Synthesis and Physicochemical Properties of Two SO3H

Article pubs.acs.org/jced

Synthesis and Physicochemical Properties of Two SO3H‑Functionalized Ionic Liquids with Hydrogen Sulfate Anion Yali Meng, Jiamei Liu, Zhen Li,* and Huanmei Wei State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China ABSTRACT: Two SO3H-functionalized ionic liquids (ILs), N-isobutyl-3-sulfopropan-1-aminium hydrogen sulfate [IBAC3S]HSO4 and N-isobutyl-4-sulfobutan-1-aminium hydrogen sulfate [IBAC4S]HSO 4 , were synthesized and characterized by NMR and HR-MS. The thermal properties and acidities were analyzed. Density, dynamic viscosity, surface tension, conductivity, and refractive index of the IL were studied as a function of temperature. The physicochemical properties, namely molecular volume, isobaric coefficients of thermal expansion, standard entropy, critical temperature, molar free volume, interstice volume, etc. of the IL were estimated according to the reported empirical and thermodynamic equations. On the basis of the Walden rule, the ionicty of the two SO3H-functionalized ionic liquids was assessed by the product of molar conductivity and viscosity.

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INTRODUCTION Ionic liquids are ionic compounds that are liquid near room temperature. As novel media and soft functional materials, they have attracted considerable attention because of their favorable physicochemical properties, such as wide liquid temperature range, excellent thermal stability, good solubility, and negligible vapor pressure since the mid-1990s.1 Recently, significan progress in the synthesis of functionalized or “task-specific” ionic liquids (TSILs) with specific functions to provide satisfied performance has been made in chemical reactions, separations, and biomass pretreatment, etc.2,3 By introducing catalytic active groups in the cations or anions, the obtained functional ionic liquids have been proved to be high-efficiency catalysts in a number of chemical transformations. Brönsted acidic ionic liquids (BAILs), an important branch of ionic liquids, possess both the proton acidity and the features of an ionic liquid.4 As environmental-friendly catalysts, BAILs now have attracted considerable attention in industrial and academic area. With the aid of BAILs catalysis, excellent selectivity and yields were obtained in many organic reactions, including Fisher esterification, polymerization, carbonylation, aldol condensation, pinacol/benzopinacol rearrangement, Beckmann rearrangement, Koch reaction, Ritter reaction, alcohol dehydrodimerization, Mannich reaction, direct amination of alcohols, and nucleophilic addition of olefins.5,6 Although BAILs have shown great application prospect in many fields, the data of their fundamental physicochemical and thermodynamic properties are limited. In order to optimize the utilization of BAILs, the investigation on their physical and chemical properties has been significantly increased recently.7−10 In 2002, Cole et al.11 synthesized functionalized BAILs equipped with an alkyl sulfonic acid group and confirmed that they were excellent catalysts in the esterification. However, as © 2014 American Chemical Society

far as we are aware, systematic studies on the physicochemical properties of these sulfonic acid group functionalized ionic liquids are exceedingly rare, and only isolated case reports have described them, for instance, solubilities,12 corrosive,13 and thermal properties.14 As part of our interest in developing new BAILs based on isobutylaminium cations bearing alkyl sulfonic acid groups, in this paper two novel acyclic SO3H-functional Brönsted-acidic ILs, N-isobutyl-3-sulfopropan-1- aminium hydrogen sulfate [IBAC3S]HSO4 and N-isobutyl-4-sulfobutan-1- aminium hydrogen sulfate [IBAC4S]HSO4, were synthesized and characterized (Figure 1) and the viscosity, density, refractive index, ionic conductivity, and surface tension for two ILs were determined at different temperatures. The effect of temperature on the physical properties was studied and some thermodynamic parameters were estimated by using appropriate models.

Figure 1. Route of synthesis of the SO3H-functional Brönsted-acidic ILs. Received: February 13, 2014 Accepted: June 15, 2014 Published: June 25, 2014 2186

