Physicochemical properties of (1-butyl-1-methylpyrrolydinium dicyanamide + γ-butyrolactone) binary mixtures

J. Chem. Thermodynamics 91 (2015) 327–335

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Physicochemical properties of (1-butyl-1-methylpyrrolydinium dicyanamide + c-butyrolactone) binary mixtures Nebojša Zec a,b, Marija Bešter-Rogacˇ a, Milan Vraneš b, Slobodan Gadzˇuric´ b,⇑ a b

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Vecˇna pot 113, 1000 Ljubljana, Slovenia Faculty of Science, Department of Chemistry, Biochemistry and Environmental Protection, University of Novi Sad, Trg Dositeja Obradovic´a 3, 21000 Novi Sad, Serbia

a r t i c l e

i n f o

Article history: Received 3 April 2015 Received in revised form 29 July 2015 Accepted 11 August 2015 Available online 20 August 2015 Keywords: Pyrrolydinium ionic liquids Dicyanamide c-butyrolactone Volumetric properties Electrical conductivity Viscosity

a b s t r a c t Experimental densities, electrical conductivities and dynamic viscosities of the pure 1-butyl-1methylpyrrolydinium dicyanamide ionic liquid, [bmpyrr][DCA], and its binary liquid mixtures with c-butyrolactone (GBL) were measured at temperatures from (273.15 to 323.15) K and at pressure of 0.1 MPa over the whole composition range. From the experimental density data the related excess molar volumes were calculated and fitted using Redlich–Kister’s polynomial equation. Obtained values are negative in the whole range of ionic liquid mole fraction and at all temperatures. Other volumetric properties, such as isobaric thermal expansion coefficients, partial molar volumes and partial molar volumes at infinite dilution were also calculated, in order to obtain information about the interactions between GBL and the selected ionic liquid. Negative values of these properties for both components indicate stronger interactions between GBL and IL compared to the pure components and better packing due to the differences in size and shape of the studied molecules. From the viscosity results, the Angell strength parameter was calculated and found to be 5.47 indicating that [bmpyrr][DCA] is a ‘‘fragile” liquid. All the results are compared with those obtained for binary mixtures of 1-butyl-1-methylpyrrolydinium bis(trifluorome thylsulfonyl)imide, [bmpyrr][NTf2], with GBL. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In the past two decades ionic liquids (ILs) attracted the attention of many researchers as suitable chemistries for a wide range of applications. Favorable physical and chemical properties of ILs, such as high thermal [1] and electrochemical stability [2], large liquidus range [3], non-flammability [4], biodegradability and recyclability [5], make them interesting as so-called green solvents associated with little waste, risk and hazard problems. Due to suitable salvation properties, air resistivity, high electrical conductivity and negligible vapor pressure, their utilization as novel electrolytes for electrochemical devices, such as lithium-ion batteries has also been the subject of intense studies [6–8]. Exchange of common organic solvents by ILs can enhance the safety of lithium-ion batteries [9]. However, a limiting factor in the application of ionic liquids is their high viscosity which can be overcome by mixing ILs with appropriate molecular solvents. Ionic liquids with dicyanamide anion (DCA) were investigated due to their properties such as low melting point and low viscosity [10,11], efficient mass transport [12] and a high electrical conduc⇑ Corresponding author. Tel.: +381 21 485 2744; fax: +381 21 454 065. E-mail address: [email protected] (S. Gadzˇuric´). http://dx.doi.org/10.1016/j.jct.2015.08.014 0021-9614/Ó 2015 Elsevier Ltd. All rights reserved.

tivity [13]. Due to the coordinating ability of the DCA anion, metal salts are often better soluble in dicyanamide based ionic liquids [14]. Also, they are not moisture sensitive and have wide electrochemical window which makes them good candidates for electrochemical devices [15]. It has been shown that [bmpyrr][DCA] ionic liquid is suitable medium for electrodeposition of different metals [16–19]. DCA as part of an ionic liquid and electrolyte thereof is promising in the use for energy storage devices because of its affordability, availability and electrochemical stability [15]. Molecular liquids such as c-butyrolactone (GBL) have a high boiling point, low melting point and low vapor pressure. The GBL is also a non-corrosive liquid suitable for electrochemical cells operating over a wide temperature range for a long time. These properties make GBL a good solvent candidate to be used to improve volumetric and transport properties of ILs. The mixture of GBL with selected ILs can improve both the performance and thermal stability of the electrolytes [20,21]. Thus, GBL is usually applied as a solvent in the new generation of lithium-ion batteries and electrochemical devices since the polarity of GBL provides excellent solvation of lithium ions and the increasing conductivity. In this paper, density, viscosity and electrical conductivity of [bmpyrr][DCA] binary mixtures with GBL were examined in the temperature range from (273.15 to 323.15) K and at pressure of

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N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

