 and 800 cm 1 using pseudo-Voigt functions calculating the sum curve to approximate the experimental curve; water mole fractions: 0.00 (A), 0.60 (B), 0.82 (C). Bold solid lines: experimental spectra; solid lines: pseudo-Voigt profiles; dashed lines: pseudo-Voigt sum curve.
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unconventional HBs at the C4,5 moiety itself. This is because the tendency of the anion to interact with the cation by means of the C4,5 position is still very low based on the successive solvation of the anion by water molecules. For completeness sake we note that interactions between water molecules and the cation through the imidazolium ring p-electrons do in principle explain the observed phenomena as well. However, we believe they are less likely in systems with hydroxyl containing solvents and also have less influence on the shift of the force balance in the studied binary system. 3.1.2. The [EMIM][EtSO4]/alcohol systems The spectroscopic data of the [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH systems indicate similar but less pronounced phenomena compared to the aqueous solutions. Their Raman spectra in the region from 700 to 800 cm 1 are shown in Fig. 4A and B. In order to obtain a more quantitative picture, the position of the 758 cm 1 band from the neat IL is plotted as a function of co-solvent mole fraction in Fig. 5. The maximum blue-shift of the C4,5AH bending frequency decreases from water to ethanol: Dmmax,[EMIM][EtSO4]/H2O = 22 cm 1, Dmmax,[EMIM][EtSO4]/MeOH = 17 cm 1, Dmmax,[EMIM][EtSO4]/EtOH = 11 cm 1. In general, in the high concentration regime the frequency shifts displayed in Fig. 5 reveal the steepest slope for water and the least steep slope for ethanol. The methanol curve lies in between the two. These observations indicate that the strength of the molecular interactions between the RTIL and the co-solvent decrease from water to ethanol. This is in concert with what can be expected intuitively, because the co-solvents under investigation exhibit dipole moments in the sequence H2O > MeOH > EtOH reflecting their individual abilities to form IIHA. As discussed for the [EMIM][EtSO4]/water system, IIHA can in principle be formed either as HBs or as interactions between the co-solvent oxygen and the cationic ring p-electrons. In addition, the increasing molecular weight of the co-solvent further supports the smaller blue-shifts of the C2AH and C4,5AH bends in the alcoholic systems. Furthermore, ethanol is slightly hydrophobic due to the aliphatic ethyl group and thus repulsive forces counteract hydrophilic dispersion and electrostatic forces resulting in weaker interactions between the solvents. Interestingly, when we look at the frequency shifts in the regime of low co-solvent concentration it appears that each binary system has an individual composition at which the co-solvent induced frequency shifting starts to develop. In contrast to the intuitively expected trend, i.e. that water molecules interact with the RTIL at very low concentration due to the strongest dipole mo-
Fig. 5. C2-, C4,5AH bending vibration of the imidazolium ring of the cation of [EMIM][EtSO4] in binary mixtures with water, methanol and ethanol in dependence of the mole fraction of the co-solvent.
ment, the [EMIM][EtSO4]/MeOH system shows a frequency shift already at the lowest mole fraction xMethanol = 0.10 studied. Water and ethanol addition leads to first observable frequency changes around 0.25–0.3 co-solvent mole fraction. This indicates that for water and ethanol the solvation of the ions requires a certain minimum amount of co-solvent in the system in order to effectively replace interionic interactions by ion-co-solvent interactions, which goes along with a dissociation of ion pairs. However, in the case of methanol our results indicate that adding a small amount of co-solvent already results in significant interactions with the ions. In contrast, ethanol as co-solvent does not affect the [EMIM][EtSO4] vibration frequency at ethanol mole fractions below 0.3, and the maximum blue-shift of the 758 cm 1 band is 11 cm 1. Another, but less frequently utilized indicator for intermolecular interactions and non-idealities due to the formation of a solvation shell is the Raman signal intensity, especially when it is normalized with respect to concentration. In an ideal system, the Raman intensity of a vibrational mode would scale proportionally with concentration, hence the concentration-normalized intensity would be constant. Fig. 6 displays the Raman intensities and concentration-normalized Raman intensities as functions of co-solvent mole fraction. The Raman intensities in the two RTIL/alcohol systems show a very similar and linear behavior until mole fractions as high as 0.83 and 0.88 for ethanol and methanol, respectively.
