(308, 313, and 318) K at 0.1 MPa - ACS Publications - American

Dec 7, 2012 - Department of Chemistry & Biochemistry, Presidency University, 86/1 College Street, Kolkata 700 073, India. J. Chem. Eng. Data , 2013, 5...
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Electrical Conductances of 1‑Butyl-3-propylimidazolium Bromide and 1‑Butyl-3-propylbenzimidazolium Bromide in Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPa Sumanta Gupta, Amritendu Chatterjee, Sajal Das, and Basudeb Basu Department of Chemistry, North Bengal University, Darjeeling 734013, India

Bijan Das*,† Department of Chemistry & Biochemistry, Presidency University, 86/1 College Street, Kolkata 700 073, India ABSTRACT: Two ionic liquids (ILs), 1-butyl-3-propyl imidazolium bromide ([BPim][Br]) and 1-butyl-3-propyl benzimidazolium bromide ([BPbim][Br]), have been synthesized from their appropriate imidazole and benzimidazole precursors and were characterized by NMR spectroscopic technique. Their electrical conductances have been measured as a function of their concentrations in water, methanol, and acetonitrile at three different temperatures. The limiting molar conductances (Λ0), the association constants (KA), and the values of the association diameters (R) of these ILs have been obtained by analysis of the conductance data using the Fuoss conductance equation. These ILs have been found to remain unassociated in water and methanol, whereas these exhibit slight ionic association in acetonitrile within the investigated temperature range. The temperature elevation causes an increase in their limiting molar conductances in the three solvents under investigation. The IL cations [BPim]+ and [BPbim]+ exist as unsolvated species in aqueous solutions, whereas substantial solvation was noticed for the [BPim]+ ion in methanol and acetonitrile solutions.

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INTRODUCTION

EXPERIMENTAL SECTION Chemicals. Acetonitrile (purity: 99 %) was purchased from E. Merck, India; it was purified by distillation with P2O5 followed by redistillation over CaH2. Methanol (purity: 99.9 %) was procured from Acros Organics and was distilled three times. Deionized triply distilled water was used for the preparation of solutions; its specific conductivity was of the order of 0.1 mS·m−1 at 308 K. The densities (ρ0) and the viscosity coefficients (η0) of the solvents thus purified are shown in Table 1, and these properties agree well with those reported in the literature.20−24 The relative permittivities (ε) of these solvents were obtained from the literature,20−23 and these are also listed in Table 1. The relative permittivity of methanol at 313 K is not available in the literature, and this was obtained from an interpolation of the available experimental relative permittivity values in the temperature range (278 to 328) K.21 1-Bromopropane (purity: 98.5 %), 1-bromobutane (purity: 98 %), and benzimidazole (purity: 99.5 %) were purchased from Loba Chemie, India, and imidazole (purity: 99 %) was purchased from SRL, India; these were used without further purification.

Ionic liquids (ILs) constitute a group of molten organic electrolytes whose chemical and physical properties could be conveniently engineered by judicious choice of their anions, cations, and the substituents. They possess specific properties, for example, negligible vapor pressures, noninflammability, good thermal stability under ordinary conditions, and outstanding catalytic properties, and above all they offer excellent solubility for inorganic and organic compounds.1−4 Recent years have, therefore, witnessed an upsurge of interest in ILs in the field of scientific research opened up by the possibility of their applications in novel eco-friendly and benign industrial processes.5−11 Plenty of physicochemical investigations have, so far, been performed on pure ILs as well as on their mixtures with molecular solvents.12−15 However, only in a limited number of cases, the infinite dilution molar properties16−19 were obtained despite their importance in understanding the ion−ion and ion−solvent interactions and the possibility of prediction of ILs in specific applications such as IL-based chemical reactions. This article reports the electrical conductances measured for 1-butyl-3-propyl imidazolium bromide ([BPim][Br]) and 1-butyl3-propyl benzimidazolium bromide ([BPbim][Br]) dissolved in water, methanol, and acetonitrile at (308, 313, and 318) K at 0.1 MPa as a function of concentration of these ILs with a view to unravel their association and solvation behavior. © XXXX American Chemical Society

