Isothermal (vapour + liquid) equilibrium and thermophysical properties for (1-butyl-3-methylimidazolium iodide + 1-butanol) binary system

J. Chem. Thermodynamics 87 (2015) 58–64

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Isothermal (vapour + liquid) equilibrium and thermophysical properties for (1-butyl-3-methylimidazolium iodide + 1-butanol) binary system Mariana Teodorescu ⇑ ‘‘Ilie Murgulescu’’ Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, 060021 Bucharest, Romania

a r t i c l e

i n f o

Article history: Received 15 January 2015 Received in revised form 20 March 2015 Accepted 22 March 2015 Available online 1 April 2015 Keywords: Low pressure VLE Refractive index 1-Butyl-3-methylimidazolium iodide ([bmim]I) 1-Butanol

a b s t r a c t Experimental isothermal (vapour + liquid) equilibrium (VLE) data are reported for the binary mixture containing 1-butyl-3-methylimidazolium iodide ([bmim]I) + 1-butanol at three temperatures: (353.15, 363.15, and 373.15) K, in the range of 0 to 0.22 liquid mole fraction of [bmim]I. Additionally, refractive index measurements have been performed at three temperatures: (293.15, 298.15 and 308.15) K in the whole composition range. Densities, excess molar volumes, surface tensions and surface tension deviations of the binary mixture were predicted by Lorenz–Lorentz (nD-q) mixing rule. Dielectric permittivities and their deviations were evaluated by known equations. (Vapour + liquid) equilibrium data were correlated with Wilson thermodynamic model while refractive index data with the 3-parameters Redlich–Kister equation by means of maximum likelihood method. For the VLE data, the real vapour phase behaviour by virial equation of state was considered. The studied mixture presents S-shaped abatement from the ideality. Refractive index deviations, surface tension deviations and dielectric permittivity deviations are positive, while excess molar volumes are negative at all temperatures and on whole composition range. The VLE data may be used in separation processes design, and the thermophysical properties as key parameters in specific applications. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Recently, the ionic liquids (ILs) have become solvents with large applicability because of their very interesting properties. The low melting points (<100 °C) and the wide range of temperature in which they exists in liquid state (about 300 °C), the reasonable viscosity and the chemical stability up to very high temperatures, the high solubility in nonpolar or polar organic solvents and the high conductivity and negligible vapour pressures and non-flammability, make IL’s be used in so-called green chemistry, for partial or total replacement of volatile organic solvents which are, as it is known, flammable and toxic. They can be very interesting solvents for a variety of industrial applications [1]. Beyond these, they are much discussed as selective solvents (entrainers) for various separation processes [2,3]. The methods of their synthesis are well developed [4], but for various scopes it should be well known their properties, in many cases in their mixtures with organic solvents. For a better understanding of their thermodynamic behaviour, phase equilibria data and thermodynamic and thermophysical properties are required.

⇑ Tel.: +40 213167912; fax: +40 213121147. E-mail address: mateodorescu@chimfiz.icf.ro http://dx.doi.org/10.1016/j.jct.2015.03.023 0021-9614/Ó 2015 Elsevier Ltd. All rights reserved.

1-Butyl-3-methylimidazolium halides [bmim]Hal (Hal = Br, Cl, I) are often used as precursors for synthesis of imidazolium ionic liquids with various anions [5]. These liquids are also interesting for theoretical analysis of ILs properties [6]. Containing the simplest possible anions as halides, they can serve as model compounds for various theoretical calculations and molecular simulation [6]. The iodide IL is also considered as electrolyte for solar cells [7]. Thermodynamic properties of the solutions of these ILs in various solvents such as vapour pressures of the solutions and activity coefficients of the components have been rarely studied [8]. Thermodynamic data for IL-containing systems are essential for separation design purpose and for the development of thermodynamic models. The refractive index dependence on composition is generally used as calibration curves for phase composition determinations at pressure–temperature equilibrium data sets, by ebulliometry, when VLE data are measured. Since the refractive index of a liquid, nD, is a property easy to measure with good accuracy, it was related with other thermophysical or electrical properties, such as density, surface tension, and dielectric permittivity, by numerous empirical and theoretical equations as referenced in literature [9]. Experimental measurements and prediction for these physical properties of IL mixtures are essential for application to many processes [10].

