Volumetric Properties of Binary Mixtures of Two 1-Alkyl-3

Article pubs.acs.org/jced

Volumetric Properties of Binary Mixtures of Two 1‑Alkyl-3Methylimidazolium Tetrafluoroborate Ionic Liquids with Molecular Solvents Olga Iulian and Oana Ciocirlan* Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, Politehnica University of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania ABSTRACT: The densities have been determined for the binary systems of ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate, [Emim][BF4] and 1-hexyl3-methylimidazolium tetrafluoroborate, [Hmim][BF4], with the molecular solvents dimethyl sulfoxide (DMSO) and acetonitrile (ACN) at atmospheric pressure over the entire range of composition and temperatures from (298.15 to 353.15) K. From experimental densities, the excess molar volumes, VE, were calculated and fitted to a Redlich−Kister equation. Moreover, the VE results have been discussed in terms of the Prigogine−Flory−Patterson (PFP) theory. The VE variation was interpreted in terms of intermolecular interactions and structural effects on mixing.

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INTRODUCTION Room-temperature ionic liquids (ILs) have caught the attention of researchers due to their extraordinary properties as roomtemperature molten salts. Some of these, like negligible vapor pressure, wide liquid range, and an ionic conductivity−viscosity ratio, make them potential green solvents or reaction media for many applications.1−4 Generally, to design an industrial process involving ILs, it is important to know their physicochemical properties. One attraction of ILs is the possibility to obtain the desired properties by modifying the nature and size of the cations and anions, and a strategy in the application of ILs is to use mixtures of IL + organic (or inorganic) solvent to generate the targeted properties. In addition, by knowing the excess properties we can better understand the structure−property relation, making it easier choosing an appropriate IL. Therefore, IL + solvent mixtures have received growing attention in the past years. Up to now, the most investigated ionic liquids are those containing 1,3-dialkylimidazolium cations. In this context, the thermophysical properties of binary systems of 1-alkyl-3methylimodazolium tetrafluoroborates with polar organic solvents, like dimethyl sulfoxide and acetonitrile, have been studied in literature.5−10 As a continuation of our studies involving physicochemical properties of pure 1-alkyl-3-methylimodazolium tetrafluoroborates and its mixtures with traditional organic solvents,11 we report here the densities of the ionic liquids 1-ethyl-3methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium tetrafluoroborate and their binary mixtures with dimethyl sulfoxide (DMSO) and acetonitrile (ACN) from (293.15 to 353.15) K over the whole composition range. © 2012 American Chemical Society

To our best knowledge, for the binary systems of both studied ionic liquids with ACN only density data is reported at temperature of 298.15 K5; for the other two studied systems no literature data concerning volumetric properties are available.

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EXPERIMENTAL SECTION Materials. We used the reagents: 1-ethyl-3-methylimidazolium tetrafluoroborate (mass fraction > 0.98) from Sigma

Figure 1. Comparison of our density data at 298.15 K for the binary systems [Emim][BF4] (1) + ACN (2) (▲) and [Hmim][BF4] (1) + ACN (2) (■) with literature data, ref 5 (×). Received: February 26, 2012 Accepted: September 24, 2012 Published: October 1, 2012 2640

dx.doi.org/10.1021/je300316a | J. Chem. Eng. Data 2012, 57, 2640−2646

Journal of Chemical & Engineering Data

Article

measurement was ± 5·10−5 g·cm−3. For excess molar volume calculation, the uncertainty was estimated to be less than ± 10−2 cm3·mol−1.

