Thermodynamic and spectroscopic studies on binary mixtures of imidazolium ionic liquids in ethylene glycol

J. Chem. Thermodynamics 44 (2012) 121–127

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Thermodynamic and spectroscopic studies on binary mixtures of imidazolium ionic liquids in ethylene glycol Bhupinder Kumar a, Tejwant Singh b, K. Srinivasa Rao b, Amalendu Pal a,⇑, Arvind Kumar b,⇑ a b

Department of Chemistry, Kurukshetra University, Kurukshetra 136119, Haryana, India Salt and Marine Chemicals Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar 364002, India

a r t i c l e

i n f o

Article history: Received 28 June 2011 Received in revised form 8 August 2011 Accepted 9 August 2011 Available online 18 August 2011 Keywords: Ionic liquids Ethylene glycol Density FTIR 1 H NMR

a b s t r a c t The thermodynamic behaviour of imidazolium based ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride [C4mim][Cl]; 1-octyl-3-methylimidazolium chloride [C8mim][Cl], and 1-butyl-3-methylimidazolium methylsulfate [C4mim][C1OSO3] in ethylene glycol [HOCH2CH2OH] (EG) have been investigated over the whole composition range at T = (298.15 to 318.15) K to probe the interactions in bulk. For the purpose, volumetric properties such as excess molar volume, V Em , apparent molar volume, V/,i, and its limiting values at infinite dilution, V 1 /;i , have been calculated from the experimental density measurements. The molecular scale interactions between ionic liquids and EG have been investigated through Fourier transform infrared (FTIR) and 1H NMR spectroscopy. The shift in the vibrational frequency for C–H stretch of aromatic ring protons of ILs and O–H stretch of EG molecules has been analysed. The NMR chemical shifts for various protons of RTILS or EG molecules and their deviations show multiple hydrogen bonding interactions of varying strengths between RTILs and EG in their binary mixtures. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Air- and moisture-stable imidazolium-based ionic liquids (ILs) are a popular, and perhaps the most studied, class of ILs [1,2]. Due to their unique physicochemical properties, they are rapidly gaining attention from the scientific world in various chemical applications [1–15]. However, due to the relatively higher viscosity of some ILs, their use as solvent in technical applications, for example liquid thermal storage media or heat transfer fluids, is restricted. Further, addition of a suitable co-solvent to an IL can modify the physicochemical properties, thereby enlarging the range of application and versatility of these fluids. Therefore, understanding of mixing behaviour, thermodynamic properties and molecular level interactions in the binary mixtures of room temperature ionic liquids in the organic solvents are of relevance both for understanding their solution behaviour and providing accurate data for various industrial applications. Thermodynamic studies on binary mixtures of RTILs in organic solvents [16–31] are gaining importance day by day for a potential substitute to the mixtures of conventional volatile organic liquids used in different chemical processes. In this regard, our group has also initiated systematic studies on the solvation and mixing behaviour of RTILs in industrially important solvents, such as ethylene glycol (EG) and ⇑ Corresponding authors. Tel.: +91 278 256 7039; fax: +91 278 256 7562/256 6970 (A. Pal). E-mail addresses: [email protected] (A. Pal), [email protected] (A. Kumar). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.08.008

its derivatives, at both the microscopic and macroscopic level [32–36]. Ethylene glycol (EG) is an important organic solvent with extensive use in many industrial applications which include antifreeze in cooling and heating systems, in hydraulic brake fluids, and as a solvent in the paint and plastic industries [37]. Binary mixtures of RTILs and EG can serve as materials with better heat-transfer properties than that of individual components, and can be used in various chemical processes. Such systems need to be thoroughly investigated for their bulk and molecular scale behaviour. Our earlier findings suggest that various types of interactions prevail in the binary mixtures of RTILs and organic solvents. Hence, thermodynamic studies alone are not enough to understand the exact nature of the solute–solvent interactions [32–36]. In this study, we have investigated the bulk behaviour and molecular level interactions of imidazolium-based ILs in EG over the entire composition range using thermodynamic (volumetry), and spectroscopic (FTIR and 1H NMR) techniques. The FTIR spectroscopy which is very sensitive to changes in the dipole moment, or rather the polarizability, is used to determine the intermolecular interactions in solvent mixtures of ILs [38–42]. We have employed FTIR technique to monitor the shifts in vibrational frequencies in the binary mixtures of ILs at various compositions. 1H NMR spectroscopy, which is a direct molecular approach, has also been used for determining the specific interactions between ILs and EG molecules at molecular level. It is shown that ILs forms hydrogen bonds of varying strengths with EG molecules whose relative