Fig. 4. (A) Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/MeOH Increasing methanol mole fractions from top to bottom:0.00, 0.10, 0.29, 0.45, 0.71, 0.88, 0.93, 0.96, 0.98, 0.99, 1.00. (B) Raman spectra of the band at 758 cm 1 recorded in [EMIM][EtSO4]/EtOH (B) mixtures. Increasing ethanol mole fractions from top to bottom: 0.00, 0.04, 0.21, 0.36, 0.63, 0.84, 0.90, 0.94, 0.98, 0.99, 1.00.
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K. Noack et al. / Journal of Molecular Structure 1018 (2012) 45–53
Fig. 6. Raman intensities and normalized Raman intensities in dependence on co-solvent mole fraction.
At these compositions the Raman intensities suggest an additional transition point for the individual binary systems. However, looking at the normalized intensities no significant deviations from an ideal curve progression are observed below mole fractions as high as 0.97. Especially in the high concentration range the normalized intensities represent a better relation between ideal and non-ideal concentration-dependent changes based on solvation and dissociation effects. The strong change in the curve progression in the highly diluted RTIL/co-solvent systems indicates a very strong non-ideality, which can be due to complete dissociation of the ions. For the alcoholic systems this would be around a mole fraction of 0.98 and for water around 0.96. In the aqueous system, the Raman intensity is relatively constant at water mole fractions below 0.4. Then a linear decrease is found with a transition point suggested at 0.93. In this case, also the normalized Raman intensities reveal deviations from the ideal behavior at this point and beyond. This indicates strong non-idealities in the mixtures at high water content, suggested as a result of ion pair dissociation due to solvation effects. Especially, when only a small amount of RTIL is present in the RTIL/water system, the formation of a complete hydration shell is possible indicating strong non-ideality. Concluding from our spectroscopic data, we must note that there is no simple and intuitive rule of thumb that a lower dipole moment associated with a lower proton donor ability compared to water does necessarily mean weaker effects on the intermolecular interaction structure as can be seen in the case of the co-solvent methanol. 3.2. Physicochemical and thermophysical properties In order to obtain a better understanding of the intermolecular interactions evident in the Raman data and influencing the macroscopic behavior of the investigated binary [EMIM][EtSO4] systems, we discuss in this section our spectroscopic findings against the physicochemical properties, i.e. excess enthalpy HE, excess molar volume VE, and excess viscosity Dg taken from the literature. Note that for the alcoholic mixtures no excess enthalpy HE data were available in the literature, and their determination would go beyond the scope of this study. 3.2.1. Excess enthalpy, HE The excess enthalpy is a measure for the difference in interaction strength between like species, i.e. in our case RTIL–RTIL and water–water, and unlike species, i.e. RTIL-water [52,53]. Fig. 7 presents the excess enthalpy HE of the aqueous [EMIM][EtSO4] solutions as function of water mole fraction. The experimental data were taken from Ficke et al. [53] and fitted based on the Red-
Fig. 7. Excess Enthalpy HE in dependence of the water mole fraction of the binary system [EMIM][EtSO4]/H2O. The diamonds represent experimental data taken from [53], the solid line is a fitted Redlich–Kister function.
lich–Kister-type equation given therein. According to Ficke et al. [52,53], a negative or exothermic excess enthalpy can initially be interpreted as an increase in the interaction strength upon mixing. This means that the interactions between [EMIM][EtSO4] and water are stronger than those in pure water respectively the pure RTIL. The excess enthalpy exhibits a minimum at water mole fraction 0.8. Interestingly, the CAH bending mode frequencies in the aqueous system (see Fig. 5) show the steepest slope in this region indicating strong intermolecular interactions and thus are in concert with the findings of Ficke et al. [52,53]. As an additional measure for strong interactions between unlike species, the acid dissociation constant of [EtSO4] (pKa = 3.14) can be utilized to support the hypothesis of strong interactions between water and the IL anion, whereas the glass transition temperature of [EMIM][EtSO4] at 192.85 K can be used as indicator for strong like-like interactions in the neat RTIL [79]. If the IIHA are dominated by HBs, increasing interactions upon water addition would support either the theory of Spickermann et al. or the theory of unconventional blue-shifting hydrogen bonding [26,27,29]. Spickermann et al. investigated the phenomenon of associated RTIL ions in aqueous solution. They found the water molecules being orientated tangential. This supports hydrogen bonding and ion pairing, instead of ion pair dissociation. However, in this case, the increase in vibration frequency and decrease in excess enthalpy would be only explained by increasing like-like interactions. This would be contrary to the basicity of [EtSO4]expecting strong anion-water interactions as well as the finding
K. Noack et al. / Journal of Molecular Structure 1018 (2012) 45–53