dx.doi.org/10.1021/je5001507 | J. Chem. Eng. Data 2014, 59, 2186−2195

Journal of Chemical & Engineering Data

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EXPERIMENTAL SECTION Materials and Apparatus. Isobutylamine (99.5%) and 1,4butane sultone (99.0%) were purchased from J&K China Chemical Ltd. 1,3-Propane sultone (>99.0%) was obtained from TCI Shanghai, China. Diethyl ether (≥99.5%), toluene (≥99.5%) and concentrated sulfuric acid (98.0%) were commercially available from XiLong Chemical Co., Ltd., China, and used without further purification. NMR spectra was measured on a Varian 400 MHz spectrometer. High resolution-mass spectra (HR-MS) were recorded on a Bruker MicroTOF-QII mass instrument (ESI). Elemental analysis (C, H, N, S (wt %)) was performed on a vario Elcube elemental analyzer (EA). Preparation of [IBAC3S]HSO4 and [IBAC4S]HSO4. Isobutylamine (0.2 mol) and 1,3-propane sultone or 1,4-butane sultone (0.22 mol) were reacted at 50 °C for 6 h in a roundbottomed flask equipped with a reflux condenser and magnetic stirrer in the absence of solvent and then allowed to cool at room temperature. The obtained solid was filtered, washed with toluene and ether, and dried in vacuo for 24 h, affording Npropyl-3-sulfonate isobutylamine or N-butyl-4-sulfonate isobutylamine as a white powdery solid zwitterion in good yield of 85 % ∼ 91 %. A stoichiometric amount of sulfuric acid was added dropwise to a solution of the above obtained zwitterion in toluene, and the mixture was stirred at 60 °C for 24 h and then allowed to cool at room temperature. The upper phase was decanted, and the lower phase of product was washed thoroughly with toluene (2 × 20 mL) and ether (3 × 20 mL). The volatiles were removed by evaporation to leave a liquid, which was dried in vacuo at 80 °C over 48 h to give N-isobutyl3-sulfopropan-1-aminium hydrogen sulfate [IBAC3S]HSO4 or N-isobutyl-4-sulfobutan-1-aminium hydrogen sulfate [IBAC4S]HSO4 as a pale yellow viscous liquid with the yields of more than 72%. All ionic liquids were dried in a vacuum oven at 80 °C for more than 24 h before physicochemical properties measurements. Dried samples were stored in a Elecotronic Dry Cabinet. It has been reported that water existing in ILs would have influence on the physicochemical properties of the ILs, the water content of the abovesynthesized [IBAC3S]HSO4 and [IBAC4S]HSO4, measured by a Karl Fischer titrator, DL 31 (Mettler Toledo), was found to be less than 270 ppm with an estimated uncertainty of 0.3% before the properties were measured. The purities of the present ILs are found to be greater than 0.99 in mass fraction confirmed through 1H NMR spectroscopy. N-Isobutyl-3-sulfopropan-1-aminium hydrogen sulfate [IBAC3S]HSO4. 1H NMR (400 MHz, D2O, ppm, δ) 2.91 (t, J = 8.0 Hz, 2H), 2.72 (t, J = 4.0 Hz, 2H), 2.62 (d, J = 4.0 Hz, 2H), 1.85 (m, 2H), 1.72 (m, 1H), 0.70 (d, J = 8.0 Hz, 6H). HR-MS (ESI): [m/z]+ = 196.1002. Elemental analysis for CHNS, analysis (% calculated): C, 28.71 (28.66); H, 6.81 (6.53); N, 4.71 (4.77); S, 21.46 (21.86). N-Isobutyl-4-sulfobutan-1-aminium hydrogen sulfate [IBAC4S]HSO4. 1H NMR (400 MHz, D2O, ppm, δ) 2.79 (t, J = 8.0 Hz, 2H), 2.66 (t, J = 8.0 Hz, 2H), 2.61 (d, J = 8.0 Hz, 2H), 1.72 (m, 1H), 1.63 (m, 4H), 0.70 (d, J = 8.0 Hz, 6H). HRMS (ESI): [m/z]+ = 210.1158. Elemental analysis for CHNS, analysis (% calculated): C, 30.45 (31.26); H, 6.08 (6.89); N, 4.19 (4.56); S, 20.45 (20.86). Acidity Determination. Acidity of BAILs was determined using a Perkin Elmer lambda 650 s UV−vis spectrophotometer using a basic indicator by the method reported in literature.15,16