Received: December 20, 2011 Accepted: November 29, 2012

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method to that used for synthesizing IL 1, with imidazole (instead of benzimiazole) as the starting material. This was characterized by NMR spectroscopy [1H NMR (300 M Hz, CDCl3) δ (ppm): 10.30 (s, 1H), 7.64 (s, 1H), 7.61 (s, 1H), 4.40−4.33 (m, 4H), 2.02−1.87 (m, 4H), 1.42−1.32 (m, 2H), 1.01−0.92 (m, 6H); 13C NMR (75 M Hz, CDCl3) δ (ppm): 137.27, 123.38, 123.26, 52.09, 50.47, 32.94, 24.46, 20.16, 14.23, 11.46]. The structure of this ionic liquid is shown in Figure 1. Electrical Conductance Measurements. Electrical conductances were measured on a calibrated Orion 3-Star conductivity meter at 2 kHz using a cell (dip-type) having a cell constant of 115 m−1. Uncertainties of the measurements were always within 0.01 %. The conductivity cell was calibrated following the methods described in the literature27,28 by using KCl (aq) solution. All measurements were carried out in a water bath controlled to ± 0.05 K. The necessary correction for the solvent specific conductance was done. Details of the experimental method are available in the literature.29 The IL solutions were prepared on molality basis, and the molalities of the solutions were converted into molarities by using their densities. The reproducibility of the experimental data was checked by repeating the measurements with different stock solutions. Density Measurements. Density measurements were carried out on an Anton Paar DMA-4500 M digital precision densimeter. The precision of the density measurements was 3·10−5 g·cm−3. Calibration of the densimeter was done at each temperature using dry air under ambient pressure and deionized triply distilled water. Viscosity Measurements. The kinematic viscosities (ν) were determined with an Ubbelohde viscosimeter. The viscosimeter was kept in a water bath maintained within ± 0.05 K of the experimental temperatures. The absolute viscosities (η) have then been calculated from the kinematic viscosity values by using the density values. In each case, an average of triplicate measurements was taken into account.

Table 1. Physical Properties of Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPaa ρ0/g·cm−3 T/K

this work

308 313 318

0.99407 0.99221 0.99024

308 313 318

0.77718 0.77250 0.76770

308

0.76564

313 318

0.75970 0.75501

η0/mPa·s lit.

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Water 0.9940622 0.7194 0.9922422 0.6533 0.9902422 0.5966 Methanol 0.7772023 0.4747 0.4440 0.7677023 0.4185 Acetonitrile 0.7658124 0.3125 0.766225 0.7656026 0.3042 0.7527124 0.2903 0.755925

lit.

ε

0.719422 0.653122 0.596322

74.8222 73.1522 71.5122

0.474223

30.7423 29.81b 28.9223

0.417423 0.31424

24

0.293 0.28925

34.124 34.5425 33.8525 33.1225

a The uncertainty u is u(T) = 0.05 K. Combined expanded uncertainties are Uc(ρ0) = 0.00003 g·cm−3 and Uc(η0) = 0.005 mPa·s (level of confidence = 0.95). bObtained from interpolation of the literature values from ref 21 (please see text).

Synthesis of 1-Butyl-3-propylbenzimidazolium Bromide.25 To a round-bottomed flask containing benzimidazole (5.9 g, 50 mmol) in acetonitrile (30 mL) were added KOH (5.6 g, 100 mmol) and 1-bromobutane (7 g, 51 mmol). This mixture, after refluxing for about 4 h, was cooled down to ambient temperature. The solvent was then evaporated under vacuum; 30 mL of dichloromethane was added to it, and potassium bromide was removed by washing the material with water using a separatory funnel. The nonaqueous layer was dehydrated with Na2SO4, which on evaporation left behind the crude product (1-butyl-benzimidazole). To the crude material taken in a dry flask containing dry ethanol (20 mL) was added 1.1 equivalent amount of propyl bromide. The content of the flask was then refluxed for 24 h under nitrogen atmosphere, and after cooling down to ambient temperature, evaporation of the solvent was done under vacuum. The crude material, 1-butyl-3propyl benzimidazolium bromide, was finally recrystallized from ethanol in the form of a white solid (1). This was characterized by NMR spectroscopy [1H NMR (500 M Hz, CDCl3) δ (ppm): 11.32 (s, 1H), 7.66−7.19 (m, 4H), 4.56 (t, 2H, J = 12.4), 4.53 (t, 2H, J = 12.3), 2.06−1.91 (m, 4H), 1.41−1.34 (m, 2H), 0.99−0.88 (m, 6H); 13C NMR (75 M Hz, CDCl3) δ (ppm): 143.74, 132.24, 132.20, 128.05, 114.08, 114.04, 49.90, 48.35, 32.26, 23.75, 20.68, 14.39, 11.90]. The structure of this ionic liquid is depicted in Figure 1.