0.1 MPa. Calculated volumetric parameters and transport properties are compared with those obtained in our previous studies of the bis(trifluoromethylsulfonylimide) based ionic liquid containing the same cation. 2. Experimental section 2.1. Materials Ionic liquid [bmpyrr][DCA] and GBL were purchased from IOLITEC (x P 0.98) and Aldrich (x P 0.99), respectively. The summary of the provenance and purity of the samples is given in table 1. Chemicals were used as received and kept in the dry box in the nitrogen atmosphere. Water content specified by the supplier was 846 ppm. Prior to use, ionic liquid was additionally kept in vacuum desiccator over P2O5 for 24 h. After that water content was found to be 211 ppm determined by Karl Fischer titration. Binary mixtures covering the whole composition range of [bmpyrr][DCA] and GBL were prepared by measuring appropriate amounts of the components on a Sartorius analytical balance. The standard uncertainty of mass fraction is estimated 7.5
104. 2.2. Apparatus and procedure 2.2.1. Density measurements The density of pure components and binary mixtures were measured at pressure of 0.1 MPa using a vibrating tube densimeter, Anton Paar DMA5000 with a declared reproducibility of 1
103 kg
m3. The calibration of densimeter was performed using the density data for dry air and water. The instrument was thermostated within T = ±0.001 K and viscosity related errors in the density were automatically corrected over full viscosity range, thus the major source of measuring error were gas bubbles entrapped in the measuring cell filled with a sample. Therefore, the measuring cell was filled very carefully to minimize the probability of such error. The total volume of the samples used for density measurements was approximately 1 cm3. The relative standard uncertainty of determining the density was estimated to be 0.4%. 2.2.2. Viscosity measurements The viscosity of solutions was determined with a micro Ubbelohde viscometers (SI Analytics GmbH, Mainz, Germany, type No. 536 10 capillary I and type No. 536 20 capillary II) and an automatic flow time measuring system ViscoSystemÒ AVS 370. The viscometer was immersed in a transparent thermostat bath where the temperature was maintained to 298.15 ± 0.01 K. Each measurement was automatically repeated at least five times and yielded a reproducibility of the flow time of <0.02%. The kinematic viscosity of solutions, m (m2
s1), was calculated from the equation m = K
t [22], where t (in s) is the flow time and K is the constants characteristic for the viscometer determined by the manufacturer. This value was checked measuring the viscosity of ultra pure Millipore water. The absolute (dynamic) viscosity, g (Pa
s = kg
m1
s1), was obtained from the relation g = m
d, where d (in kg
m3) is the density of investigated solution. The

errors from calibration and temperature control yield an uncertainty of 1% of absolute viscosity. 2.2.3. Electrical conductivity measurements Conductivity was measured with a set of capillary cells with different cell constants, B0 , as these are required for concentrated solutions [23], B0 = (3 to 85) cm1. An assembly lid equipped with nine conductivity cells and switch equipment connecting them to the PC-interfaced LCR Meter Agilent 4284 A permits conductivity to be measured at nine different concentrations at each temperature. The cells were calibrated with diluted potassium chloride solutions [24] and immersed in the high-precision thermostat described previously [25]. The oil bath was set to each temperature of a temperature program with the reproducibility within 0.005 K. The temperature was checked with a calibrated Pt-100 resistance thermometer (MPMI 1004/300 Merz) connected to an HP 3458 A multimeter. Solutions of different mole fractions, known by weight, were transferred under nitrogen into the capillary cells and measurements were carried out over a temperature cycle beginning and ending at 273.15 K. The cell arrangement permits conductivity to be measured at nine concentrations at each temperature. A home-developed software package was used for temperature control and acquisition of conductance data. The measuring procedure and the extrapolation of the sample conductivity to infinite frequency are as described [25]. 3. Results and discussion 3.1. Densities, viscosities and electrical conductivities of pure components Densities of pure components, [bmpyrr][DCA] and GBL were measured in the temperature range from (273.15 to 333.15) K in steps of 5 K and the results are presented in table 2 and plotted in figure 1, together with the available literature values for these compounds [26–39]. It is found that deviations of our experimental results obtained for IL (figure 1a) from the literature values are less than 0.05% in the whole range of temperature, except in the case of the values measured by França et al. [28] where deviations are between 0.1% and 0.2%. In the case of the values reported by Krolikowska et al. [30] deviations are around 0.3%. However, we believe that density values in the work of Krolikowska et al. are misprinted and shifted for T = 5 K. Experimental density values in work of Galan-Sanchez et al. [31] are erroneous and deviate around 10% from all literature values as already discussed by other authors [26,27]. Experimental densities for GBL (figure 1b) deviate from the literature values up to 0.075%, except in the case of the values obtained by Yang et al. [36]. It has to be said that there is no available (or very few) literature data for the experimental densities measured at temperatures below 288.15 K and above 313.15 K for both IL and GBL. Viscosity of the pure components (table 2) was measured in the temperature range from (273.15 to 323.15) K. Our viscosity results for pure IL are in a good agreement with the results obtained by

TABLE 1 Provenance and purity of the samples. Chemical name

[bmpyrr][DCA] c-butyrolactone a b

Provenance

IOLITEC Aldrich

Purification method

None None

Specified by the supplier. Water content after keeping in vacuum desiccator over P2O5 for 24 h.