The ionic liquid samples and the basic indicator 4-nitroaniline (3.62 × 10−3 mol·kg−1) were dissolved in distilled water at a concentration of 2.5 × 10−2 mol·kg−1. Density and Viscosity Measurements. Density was measured by using a Mettler Toledo DM45 Deltarange Density meter from (293.15 to 363.15) K. The temperature was controlled internally with an uncertainty of ±0.02 K. The viscosity of the sample can be corrected automatically by the densimeter. The calibration of the densimeter was carried out using doubly distilled and degassed water and dried air at atmospheric pressure. The manufacturer suggests the repeatability and the accuracy in the density measurements are 5 × 10−6 g·cm−3 and 5 × 10−5 g·cm−3. The standard uncertainty (k = 2) of determining the density is less than 3 × 10−5 g·cm−3. The viscosity measurements were performed with a Brookfield DV-III Ultra Viscometer, with a nominal uncertainty of ± 1% according to the manufacturer’s specifications. The temperature was regulated within ± 0.02 K by means of a Brookfield TC 602P thermostated bath. Refractive Indices Measurements. The refractive indices were measured with an ATAGO programmable digital refractometer (RX-5000 alpha) over the temperature range from (293.15 to 333.15) K. The temperature control accuracy is ± 0.05 K. Millipore water was used to calibrate the refractometer before each experiment. Reproducibility was confirmed by taking at least three experiments for each IL at every temperature point studied in the present work. The standard uncertainty was less than 5 × 10−4. Surface Tension Measurements. A QBZY series full automatic surface/interface tensiometer (ShangHaiFangRui Instrument Co., Itd) was used in the surface tension measurements by using a platinum plate method. The sample cell and platinum plate must be cleaned to remove any surface active impurities before each measurement. The measurements were performed from (293.15 to 328.15) K with an uncertainty of ± 0.02 K at atmospheric pressure. A double-jacketed glass vessel equipped with a measuring device for temperature was used to keep the sample maintained at constant temperature. Before sample measurement, the test calibration should be accomplished by using double distilled water and pure ethanol. The overall standard uncertainty is estimated to be 0.04 mN· m−1. Ionic Conductivity Measurements. The ionic conductivities of ionic liquids were measured by means of a MettlerToledo FE30 Conductivity meter, which was calibrated using standard aqueous KCl solutions, and the standard uncertainty was ±0.5%. The cell was placed in a thermostatic silicone oil bath (501A), in which the temperature was kept constant to within ± 0.05 K. A Pt100 probe was used to measure the temperature with an evaluating standard uncertainty of ±0.02 K. Thermal Analysis. NETZSCH 200 F3 Differential Scanning Calorimeter was used to evaluate the phase behavior of ionic liquids by differential scanning calorimetry (DSC). The DSC data were collected with a scan rate of 10 K·min−1, using the liquid nitrogen cooling system. The samples were sealed in aluminum pans. The glass transition temperature (Tg) was defined as the midpoint of the glass transition temperature range, bounded by the tangents to the two flat regions of the heat flow curve. The standard reference indium and zinc samples were used to calibrate the DSC instrument and the uncertainty was ± 0.1 K. The decomposition temperature (Td) was measured by thermogravimetric (TG) analysis performed 2187



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15) °C at a rate of 10 K·min−1. This means that the tendency for crystallization is weak for both ionic liquids because of the nonthermodynamic equilibrium below glass transition temperature. Acidity. Acidity of BAILs was determined in water using 4nitroanline as a basic indicator by using UV−visible spectroscopy. The concentrations of BAILs and the indicator were 2.5 × 10−2 mol·kg−1 and 3.62 × 10−3 mol·kg−1, respectively. As shown in Figure 4, a decrease can be seen for the absorbance of

on a NETZSCH Simultaneous Thermal Analysis 449F3 under N2 atmosphere with a heating rate of 10 K·min−1. The accuracy of temperature is ± 0.1 K.

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RESULTS AND DISCUSSION Thermal Properties of the BAILs. With TG analysis, the thermal stability of BAILs was examined. In accordance with the conventional method, Td was defined by the cross point of two extrapolated lines. The TG traces related to the decomposition of [IBAC3S]HSO4 and [IBAC4S]HSO4 are shown in Figure 2.

Figure 4. Absorbance spectra of 4-nitroaniline in water after addition of BAILs.

Figure 2. TG curves for BAILs: solid, [IBAC3S]HSO4; dot, [IBAC4S]HSO4.

the unprotonated form (I) of the indicator along with the increase of acidity of BAILs. On the basis of the absorptions measured, the ratio [I]/[IH+] (I represents indicator) can be determined, therefore Hammett function (H0) can be calculated using H0 = pK(I)aq + log([I]/[IH+]), where pK(I)aq = pKa = 0.99 for 4-nitroanaline. The calculated values are presented in Table 1, and the H0 values follows the order:

The TG curves clearly indicated that all BAILs were stable at temperature of at least 270 °C. However, the thermal decompositions of [IBAC3S]HSO4 and [IBAC4S]HSO4 are not very similar. Two distinct decomposition temperatures between 270 and 337 °C were observed for [IBAC4S]HSO4. The decompositions at lower temperatures are likely due to the decomposition of the −C2H4SO3H group with a weight loss percentage of ≈36%. As shown in Figure 3, the samples exhibit only a glass transition temperature (Tg), −39.6 °C for [IBAC3S]HSO4 and −48.1 °C for [IBAC4S]HSO4 during heating from (−120 to

Table 1. H0 Value of BAILs in Water at 298 Ka no.