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M. Teodorescu / J. Chem. Thermodynamics 87 (2015) 58–64

The present work is a continuation of a project dealing with phase equilibria and thermophysical properties of ILs with classical solvent mixtures. In previous papers [11,12], the systems of two prototype ILs (1-butyl-3-methylimidazolium bromide and chloride) with 1-butanol have been investigated. For now, another prototype IL, 1-butyl-3-methylimidazolium iodide, with the same alcohol binary system is under focus. No (vapour + liquid) equilibrium (VLE) data or thermophysical properties are available for it [13]. Experimental isothermal VLE data are reported at three temperatures: (353.15, 363.15, and 373.15) K, in the range of 0 to 0.22 liquid mole fraction of [bmim]I. The composition range was limited due to the reduced quantity of IL which it was at disposal. Additionally, refractive index measurements have been performed at three temperatures: (293.15, 298.15 and 308.15) K in the whole composition range. Densities, excess molar volumes, surface tensions and surface tension deviations of the binary mixture were predicted by Lorenz–Lorentz (nD-q) mixing rule [14]. Dielectric permittivities and their deviations were evaluated by known equations [15,16]. The phase equilibria, thermodynamic and thermophysical data reported here will bring new information required for the design of the separation processes of 1-butanol from its mixtures using an organic salt as [bmim]I and, at the same time, they will allow an insight into specific inter and intra molecular interactions or structural arrangements existing in the binary (IL + 1-butanol) system. 2. Experimental 2.1. Materials The providers and characteristics of the used compounds are summarized in Table 1. After purification, both chemicals were stored in closed system to dried atmosphere, on calcium chloride. A good comparison with literature values has been obtained for all physical properties of 1-butanol. This is shown in Table 2. As for the [bmim]I, a rather good comparison is shown for the density values vs. data of Sastry et al. [18] and worse comparison vs. data of Kim et al. [17]. The Kim et al. [17] values for both refractive indexes and densities are lower than our data probably due to a higher content of water in the IL. In [17] the water content of [bmim]I is not declared. 2.2. Apparatuses and procedures The vapour pressures measurements of pure 1-butanol and of the binary mixtures were carried out by dynamic method using a modified Swietoslawski ebulliometer [22]. The apparatus, is described in details previously together with the experimental procedure [23] commonly used [22,24,25]. The temperatures at the thermodynamic equilibrium in the apparatus were measured with an uncertainty of ±0.1 K, by means of mercury thermometers

TABLE 1 Commercial sources, purities and methods of purification of the used chemical compounds. Compound

Commercial source

Purity/mass fraction

Purification method

[bmim]I

Aldrich

>0.990 with 60.005 water content

1-Butanol

Riedel de Häen

>0.995

Dried in oven under vacuum of 0.1 kPa at T = 343.15 K for 48 h; Water content after purification: 0.0007 mass fraction by Karl-Fischer titration Dried and stored on molecular sieves 4Å

calibrated in advance at National Institute of Metrology, Bucharest. The vapour pressure measurements were performed by using a mercury manometer. The pressure readings were made with a cathetometer with uncertainty of ±0.1 mmHg, and its reproducibility was estimated to be better than 50 Pa. The uncertainty of the pressure measurements is estimated to be 0.1% of measured value. The composition of the liquid phase in equilibrium with the vapour phase was analyzed by the refractometric method making use of a calibration curve obtained by measurements of the refractive index of weighed mixture samples (uncertainty ±0.2 mg by GH-252 A&D Japan balance) at T = 298.15 K and data correlation with Redlich–Kister polynomials [26] with three parameters in the form:

nED ¼ nD  ½xnD;1 þ ð1  xÞnD;2  ¼ xð1  xÞ½a0 þ a1 ð1  2xÞ þ a2 ð1  2xÞ2 

ð1Þ

The Redlich–Kister parameters a0, a1, a2 of equation (1) were obtained by maximum likelihood optimization method using the following objective function:

2  2 3 E E 2 N n  n X D;i;exp D;i;calc ðx  x Þ 6 i;exp 7 i;calc OF ¼ þ 4 5 2 2 i¼1

rx

rnE

ð2Þ

D

The refractive index was measured by a digital refractometer Abbemat RXA 170 from Anton-Paar (Austria) at the wavelength of the D line of sodium, 589.3 nm, with uncertainty ±0.0001. The temperature of the Safire prism was controlled by a Peltier element to within ±0.01 K and the calibration of the apparatus was carried out with bidistilled and deionized water and by determining refractive indexes and its deviations at T = 298.15 K for the binary (cyclooctane + toluene) system at several compositions. The difference resulted from the comparison of our refractive index deviation values, nED , and those correlated on experimental data from literature [27] was 2  104 in mean absolute average, which it means a good comparison. The nED , which is defined as

nED ¼ nD  xnD;1  ð1  xÞnD;2

ð3Þ

has an estimated uncertainty of 2  104. In equations (1) and (3), nD;1 and nD;2 represent the refractive indexes of the pure components 1 and 2, nD the refractive index for the mixture 1 + 2, and x denotes the mole fraction of component 1 in the binary mixture. Each measurement of refractive index consisted in 4 readings for the same sample. Their averaged value is reported here as one experimental point. Special attention was devoted to avoid extra moisture absorption from the atmosphere in pure chemicals and in their binary mixtures. The uncertainty in the determination of the composition was 0.001 mole fraction. The VLE apparatus and experimental procedure were successfully checked and used for investigation of different other mixtures as mentioned before [28]. The density and ultrasonic speed of sound (at 5000 lm original path length and frequency of approximately 3 MHz [29,30] of the sound waves) measurements of the pure compounds at the three temperatures (293.15, 298.15, and 308.15) K were carried out by using a density and speed of sound metre Anton Paar DSA-5000M with precision of ±0.005 kg  m3 and ±0.01 m  s1. Dried air and distilled deionized ultra pure water at atmospheric pressure were used as calibration fluids for the cell. The probes thermostating was maintained constant at ±0.01 K. The experimental measurement uncertainty for density was 0.1 kg  m3 and for the speed of sound 0.1 m  s1.

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M. Teodorescu / J. Chem. Thermodynamics 87 (2015) 58–64

TABLE 2 Refractive indices, nD, densities, q, and ultrasonic speeds of sound, s, at atmospheric pressure (101.22 ± 0.26 kPa) and vapour pressures, P, of pure compounds. nDa

Compound/Te/K

qb/(kg  m3)

This work

Lit.

sc/(m  s1)

This work

Lit.

Pd/kPa

This work

Lit.

This work

Lit.

21.19 33.80 51.73

21.80j 34.19j 51.89j

[bmim]I

a b c d e f g h i j

293.15 298.15

1.5825 1.5810

1.5695f

1483.5 1479.6

308.15

1.5781

1.5670f

1471.7

293.15 298.15 308.15 353.15 363.15 373.15

1.3993 1.3973 1.3932

1.39924i 1.39747h 1.39342h

809.8 805.9 798.2

1508.1 1497.5

1484.210g 1460f 1480.731g 1450f

1473.3

1-Butanol 809.5i 805.93h 798.21h

1257i 1239.28h 1206.26h

1256.3 1239.5 1205.9

Standard uncertainties: unD = 0.0001. Standard uncertainties: uq = 0.1 kg  m3. Standard uncertainties: us = 0.1 m  s1. Standard uncertainties: uP = 0.1% of measured value. Standard uncertainties: uT = 0.01 K for density, refractive index and speed of sound measurements, uT = 0.1 K for vapour pressure measurements. Reference [17]. Reference [18]. Reference [19]. Reference [20]. Reference [21].

60

3. Results and discussions

S¼

N h X

2

2

2 p

2

2 T

2 x

ðPie  Pic Þ =r þ ðT ie  T ic Þ =r þ ðxie  xic Þ =r

i

ð3Þ

50

40

P / kPa

The experimental isothermal (P, x) data measured for the binary system at T = (353.15, 363.15, and 373.15) K are shown in table 3 and figure 1. The data were correlated with Wilson thermodynamic model [31] with the basic equations shown recently [11]. The model parameters (kij–kij) expressed in J  mol1 are shown in table 4. The regression for their obtaining was performed by means of maximum likelihood method employing a program described by Hala et al. [32]. The objective function is defined as it follows:

30

i¼1

20

where N is the number of experimental points, Pie, Tie, and xie are the experimental data and Pic, Tic and xic are the corresponding