Purity specified by the supplier Mass fraction purity

Halides

Water content by Karl–Fischer titration

x > 0.98 x P 0.99

<2%

846 ppma (211 ppm)b 176 ppm

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N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335 TABLE 2 Experimental densities (d), dynamic viscosities (g) and electrical conductivities (j) of pure [bmpyrr][DCA] and GBL at the specified temperatures and at 0.1 MPa.

273.15 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

g/(mPa
s)

d/(g
cm3)

j/(mS
cm1)

[bmpyrr] [DCA]

GBL

[bmpyrr] [DCA]

GBL

[bmpyrr] [DCA]

1.027694 1.024821 1.021962 1.019114 1.016277 1.013449 1.010638 1.007840 1.005058 1.002285 0.999525 0.996781 0.994049

1.149155 1.144229 1.139314 1.134403 1.129493 1.124588 1.119684 1.114782 1.109874 1.104965 1.100053 1.095137 1.090216

93.95 74.27 59.60 48.60 40.25 33.71 28.53 24.53 21.07 18.37 16.15

2.69 2.43 2.21 2.02 1.86 1.71 1.58 1.47 1.37 1.28 1.20

5.23 6.45 7.86 9.43 11.23 13.17 15.27 17.54 19.97 22.54 25.29

0.2

100⋅(dexp−dlit)/dexp

T/K

0.4

a)

-0.4

3.2. Volumetric parameters Densities of ([bmpyrr][DCA] + GBL) mixtures were measured in the temperature range from (273.15 to 323.15) K and presented as a function of temperature in table 3 and also in figure S1 in the supporting information. From these results it can be concluded that the density increases with the ionic liquid mole fraction in the binary mixture and decreases with the increasing temperature. Obtained data were fitted as a function of temperature using a linear fit. These parameters are tabulated in table S1 in the supporting information. Values of the excess molar volume, VE, were calculated from the experimental density of the mixture, d, densities of the pure components, di, the corresponding mole fractions, xi, and molar masses, Mi, using the following equation:

ð1Þ

where x1, M1 and d1 relate to [bmpyrr][DCA], and x2, M2 and d2 relate to GBL. Values of the excess molar volumes are given in table

100⋅(dexp − dlit)/dexp

0.4

0.2

0.0 6

c)

4 2 100⋅(ηexp−ηlit)/ηexp

Gonzalez et al. [26] with the similar instrumentation and Krolikowska et al. [30]. Deviations of our viscosity from Gonzales and Krolikowska are approximately 3%, which is in agreement with the uncertainty of 5% reported in their papers. Only three more values for [bmpyrr][DCA] were found in literature (50 and 41) mPa
s at T = 293.15 K [11,40] and 36.5 mPa
s at T = 298.15 K [41]. Viscosities of pure GBL are graphically presented in figure 1c, and compared with those available in literature [32,33,35,38,42–45]. The relative viscosity deviation of [bmpyrr][DCA] from the literature data was found to be 2% in the entire range of temperatures, except in the case of literature [32] at higher temperatures, literature [45] at lower temperatures, and in literature [33] at 318.15 K where deviation is 4% or more. Density and viscosity of pure GBL deviate from our previous work due to different instrumentation used [32]. However, results presented in this work are more accurate. The electrical conductivity of pure [bmpyrr][DCA] was measured in the temperature range from (273.15 to 323.15) K. There is an obvious lack of available experimental data in the literature for the electrical conductivity of pure [bmpyrr][DCA]. To the best of our knowledge and according to available literature data, systematic investigation of [bmpyrr][DCA] electrical conductivity have not been performed yet. Our conductivity results of pure IL obtained for the first time are listed in table 2.



 1 1 1 1 þ x2 M 2 ;   d d1 d d2

-0.2

b)

Relative standard uncertainties are: u(d) = 0.4%, u(g) = 1%, u(k) = 0.5%, RSD(p) = 1.5% standard uncertainty: u(T) = 0.015 K.

V E ¼ x1 M 1

0.0

0 -2 -4 -6 -8 -10

270

280

290

300

310

320

330

340

T/K FIGURE 1. Plot of relative deviations plotted against temperature between our experimental density and viscosity data (- - -) and those from the literature for: (a) [bmpyrr][DCA] density: (j) González et al. [26], (h) Blahut et al. [27], (d) França et al. [28], (s) González et al. [29], (–) Krolikowska et al. [30] and (b) GBL density and (c) GBL viscosity: (N) Vraneš et al. [32], (4) Boodida et al. [33], (.) Krakowiak et al. [34], (5) Moumouzias et al. [35], (w) Yang et al. [36], ( ) Abdullah et al. [37], (g) Lu et al. [38], ( ) Mathuni et al. [39], () Chagnes et al. [42], ( ) Kuratani et al. [43], ( ) Afans’yev et al. [44], () Ramkumar et al. [45].