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RESULTS AND DISCUSSION To verify the accuracy of our experimental data, the densities of pure substances were compared with literature data on temperature ranges (Table 1). For the two ILs the relative deviations of literature density data12,13,15 from our experimental data were calculated. The obtained values are less than 0.35 % for [Emim][BF4] and 0.04 % for [Hmim][BF4], so we can say that our data compare well with those reported by different authors.12,13,15 The obtained densities, ρ, for the binary systems of [Emim][BF4] and [Hmim][BF4] with DMSO and ACN, respectively, at temperatures between (293.15 and 353.15) K over the whole composition range are given in Tables 2 and 3. The density data at 298.15 K for the binary systems of [Emim][BF4] + ACN and [Hmim][BF4] + ACN correspond well with those found in literature (Figure 1).5 From the measured density data, the experimental excess molar volumes, VE, for the binary mixtures were obtained and fitted to the Redlich−Kister equation:

Figure 2. Excess molar volumes, VE, for the [Emim][BF4] (1) + DMSO (2) mixtures at different temperatures: ○, 293.15 K; ◆, 298.15 K; △, 303.15 K; ∗, 313.15 K; +, 323.15 K; −, 333.15 K; □, 343.15 K; ×, 353.15 K; solid line, Redlich−Kister correlation.

Aldrich, 1-hexyl-3-methylimidazolium tetrafluoroborate (mass fraction > 0.995), DMSO (mass fraction > 0.999), and ACN (mass fraction > 0.995), from Merck. DMSO and ACN were stored overnight on 3A molecular sieves to remove the water traces. The ionic liquids, being hygroscopic liquid substances, were dried into a Schlenk tube under vacuum at 333 K for 11 h. To minimize contact with the atmosphere, sample preparation was performed under nitrogen and for loading the samples into the apparatus syringes were used equipped with a Luer Lock valve. Apparatus and Procedure. Preparation of binary mixtures was carried out using an electronic balance HR-120 (A&D Japan) with a precision of ± 10−4 g. The uncertainty of the mole fraction did not exceed ± 0.0002. Measurement of Density. For the experimental determination of density on the investigated temperature range, an Anton Paar DMA 4500 densimeter with a precision of ± 0.05 kg·m−3 was used. The densimeter included an automatic viscosity correction. The apparatus was calibrated with dry air and bidistilled water at atmospheric pressure. The uncertainty in temperature determination was ± 0.01 K and in density

V E = x1x 2 ∑ Ak (x1 − x 2)k

(1)

k

where x1 and x2 are the mole fractions of the IL and organic compounds, respectively, and Ak are the adjustable parameters. These fit parameters are obtained by using the method of leastsquares and are given in Table 4, along with the corresponding standard deviation, σ, calculated as: n E E 2 0.5 σ = [ ∑ (Vexp , i − Vcalc, i) /(n − m)]

(2)

i=1

where n is the number of experimental points and m is the number of parameters. As can be seen in Tables 2 and 3 and Figures 2 to 5, the excess molar volume values are negative over the entire range of composition at all studied temperatures for the binary systems, [Emim][BF4] and [Hmim][BF4], with DMSO and ACN,

Table 1. Comparison of Measured Densities with Literature Values for Pure Components at T = (293.15 and 353.15) K ρ/g·cm−3 [Emim][BF4] T/K

exptl

293.15

1.29063

298.15

1.28682

303.15

1.28302

313.15

1.27545

323.15

1.26797

333.15 343.15 353.15

1.26055 1.25318 1.24587

[Hmim][BF4] lit.

exptl 12

1.2884 1.2893613 1.282312 1.2855113 1.2800714

1.14881

1.278712 1.2817413 1.271412 1.2741713 1.264212 1.2666913 1.256912 1.249712 1.242412

1.14185

1.14529

1.13504 1.12824 1.12148 1.11475 1.10805

ACN lit.

exptl 13

1.14892 1.1486715 1.1454713 1.1461814 1.1451015 1.1453216 1.1419913

0.78195

1.1351013 1.1346415 1.1283013

0.76018

DMSO lit.

exptl

lit.