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B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127

strength depends on the structure of ILs (length of alkyl chain or the nature of anion). In short, the aim of the present article is to shed light on bulk and molecular scale interactions between RTILs (1-butyl-3-methylimidazolium chloride [C4mim][Cl]; 1-octyl-3-methylimidazolium chloride [C8mim][Cl] or 1-butyl-3-methylimidazolium methylsulfate [C4mim][C1OSO3]) and ethylene glycol [HOCH2CH2OH] (EG) in a systematic way using thermodynamic and spectroscopic techniques. The RTILs used here are completely miscible with EG under ambient conditions, and offer a comparison of changes in molecular level interactions with the structural variation in ILs (alterations in alkyl chain length of the cation or nature of the anion).

controlled to within ±0.01 K by a built-in Peltier device that corresponds to an uncertainty in density of ±0.0002%. Measured density values are precise to 1  105 g  cm3. The overall uncertainty in comparison with literature data for the calibrating liquids and in the averaged density measurements of the binary mixtures is judged to be less than 0.02%. Experimental uncertainty in the estimated excess molar volume is approximately ±2  103 cm  mol1. The uncertainty estimates do not include the effects of certain minor impurities that may be present in the IL. Reproducibility of the results was confirmed by performing the measurements in triplicate.

2. Experimental

FTIR measurements of pure components and binary mixtures of ILs of different concentrations were carried out at room temperature using NICOLET 6700 FT-IR spectrometer. For recording spectra, a cell with BaF2 windows and a Teflon spacer was used: the optical path length was 0.02 mm. For each spectrum 132 scans were made with a selected resolution of 4 cm1.

2.3. FTIR measurements

2.1. Materials Ionic liquids (ILs) with mol fraction purity shown, 1-butyl-3-methylimidazolium chloride [C4mim][Cl] (>0.980), 1-octyl-3-methylimidazolium chloride [C8mim][Cl] (>0.980), and 1-butyl-3-methylimidazolium methylsulfate [C4mim][C1OSO3] (>0.980), were purchased from Solvent Innovation, Germany. The ILs received from the commercial source were further purified according the procedures described in our earlier paper [34]. Prior to the mixture preparation, the ILs were dried and degassed under vacuum at 70 °C for 48 h to remove moisture. The Karl– Fisher analysis of the samples indicated that the water content was reduced to less than 350  106 in each IL. Analytical grade ethylene glycol (>0.990 mol fraction purity) was obtained from Loba Chemie, Mumbai. Ethylene glycol was used after drying over the 0.4 nm molecular sieves and under vacuum at ambient conditions. Binary mixtures were prepared by mass, using an analytical balance with a precision of ±0.0001 g (Denver Instrument APX-200). The mole fraction of each mixture was obtained with an accuracy of 1  104 from the measured masses of the components. All the samples were kept in tightly sealed bottles to minimise the absorption of atmospheric moisture and CO2.