10

TABLE 3 (Vapour + liquid) equilibrium data for the ([bmim]I (1) + 1-butanol (2)) system. Pressure, Pe, temperature, T, and [bmim]I liquid phase composition, x1e, are experimental data and activity coefficients, c1, c2, and excess Gibbs free energy, GE, are calculated data with Wilson model by maximum likelihood method assuming real behaviour of the vapour phase. x1ea

a b c

y1,c

b

Pe /kPa

c1

E

c2

G /(J  mol

1 1.02 1.04 1.05

0 66 40 33

0 0.049 0.117 0.212

0 0.000 0.000 0.000

21.19 20.57 19.43 17.37

Tc = 353.15 K 3.40 1.08 0.82 0.80

0 0.050 0.119 0.199

0 0.000 0.000 0.000

33.80 32.80 30.97 27.46

Tc = 363.15 K 4.85 0.75 0.64 0.68

1 1.03 1.04 1.02

0 29 64 174

0 0.050 0.124 0.199

0 0.000 0.000 0.000

51.73 50.14 47.85 41.48

Tc = 373.15 K 36.78 0.64 0.58 0.64

1 1.03 1.04 1.02

0 24 104 230

Standard uncertainties: ux1 = 0.001. Standard uncertainties: uP = 0.1% of measured value. Standard uncertainties: uT = 0.1 K.

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x1

1

)

FIGURE 1. Bubble curves for ([bmim]I (1) + 1-butanol (2)) system at T = 353.15 K (d), T = 363.15 K (N), and T = 373.15 K (j); filled symbols: experimental bubble curves (pressure P vs. liquid composition in [bmim]I x1); (—) Wilson correlation of the bubble curves with real behaviour assumption of the vapour phase.

TABLE 4 Parameters of the Wilson model, (k12–k11) and (k21–k22), and standard deviations for the liquid composition rx1, pressure rp, and temperature rT obtained from the correlation of the VLE data for the ([bmim]I (1) + 1-butanol (2)) system. T/K

(k12–k11)/ (J  mol1)

(k21–k22)/ (J  mol1)

rx1

rP/ kPa

r T/

353.15 363.15 373.15

8589.505 11718.74 19084.05

1632.703 2183.458 2436.633

0.0001 0.0004 0.0016

0.002 0.010 0.059

0.05 0.14 0.53

K

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M. Teodorescu / J. Chem. Thermodynamics 87 (2015) 58–64

calculated values for pressure, temperature, and liquid composition of component 1, respectively. In this work, the standard deviations for the measured quantities were set to rP = 0.1% of measured value, rT = 0.1 K, and rx = 0.001, respectively. All standard deviations of correlations were calculated using the P 1=2 expression: rZ ¼ ½ ðZ i;e  Z i;c Þ2 =ðN  mÞ , where Z is the value of the property P, nED , and x, N is the number of experimental points and m = 2 in the case of Wilson model or m = 3 in the case of Redlich–Kister equation. In the case of VLE data, the real behaviour of the vapour phase was described with the virial equation of state. The second virial coefficients for both components and for binary mixture were evaluated by means of the Hayden and O’Connell method [33], while the molar volumes were calculated by using a generalised Watson relation [34]. For the calculations, the experimentally determined vapour pressures of pure 1-butanol (as given in tables 2 and 3) and those estimated (from http://ilthermo.boulder.nist. gov/index.html) as 1  106, 2  106 and 4  106 kPa for the [bmim]I were used. The results of correlations are summarized in table 4. In the range of the liquid composition measurements, in figure 1 and table 4 it can be seen a good agreement between experimental and calculated bubble curves. Insignificant changing of the model parameters and of the bubble curves takes place if the vapour pressures of the IL are considered as two orders of magnitude higher. The higher standard deviation of temperature obtained for the 3rd set of data (at T = 373.15 K) is due probably to the fact that temperature of equilibrium was not perfectly constant at T = 373.15 K. It was varying around this temperature with ±0.2°. The variation of the excess Gibbs free energy GE with [bmim]I liquid composition in the binary system is given in figure 2. It can be observed a S-shaped variation. Analyzing the GE values obtained for [bmim]Br or [bmim]Cl with 1-butanol binary systems [11,12], it results that at constant composition and temperature the GE is varying with the nature of the IL in the order: [bmim]Cl > [bmim]Br > [bmim]I. This variation can be explained by higher strength of the interactions between unlike molecules