ILs

Amax

[I] (%)

[IH+] (%)

H0

1 2 3

[IBAC3S]HSO4 [IBAC4S]HSO4 No BAIL

0.482 172 0.443 118 0.621 384

77.6 71.3 100

22.4 28.7 

1.530 1.386 

a

Indicator: 4-nitroanline.

[IBAC3S]HSO4 > [IBAC4S]HSO4. The H0 values of the two BAILs are comparable with other single sulfonic acidfunctionalized ILs with different cations.17−19 However, the Hammett acidities are lower than bifunctional BAILs with two sulfonic acid groups in cation.20 Density, Viscosity, Surface Tension, Refractive Index, and Ionic Conductivity of the BAILs. Table 2 shows the density, viscosity, surface tension, refractive index, and ionic conductivity experimental values of [IBAC3S]HSO4 and [IBAC4S]HSO4 in the temperature range of (293.15 to 373.15) K at atmospheric pressure. The density, ρ, the refractive index, nD, and the ionic conductivity, σ, values were fitted using the following equation21 Figure 3. DSC curves for BAILs (exo. down): solid, [IBAC3S]HSO4; dot, [IBAC4S]HSO4.

z = A 0 + A1T + A 2 T 2 2188

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Article nDAT

Table 2. Density, ρ, Viscosity, η, Surface Tension, γ, Refractive Index, nD, and Ionic Conductivity, σ, of BAILs as a Function of Temperature at Pressure p = 0.1 MPaa T (K)

ρ

η −3

(g cm )

(mPa·s)

γ

nD −1

SD = [ ∑ (zexp − zadjust)2 /nDAT]1/2

where zexp is the experimental property value, zadjust is the adjustable data value, and nDAT denotes the number of experimental points. As Table 3 shows, eq 1 can fit the experimental results for density, ionic conductivity, and refractive index with good accuracy. Figures 5, 6, and 7 show the temperature dependence of the physical properties graphically.

σ −1

(mN·m )

(ms·cm )

[IBAC3S]HSO4 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

1.387 92

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

1.359 08

1.383 65 1.377 60 1.371 59 1.365 64 1.359 71 1.353 80 1.347 97

1.353 21 1.347 39 1.341 63 1.335 95 1.330 29 1.324 66 1.319 09

52.18 51.64 51.14 50.69 50.15 49.58 48.98

1.505 59 1.504 36 1.503 14 1.502 05 1.500 80 1.499 74 1.498 51 1.497 56 1.496 30

47.15 46.79 46.48 46.12 45.74 45.30 44.93

1.481 43 1.480 30 1.479 18 1.478 04 1.476 89 1.475 76 1.474 61 1.473 40 1.472 12

14 383 9083 5533 3833 2867 1983 1333 1033 766.7 616.7 483.3 416.7 333.3 283.3 [IBAC4S]HSO4 14 433 9450 6033 4267 2967 2117 1550 1033 766.7 633.3 483.3 383.3 316.7 266.7 200.0 166.7

(2)

i

0.0856 0.1447 0.2350 0.3670 0.5260 0.7130 1.039 1.396 1.766 2.210 2.800 3.530

5.93 7.57 10.45 13.17 16.53 21.60 25.30 30.10 36.70 42.90 49.60 56.10

Figure 5. Plot of density, ρ vs T and fitted curves (−) for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

a Stanard uncertainty u are u(T) = 0.02 K, u(p) = 0.01 MPa, u(ρ) = 3 × 10−5 g·cm−3, u(nD) = 5 × 10−4, u(η) = 1 %, u(γ) = 0.04 mN·m−1, and u(σ) = 0.5 %.

where z denotes ρ, nD, or σ of the BAILs, A0, A1, and A2 are the fitting parameters, and T is the temperature. The values of the fitting parameters are shown along with the standard deviation values (SD) in Table 3. The standard deviation was calculated by eq 2:

Figure 6. Plot of refractive index, nD vs T and fitted curves (−) for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

Table 3. Fitting Parameter Values of Eq 1 to Correlate Physical Properties of BAILs and the Standard Deviations (SDs) physical property

A0

ρ (g·cm−3) nD σ (ms·cm−1)

1.499 1.602 110.109

ρ (g·cm−3) nD σ (ms·cm−1)

1.553 1.519 837.096

A1 [IBAC3S]HSO4 −2.144 × 10−4 −4.129 × 10−4 −0.731 [IBAC4S]HSO4 −7.320 × 10−4 −3.748 × 10−5 −5.924 2189

A2

SD

−5.595 × 10−7 2.913 × 10−7 0.0012

3.70 × 10−4 5.61 × 10−5 0.050

2.452 × 10−7 −3.091 × 10−7 0.0105

1.26 × 10−5 2.53 × 10−5 0.38



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Volumetric and Surface Properties. Other important thermodynamic properties such as the isobaric coefficient of thermal expansion, α, which expresses the change of the molar volume with temperature can be derived from the density values by using the following equation15 α = (1/V )(∂V /∂T )P = − 1/ρ(∂ρ /∂T )P

(4)

where V is the molar volume of the BAIL, ρ and T are the density and temperature, respectively. With the use of the adjustable parameters for eq 1, α can be calculated. The thermal expansion coefficients for [IBAC3S]HSO4 and [IBAC4S]HSO4 are compared with that of other liquids, such as an acidic ionic liquid with the same anion, [TMGBSA]HSO4,22 decane,23 methanol,23 and water.24 As seen from Figure 9, [IBAC3S]-

Figure 7. Plot of ionic conductivity, σ vs T and fitted curves (−) for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

The Vogel−Fulcher−Tammann equation (VFT) is used to fit the experimental data of viscosity to temperature for the synthesized BAILs: ⎛ k ⎞ η = AT 0.5exp⎜ ⎟ ⎝ T − T0 ⎠

(3)

where A, k, and T0 are fitting parameters. The parameter T0 is defined as the ideal glass transition temperature. The values of parameters are listed in Table 4 together with the standard relative deviations (SD). The VFT fits for two BAILs are depicted in Figure 8. Figure 9. Dependence of thermal expansion coefficient on temperature of ILs at 0.1 MPa compared with several other common fluids. ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4; ●, decane;23 ▲, methanol;23 Δ, water;24 and □, [TMGBSA]HSO4.22

Table 4. Adjustable Parameters of the VFT Equation (A, k, and T0) Together With the Standard Relative Deviations of the Fit (SD) for the Viscosity of [IBAC3S]HSO4 and [IBAC4S]HSO4 A [IBAC3S]HSO4 [IBAC4S]HSO4

k

HSO4 and [IBAC4S]HSO4 have similar values of thermal expansion coefficients, but the tendency of α related to the temperature is slightly different. The thermal expansion coefficient of [IBAC3S]HSO4 increases slightly with the increment of temperature. This is true for most of the fluid and ionic liquid behavior. On the contrary, the variation tendency of the thermal expansion coefficient for [IBAC4S]HSO4 along the temperature is a little different with that of [IBAC3S]HSO4. They keep invariant with temperature, however, at higher temperature, they decrease slightly. Furthermore, as shown in Figure 9, the difference of the thermal expansion coefficient change trend with temperature between the two Brönsted acidic ionic liquids and other liquids, such as decane, methanol, or water is obvious. The reasons accounting for the indistinctive changing trend of α for the BAILs are probably that the dipole−dipole interaction energies are not stronger than the Coulombic energies between cation and anion, which shows that the intermolecular attraction counterbalances the repulsive forces between molecules at ambient pressure.25 The thermal expansion coefficient of [TMGBSA]HSO4 containing SO3Hguanidinium-based cation, decreases slightly at higher temperatures. The coefficient of thermal expansion of a fluid near its thermodynamic critical point approaches infinity, therefore, the coefficient of thermal expansion of [TMGBSA]HSO4 exists a

T0

(mPa·s·K−0.5)

(K)

(K)

SD

0.026 10 0.0069

1042.76 1374.22

207.39 181.50

0.017 0.026

Figure 8. Dynamic viscosity, η, and fitted curves with VFT equation (−) as a function of temperature. Experimental points: ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

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minimum at higher temperatures. As mentioned in literature,26 there is no clear explanation for this phenomenon. The molecular volumes of [IBAC3S]HSO4 and [IBAC4S]HSO4 were given by eq 5 using the experimental values of the density: Vm =