2.4. 1H NMR measurements The 1H NMR spectra of the (ILs + EG) mixtures over the whole composition range were recorded using a Bruker 500 MHz spectrometer at T = 298 K. The proton chemical shifts were referenced with respect to external standard TMS (d = 0.000  106) in C6D6 (deuterated benzene). The chemical shifts of the peaks of interest were determined using peak pick facility. 3. Results and discussion 3.1. Volumetric behaviour Experimental density at T = 298.15 K of pure ILs and ethylene glycol used in this study has been compared with the recent literature values [43–48] in table 1 along with the molecular structure of the liquids used. The densities for different binary mixtures as a function of composition at T = (298.15 to 318.15) K are presented in table 2. Experimental densities were used to estimate excess molar volume V Em using the relation

2.2. Density measurements Density of the pure liquids and their binary mixtures were measured with an Anton Paar (Model DMA 5000) vibrating-tube densimeter with a resolution of 5  106 g  cm3. The densimeter was calibrated with doubly distilled, degassed water, and with dry air at atmospheric pressure. The temperature of the apparatus was

V Em ¼ V m  V id m ¼

ðx1 M 1 þ x2 M2 Þ

q

 

x1 M 1

q1

þ

x2 M 2

q2

 ð1Þ

;

TABLE 1 Observed and literature values of density q of pure components at T = 298.15 K. Compound

q/(g  cm3)

Structure

[C4mim][Cl]

N

[C8mim][Cl]

N

Lit.

1.08201

1.080a

Cl-

1.00956

1.00882b 1.0070c

O

1.20584

1.21222d 1.2124e

1.10980

1.11002f

Cl-

N

N

[C4mim][C1OSO3] N

Obs.

N

-

O

S

OCH3

O

Ethylene glycol

HO OH

a b c d e f

Reference [43]. Reference [44]. Reference [45]. Reference [46]. Reference [47]. Reference [48].

123

B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127 TABLE 2 Experimental density q, for the various binary mixture of ILs and EG at T = (298.15 to 318.15) K.

q/(g  cm3)

q/(g  cm3)

q/(g  cm3)

T = 298.15 K

T = 308.15 K

T = 318.15 K

1.10980 1.10607 1.08593 1.06870 1.05079 1.03953 1.03040 1.02363 1.01949 1.01621 1.01392 1.01275 1.01027 1.00956

{[C8mim][Cl] + EG} 1.10270 1.09889 1.07894 1.06198 1.04434 1.03327 1.02426 1.01752 1.01341 1.01014 1.00787 1.00675 1.00434 1.00374

1.09550 1.09183 1.07221 1.05533 1.03795 1.02692 1.01791 1.01120 1.00718 1.00394 1.00174 1.00065 0.99843 0.99792

0 0.0052 0.0512 0.1106 0.2135 0.3018 0.4059 0.5066 0.6163 0.7000 0.8408 0.9244 1

1.10980 1.11066 1.11906 1.12896 1.14536 1.15655 1.16763 1.17690 1.18567 1.19166 1.19942 1.20297 1.20584

{[C8mim][C1OSO3] + EG} 1.10270 1.10350 1.11191 1.12184 1.13827 1.14950 1.16069 1.17001 1.17875 1.18471 1.19262 1.19625 1.19925

1.09550 1.09612 1.10420 1.11404 1.13075 1.14235 1.15359 1.16297 1.17177 1.17789 1.18585 1.18952 1.19270

n X

Ai ð2x1  1Þi :

A4

298.15 308.15 318.15

1.9395 2.1025 2.2285

0.3623 0.6712 0.5229

{[C4mim][Cl] + EG} 0.0268 0.9087 0.0019 0.1230

0.004 0.007 0.002

298.15 308.15 318.15

2.7895 2.8405 2.9987

1.1040 0.9851 0.7490

{[C8mim][Cl] + EG} 0.1603 0.4148 0.4150

0.004 0.006 0.006

298.15 308.15 318.15

2.0445 2.1136 2.2209

{[C8mim][C1OSO3] + EG} 1.4333 0.5375 0.4152 1.3895 0.3825 0.4555 1.4015 0.5073 0.5977

1.4515 1.3222 1.8688

0.003 0.004 0.006

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 1. Plot of V Em against mole fraction for binary mixtures of EG with [C4mim][Cl] (j); [C8mim][Cl] (d); and [C4mim][C1OSO3] (N) at T = 298.15 K. Lines are just a guide to the eye.