in the case of [bmim]Cl than in the case of the [bmim]Br and in the case of [bmim]I, in the same order of decreasing electronegativity of the halogen ion, thing which is normal. At 0.15 mole fraction of [Bmim]I in the binary mixture and at average temperature of 363.15 K, the excess enthalpy, HE, calculated from the Gibbs–Helmholtz equation, is 3125 J  mol1. No experimental calorimetric value has been found in the literature for comparison. However, since the VLE data were correlated successfully with Wilson thermodynamic model without showing systematic errors, we can say that the VLE data are thermodynamic consistent. The experimental isothermal (x1, nD) data for the binary system have been determined at T = (293.15, 298.15 and 308.15) K and they are shown in table 5 with the calculated refractive index deviations nED . The correlation of these data at each temperature has been made by 3-parameters Redlich–Kister model by using maximum likelihood method, following the procedure described for T = 298.15 K in the section Apparatuses and procedures. The correlation results are presented in table 6 and the variation of the refractive index deviation vs. composition is shown in figure 3. Due to the small water content of the [bmim]I ionic liquid, expected values for the standard deviations of the liquid compositions have been obtained. It should be mentioned that not complete purification of the organic salt is possible. Similar situation was reported in the literature [18]. From figure 3 it can be observed that excess refractive indexes are positive on whole composition interval and increase slightly with increasing temperature. Similar behaviour was reported previously for [bmim]Cl [12] and [bmim]Br [11] with 1-butanol system where the same type of interactions are present. From refractive index vs. composition data and densities of the pure compounds at T = (293.15, 298.15 and 308.15) K, densities, excess molar volumes, surface tensions and dielectric permittivities at optical frequency and their deviations vs. composition of the binary mixture were determined by Lorenz–Lorentz mixing rule [14] or by known equations [15,16]. The selection of the mixing rule was made after analyzing of ten different mixing rules

100 TABLE 5 Experimental refractive indexes, nD , and its deviations, nED , vs. [bmim]I liquid mole fraction x1, for ([bmim]I (1) + 1-butanol (2)) binary system at atmospheric pressure of (101.22 ± 0.26 kPa).

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x1a

nD b

x1a

nED

nD b

nED

1.5410 1.5634 1.5686 1.5825

0.0277 0.0172 0.0136 0

c

-1

G / (J mol )

-100

E

-200

-300

-400

-500

x1 FIGURE 2. Excess Gibbs free energy, GE, variation with liquid mole fraction of [bmim]I, x1 at T = 353.15 K (—),T = 363.15 K (- - - -) and T = 373.15 K (— - - —) according to Wilson model for the ([bmim]I (1) + 1-butano (2)) binary system.

a b c

0 0.311 0.381 0.510 0.574

1.3992 1.4864 1.5007 1.5237 1.5339

T = 293.15 K 0 0.623 0.0302 0.802 0.0317 0.850 0.0310 1 0.0294

0 0.108 0.171 0.311 0.381 0.510

1.3973 1.4332 1.4508 1.4848 1.4991 1.5221

c T = 298.15 K 0 0.574 0.0161 0.623 0.0221 0.802 0.0303 0.850 0.0318 1 0.0311

1.5322 1.5393 1.5618 1.5670 1.5810

0.0294 0.0276 0.0171 0.0135 0

0 0.108 0.171 0.311 0.381 0.510

1.3932 1.4294 1.4472 1.4815 1.4958 1.5189

c T = 308.15 K 0 0.574 0.0163 0.623 0.0224 0.802 0.0308 0.850 0.0322 1 0.0313

1.5289 1.5361 1.5587 1.5639 1.5781

0.0295 0.0278 0.0172 0.0135 0

Standard uncertainties: ux1 = 0.001. Standard uncertainties: unD = 0.0001. Standard uncertainties: uT = 0.01 K.

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M. Teodorescu / J. Chem. Thermodynamics 87 (2015) 58–64

0.0

TABLE 6 Redlich–Kister coefficients aj (equation (1)) and the standard deviations of the [bmim]I liquid composition rx1 and of the refractive index deviation rnE for ([bmim]I D (1) + 1-butanol (2)) binary system.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

a0

a1

a2

rx

rnED  104

-0.2

293.15 298.15 308.15

0.1251 0.1254 0.1264

0.0354 0.0379 0.0394

0.0117 0.0150 0.0151

0.0004 0.0005 0.0005

1.0 1.0 0.9

-0.4 -1

V 10 / (m mol )