of Ficke et al., that [EMIM][EtSO4] decomposes in aqueous solutions to [EMIM][HSO4] and EtOH supporting the idea of dissociation, at least for the anion. Based on that, the presence of blue shifting HBs seems more likely and in concert with our spectroscopic findings discussed in the preceding section. 3.2.2. Excess volume, VE The excess molar volume can provide a measure of how the packing in the system rearranges upon mixing. Negative values express a decreased overall volume compared to an ideal mixture, for example due to shorter distances between atoms and molecules based on stronger interactions or geometrical rearrangements. On the other hand, positive values indicate a volume increase owing to increasing repulsive interactions or the replacement of short range by long range intermolecular interactions (e.g. van der Waals forces) leading to longer mean free paths, for instance. In Fig. 8, the excess volumes based on density measurements as well as fitted curves (Redlich–Kister-type equation) are displayed. The experimental data were taken from Gomez et al. [70] and Gonzales et al. [71]. Generally speaking, all binary systems under investigation exhibit negative excess molar volumes over the entire composition range [75]. Water shows the smallest negative excess volume while the largest is found for methanol. The spectroscopic data discussed above have already suggested a kind of exceptional behavior of the [EMIM][EtSO4]/MeOH system as even small amounts of co-solvent resulted in significant intermolecular interactions. This is in concert with the macroscopic observations regarding the excess volume. The enhanced packing of methanol molecules in between the interstitials of [EMIM][EtSO4] provides an explanation of this phenomenon, although the interactions between the methanol molecules are not as strong as they are between water molecules. Moreover, water has two protons which can form IIHA and consequently it is possible that rather bulky trimer structures form in which one water molecule interacts with two ions. The same explanation can be used for ethanol in comparison to water. The differences between the two alcoholic systems can be explained by the slightly increasing hydrophobic forces, which influence the balance between polar and non-polar interactions, along with the even lower dipole moment and ability of donating protons in the case of ethanol. Assuming the presence of blue-shifting hydrogen bonding with the background of Hobza’s theory of an accompanying geometric rearrangement and subsequent contraction of bonds or molecular
Fig. 8. Excess molar volume VE of the binary systems plotted against the co-solvent mole fraction. Experimental points for [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH, [EMIM][EtSO4]/EtOH taken from [70,71]; solid lines represent fitted Redlich–Kister functions.
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parts, the packing respectively excess molar volume is also strongly influenced by the strength of such weak and blue-shifting HBs. Furthermore, it appears that strong HBs, like those in the [EMIM][EtSO4]/H2O system, no matter whether of conventional or unconventional nature, oppose a decreasing excess molar volume and therefore a less non-ideal mixing behavior is observed. Water possesses the strongest IIHA and the greatest dipole moment and thus supports ion association, which leads to a rather bulky packing. In case of methanol and ethanol, which are less polarized, the molecules fit better into the interstitial space of the ionic liquid and therefore show a more negative excess molar volume. The excess molar volume of ethanol solutions is less negative than that of methanol solutions due to the presence of slightly hydrophobic forces.
3.2.3. Excess viscosity, Dg Viscosity can also be interpreted with respect to the strength of intermolecular interactions. It depends on attractive forces and cohesion as well as ‘internal friction’. Basically, high viscosity values can be explained by strong interactions and hence high internal friction. A strong viscosity decrease upon co-solvent addition can be observed for all binary systems studied. The non-ideal behavior becomes obvious in Fig. 9 showing the excess viscosity, i.e. the viscosity deviation Dg from the ideal case, in dependence on the co-solvent mole fraction. The experimental data displayed were again taken from Gomez et al. [70] and Gonzales et al. [71]. As for the excess molar volume and enthalpy, the aqueous mixtures show the lowest deviation from the ideal mixing behavior followed by ethanol and, again, methanol exhibits the strongest deviations. However, the differences between methanol and ethanol (Fig. 9) are relatively small in contrast to the observations concerning the excess volumes discussed before (Fig. 8). Interestingly, only a small difference between the alcoholic systems was also found for the normalized Raman intensities (see Fig. 6), while the aqueous solutions revealed a stronger deviation, especially in the high concentration regime. This indicates that the co-solvent polarization plays a more important role for the viscosity than for the excess volume as can be seen from the relative permittivity values: water (78.5), methanol (32.6), ethanol (24.6). By considering the mass of the co-solvent molecules, which successively hydrate the ions, and the fact that the strong, short-range electrostatic interactions get partially replaced by weaker longrange dispersion forces as well as the increase in mean free path,
Fig. 9. Viscosity deviation Dg of the binary systems [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH. Symbols represent experimental data taken from [70,71]. The solid lines are fitted Redlich–Kister functions.