where Vm and V id m are the real and ideal molar volumes of the solutions; q, q1 and q2 are the density of mixture, component 1 and component 2 of the mixture; M1, M2 and x1, x2 are the molar masses and mole fractions of component 1 and component 2, respectively. The composition dependence of V Em was correlated by the conventional curve-fitting strategy for the excess molar thermodynamic properties of binary mixtures by using the Redlich–Kister type polynomial equation [49]

FðxÞ ¼ x1 x2

A2

-1

0 0.0077 0.0589 0.1190 0.2112 0.2985 0.4021 0.5118 0.6041 0.6999 0.7837 0.8328 0.9583 1

1.09550 1.09397 1.09221 1.09029 1.08561 1.07922 1.07558 1.07298 1.07200 1.07111 1.07076 1.07075 1.07088 1.07104

A1

3

1.10980 1.10821 1.10610 1.10402 1.09895 1.09239 1.08858 1.08581 1.08446 1.08324 1.08247 1.08235 1.08227 1.08201

A0

E

0 0.0132 0.0294 0.0484 0.1028 0.2101 0.3094 0.4202 0.4947 0.5992 0.7117 0.8143 0.9126 1

{[C4mim][Cl] + EG} 1.10270 1.10108 1.09921 1.09719 1.09229 1.08544 1.08193 1.07923 1.07812 1.07718 1.07669 1.07651 1.07649 1.07651

A3

r/(cm3  mol1)

T/K

Vm / (cm .mol )

x1

TABLE 3 Parameters of equation (2) with standard deviations r of V Em (cm3  mol1) at different temperatures.

ð2Þ

i¼0

The coefficients Ai of equation (2) were calculated by the method of least squares. The values of coefficients Ai along with the standard deviations r are reported in table 3. Comparison of V Em for the different binary mixtures of [C4mim][Cl], [C8mim][Cl] and [C4mim][C1OSO3] with EG as a function of IL mole fraction at T = 298.15 K is shown in figure 1. The V Em is positive for all the mixtures and increases in the order: [C4mim][Cl] < [C4mim][C1OSO3] < [C8mim][Cl]. The positive V Em in these mixtures shows the dominance of dispersion forces over the specific interactions, which increases with the increase of molecular size of the IL in the mixture. The maximum

in V Em shifts from x1  0.34 for [C4mim][C1OSO3] to x1  0.42 for [C4mim][Cl] and [C8mim][Cl]. When the volumetric behaviour of binary mixtures of ILs and EG is compared with respect to nature of anion and alkyl chain length of the cation, it is found that for a common anion [Cl], V Em increase with the increase of alkyl chain length of the cation, whereas for the ILs with a common cation [C4mim], the V Em increase in the order: [Cl] < [C1OSO3]. However, in IL rich region (x1 > 0.50), V Em becomes less positive in the mixture [C4mim][C1OSO3] as compared to the mixture [C4mim][Cl], indicating the decreased importance of the dispersion type interactions, that is, interactions between unlike molecules increased with increase of IL concentration in the former system. Temperature dependence of V Em for the investigated systems is shown in figures 2 to 4. A comparison of data at different temperatures reveals that the temperature coefficient ð@V Em =@TÞp is increasing in values for all the binary mixtures with increasing temperature. The increase in coefficient ð@V Em =@TÞp for various binary mixtures varies in the order: [C4mim][Cl] + EG > [C4mim][C1OSO3] + EG > [C8mim][Cl] + EG. Further, the apparent molar volumes of the various ILs in the EG were derived using the equations

V /;1 ¼

M 2 ð1  x1 Þðq2  qÞ M1 þ : x1 qq2 q

ð3Þ

124

B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127

Figure S1(a) to (c) (supporting information) show the comparison of variation of apparent molar volumes as a function of composition for [C4mim][Cl], [C8mim][Cl] and [C4mim][C1OSO3], respectively. Apparent molar volumes of ILs increase with the increase of EG concentration and also with the temperature in all the binary mixtures.