3 and graphically presented in figure 2 using a Redlich–Kister’s polynomial equation [46] where YE represents the excess property:

Y E ¼ x1 x2

n X Ai ð1  2x1 Þi :

ð2Þ

i¼0

Values of the excess molar volumes were fitted by a method of the least squares. Here, Ai refers to the adjustable parameters and n is the number of the coefficients in the equation. The coefficients of the Redlich–Kister’s equation, as well as the standard deviations of the fit are given in table S2 in the supporting information. Negative values of excess molar volumes (figure 2) in the whole range of IL mole fraction and at all temperatures indicate stronger

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N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

TABLE 3 Density (d), excess molar volume (VE), apparent molar volume (V/i), and partial molar volume (Vi) at different temperatures and mole fraction (x1) compositions of the ([bmpyrr] [DCA] (1) + GBL (2)) mixtures at 0.1 MPa. x1

d/(g
cm3)

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.149155 1.124753 1.107123 1.087917 1.063962 1.046522 1.032974 1.027694

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.139314 1.116136 1.099190 1.080611 1.057342 1.040350 1.027116 1.021962

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.129493 1.107563 1.091307 1.073370 1.050781 1.034233 1.021315 1.016277

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.124588 1.103289 1.087383 1.069767 1.047527 1.031193 1.018433 1.013449

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.119684 1.099021 1.083471 1.066180 1.044281 1.028171 1.015566 1.010638

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.109874 1.090512 1.075667 1.059035 1.037833 1.022161 1.009872 1.005058

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

1.100053 1.082021 1.067900 1.051931 1.031434 1.016205 1.004229 0.999525

VE/(cm3
mol1)

V1/(cm3
mol1)

V2/(cm3
mol1)

V/1/(cm3
mol1)

V/2/(cm3
mol1)

74.856 74.736 74.533 74.226 74.043 73.897

200.12 200.70 201.26 201.91 202.36 202.67

74.62 74.44 74.25 74.04 73.93 73.87

75.497 75.366 75.147 74.825 74.642 74.485

201.04 201.68 202.30 202.99 203.47 203.80

75.247 75.050 74.858 74.639 74.522 74.464

76.148 76.004 75.767 75.428 75.245 75.072

201.94 202.65 203.33 204.08 204.58 204.94

75.878 75.668 75.465 75.237 75.112 75.052

76.477 76.326 76.080 75.731 75.547 75.363

202.38 203.13 203.84 204.61 205.13 205.50

76.196 75.980 75.770 75.535 75.407 75.339

76.808 76.651 76.395 76.037 75.852 75.657

202.82 203.60 204.34 205.15 205.69 206.07

76.517 76.293 76.077 75.837 75.703 75.630

77.480 77.309 77.032 76.652 76.465 76.251

203.68 204.55 205.35 206.22 206.80 207.21

77.165 76.927 76.698 76.443 76.302 76.219

78.165 77.978 77.678 77.275 77.084 76.846

204.51 205.47 206.35 207.29 207.91 208.34

77.823 77.569 77.325 77.055 76.903 76.805

T/K = 273.15 0.262 0.387 0.459 0.436 0.299 0.101

200.65 201.39 202.03 202.51 202.65 202.68 T/K = 283.15

0.284 0.416 0.490 0.462 0.316 0.107

201.64 202.45 203.14 203.65 203.78 203.82 T/K = 293.15

0.308 0.448 0.525 0.491 0.336 0.115

202.61 203.50 204.24 204.78 204.92 204.96 T/K = 298.15

0.321 0.465 0.544 0.509 0.347 0.119

203.10 204.02 204.80 205.35 205.49 205.53 T/K = 303.15

0.334 0.483 0.563 0.526 0.360 0.124

203.58 204.54 205.35 205.92 206.05 206.10 T/K = 313.15

0.362 0.520 0.605 0.562 0.384 0.133

204.52 205.57 206.45 207.06 207.19 207.24 T/K = 323.15

0.393 0.562 0.650 0.603 0.412 0.143

205.45 206.60 207.55 208.19 208.33 208.39

Standard uncertainties are: u(x) = 7.5
104, u(VE) = 5.2
103 cm3
mol1, u(VU) = 8.9
103 cm3
mol1, u(T) = 0.015 K, Relative standard uncertainty: u(d) = 0.4%, RSD (p) = 1.5%.

interactions between GBL and IL compared to the pure components. In other words, IL is solvated by GBL molecules due to a strong ion–dipole interactions weakening ion–ion interactions between [bmpyrr]+ and [DCA]. This is in accordance with conclu-

sions obtained from the experimental viscosity and electrical conductivity. By comparing the excess molar volumes obtained for the ([bmpyrr][DCA] + GBL) mixture with those reported for ([bmpyrr] [NTf2] + GBL) [47] (figure 2), it can be noticed that the V E values

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N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

The values for the partial molar volumes V1 and V2 are given in table 3and plotted in figure S2. The values of the partial molar volumes of components increase with increasing their mole ratio in the mixture. In figure S3 values of partial molar volumes are graphically compared with the values calculated for the system containing [bmpyrr][NTf2] [47]. In the case when x1 = 0 or x2 = 0, equations (3) and (4) may be transformed into:

0.2 0.1 0.0

VE/cm3⋅mol-1

-0.1 -0.2 -0.3 -0.4

o V1 1 ¼ V1 þ

-0.5

ð5Þ

i¼0

-0.6 o V1 2 ¼ V2 þ

-0.7 0.2

0.4

0.6

0.8

1.0

x1 FIGURE 2. Excess molar volumes of the ([bmpyrr][DCA] (1) + GBL (2)) binary mixtures as a function of [bmpyrr][DCA] mole fraction, (x1), at different temperatures: (j) 273.15 K, (s) 283.15 K, (N) 293.15 K, (5) 298.15 K, (J) 303.15 K, (.) 313.15 K and (r) 323.15 K. The lines represent the Redlich–Kister-type fittings (Equation (2)) with the parameters indicated in the table S2. For comparison the values for ([bmpyrr][NTf2] + GBL) binary mixture (w) at T = 298.15 K are presented [45].

are negative in the whole composition range in the case of [bmpyrr][DCA], while they are positive for [bmpyrr][NTf2] binary mixtures with GBL. These differences depending solely on type of the IL anion are caused by the stronger ion–dipole interactions and possible hydrogen bonding between GBL and DCA with pronounced hydrogen bond basicity [48]. These facts provide negative excess molar volumes in the case of [DCA] based ionic liquid. Similar observation were made by González et al. [26], who investigated binary mixtures of [bmpyrr][DCA] with alcohols. By comparing the V E values of ([bmpyrr][DCA] + GBL) mixture with the mixture of the same IL with alcohols, it can be noticed that in both cases negative V E values were observed, showing asymmetrical curves typical in systems containing components with a large molar volume difference [26]. It is worth mentioning that V E curves have the same shape and minimum at molecular solvent rich region for both GBL and alcohols. Knowing the molar volumes of the pure components, V o1 for [bmpyrr][DCA] and V o2 for GBL, the partial molar volumes of the components, V1 and V2 can be calculated from the expressions (3) and (4), using the parameters Ai obtained from equation (2) and presented in table S2:

V 1 ¼ V o1 þ ð1  x1 Þ

i¼n X Ai ð1Þi ðx2 ! 0Þ;

ð6Þ

i¼0

-0.8 0.0

i¼n X 2

i¼n X Ai ð1  2x1 Þi  2x1 ð1  x1 Þ2 Ai ðiÞð1  2x1 Þi1 ;

i¼0

i¼0

ð3Þ V 2 ¼ V o2 þ x21

i¼n X Ai ðx1 ! 0Þ;

i¼n i¼n X X Ai ð1  2x1 Þi þ 2x21 ð1  x1 Þ Ai ðiÞð1  2x1 Þi1 : i¼0

i¼0

ð4Þ

1 where V 1 1 and V 2 are the partial molar volumes of the components at infinite dilution. Useful information about solute–solvent interactions can be obtained knowing the partial molar properties at infinite dilution. At infinite dilution solute–solute interactions can be neglected. Thus, rearrangement of equations (5) and (6) gives partial excess molar volumes at infinite dilution of the components

ðV E1 Þ

1

1

and ðV E2 Þ :

1

ðV E1 Þ ¼

i¼n X

Ai ;

ð7Þ

Ai ð1Þi ;

ð8Þ

i¼0

1

ðV E2 Þ ¼

i¼n X i¼0

The partial molar volumes at infinite dilution and partial excess molar volumes at infinite dilution are listed in table 4. Negative values of these properties for both components also indicate a stronger ion–dipole interactions between GBL and IL compared to the pure components. This is the opposite trend to the one obtained in ([bmpyrr][NTf2] + GBL) binary mixtures [47] and similar to the one obtained in ([bmim][NTf2] + GBL) system [32]. In addition to excess molar volumes, other volumetric properties have been calculated. The apparent molar volumes, V /1 and V /2 , were obtained using the following expressions:

V /1 ¼

ðd2  dÞ M 1 þ ; m1 dd2 d

ð9Þ

V /2 ¼

ðd1  dÞ M 2 þ : m2 dd1 d

ð10Þ

Here, V /1 and V /2 are the apparent molar volumes of [bmpyrr][DCA] and GBL, respectively. The m1 and m2 are molalities related to IL and GBL, respectively. The apparent molar volumes are reported in table 3 and plotted in figure S4. As in the case of partial molar volumes, values of the apparent molar volumes of components increase with increasing their mole ratio in the mixture. It can be seen from figure 3 that addition of small amounts of GBL leads to a much larger change in volume in the system containing [NTf2] anion [47] compared to the system containing [DCA].