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the shear stress in the mixture decreases leading to reduced viscosity values and increased ion mobility. Furthermore, the conductivity increases, because more free charge carriers are present since the neutral ion pairs bonded by Coulombic forces are now replaced by charged ion pairs bonded by van der Waals interactions [54]. Moreover, owing to the weakness and the unsteady nature of HBs, the conformational isomerism increases and thus the entropy of the system supporting higher ion mobility and thus a much lower viscosity, even so the molar volume, decreases. This is in concert with our previous work on interionic interactions in neat ionic liquids: Based on the theories of Fumino et al. [80] and Hunt [65], we have recently discussed the effects of a decrease in Coloumbic interactions and the replacement of electrostatic interactions by dispersion forces [18].
As water, methanol and ethanol molecules arrange themselves inside the interstitials of the ionic liquid, even the negative excess values for the molar excess volume as well as for the viscosity can be explained. The negative excess enthalpies for the aqueous [EMIM][EtSO4] solutions indicate stronger interactions between unlike species and hence would suggest that the interactions between water and the individual ions are stronger than the interionic interactions in [EMIM][EtSO4]. However, this is not inevitably true, if we consider the tendency of water to induce stronger ion pair associations in the low concentration range, the effect of the low size and mass of water molecules as well as the presence of blue-shifting hydrogen bonding, which lead to shortened bonds.
4. Conclusion
Parts of the work have been supported by the German Research Foundation (DFG) which funds the Erlangen Graduate School of Advanced Optical Technologies (SAOT) and the Cluster ‘‘Engineering of Advanced Materials’’ (EAM) in the framework of the German Excellence Initiative as well as the Priority Program DFG SPP-1191. In addition, J.K. acknowledges support from the British Council (ARC 1378).
In this work, we have carried out a study on the molecular interactions involving hydrogen atoms (IIHA) and the mixing behavior of binary [EMIM][EtSO4] systems. For this purpose, we have analyzed the Raman spectra of the binary [EMIM][EtSO4]/H2O, [EMIM][EtSO4]/MeOH and [EMIM][EtSO4]/EtOH systems in detail and compared these results to the macroscopic behavior of these mixtures in terms of excess data of the mixture enthalpy, molar volume and viscosity. The excess data reflect the macroscopic non-idealities of the mixtures which have been investigated at a molecular level by means of concentration-normalized Raman intensities. Eventually, we can conclude, that blue-shifting CH vibrations, a decrease in viscosity, negative excess enthalpies, and excess molar volumes all indicate the same: Upon co-solvent addition the strong interactions between the anion and the C2 position of the imidazolium cation are weakened, and at high dilution they are replaced by cation-co-solvent interactions. In other words, the ion pairs dissociate. The normalized Raman intensities indicate the start of the ion dissociation leading to non-ideal mixing behavior at mole fractions exceeding 0.4 and a complete dissociation above a mole fraction of 0.96. The blue-shifts of the C2AH bend are based both on the weakening of interactions between the RTIL counter ions and on the lower molar mass of the co-solvent molecules compared to the RTIL anion. The molecular interactions are dominated by IIHA including blue- and red-shifting HBs, but may also involve interactions between the co-solvent oxygen and the cationic ring p-electrons. The latter is possible when the co-solvent molecules lie above or below the imidazolium ring. Both interaction types lead to an increase in electron density at the C2AH bond. In contrast, the blue-shifting C4,5AH bending vibrations are dominated by IIHA which mainly result from unconventional hydrogen bonding. This is supported by an increase in IR absorption of the C4,5AH vibration relative to the decrease upon dilution. In turn, this is an indicator for an increased change in the dipole moment respectively the polarization as compared to the neat RTIL. This would lead to stronger interactions between the different solvent molecules through this position, which is further supported by the fact that the excess volume decreases (although the mean free path should increase, which should be reflected by positive excess volumes when the force balance is shifted into the direction of long range dispersion forces). Our findings are partially in concert with Hobza’s theory of bond contractions as a result of improper hydrogen bonding. However, it should be noted that the molecular interactions of co-solvent molecules above/below the imidazolium ring may play a role in shifting the force balance, too. Therefore theories that only include IIHA are not sufficient to explain all aspects of the observations made.
Acknowledgment
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