0.6

3

0.4 0.3

3.2. FTIR studies

E

-1

Vm / (cm .mol )

0.5

0.2

Figure 5(a) shows the enlarged FTIR spectra in the range 3000 to 3650 cm1 recorded for EG-[C4mim][C1OSO3] mixtures as representative. In general, the absorption intensity of the broad OH stretching band of EG decreases with increasing mole fraction of the ILs for all binary mixtures investigated, thus reflecting the quantitative nature of spontaneous IR absorption. Contrary to the binary mixtures of {EG + [C4mim][BF4]} and {EG + [C8mim][BF4]}, no band splitting has been observed at higher mole fractions of the ILs investigated in the present study[36]. The spectra of EG  [C4mim][Cl], EG  [C8mim][Cl], and EG  [C4mim][C1OSO3] are dominated by only a strong broad band over the whole composition range similar to that of {EG + [C8mim][C1OSO3]} binary mixtures [36]. A significant blue shifting in the band position of OH stretch band of EG is observed with the addition of [C4mim][C1OSO3] whereas such shifting is absent for the [C4mim][Cl] and [C8mim][Cl] as can be seen from figure 5(b). The magnitude of peak shifting is indicative of changes in intermolecular hydrogen bonding between EG and ILs depending on the nature of cation or anion. Presumably, the frequency changes are induced by the formation of hydrogen bonds between hydroxyl group of EG and constituent ions of the ILs and the bearing of inter- and intramolecular hydrogen bonding in EG. Figure 6(a) and (b) shows the variation in stretching frequency for the C(2)–H and C(4,5)–H stretch of the imidazolium cation of IL. The characteristic individual peaks around 3120 cm1 and 3160 cm1 are assigned to C(2)–H and C(4,5)–H stretching bands, respectively [36,41,50,51]. Figure 6a shows the changes in the C(2)–H stretching frequency over the whole composition range for all the binary mixtures investigated. With the increase in concentration of EG in binary mixtures, the C(2)–H frequency is blue shifted for all the systems. A gradual blue shifting is observed in the case of {EG + [C4mim][Cl]} mixtures whereas in the case of {EG + [C8mim][Cl]} and {EG + [C4mim][C1OSO3]} mixtures, no blue shifting is observed till 1 > x1 > 0.60, and then a sharp blue shift in the EG rich region is observed between 0.60 > x1 > 0. A blue shifting of 22 cm1 and 20 cm1 is observed for EG  [C4mim][Cl] and EG  [C8mim][Cl], respectively whereas it is 5 cm1 in case of EG  [C4mim][C1OSO3]. Blue shifting is the consequence of an increase of force constant due to strengthening the C–H bond due to weak electrostatic interaction between the C(2)–H of IL cation and the lone pair electrons of the oxygen atom in EG [52]. On comparing the data with our previous results, it is found that the interaction between C(2)–H of IL cation and the lone pair electrons of the oxygen atom in EG for various ILs varies in the order: [C4mim][Cl] > [C8mim][Cl] > [C4mim][C1OSO3]  [C4mim][C8OSO3] > [C4mim][BF4]  [C8mim][BF4] indicting the interaction between C(2)–H and various anions as: [Cl] < [C1OSO3]  [C8OSO3] < [BF4]. It is found that with the increase in alkyl chain length of the cation, the interactions between C(2)–H and EG decreases. The behaviour of the C(4,5)–H stretch as a function of IL mole fraction is shown in figure 6b. Similar to the C(2)–H stretch, a blue shift (10 cm1 to 12 cm1) with the addition of EG to ILs is observed. The small change in C(4,5)–H stretch frequency by comparison to C(2)–H stretch indicates weaker interactions between C(4,5)–H and EG. Blue shifting for the C(4,5)–H stretch follows the similar pattern as followed by C(2)–H stretch with addition of EG.

0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 2. Plot of V Em against mole fraction for the binary mixture {[C4mim][Cl] + EG}: (j) at T = 298.15 K; (d) at 308.15 K; and (N) at 318.15 K.