T/K

0.035

6

3

-0.6

-0.8

E

results in giving the refractive indices from experimental densities for twelve binary mixtures of various polarity at T = 298.15 K [9]. The predicted variation of the excess molar volume with composition and temperature for the binary ([bmim]I (1) + 1butanol (2)) system is shown in figure 4. As can be observed in this figure, the VE is negative on whole composition interval, it is more negative when the temperature increases, and similar behaviour is given by applying other good mixing rules like Gladstone–Dale, Edwards and Eykman [35]. [Bmim]I ionic liquid can act both as a hydrogen-bond acceptor ([I]) and donor ([bmim]+). It is expected to interact with 1-butanol which has both accepting and donating sites. On the other hand, it is well known that 1-butanol is hydrogen-bonded solvent with both high enthalpies of association and association constant. Hence, it is expected to stabilize [bmim]I with hydrogen-bonded donor sites. Comparing with [bmim]Cl and [bmim]Br, the [bmim]I is apparently less interacting molecule, especially when the other compound of IL mixture is water or alcohol [36]. At constant temperature, the excess molar volumes for ([bmim]I + 1-butanol) system calculated by Lorenz–Lorentz mixing rule are smaller than those of [bmim]Br which are smaller than those of [bmim]Cl with the same alcohol [11,12]. This is due probably to the fact that I is less electronegative than Br and Cl. For each mentioned binary system, the temperature effect is not specific to new hydrogen bonds between unlike molecules formation; it is most likely due to the packing between hydrogen acceptor sites of 1-butanol and donor sites of imidazolium ([bmim]+) cycle. These packing effects became more dominant and increase with temperature, as it was observed for other systems cited in literature [37].

-1.0

-1.2

-1.4 x1 FIGURE 4. Prediction of excess molar volumes VE vs. composition in [bmim]I x1 for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (—), T = 298.15 K ( ) and T = 308.15 K (- - -) by different mixing rules: (s) Lorenz–Lorentz, (4) Gladston-Dale, (h) Edwards, and (}) Eykmann.

The surface tension of a liquid is a property of great importance in mass transfer processes such as distillation, extraction, or absorption. It is not easily measured, and considerable attention has been paid to the development and analysis of equations allowing its prediction from properties for which data are more readily available as refractive index, for example. The surface tension r is related to the densities of the liquid qL and of the vapour qV phases of the substance by using the Macelod equation [15]:

r ¼ ctðqL  qV Þ4 :

ð4Þ

For a pure liquid compound, multiplying both sides by the molar mass M and ignoring qV in comparison with qL affords the Sugden equation [16]:

r1=4 M=qL ¼ r1=4 V m ¼ Parachor

0.030

from which it results :

0.025

n DE

0.020 0.015 0.010 0.005 0.000 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 3. Refractive index deviations nED vs. composition in [bmim]I, x1, for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (d), T = 298.15 K (N) and 308.15 K (j); Filled symbols are calculated values by equation (3); Lines are 3parameters Redlich–Kister correlation for T = 293.15 K (—), T = 298.15 K (- - -) and T = 308.15 K (— - —).

r¼

 4 Parachor Vm

ð5Þ ð6Þ

In equations (5) and (6), the Parachor is assumed as mole-wise additive and molar volume, Vm, is calculated from experimental densities of pure compounds and those predicted for the mixtures by the Lorenz–Lorentz relation [14] (from Redlich–Kister correlated refractive indices). So, we used equation (6) to predict the surface tensions of binary liquid mixtures at three temperatures (293.15, 298.15 and 308.15) K and after that to calculate the surface tension deviation from ideality by applying a similar equation with equation (3). Surface tension variation with composition and temperature is shown in figure 5 and surface tension deviation variation with the same two variables appears in figure 6. The parachors of the pure compounds have been calculated from the surface tension of the pure components from literature at T = (298.15 and 308.15) K [38,39] and molar volumes at the same temperatures determined experimentally in this work. As can be seen in figure 6, the surface tension deviations predicted here are positive on whole composition range being slightly higher at higher temperature in the range of 0 to 0.55 mole fraction of [bmim]I and slightly smaller at higher temperature in the range of 0.55 to 1 mole fraction of [bmim]I. The positive values are explained by the volume expansion resulted from the new H-bonds and packing between unlike molecules in

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M. Teodorescu / J. Chem. Thermodynamics 87 (2015) 58–64

50

23

22

45

21 12 -1 ε 10 / (F m )

-1 σ 10 3 / (N m )