1.10076

1.10042418 1.1005319 1.0956220 1.095721

0.77655

0.776937 0.7765338 0.7765317

1.09574

0.77113

0.771447 0.7711217 0.760687 0.7601617

1.09073 1.08069 1.07066

1.09046718 1.0904919 1.08049018 1.0803219 1.0705722

1.1211615 1.1079215 2641



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Table 2. Experimental Densities (ρ/g·cm−3 ± 5·10−5) for the Binary Systems [Emim][BF4] + DMSO and [Emim][BF4] + ACN at Different Temperatures (T ± 0.01/K) and Atmospheric Pressure (101 ± 2 kPa) T/K x1

293.15

298.15

0.0000 0.1041 0.2134 0.3311 0.4282 0.5219 0.5902 0.7069 0.7979 0.9128 1.0000

1.10076 1.14204 1.17677 1.20590 1.22486 1.23984 1.24920 1.26306 1.27255 1.28299 1.29063

1.09574 1.13739 1.17235 1.20164 1.22069 1.23574 1.24516 1.25909 1.26864 1.27913 1.28682

0.0000 0.1699 0.2884 0.4247 0.5097 0.6318 0.7197 0.8213 0.9203 1.0000

0.78195 0.98500 1.07335 1.14325 1.17648 1.21340 1.23656 1.25831 1.27703 1.29063

0.77655 0.98032 1.06894 1.13905 1.17238 1.20939 1.23261 1.25441 1.27317 1.28682

303.15

313.15

323.15

[Emim][BF4] (1) + DMSO (2) 1.09073 1.08069 1.07066 1.13273 1.12343 1.11416 1.16792 1.15912 1.15036 1.19738 1.18892 1.18051 1.21654 1.20827 1.20008 1.23167 1.22357 1.21554 1.24114 1.23314 1.22522 1.25514 1.24730 1.23953 1.26473 1.25699 1.24933 1.27528 1.26765 1.26009 1.28302 1.27545 1.26797 [Emim][BF4] (1) + ACN (2) 0.77113 0.76018 0.74911 0.97564 0.96628 0.95692 1.06449 1.05578 1.04705 1.13480 1.12651 1.11822 1.16821 1.16012 1.15203 1.20538 1.19743 1.18954 1.22867 1.22083 1.21307 1.25053 1.24281 1.23516 1.26933 1.26170 1.25415 1.28302 1.27545 1.26797

333.15

343.15

353.15

1.06063 1.10490 1.14162 1.17214 1.19194 1.20757 1.21736 1.23184 1.24174 1.25261 1.26055

1.05058 1.09567 1.13293 1.16382 1.18384 1.19966 1.20956 1.22421 1.23420 1.24518 1.25318

1.04051 1.08642 1.12424 1.15554 1.17580 1.19180 1.20181 1.21662 1.22673 1.23781 1.24587

0.73786 0.94750 1.03832 1.10997 1.14398 1.18171 1.20537 1.22757 1.24665 1.26055

Table 3. Experimental Densities (ρ/g·cm−3 ± 5·10−5) for the Binary Systems [Hmim][BF4] + DMSO and [Hmim][BF4] + ACN at Different Temperatures (T ± 0.01/K) and Atmospheric Pressure (101 ± 2 kPa) T/K x1

293.15

298.15

0.0000 0.1160 0.2495 0.2831 0.3857 0.5036 0.5714 0.6702 0.7517 0.7883 0.8976 1.0000

1.10076 1.11662 1.12754 1.12963 1.13464 1.13894 1.14080 1.14300 1.14454 1.14520 1.14713 1.14881

1.09574 1.11215 1.12342 1.12556 1.13072 1.13514 1.13707 1.13936 1.14095 1.14161 1.14354 1.14529

0.0000 0.1040 0.2049 0.2948 0.4066 0.4616 0.5697 0.6626 0.8083 0.9044 1.0000

0.78195 0.91251 0.98551 1.02785 1.06355 1.07762 1.09930 1.11367 1.13167 1.14151 1.14881

0.77655 0.90782 0.98118 1.02373 1.05963 1.07377 1.09563 1.11003 1.12814 1.13801 1.14529