0.8 0.7

E

3

-1

Vm / (cm .mol )

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 3. Plot of V Em against mole fraction for the binary mixture {[C8mim][Cl] + EG}: (j) at T = 298.15 K; (d) at 308.15 K; and (N) at 318.15 K.

0.6

0.4

3

-1

Vm / (cm .mol )

0.5

E

0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 4. Plot of V Em against mole fraction for the binary mixture {[C4mim][C1OSO3] + EG}: (j) at T = 298.15 K; (d) at 308.15 K; and (N) at 318.15 K.

125

B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127

3460 1.0

Increasing IL concentration

a

-1

0.8

Intensity / (a.u)

b

3440

wavenumber / (cm )

0.9

0.7 0.6 0.5 0.4 0.3

3420 3400 3380 3360

0.2 3340 3200 3400 -1 wavenumber / (cm )

0.0

3600

0.2

0.4

0.6

0.8

1.0

x1

FIGURE 5. Normalized FTIR spectra of {EG + [C4mim][C1OSO3]} binary mixture over the range 3000 to 3700 cm1 (x1 = 0.0, 0.11, 0.21, 0.30, 0.41, 0.51, 0.61, 0.70, 0.84, and 0.93). (b) Frequency shift for OH stretch as a function of ILs concentration in (j) [C4mim][Cl]; (d) [C8mim][Cl]; and (N) [C4mim][C1OSO3].

3125

3162

3111 3110

3160

3109

-1

-1

3115

3112

3108 0 .2

3110

0 .4

0 .6

0 .8

1.0

x1

3105 3100 3095

a

3090 3085

wavenumber / (cm)

-1

wavenumber / (cm )

3120

wavenumber / (cm )

3164

3113

3158 3156 3154 3152

3148 3146

0.2

b

3150

0.4

0.6

0.8

1.0

x1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

x1

FIGURE 6. Peak shift for absorption frequency of (a) C(2)–H stretch; (b) C(4,5)–H for various (EG + IL) binary mixtures as a function of IL concentration: (j) [C4mim][Cl]; (d) [C8mim][Cl]; and (N) [C4mim][C1OSO3].

3.3. 1H NMR studies When an organic solvent or water is mixed with an IL, competition between various moeities present in the mixture such as cation, anion and solvent molecules for hydrogen and ionic bonding interactions is expected. In 1-alkyl-3-methyimidazolium based ILs, multiple hydrogen bonding between protons of the cationic head group with even weak hydrogen bond acceptor anions prevails along with other ionic and van der Waals interactions. Addition of hydrogen bond donors such as EG can bring about significant changes in the magnitude of hydrogen bonding interactions through formation of new hydrogen bonds or breaking/weakening of the existing hydrogen bonds in the pure ILs [36]. The 1H NMR, which is a direct molecular probe, has been used to elucidate the change in electronic environment of various protons of different ILs and EG in their binary mixtures. The 1H NMR chemical shift (d), has been measured in the binary mixtures over the whole composition range. Figure 7(a) and (b) shows the variation in observed chemical shift for OH (dOH) and CH2 (dCH2 ) of EG in the binary mixtures as a function of mole fraction of ILs. An up field shift in dOH has been observed for the {EG + [C4mim][C1OSO3]} system with the increase in IL concentration whereas initially an up field shift to x1 = 0.35 followed by a down field shift is observed for {EG + [C4mim][Cl]} and {EG + [C8mim][Cl]} systems. An up field