40

35

20

19

30 18

25

17

20 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

16

1

0.0

0.1

0.2

0.3

0.4

0.6

0.7

0.8

0.9

1.0

the binary mixture of ([bmim]I + 1-butanol). It seems that packing of the molecules is dominant in the range 0 to 0.55 since excess surface tension is higher at higher temperature. In the other range, the H-bonds became dominant probably. Similar but less obvious behaviour was previously found for the [bmim]Br and [bmim]Cl + 1-butanol binary systems where the same type of interactions and structural arrangements were expected [11,12]. From refractive index data, the relative permittivity at optical frequency can be calculated by squaring the refractive index determined at the wavelength of 589.3 nm. For 1-butanol, the relative permittivities agree well with those extrapolated at the two temperatures of (298.15 and 308.15) K from experimental 4.0

FIGURE 7. Prediction of dielectric permittivities at optical frequency e vs. composition in [bmim]I x1 for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (-s-), T = 298.15 K (-4-) and T = 308.15 K (-h-).

0.9 0.8 0.7

12 E -1 ε 10 / (F m )

FIGURE 5. Prediction of surface tensions r vs. composition in [bmim]I x1 for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (-s-), T = 298.15 K (-4-) and T = 308.15 K (-h-).

E 3 -1 σ 10 / (N m )

0.5

x1

x1

0.6 0.5 0.4 0.3

3.5

0.2

3.0

0.1

2.5

0.0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x1

2.0

FIGURE 8. Prediction of excess dielectric permittivities at optical frequency eE vs. composition in [bmim]I x1 for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (-s-), T = 298.15 K (-4-) and T = 308.15 K (-h-).

1.5 1.0 0.5 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

x1 FIGURE 6. Prediction of excess surface tensions rE vs. composition in [bmim]I x1 for ([bmim]I (1) + 1-butanol (2)) system at T = 293.15 K (-s-), T = 298.15 K (-4-) and T = 308.15 K (-h-).

literature data [40] with mean absolute relative deviation of 0.35%. No data have been found for [bmim]I for comparison. The dielectric permittivity (figure 7) at optical frequency is obtained by multiplying relative permittivity by vacuum permittivity. The excess permittivity (figure 8) calculated by a similar equation like (3) is found to be positive on whole composition interval and slightly increase with increasing temperature with a maximum around 0.77 1012 F  m1 and 0.40 mole fraction of [bmim]I. This indicates again that both unlike molecular species interact in such a way that they act as more H-bonded structure than those of pure compounds themselves.

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4. Conclusions The binary (1-butyl-3-methylimidazolium iodide + 1-butanol) system has been investigated isothermally at (vapour + liquid) equilibrium at three temperatures: (353.15, 363.15, and 373.15) K. By correlation of the experimental (P, T, x) data with Wilson thermodynamic model it was found that the system is zeotropic with small positive and negative deviations from ideality (S-shaped). Excess molar enthalpy calculated from the excess molar Gibbs free energy temperature dependence by Gibbs– Helmholtz equation is positive and high, indicating specific interactions between IL and organic solvent molecules. The GE values are smaller than those of the binary [bmim]Br and [bmim]Cl + 1-butanol systems [11,12] due to the smaller strength of the intermolecular interactions between [bmim]I and 1-butanol molecules at mixing. For the same system, refractive index measurements have been performed at three temperatures (293.15, 298.15 and 308.15) K in whole composition interval. Excess refractive index is positive at all temperatures on whole composition range and increase slightly with increasing temperature. Using the Lorenz–Lorentz (nD-q) mixing rule, the densities, excess molar volumes, surface tensions and surface tension deviations have been predicted. By other known relations, dielectric permittivity and its deviations have been also calculated at the same temperatures of (293.15, 298.15 and 308.15) K. Structural effects for the binary investigated mixture have been explained in terms of excess thermodynamic and thermophysical properties. Acknowledgements This contribution was carried out within the research programme ‘‘Chemical thermodynamics and kinetics. Quantum chemistry’’ of the ‘‘Ilie Murgulescu’’ Institute of Physical Chemistry, financed by the Romanian Academy. Support of the EU (ERDF) and Romanian Government, which allowed for the acquisition of the research infrastructure under POS-CCE O 2.2.1 Project INFRANANOCHEM – Nr. 19/01.03.2009, is gratefully acknowledged. References [1] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. [2] J. Gmehling, Ionic Liquids in Separation Processes, in: T.M. Letcher (Ed.), Chemical Thermodynamics for Industry, The Royal Society of Chemistry, Cambridge, UK, 2004 (Chapter 7). [3] A.B. Pereiro, J.M.M. Araujo, J. Chem. Thermodyn. 46 (2012) 2–28.

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JCT 15-36