TABLE 4 Partial molar volume at infinite dilution (Vi1) and partial molar excess volume at infinite dilution (ViE)1 for the components of the mixtures at different temperatures. T/K

3 1 V1 ) 1 /(cm
mol

(VE1)1/(cm3
mol1)

3 1 V1 ) 2 /(cm
mol

(VE2)1/(cm3
mol1)

273.15 283.15 293.15 298.15 303.15 313.15 323.15

199.60 200.42 201.26 201.68 202.08 202.87 203.64

3.227 3.501 3.802 3.961 4.128 4.480 4.865

73.758 74.318 74.870 75.141 75.414 75.971 76.519

1.157 1.245 1.350 1.411 1.473 1.596 1.741

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N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

0.5

79.0

0.0

78.0

-0.5

77.5

-1.0

76.5

-1.5 -2.0

76.0

-2.5

75.5

-3.0 -3.5

75.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

x1 FIGURE 3. Apparent molar volumes of GBL (j) ([bmpyrr][DCA] + GBL) and (s) ([bmpyrr][NTf2] + GBL) [45] as a function of ionic liquid mole fraction, (x1), at T = 298.15 K.

On the basis of the volumetric data, one can also calculate the isobaric thermal expansivity. Thermal expansion coefficients, aip and ap , for the pure components and mixtures, respectively, are defined as:

aip ¼


 1 @V oi ; V oi @T P

ð11Þ

ap ¼


 1 @V : V @T P

ð12Þ

At constant molality, the expression (12) can be written as:

ap ¼ 


 1 @d ; d @T P

ð13Þ

and the corresponding isobaric thermal expansion coefficients were calculated using equation (13). The values of these coefficients aip and ap are given in table 5 at different temperatures. Also the excess thermal expansion, aEp , was calculated using equation (14): i¼2 X a ¼ ap  /i aip ;

ð14Þ

E p

i¼1

where /i is the volume fraction of component i, defined as:

,

/i ¼

E

77.0

5

10 ⋅ αp / K-1

3

VΦ2 / cm ⋅ mol

-1

78.5

xi V oi

i¼2 X xi V oi :

ð15Þ

i¼1

TABLE 5 Thermal expansion coefficients, (ap), as a function of mole fraction of ionic liquid, (x1), in the temperature range from (273.15 to 323.15) K for the investigated mixtures. x1

T/(K) 273.15

283.15

293.15

298.15

8.544 7.597 7.085 6.614 6.114 5.794 5.565 5.482

8.618 7.656 7.136 6.659 6.152 5.828 5.597 5.513

8.693 7.715 7.187 6.704 6.190 5.863 5.629 5.544

8.731 7.745 7.213 6.726 6.210 5.880 5.645 5.559

0.4

0.6

0.8

1.0

φ1 FIGURE 4. Variation of excess thermal expansion coefficients with volume fraction of ionic liquid, /1, at different temperatures: (j) 273.15 K, (s) 283.15 K, (N) 293.15 K, (5) 298.15 K, (J) 303.15 K, (.) 313.15 K and (r) 323.15 K.

The obtained results are presented in figure 4. As shown in figure 4, the aEp deviation is negative over the whole composition range at all temperatures. Expansivity coefficients are related to the fluctuation of the cross term of enthalpy interaction and interaction volume of liquids and mixtures. The excess amount may reflect the molecular orientation and packing of mixtures. Positive values of aEp , typical for the systems containing molecules capable to self-associate [49–52], were observed in the ([bmpyrr][NTf2] + GBL) [47]. Negative values of excess thermal expansion coefficient obtained in our case indicate the absence of GBL selfassociation in the mixture. This means that the addition of small amount of ionic liquid leads to re-orientation of GBL molecules and termination of weak hydrogen bonds present in pure GBL [53].

3.3. Viscosity Experimental viscosity values of the ([bmpyrr][DCA] + GBL) binary mixtures are listed in table 6 and graphically presented in figure S5. Viscosity of pure ionic liquid decreases with temperature from 93.92 mPa
s at 273.15 K to 16.14 mPa
s at 323.15 K. In figure S5 it can be seen that the addition of GBL reduces viscosity of the mixture as expected. The addition of molecular solvents with high relative permittivity such as GBL, increase ion solvation weakening ion–ion interactions resulting in reduced viscosity of the mixture. Compared with the ([bmpyrr][NTf2] + GBL) [47], [bmpyrr][DCA] mixtures with GBL have a similar viscosity up to x(IL) = 0.4. After this mole fraction, higher values of the viscosity can be observed in the system with [bmpyrr][NTf2] (figure S6) [47]. The experimental results of viscosity at different temperatures were fitted using the Vogel–Fulcher–Tammann (VFT) equation:

303.15

313.15

323.15

g ¼ a expðb=ðT  T o ÞÞ;

8.769 7.775 7.239 6.749 6.229 5.897 5.661 5.575

8.846 7.836 7.292 6.795 6.268 5.932 5.693 5.606

8.925 7.897 7.345 6.840 6.307 5.967 5.725 5.637

where g is the viscosity, T is the temperature in K, and a, b, To are the coefficients of VFT equation whose values are given in table S3 together with the standard deviations. The Angell strength parameter D (b/To) was calculated and also presented in table S3. This parameter is large for ‘‘strong” liquids where the viscosity approaches Arrhenius temperature dependence and is small for ‘‘fragile” liquids. As can be seen from table S3, the Angell parameter

ap
104/(K1) 0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

0.2

ð16Þ

333

N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335 TABLE 6 Viscosity, (g), and viscosity deviations, (Dg), of ([bmpyrr][DCA] + GBL) binary mixtures as a function of [bmpyrr][DCA] mole fraction, (x1), in the temperature range from (273.15 to 323.15) K and at 0.1 MPa. x1