shift is indicative of an increase in electron density around the 1 H nuclei of OH of EG which is due to (i) breaking the intermolecular hydrogen bonding in EG (ii) transfer of charge from the nitrogen atoms of the imidazolium cation to the hydroxyl proton which is comparatively greater in the case of [C4mim][C1OSO3] and (iii) less hydrogen bonding type interactions between the hydroxyl proton of EG and IL anions except that of [C1OSO3] where such interactions are comparatively higher. In the case of {EG + [C4mim][Cl]} and {EG + [C8mim][Cl]} binary mixtures, a down field shift has been observed at higher IL concentration which may be due to decreased IL–EG interactions at the cost of increased cation–anion interactions and gradually leads to the hydrogen bonded environment of EG in binary mixtures similar to that of pure EG. For all the binary mixtures investigated, dCH2 goes up field. A comparatively large change in dOH (ca. 5.2 to 4.1)  106 indicates the greater polarisation in the OH bond than in that of the CH bond in the EG-[C4mim][C1OSO3] system. A slight monotonic up field shift was observed in dCH2 (ca. 3.5 to 2.9)  106 in all the mixtures indicating a small enhancement of electron density around CH2 in the EG. The dCH2 for various ILs in their mixtures with EG varies in the order: [C4mim][C1OSO3] > [C8mim][Cl] > [C4mim][Cl]. Estimates of deviations from ideality in the 1H NMR chemical shift 1 (Dd) from the additivity rule (Dd ¼ d  x1 d1 1  x2 d2 ) have been made and provide the extent of solute–solvent interactions (see

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B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127

3.6

a

3.5

5.0

3.4

4.8

3.3

δobsd 10 6

δobsd 10 6

5.2

4.6 4.4

b

3.2 3.1 3.0

4.2

2.9

4.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

x1

0.6

0.8

1.0

x1

FIGURE 7. Variation of dobsd for different protons of EG (a) OH and (b) CH2–CH2 in (EG + IL) mixtures as a function of IL concentration: (j) [C4mim][Cl]; (d) [C8mim][Cl]; and (N) [C4mim][C1OSO3].

supporting information, figure S2(a) and (b)) [34,36]. The Dd for OH of EG was found to be negative for all the binary systems investigated over the whole composition range. The position of minima in Dd, which indicates the composition of maximum interaction between components of the binary system for different ILs, varies in the order: [C4mim][Cl] (x1 = 0.31) > [C8mim][Cl] (x1 = 0.35) > [C4mim][C1OSO3] (x1 = 0.40). The minima for the Dd for CH of EG varies in the order: [C4mim][Cl] (x1 = 0.30) > [C4mim][C1OSO3] (x1 = 0.32) > [C8mim][Cl] (x1 = 0.40). To gather information about the changes in the electronic environment of different protons of various ILs as a consequence of the EG addition, we monitored the change in 1H chemical shift for various protons of IL (ring protons, C(2)H (d2H), C(4,5)H (d4,5H), and terminal alkyl chain proton, C(t)H(dtH) of alkyl imidazolium cations) in their binary mixtures with EG [figure 8 (a), and (b)]. As can be seen from figure 8a, with the addition of EG to the ILs ([C4mim][Cl] or [C8mim][Cl]), d2H shifts downfield. A similar pattern was observed in d4,5H for these systems (supporting information figure S3). Similar to that of the binary mixture {EG + [C4mim][C8OSO3]}, addition of EG to [C4mim][C1OSO3] brought both the d2H and d4,5H up field till 0 > x1 > 0.20 and then a shift down field began with the further addition of EG. The chemical shift for the terminal alkyl chain protons, C(t)H(dtH), pro-

0.9

10.2

a

δ obsd / (ppm)