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RESULTS AND DISCUSSION Table 2 lists the molar conductivity values obtained experimentally as functions of the molality (m) of the IL solutions at (308, 313, and 318) K at 0.1 MPa. These were analyzed by using the Fuoss conductance equation.30,31 A given set of conductance data (cj, Λj; j = 1, ..., n) can provide three parameters, namely, the limiting molar conductivity (Λ0), the equilibrium constant for ionic association (KA), and the distance of the closest approach of ions (R). For a given data set these three parameters were obtained by an iterative solution of the following set of equations: Λ = p[Λ0(1 + RX + EL]

(1)

p = 1 − α(1 − γ )

(2)

KA = (1 − γ )/cγ 2f 2

(3)

−ln f =

β=

Figure 1. Structures of the ionic liquids -1-butyl-3-propylbenzimidazolium bromide (1) and 1-butyl-3-propylimidazolium bromide (2).

βk 2(1 + kR )

e2 εkBT

(4)

(5)

KA = KR (1 + KS)

(6)

where RX is the relaxation field effect, EL is the electrophoretic countercurrent, γ is the fraction of unpaired ions, α is the

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Synthesis of 1-Butyl-3-propylimidazolium Bromide. Synthesis of this IL (2) was accomplished by following a similar

B

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Table 2. Molalilty Dependence of the Molar Conductances of 1-Butyl-3-propylimidazolium Bromide and 1-Butyl-3propylbenzimidazolium Bromide in Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPaa 104m/mol·kg−1

T = 308 K

T = 313 K

T = 318 K

Λ/S·cm2·mol−1

Λ/S·cm2·mol−1

Λ/S·cm2·mol−1

104m/mol·kg−1

1-Butyl-3-propylimidazolium Bromide in Water 99.60 108.00 117.70 100.84 109.40 118.98 103.04 111.65 120.94 106.50 115.18 124.01 109.50 118.24 126.69 112.12 120.91 129.05 114.41 123.20 131.16 116.42 125.31 133.05 118.18 127.12 134.74 119.72 128.72 136.25 121.06 130.13 137.60 122.24 131.36 138.70 1-Butyl-3-propylimidazolium Bromide in Methanol 1001.93 52.38 55.40 59.23 901.74 53.04 56.11 60.16 766.46 53.72 57.03 61.23 574.86 54.68 58.32 62.57 431.14 55.61 59.36 63.71 323.38 56.68 60.52 64.91 242.51 57.92 61.85 66.27 181.88 59.32 63.33 67.81 136.42 60.83 64.93 69.46 102.31 62.39 66.58 71.19 76.73 63.95 68.15 72.90 57.55 65.46 74.80 1-Butyl-3-propylimidazolium Bromide in Acetonitrile 998.72 59.20 61.99 64.35 898.24 61.24 63.93 66.48 762.82 64.33 67.01 69.68 656.05 67.07 69.56 72.55 571.39 69.44 72.22 75.23 487.52 74.67 78.07 428.13 74.24 77.24 80.54 320.88 78.83 82.13 85.95 1-Butyl-3-propylimidazolium Bromide in Acetonitrile 274.96 81.23 240.52 83.25 86.91 91.14 180.33 87.51 91.55 96.27 135.20 91.60 96.04 101.28 101.38 95.50 100.75 106.30 1-Butyl-3-propylbenzimidazolium Bromide in Water 1330.01 95.87 102.58 1200.00 97.37 103.93