Mm M = 1.66 × 10−3 m ρNA ρ

temperatures of [IBAC3S]HSO4 and [IBAC4S]HSO4 are presented in Table 5. Table 5. Surface Excess Entropy, Sa, Surface Excess Enthalpy, Ha, Empirical Constant of Eötvos Equation, k, Estimated Critical Temperature, Tc, of [IBAC3S]HSO4 and [IBAC4S]HSO4

(5)

Sa −2

(mJ·m ·K )

where Mm and NA are molar mass and Avogadro’s constant, respectively. The molecular volume values calculated for [IBAC3S]HSO4 and [IBAC4S]HSO4 are 0.3514 nm3 and 0.3763 nm3, respectively, at 298.15 K. Glasser and Jenkins27,28 established a linear relationship between the standard entropy, S0, and the molecular volume, which can be used to calculate the standard entropy in the ILs as follows:29−34 S °/(J·K−1·mol−1) = 1246.5(Vm /nm 3) + 29.5

0.105 0.0741

Ha

k

Tc

(mJ·m−2)

(J·K−1)

(K)

[IBAC3S]HSO4 83.49 3.25 × 10−7 [IBAC4S]HSO4 69.24 2.28 × 10−7

868.84 1066.94

Generally, k is considered as a polarity correlation physical quantity. For instance, k is about 2.2 × 10−7 J·K−1 for most organic liquids,41 whereas in the case of strong polarity fused salts, it is extremely small. One example is the fused NaCl, k = 0.4 × 10−7 J·K−1.36 As can be seen from Table 5, the polarity of ionic liquid [IBAC3S]HSO4 and [IBAC4S]HSO4 is close to that of organic liquids. Experimental values of surface tension for IBAC3S]HSO4 and [IBAC4S]HSO4 against (T − 298.15) K were fitted to a linear equation as illustrated in Figure 11. The correlation

(6)

For [IBAC3S]HSO4 and [IBAC4S]HSO4, the calculated standard entropy are 467.5 J·K−1·mol−1 and 498.6 J·K−1· mol−1, respectively. The surface tensions of two BAILs at different temperatures were measured, and the data were listed in Table 2. The surface tension of [IBAC3S]HSO4 is lower than that of [IBAC4S]HSO4, which is attributed to a weakening of the Coulomb interaction with the alkyl chain length increase.35 The values of γ at T = 298.15 K are 52.1 mN·m−1 and 47.15 mN·m−1 for [IBAC3S]HSO4 and [IBAC4S]HSO4, respectively, which is much higher than those for typical organic solvents. Generally speaking, surface tension of many liquid exhibits a nearly linear decrease with the increase of temperature. The relationship between surface tension and temperature is expressed in the Eötvos equation36−40 γV 2/3 = k(Tc − T )

−1

(7)

where V is the molar volume of the liquid, according to Eötvos, k is an empirical constant, and Tc is the critical temperature. From the linear regression of γV2/3 − T, as shown in Figure 10, we can obtain k, which is the negative value of the slope. The empirical constant of Eötvos equation and estimated critical Figure 11. Plot of surface tension, γ vs T and fitted curves (−) for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

coefficients are 0.998 and 0.997, respectively. From the slope (∂γ/∂T)p, the surface excess entropy, Sa = −(∂γ/∂T)p at 298.15 K, was obtained, Sa = 0.105 mJ·m−2·K−1 for [IBAC3S]HSO4 and Sa = 0.0741 mJ·m−2·K−1 for [IBAC4S]HSO4, respectively. For most ionic liquids, the decrease of γ with increasing temperature, that is Sa, is smaller than that of water42 (0.138 mJ·m−2·K−1). Unlike water (where, as temperature increases, there are profound structural modifications of the fluid due to the disappearance of its hydrogen-bond network), the electrostatic and van der Waals interactions responsible for most of the internal cohesive energy of ionic liquids remain active throughout the analyzed temperature ranges and yield the rather modest values of Sa, which indicated that the organization level of the ionic liquids structure is high.43 Additionally, the surface excess enthalpy can be obtained by equation: Ha = γ −T(∂γ/∂T)p.44 The values of Ha at 298.15 K for the BAILs are also listed in Table 5.