8.86

9.9

b

0.8

8.84

0.7

8.82

8.80 0.0

0.6 0.2

0.4

9.6

0.6

0.8

1.0

x1

δobsd 10 6

δobsd 10 6

gressed continuously up field with the addition of IL in all the investigated ILs over the whole composition range as can be seen in figure 8b. For the (EG + IL) mixtures, the C(t)H(dtH) follows the order: [C8mim][Cl] > [C4mim][Cl] > [C4mim][C1OSO3]. Again, the estimates of deviations from ideality in the 1H NMR chemical shift Dd provided important information on relative strengths of chemical interactions between various protons of ILs and EG. The Dd is negative for all the imidazolium protons in all the mixtures with greatest magnitude for C(2)H protons and least magnitude for the terminal protons of imidazolium alkyl chain similar to our earlier findings [supporting information figure S4(a) to (c)] [36]. When different ILs are compared, we found the magnitude of interactions between C(2)H or C(4,5)H protons and EG in the order: [C4mim][Cl] > [C8mim][Cl] > [C4mim][C1OSO3], whereas the order of magnitude of interactions between terminal protons of imidazolium alkyl chain and EG was: [C4mim][Cl] < [C4mim][C1OSO3]  [C8mim][Cl]. On comparing the NMR results for various binary mixtures, it is found that the extent of interaction of various protons of different ILs with EG follows the order: C(2)H > C(4)H > C(5)H > C(t)H(dtH) for EG  [C4mim][Cl], for EG  [C8mim][Cl], it follows: C(5)H > C(4)H > C(t)H(dtH) > C(2)H and for EG  [C4mim][C1OSO3] it is as: C(t)H(dtH) > C(2)H > C(5)H > C(4)H. In the system (EG + [C4mim][C1OSO3]),

9.3

0.5 0.4 0.3 0.2

9.0

0.1 8.7 0.0

0.2

0.4

0.6 x1

0.8

1.0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

x1

FIGURE 8. Variation of dobsd for different protons (a) C(2)H; (b) terminal methyl group of imidazolium cation in (EG + IL) mixtures as a function of IL concentration: (j) [C4mim][Cl]; (d) [C8mim][Cl]; and (N) [C4mim][C1OSO3].

B. Kumar et al. / J. Chem. Thermodynamics 44 (2012) 121–127

the maximum interaction between C(t)H(dtH) and EG is indicative of strong interaction between aromatic ring protons and the [C1OSO3] anion which weakens the interaction between ring protons and EG. The findings from the molecular scale studies conducted using NMR technique are contrary to those from thermodynamic studies probed volumetrically. Unlike Dd, the excess volume (V Em ) was positive with a large magnitude in all the mixtures and followed the order: [C4mim][Cl] < [C4mim][C1OSO3] < [C8mim][Cl] whereas different pattern are observed for different EG or IL protons regarding the interactions between EG and ILs. Various factors such as solvation, polarity, hydrogen bonding capacity, non-bonded interactions, etc. affect the nature and extent of specific interactions between unlike molecules whereas V Em includes all the effects i.e. physical, geometrical and chemical interactions present in mixture. The opposite trend with a large difference in the magnitude clearly indicates the dominance of packing efficiency depending on the differences in shape and size of the unlike molecules as well as dispersion type interactions over the specific interactions in the studied mixtures. The contradiction in the position of minima in V Em and Dd for various protons of different ILs can be explained on the basis of the large dominance of physical interactions in these mixtures over the specific interactions which hardly affect the minima. 4. Conclusions The bulk scale mixing behaviour of the imidazolium based-ILs in EG was investigated volumetrically. Excess molar volumes are positive over the whole composition range and increases with the increase in temperature for all the mixtures investigated. The values of V Em in these mixtures become more positive with the increase of alkyl chain length of the imidazolium cation or by replacing the [Cl] by the anion [C1OSO3] in a similar alkyl chain length ionic liquid. The FTIR and 1H NMR spectroscopic techniques provided information about the molecular scale interactions prevailing in these systems. Non-bonding oxygen electron and hydroxyl protons of EG were found to interact strongly with the aromatic ring protons as compared to alkyl chain protons. Various ring protons show different interactional preferences towards EG depending upon the nature of the cation or anion. A comparative analysis of thermodynamic and spectroscopic results shows that the multiple hydrogen bonding interactions occurring in the systems investigated at the microscopic level are not reflected in the mixing macroscopic behaviour. Acknowledgements Financial support for this project (Grant No. SR/S1/PC-55/2008) by the Government of India through the Department of Science and Technology (DST), New Delhi is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2011.08.008. References [1] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, second ed., Wiley VCH, Weinheim, 2008. [2] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123–150.

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JCT-11-237