T = 313 K

T = 318 K

Λ/S·cm2·mol−1

Λ/S·cm2·mol−1

1-Butyl-3-propylbenzimidazolium Bromide in Water 1030.11 89.21 99.31 106.06 924.81 90.76 830.46 92.06 788.15 92.62 101.90 108.92 588.31 94.90 104.02 111.25 439.66 96.77 105.66 113.27 328.87 98.07 107.06 114.95 246.17 99.14 108.29 116.56 184.36 100.06 109.39 117.88 138.11 100.90 110.38 119.08 103.49 101.65 111.26 120.20 77.57 102.35 1-Butyl-3-propylbenzimidazolium Bromide in Methanol 1012.50 51.41 55.43 61.11 910.23 53.07 57.28 62.91 772.52 54.88 59.29 64.81 578.14 56.82 61.37 66.76 432.94 58.04 62.63 67.94 324.32 59.06 63.67 68.96 243.02 60.09 64.72 70.02 182.15 61.20 65.87 71.23 136.53 62.41 67.12 72.57 102.37 63.80 68.46 74.00 76.75 64.98 69.83 75.49 57.55 66.27 71.32 77.17 1-Butyl-3-propylbenzimidazolium Bromide in Acetonitrile 999.54 70.05 73.20 76.63 898.29 72.99 76.19 79.74 762.02 77.40 80.55 84.43 1-Butyl-3-propylbenzimidazolium Bromide in Acetonitrile 569.99 84.76 88.04 92.53 426.63 91.79 95.37 100.44 319.48 98.55 102.39 108.20 239.36 105.06 109.39 115.81 179.35 111.34 116.13 123.22 134.48 117.30 122.62 130.37 100.77 123.03 128.73 137.18 75.55 128.28 134.66 143.35 56.65 133.04 140.64

1000.40 898.74 762.10 569.63 426.16 319.02 238.91 178.98 134.14 100.54 75.36 56.51

a

T = 308 K Λ/S·cm2·mol−1

Standard uncertainties u are u(T) = 0.05 K and u(m) = 0.00002 mol·kg−1, and the combined expanded uncertainty Uc is Uc(Λ) = 0.03 S·cm2·mol−1 (level of confidence 0.95).

Following Fuoss,30 calculations were performed over a range of R values to find the Λ0 and KA values minimizing the standard deviations, σ,

fraction of contact pairs, KA is the overall pairing constant evaluated from the association constants of contact pairs (KS) of solvent-separated pairs (KR), ε is the relative permittivity of the solvent, e is the electronic charge, kB is the Boltzmann constant, k−1 is the radius of the ion atmosphere, c is the molarity of the solution, f is the activity coefficient, T is the temperature in absolute scale, and β is twice the Bjerrum distance. The initial values of Λ0 for the iteration procedure have been obtained by extrapolating the data according to Shedlovsky and Fuoss.32

σ = [∑ [Λj(calcd) − Λj(obsd)]2 /(n − 2)]1/2

(7)

of the fits; the value of R which fits the experimental data best was, therefore, identified as the minimum of a σ vs R profile. The Λ0, KA, and R values thus derived from the conductance data are recorded in Table 3. C

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Table 3. Derived Conductivity Parameters for 1-Butyl-3-propylimidazolium Bromide and 1-Butyl-3-propylbenzimidazolium Bromide in Water, Methanol, and Acetonitrile at (308, 313, and 318) K at 0.1 MPa IL