303.15

313.15

323.15

[Hmim][BF4] (1) + DMSO (2) 1.09073 1.08069 1.07066 1.10768 1.09877 1.08989 1.11928 1.11104 1.10285 1.12149 1.11336 1.10530 1.12680 1.11900 1.11123 1.13136 1.12383 1.11630 1.13335 1.12595 1.11856 1.13572 1.12847 1.12126 1.13737 1.13025 1.12316 1.13807 1.13100 1.12396 1.14009 1.13321 1.12629 1.14185 1.13504 1.12824 [Hmim][BF4] (1) + ACN (2) 0.77113 0.76018 0.74911 0.90311 0.89368 0.88423 0.97686 0.96821 0.95958 1.01962 1.01143 1.00326 1.05570 1.04789 1.04010 1.06992 1.06225 1.05462 1.09191 1.08450 1.07712 1.10639 1.09913 1.09191 1.12459 1.11753 1.11051 1.13452 1.12758 1.12069 1.14185 1.13504 1.12824

333.15

343.15

353.15

1.06063 1.08101 1.09468 1.09726 1.10352 1.10890 1.11126 1.11409 1.11610 1.11696 1.11948 1.12148

1.05058 1.07216 1.08654 1.08926 1.09583 1.10149 1.10398 1.10697 1.10909 1.10999 1.11263 1.11475

1.04051 1.06332 1.07843 1.08120 1.08818 1.09412 1.09674 1.09987 1.10210 1.10306 1.10589 1.10805

0.73786 0.87472 0.95094 0.99511 1.03235 1.04702 1.06978 1.08472 1.10352 1.11382 1.12148

0.72643 0.86517 0.94230 0.98697 1.02463 1.03947 1.06247 1.07756 1.09657 1.10703 1.11475

The VE behavior of studied mixtures of ionic liquid + organic solvent was investigated by Prigogine−Flory−Patterson (PFP) theory,23−25 which has been developed to analyze excess

respectively, more negative for the systems with ACN than DMSO. The minima in VE were observed in the range x1 = 0.25 to 0.4. 2642



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Table 4. Coefficients Ak of the Fitting eq 1 and Standard Deviations, σ T/K 293.15 298.15 303.15 313.15 323.15 333.15 343.15 353.15 293.15 298.15 303.15 313.15 323.15 333.15 293.15 298.15 303.15 313.15 323.15 333.15 343.15 353.15 293.15 298.15 303.15 313.15 323.15 333.15 343.15

A0

A1

A2

[Emim][BF4] + DMSO −2.1933 1.2414 0.9225 −2.2650 1.3096 0.8918 −2.3391 1.3723 0.8724 −2.5027 1.5028 0.7909 −2.6795 1.6352 0.7199 −2.8703 1.7656 0.6425 −3.0784 1.9096 0.5439 −3.3036 2.0590 0.4592 [Emim][BF4] + ACN −4.1658 3.3885 −0.3183 −4.3708 3.5636 −0.4074 −4.5640 3.7352 −0.5501 −5.0429 4.1452 −0.7615 −5.5499 4.5879 −1.0364 −6.1139 5.0777 −1.3257 [Hmim][BF4] + DMSO −0.8401 0.9002 0.2592 −0.9043 0.9330 0.2448 −0.9319 0.9641 0.1677 −0.9857 1.0671 0.0577 −1.0539 1.1920 −0.0537 −1.1507 1.3042 −0.1318 −1.2593 1.4383 −0.2033 −1.3728 1.5552 −0.3342 [Hmim][BF4] + ACN −3.7326 3.3702 −3.0938 −3.9173 3.4672 −3.2920 −4.0723 3.6391 −3.4408 −4.4020 4.0435 −3.7470 −4.7815 4.4727 −4.1409 −5.2161 4.9711 −4.5540 −5.7089 5.5180 −5.0738

σ 0.014 0.013 0.013 0.013 0.012 0.012 0.012 0.012 0.032 0.029 0.028 0.031 0.031 0.030

Figure 4. Excess molar volumes, VE, for the [Hmim][BF4] (1) + DMSO (2) mixtures at different temperatures: ○, 293.15 K; ◆, 298.15 K; △, 303.15 K; ∗, 313.15 K; +, 323.15 K; −, 333.15 K; □, 343.15 K; ×, 353.15 K; solid line, Redlich−Kister correlation.