T/(K) 273.15

283.15

293.15

298.15

303.15

313.15

323.15

1.71 2.80 4.02 6.19 11.38 18.62 28.31 33.71

1.58 2.56 3.64 5.53 9.91 16.03 23.88 28.53

1.37 2.18 3.04 4.52 7.85 12.31 18.01 21.07

1.20 1.87 2.58 3.77 6.36 9.69 13.92 16.15

Dg/(mPa
s) 0.00 0.00 2.57 2.06 4.53 3.64 6.52 5.21 7.92 6.27 6.80 5.33 2.87 2.23 0.00 0.00

0.00 1.68 2.95 4.22 5.10 4.29 1.99 0.00

0.00 1.13 1.98 2.82 3.33 2.76 1.11 0.00

0.00 0.80 1.39 1.95 2.29 1.91 0.75 0.00

ga/(mPa
s) 0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

2.69 4.69 7.20 12.03 24.90 45.65 76.07 93.95

2.21 3.75 5.58 8.99 17.71 30.79 49.24 59.60

0.0000 0.0987 0.1859 0.3029 0.4985 0.6954 0.9013 1.0000

0.00 7.00 12.44 18.30 23.27 20.48 8.85 0.00

0.00 4.13 7.29 10.60 13.10 11.32 4.68 0.00

1.86 3.07 4.46 6.96 13.06 21.74 33.58 40.25

Relative standard uncertainties are: u(g) = 1%, RSD(p) = 1.5% standard uncertainties: u(T) = 0.015 K, u(x) = 7.5
104.

for pure ionic liquid is 5.47, which indicates that [bmpyrr][DCA] is a ‘‘fragile” liquid [54]. Deviation of the viscosity, Dg, is calculated using equation (17):

Dg ¼ g  ðx1 g1 þ x2 g2 Þ;

ð17Þ

where x1 and x2 are the mole fractions of [bmpyrr][DCA] and GBL; g, g1 and g2 are the experimental viscosity of these mixtures, pure ionic liquid and pure GBL, respectively. Figure 5 shows the deviation of viscosity as a function of IL mole fraction for each measured temperature. All values of Dg are negative in the whole temperature range, as shown in figure 5 and table 6. With decreasing temperature, more negative values of Dg were observed. It can be seen that at all temperatures the maximum deviation of viscosity is at x(IL) = 0.55. Negative viscosity deviations and negative VE values are probably the consequence of formation of heteronuclear complexes due to intermolecular hydrogen bonding between hydrogen atom in the position C-2 of

GBL and nitrogen atom of DCA, which is in accordance with calculated volumetric data. The dependence of the viscosity deviations on IL mole fraction at different temperatures were obtained using the Redlich–Kister’s polynomial fit of the third order whose parameters are given in table S4, together with the standard deviations. 3.4. Electrical conductivity Experimental values of electrical conductivity of ([bmpyrr] [DCA] + GBL) mixtures as a function of temperature are given in table 7 and graphically presented in figure 6. The conductivity data are analyzed using the empirical Casteel–Amis four-parameter equation [55]:

j ¼ jmax




x1

x1;max

n


 x1  x1;max : exp mðx1  x1;max Þ2  n x1;max

ð18Þ

Here, jmax is the highest electrical conductance for a given mixture, xmax is the mole fraction of ionic liquid in which the electrical conductivity of the mixture at a given temperature shows maximum, n and m are parameters of the fit. The results are given in table S5 and graphically presented in figure 6. It can be observed from figure 6 that addition of [bmpyrr][DCA] leads to the rapid growth of the

TABLE 7 Electrical conductivity, (j), of ([bmpyrr][DCA] + GBL) binary mixture as a function of [bmpyrr][DCA] mole fraction, (x1), in the temperature range from (273.15 to 323.15) K and at 0.1 MPa. x1

T/(K) 273.15

283.15

293.15

298.15

303.15

313.15

323.15

15.78 23.82 27.13 28.99 28.56 27.90 26.19 23.34 19.12 16.08 15.27

17.92 27.27 31.33 33.70 33.63 33.19 31.62 28.75 24.26 20.90 19.97

20.09 30.80 35.66 38.55 38.94 38.75 37.42 34.58 29.94 26.33 25.29

j/(mS
cm1) 0.0463 0.0987 0.1449 0.1859 0.2493 0.3029 0.3845 0.4985 0.6954 0.9013 1.0000

9.77 14.25 15.65 16.34 15.17 14.22 12.52 10.28 7.42 5.62 5.23

11.69 17.26 19.25 20.28 19.26 18.41 16.58 14.07 10.70 8.49 7.86

13.70 20.49 23.09 24.50 23.74 22.94 21.16 18.42 14.57 11.92 11.23

14.74 22.15 25.10 26.72 26.13 25.39 23.63 20.83 16.78 13.92 13.17

Relative standard uncertainties are: u(k) = 0.5%, RSD(p) = 1.5% standard uncertainties: u(T) = 0.015 K, u(x) = 7.5
104.