T/K

Λ0/S·cm2·mol−1

KA/dm3·mol−1

R/Å

σ%

2.87 2.57 1.73 0.74 1.16 1.50

± ± ± ± ± ±

0.01 0.02 0.01 0.01 0.01 0.01

8.1 8.0 7.7 12.6 11.4 10.1

0.05 0.06 0.03 0.04 0.02 0.03

2.31 3.06 3.09 0.93 0.58 0.18

± ± ± ± ± ±

0.02 0.05 0.06 0.05 0.04 0.06

4.6 4.8 4.9 5.8 5.8 5.6

0.02 0.04 0.05 0.04 0.04 0.06

32.83 37.61 42.97 49.25 54.76 60.33

± ± ± ± ± ±

0.09 0.12 0.08 0.09 0.09 0.01

7.7 7.2 7.2 10.0 9.7 10.0

0.02 0.02 0.02 0.02 0.02 0.01

Water [BPim][Br]

[BPbim][Br]

[BPim][Br]

[BPbim][Br]

[BPim][Br]

[BPbim][Br]

308 313 318 308 313 318

130.22 139.89 147.50 108.83 118.87 129.03

± ± ± ± ± ±

308 313 318 308 313 318

79.35 84.75 90.61 79.21 85.09 92.03

± ± ± ± ± ±

308 313 318 308 313 318

128.10 136.41 145.89 166.92 177.04 190.64

± ± ± ± ± ±

0.03 0.05 0.02 0.03 0.02 0.03 Methanol 0.01 0.03 0.04 0.02 0.02 0.04 Acetonitrile 0.04 0.08 0.05 0.05 0.05 0.01

The representative plots (Figures 2 to 4) show the dependence of the experimental Λ on the IL molality. Also included in these figures are the calculated profiles in accordance with eqs 1 to 6.

Figure 3. Variation of molar conductivity of 1-butyl-3-propylimidazolium bromide as a function of molality in methanol at different temperatures at 0.1 MPa. The symbols represent the experimental points, whereas the lines represent the calculations according to eqs 1 through 6. ▽, 318 K; □, 313 K; ○, 308 K.

Figure 2. Variation of molar conductivity of 1-butyl-3-propylimidazolium bromide as a function of molality in water at different temperatures at 0.1 MPa. The symbols represent the experimental points, whereas the lines represent the calculations according to eqs 1 through 6. ▽, 318 K; □, 313 K; ○, 308 K.

also change in the same order as the limiting equivalent conductivities of the ILs as a whole. It is now well-established33 that some ions exist as bare species (unsolvated) in solution with sizes being equal to their crystallographic sizes, whereas others are solvated with a concomitant increase in their actual sizes in solution. The order of variation of the limiting ionic equivalent conductivities should, thus, point out the relative ionic solvodynamic dimensions and hence provide information on their solvation behavior in solutions. The ionic mobilities of the IL cations in water decrease in the order: [BPim]+ > [BPbim]+. This indicates that the larger IL cation is less mobile than the smaller one in aqueous solution. This is possible only if the ions remain unsolvated in water; had these IL cations been hydrated, the smaller ion [BPim]+ with a higher charge density on its surface would have been more hydrated compared to the bigger [BPbim]+ ion (with smaller surface charge density), and hence their ionic mobilities should

The Λ0 values for the two ILs investigated are found to increase with temperature, and these could be described by the following linear relationship: Λ0 = a0 + a1(308.15 − T )

(8)

and the values of the coefficients of eq 8, together with the correlation coefficients (r), have been listed in Table 4. An inspection of the variation of the limiting ionic equivalent conductivities of the IL cations in the investigated solvents could provide an important clue to their solvation in these media. Since the anion is common to both the ILs, the limiting ionic equivalent conductivities of the IL cations, according to the Kohlrausch’s law of independent migration of ions, would D