0.007 0.006 0.007 0.009 0.008 0.011 0.012 0.016 0.027 0.029 0.029 0.031 0.034 0.037 0.042

Figure 5. Excess molar volumes, VE, for the [Hmim][BF4] (1) + ACN (2) mixtures at different temperatures: ○, 293.15 K; ◆, 298.15 K; △, 303.15 K; ∗, 313.15 K; +, 323.15 K; −, 333.15 K; □, 343.15 K; ×, 353.15 K; solid line, Redlich−Kister correlation.

The PFP equation for VE indicates the influence of intermolecular interactions and different structural effects arising from differences in size and shape of molecules on excess property. The expression for VE, which contains an interactional term, a free volume term, and an internal pressure term, is given as E VPFP E = Vint + VfvE + VipE X1V1* + X 2V 2*

= Figure 3. Excess molar volumes, VE, for the [Emim][BF4] (1) + ACN (2) mixtures at different temperatures: ○, 293.15 K; ◆, 298.15 K; △, 303.15 K; ∗, 313.15 K; +, 323.15 K; −, 333.15 K; solid line, Redlich− Kister correlation.

(v 1/3 ̃ − 1)v 2/3 ̃ ψ1θ2χ12 −1/3 − 1]P1* [(4/3)v ̃ −

(v1̃ − v2̃ ) 2((14/9)v −̃ 1/3 − 1)ψ1ψ2

[(4/3)v −̃ 1/3 − 1]v ̃ (v1̃ − v2̃ )(P1* − P2*)ψ1ψ2 + P2*ψ + P1*ψ 1

thermodynamic functions. This model has been applied for the discussion of excess volumes behavior even for more complex mixtures, like those containing ionic liquids.8,26,27

2

(3)

where Xi is the mole fraction, ψi the contact energy fraction, θi the surface site fraction, ṽi the reduced volume; P*i and V*i are 2643



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Table 5. Physicochemical Data of Pure Components Used in the Prigogine−Flory−Patterson Theory at T/K = 298.15

[Emim][BF4] [Hmim][BF4] ACN DMSO a

α*

kT*

K−1

cm3·J−1 a

0.000582 0.000601a 0.001445a 0.000916a

0.0003374 0.000377c 0.001134d 0.000522e

b

Vi

Vi*

Pi*

Ṽ i

cm3·mol−1

cm3·mol−1

J·cm−3

T̃ i

Ti* K

1.1553 1.1597 1.3323 1.2302

153.84 221.85 52.86 71.30

133.16 191.29 39.68 57.96

686.5 639.5 674.3 791.5

0.04066 0.04155 0.06846 0.05423

7328 7172 4353 5495

This work. bReference 28. cReference 15. dReference 8. eReference 29

Table 6. Experimental and Calculated VE Using PFP Theory and the Three PFP Contributions at around x1 = 0.4 and T/K = 298.15 VEexp system [Emim][BF4] + DMSO [Emim][BF4] + ACN [Hmim][BF4] + DMSO [Hmim][BF4] + ACN

VEPFP −1

cm ·mol 3

−0.6046 −1.2476 −0.2527 −1.2354

cm ·mol 3

VEint −1

−0.5345 −1.1725 −0.2298 −1.1794

VEfv −1

cm ·mol 3

−0.5762 −0.3527 −0.4087 −0.4933

−1

cm ·mol 3

−0.1857 −0.7682 −0.1941 −0.8594

VEip

χ12

σ

cm ·mol−1

J·cm−3

cm3·mol−1

0.2274 −0.0515 0.3729 0.1733

−83.24 −49.14 −46.03 −59.77

0.0505 0.0692 0.0314 0.0468

3

Figure 7. Excess molar volume V E for the binary systems [Hmim][BF4] (1) with (a) DMSO (2) and (b) ACN (2), respectively, at T = 298.15 K calculated with the PFP theory: ·−·−, interactional contribution; ··−··−, free volume contribution; ---, internal pressure contribution; solid line, total excess molar volume predicted by PFP theory; ○, experimental results.