Figure 10. Plot of γV 2/3 vs T for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4. 2191



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The values of Ha (ca. 83 and 69 mJ·m−2, for [IBAC3S]HSO4 and [IBAC4S]HSO4, respectively) are much lower than that of fused salts, for instance, Hσ = 146 mJ·m−2 (for fused NaNO3), but can well approximate to that of organic liquids, such as benzene and n-octane with Hσ = 67 mJ·m−2 and 51.1 mJ·m−2, respectively.36 Molar Refraction. The refractive index values of BAILs against temperature are shown in Figure 6 along with the value calculated using the second-order polynomial equation plotted as a solid line. The value of refractive index decreased with temperature increase. Lorenz−Lorentz equation is usually applied to calculate the molar refraction, Rm.45,46 Thus, the molar refractions of [IBAC3S]HSO4 and [IBAC4S]HSO4, Rm, were calculated at several temperatures with eq 8: ⎛ n 2 − 1 ⎞⎛ M ⎞ ⎛ n2 − 1 ⎞ ⎟⎜ m ⎟ ⎟V = ⎜ D2 R m = ⎜ D2 ⎝ nD + 2 ⎠⎝ ρ ⎠ ⎝ nD + 2 ⎠

σ=

⎛ −B ⎞ A exp⎜ ⎟ T ⎝ T − T0 ⎠

(10)

where the adjustable parameters were A, B, and T0. B is a factor related to the activation energy, and T0 is the ideal glass transition temperature. With this VTF equation, we can neglect the effect of Tg in the discussion of ion conduction behavior by using the normalized temperature (T − T0). Many discussions related to the most suitable temperature as T0 may be found in the literature,51−54 and in the present study, Tg detected by the DSC measurement was used as T0. Figure 12 shows the VTF

(8)

where V denotes the molar volume. The calculated molar refraction together with the molar volume were summarized in Table 6. On the basis of the data, we may conclude that the Table 6. Values of Calculated Molar Volumes, V, Molar Refractions, Rm, and Molar Free Volumes, Vm,f, of [IBAC3S]HSO4 and [IBAC4S]HSO4 at Several Temperatures T

Vm

Rm

Vm,f

(K)

(cm3·mol−1)

(cm3·mol−1)

(cm3·mol−1)

[IBAC3S]HSO4 293.15 298.15 303.15 313.15 323.15 333.15

211.09 211.70 212.02 212.95 213.88 214.82

293.15 298.15 303.15 313.15 323.15 333.15

226.18 226.67 227.16 228.14 229.12 230.09

62.67 62.72 62.69 62.72 62.75 62.78 [IBAC4S]HSO4 64.41 64.43 64.44 64.45 64.46 64.44

Figure 12. VTF plot of the ionic conductivities with fitting parameters for ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4.

148.42 148.98 149.33 150.23 151.14 152.03

plots together with the fitting parameters of the ionic conductivities for [IBAC3S]HSO4 and [IBAC4S]HSO4. The solid lines in Figure 12 suggested that the temperature dependence of ionic conductivities for [IBAC3S]HSO4 and [IBAC4S]HSO4 were well-fitted by the VTF equation. As reported in the literature,55−60 the Walden rule may be applicable to ionic liquids to assess the ionicity of an IL roughly. Walden’s rule states that the product of the molar conductivity, Λ, and dynamic viscosity, η, is approximately constant. In Figure 13, Walden plots for the BAILs are presented. The molar conductivity, Λ (S·cm2·mol−1), was calculated by using the expression Λ = σMmρ−1, where Mm, σ, and ρ are the molar mass, specific conductivity, and density, respectively, of the ILs. In Walden’s rule, the data for a dilute (usually 0.01 M) aqueous solution of KCl are used to establish the “ideal” Walden line, due to the fully dissociated and ions of equal mobility. In Figure 13, the solid straight line indicates the KCl ideal line. Compared with the ideal line, the Walden plots of [IBAC3S]HSO4 and [IBAC4S]HSO4 fall above the ideal line obviously, suggesting that these SO3H-functionalized Brönsted acidic ionic liquids belong to “superionic liquids”, which means the system obeys some decoupled transport mechanism, for example, Grotthuss proton transport or small ion penetration mechanism.61 Other hydrogen sulfate-based ionic liquids, which are somewhat alike in superionic behavior have already been reported.56,62,63 Application of the Interstice Model. Related to pure ionic liquids, Yang and co-workers64,65 suggested a new theory called interstice model, which may be applied to calculate the interstice volume, υ, on the classical statistical mechanics:

161.76 162.24 162.72 163.69 164.65 165.65

temperature increase implies a small increase in the values of the molar volumes; however, the effect of temperature on the molar refractions is neglectable. The unoccupied fraction of the molar volume of an ionic liquid is defined as molar free volume Vm,f, which is estimated by eq 9.47−49 Vm,f = Vm − R m

(9)

where Vm and Rm are the molar volume and the molar refraction of the IL, respectively. Equation 9 is generally applicable in the case of spherical molecules and may only provide a qualitative measure of the free volume for ionic liquids in this work, which are built from nonspherical ions. Nevertheless, the values of calculated molar free volume at several temperatures are also shown in Table 6. Conductivity of the BAILs and Walden Rule. The conductivities of [IBAC3S]HSO4 and [IBAC4S]HSO4 were correlated as a function of temperature and illustrated in Figure 7. The VTF equation was chosen to analyze the ion conduction behavior of BAILs.50 2192



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however, the conductivity increased exponentially with the rise in temperature. The experimental data of the physical properties were well-fitted with empirical equations. The dynamic viscosity decrease as temperature increases, and the values were fitted usingthe Vogel−Fulcher−Tammann (VFT) equations. The isobaric thermal expansion coefficient was derived from the density data. The molecular volume and standard entropy at T = 298.15 K were also calculated. The present BAILs exhibit weak temperature dependency for the isobaric thermal expansion coefficient. By applying the Walden rule, these SO3H-functionalized ionic liquids can be classified as superionic liquids. With respect to the interstice model, the predicted values of thermal expansion coefficients for [IBAC3S]HSO4 and [IBAC4S]HSO4 agree well with the experimental ones within 1 order of magnitude. Thus, the results of this study enriched BAIL properties database and offered theoretical basis for the development of many chemical processes related to catalysis and biomass treatment.

Figure 13. Walden plots for BAILs studied herein: ■, [IBAC3S]HSO4; ○, [IBAC4S]HSO4; the solid straight line is the ideal Walden line, which corresponds to the data for dilute aqueous KCl solutions at ambient temperature.62

⎛k T ⎞ υ = 0.6791⎜ b ⎟ ⎝ γ ⎠

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Corresponding Author

*E-mail: [email protected].

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Funding

This work was supported financially by the National Key Basic Research Program of China (973 Program, Grant 2011CB201404) and the National Natural Science Foundation of China (Grant 21133011).

(11)

where kb and γ are the Boltzmann constant and the surface tension of ionic liquid, respectively, and T is the temperature in K. Thus, the values of υ at 298.15 K are obtained as 15.04 × 10−24 cm3 and 17.51 × 10−24 cm3 for [IBAC3S]HSO4 and [IBAC4S]HSO4, respectively. Whereafter, the total volumes of the interstice Συ = 2Nυ are 18.11 cm3 and 21.08 cm3, respectively. The volume fraction of the interstices is Συ/V = 0.086 and Συ/V = 0.093 for [IBAC3S]HSO4 and [IBAC4S]HSO4, respectively. The values agree well with that of the majority of materials, which exhibit a 10 to 15% volume expansion in transition from the solid-to-liquid state.66,67 From the interstice theory, we can draw a conclusion that the expansion of IL volume when temperature increases is only due to the expansion of the interstices. As a result, the volume expansivity, α, can be estimated by using eq 12 α=

1 ⎛⎜ ∂V ⎞⎟ 3 2Nυ 3Nυ = = ⎝ ⎠ V ∂T p 2 VT VT

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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where N is the Avogadro constant. The obtained volume expansivities estimated from eq 12 for [IBAC3S]HSO4 and [IBAC4S]HSO4 at 298.15 K were 4.304 × 10−4 K−1 and 4.679 × 10−4 K−1, respectively. In comparison with the experimental values, αexp = 3.933 × 10−4 K−1 and 4.319 × 10−4 K−1 for [IBAC3S]HSO4 and [IBAC4S]HSO4, respectively, they agree well within 1 order of magnitude, which implies that the interstice theory is reasonable for the BAILs, [IBAC3S]HSO4, and [IBAC4S]HSO4.

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CONCLUSIONS Two SO3H-functionalized ionic liquids were prepared, and the density, viscosity, surface tension, refractive index, and ionic conductivity of ILs were measured as functions of temperature at atmospheric pressure and fitted to appropriate models. Additionally, the thermogravimetric analysis, DSC, and acidity analysis were also performed. The density, surface tension, and refractive index decreased linearly as temperature increases; 2193



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Synthesis and Physicochemical Properties of Two SO3H

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