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other hand, the fractions of unpaired ions are found to be in the range of 0.47−0.86 within the investigated concentration and temperature ranges; this observation demonstrates that a substantial proportion of these ILs are associated and exist as ion pairs in acetonitrile. Further, the extent of ion association in acetonitrile increases as the temperature is elevated (cf. Table 3). Now, an increase in temperature causes ion desolvation as well as an attenuation of the product of the relative permittivity and the absolute temperature (εT) for this medium. Desolvation promotes ion association, and a decrease in the εT value also enhances the value of the association constant according to eqs 3 to 5. Hence both these effects might have been contributed to the increase in the KA values with temperature in acetonitrile. The existence of free ions for these two ILs in water might be attributed to the high values of the relative permittivity of the medium (ranging from 71.51 to 74.82 over the investigated range of temperature), thus promoting the dissociation of these electrolytes. Since the relative permittivities of methanol and acetonitrile are not very much different, one expects close correspondence in the KA values of these ILs in these solvent media if Coulombic forces are only responsible for ionic association governed by the relative permittivities of the media. The large differences between the KA values of the ILs in methanol and acetonitrile indicate that the association/dissociation behavior of these ILs in methanol and acetonitrile could not be accounted for only on the basis of the relative permittivities of these two solvents. Non-Coulombic forces might, therefore, play a decisive role in the ionic association phenomena in acetonitrile. Further studies in this direction are, therefore, necessary.

Figure 4. Variation of molar conductivity of 1-butyl-3-propylbenzimidazolium bromide as a function of molality in acetonitrile at different temperatures at 0.1 MPa. The symbols represent the experimental points, whereas the lines represent the calculations according to eqs 1 through 6. ▽, 318 K; □, 313 K; ○, 308 K.

Table 4. Coefficients of Equation 8 IL [BPim][Br] [BPbim][Br] [BPim][Br] [BPbim][Br] [BPim][Br] [BPbim][Br]

a0 Water 130.56 108.81 Methanol 79.27 79.03 Acetonitrile 127.91 166.34

a1

r2

1.7280 2.0200

0.9953 0.9996

1.1260 1.2820

0.9994 0.9977

1.7790 2.3720

0.9986 0.9929

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CONCLUSIONS Two ionic liquids, 1-butyl-3-propyl imidazolium bromide ([BPim][Br]) and 1-butyl-3-propyl benzimidazolium bromide ([BPbim][Br]), have been synthesized respectively from their appropriate imidazole and benzimidazole precursors and characterized by NMR spectroscopic technique. The electrical conductances of these two ionic liquids were measured as functions of their concentrations in water, methanol, and acetonitrile at different temperatures. Analyses of the conductance data on the basis of the Fuoss conductance equation provided important insight into the ion association and solvation behavior on the investigated ionic liquid solutions.

have followed an opposite order. This observation thus suggests that the IL ions [BPim]+ and [BPbim]+ exist as unsolvated species in aqueous solutions. The ionic mobilities of the IL cations [BPim]+ and [BPbim]+ in methanol are approximately the same. This suggests that the solvodynamic dimensions of these IL cations in methanol are similar. Thus it can be inferred that at least the smaller cation [BPim]+ is solvated in methanol. A comparison of the trend in the IL cationic mobilities in acetonitrile with their bare sizes indicates that the bigger the bare size of cation, the more mobile it would be. Therefore, the solvodynamic dimensions of the cations in acetonitrile solution decrease in the order: [BPbim]+ > [BPim]+. Thus there is apparent significant solvation of the [BPim]+ ion by the acetonitrile molecules so that it turned into a bigger solvodynamic entity than the [BPbim]+ ion. The present study, however, does not rule out the possibility of solvation of the [BPbim]+ ion in methanol and acetontrile solutions, although weaker solvation is expected for this ion compared to [BPim]+ ion since the former with a bigger size would possess a smaller surface charge density compared to the latter. The association constants of these two ILs in the three media investigated are listed in Table 3. Now, the fractions of unpaired ions (γ) can be obtained from the KA values using eqs 3 to 5. The fractions of unpaired ions are found to be in the range of 0.87−1.00 for the two ILs in water and methanol within the investigated concentration and temperature ranges indicating that a preponderant proportion of these ILs remain in the form of free ions. For these ILs in acetonitrile, on the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes †

Previous address: Department of Chemistry, North Bengal University, Darjeeling 734013, India. Funding

Financial assistance by the Department of Science and Technology, New Delhi, Government of India (SR/S1/PC-67/2010 and SR/ FT/CS-017/2010) and the Department of Chemistry, North Bengal University are gratefully acknowledged. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071−2084.

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dx.doi.org/10.1021/je300339q | J. Chem. Eng. Data XXXX, XXX, XXX−XXX