Figure 6. Excess molar volume V E for the binary systems [Emim][BF4] (1) with (a) DMSO (2) and (b) ACN (2), respectively, at T = 298.15 K calculated with the PFP theory: ·−·−, interactional contribution; ··−··−, free volume contribution; ---, internal pressure contribution; solid line, total excess molar volume predicted by PFP theory; ○, experimental results.

the characteristic pressure and characteristic volume, respectively, and χ12 is the interaction parameter.

Table 5 presents some physicochemical data and parameters of the pure components used in PFP theory needed for the VE 2644



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estimation. They include the thermal expansion coefficient, αi, and isothermal compressibility coefficient, kTi, which are used to calculate the reduced volume and the characteristic pressure.11 The differences in the molar volume of ILs versus organic solvents, bigger for the systems with ACN, and in characteristic pressure, bigger for the systems with DMSO, can be observed. The results of the VE calculation with PFP theory, like the experimental VE values at around x1 = 0.4 and T = 298.15 K, are given in Table 6. In Figures 6 and 7 are presented the three contributions to VE due to interactional contribution, free volume effect, and internal pressure contributions, over the whole composition range. A good correlation is observed for our experimental data with those predicted by PFP theory as shown by the solid line, with standard deviations in the range of (0.03 to 0.07) cm3·mol−1, as is indicated in Table 6. The interaction parameter, χ12, which minimized deviations from the VE experimental data, was evaluated over the entire range of composition, and it is given in Table 6 for each system. From Table 6 it can be observed that for both binary investigated systems with ACN the free volume effect is the most important factor in deciding the sign of VE, while for the systems with DMSO, the interactional contribution is dominant. For the systems with ACN, this behavior suggests geometrical accommodation in the binary mixture, which is supported by the big differences between the molar volumes of the pure components (Table 5), bigger for the [Hmim][BF4] + ACN system. For the systems with DMSO, this behavior suggests a relative strong intermolecular specific interaction between the 1,3dialkyl substituted imidazolium ionic liquids and DMSO on mixing. This is based on spectroscopic studies in IR, which showed the existence of hydrogen-bond interactions between the H atom from position 2 of the imidazolium ring and the functional group SO of DMSO in the binary system [Bmim][BF4] + DMSO.6 Moreover, the systems with ACN present an interactional term which cannot be neglected, as the systems with DMSO have an important geometrical, structural constituent (VEfv and VEip). Actually, we can attribute the negative values for VEint in the systems with ACN to the same hydrogen bond interactions between the H atom from position 2 of the imidazolium ring and the functional group CN from ACN. The internal pressure term is important for the [Hmim][BF4] + DMSO system, where the differences between the characteristic pressures of components are bigger than for the other systems (Table 5). This explain less negative values for VE for this system than for the [Emim][BF4] + DMSO system. The negative values obtained for the interaction parameter listed in Table 6 are in accordance with the interactional contribution to excess molar volume. For the four binary systems, the variation with temperature indicates that as the temperature increases from (293.15 to 353.15) K the VE values become more negative.

investigated temperatures. The PFP theory gives a good representation of our experimental data. The analysis of different contributions to VE suggests that the free volume effect is the most significant term in deciding the sign and size of the excess molar volume for the systems with ACN, and the interactional contribution is significant for the systems with DMSO, suggesting strong intermolecular interactions between components on mixing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +40214023855. Funding

This work was supported by the Sectorial Operational Programme for Human Resources Development, under project “Postdoctoral program for advanced research in the nanomaterials field” ID 54785. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSION This paper reports densities of binary systems of [Emim][BF4] + DMSO, [Emim][BF4] + ACN, [Hmim][BF4] + DMSO, and [Hmim][BF4] + ACN, in the temperature range from (293.15 to 353.15) K, most of them for first time. The obtained excess molar volumes are negative for all investigated systems over the entire range of composition at all 2645



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Volumetric Properties of Binary Mixtures of Two 1-Alkyl-3

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