0

40

-5

κ / mS cm-1

Δη / mPa⋅s

30

-10 -15

10

-20 -25 0.0

20

0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 5. Viscosity deviations from the ideality of the ([bmpyrr][DCA] (1) + GBL (2)) binary mixtures as a function of [bmpyrr][DCA] mole fraction, (x1), at: T = (j) 273.15 K, (s) 283.15 K, (N) 293.15 K, (5) 298.15 K, (J) 303.15 K, (.) 313.15 K and (r) 323.15 K. The lines represent the Redlich–Kister-type fittings (Equation (2)) the parameters indicated in table S4 as the supporting information of this manuscript.

0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 6. Electrical conductivity of the ([bmpyrr][DCA] (1) + GBL (2)) binary mixture as a function of [bmpyrr][DCA] mole fraction, (x1) at: T = (j) 273.15 K, (s) 283.15 K, (N) 293.15 K, (5) 298.15 K, (J) 303.15 K, (.) 313.15 K and (r) 323.15 K. The lines represent the Casteel–Amish-type fitting (Equation (18)) of the experimental data with parameters reported in table S6 of the supporting information.

334

N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

electrical conductivity of the mixture and reaches its maximum at a value of x1  0.2 in the whole temperature range. This occurs due to a pronounced ion solvation with GBL molecules. The ion–ion interactions in the pure ionic liquid are replaced by the ion–dipole interactions between the [bmpyrr]+ and [DCA] with GBL. Electrical conductivity of the mixture gradually decreases almost linearly, due to the reduced mobility of the ions and strong ion–ion interactions, reaching the lowest value in pure ionic liquids where the cation–anion interactions are the most pronounced [56]. Electrical conductivity values are compared for ([bmpyrr][DCA] + GBL) with ([bmpyrr][NTf2] + GBL) [47] at T = 298.15 K and presented in figure 7. From these figure it can be noticed that electrical conductivity values are higher in the [bmpyrr][DCA] mixtures with GBL comparing to those containing [bmpyrr][NTf2] in the whole composition range. Electrical conductivity of pure [bmpyrr][DCA] (13.17 mS
cm1 at T = 298.15 K) is almost five times higher than [bmpyrr][NTf2] (2.77 mS
cm1 at T = 298.15 K [47]). At the compositions where maximum of conductivity is observed, difference in conductivity is around 70%. It is well known that the electrical conductivity of the IL mixtures with molecular solvents largely depends on the viscosity of the system. However, the viscosity values of both mixtures is almost identical up to x1  0.4, suggesting that in addition to viscosity, conductivity is influenced by other factors, such as the ion aggregation [54,57]. Based on the experimental density and electrical conductivity data, molar conductivity (Km) was calculated. The calculated molar conductivity is given in table 8 and presented in figure S7. From figure S7, it can be observed that the molar conductivity increases with increasing temperature for all investigated mixtures. Molar conductivity values for pure [bmpyrr][DCA] are in the range from (0.44 to 2.18) S
cm2
mol1 at temperatures from (273.15 to 323.15) K, which is a consequence of the better ions mobility at higher temperature [58]. The molar conductivity data were fitted as a function of temperature using the VFT equation (19):

from matrix modes in superionic conductors [56]. This exponent is the slope of the logarithmic form of equation (21), while lg K is the graph’s intercept:

Km ¼ A expðB=ðTT o ÞÞ;

lgKm ¼ a
lgðg1 Þ þ lgK:

ð19Þ

where Km is the molar conductivity, T is the temperature in K, and A, B, To are the coefficients of VFT equation whose values are given in table S6 as the supporting information.

30

TABLE 8 Molar conductivity, (Km), of ([bmpyrr][DCA] + GBL) binary mixture as a function of [bmpyrr][DCA] mole fraction, (x1), in the temperature range from (273.15 to 323.15) K. x1

T/(K) 273.15

0.0463 0.0987 0.1449 0.1859 0.2493 0.3029 0.3845 0.4985 0.6954 0.9013 1.0000

37.58 25.18 18.46 14.73 9.87 7.39 4.88 2.86 1.26 0.59 0.44

283.15

293.15

298.15

Km/(S
cm2
mol1) 53.56 57.84 36.77 39.90 27.65 30.17 22.42 24.54 15.66 17.30 12.09 13.42 8.35 9.36 5.18 5.88 2.50 2.89 1.27 1.49 0.95 1.12

45.33 30.74 22.87 18.42 12.62 9.63 6.50 3.93 1.82 0.90 0.66

303.15

313.15

323.15

62.20 43.08 32.73 26.72 18.97 14.79 10.41 6.61 3.30 1.72 1.30

71.20 49.71 38.08 31.29 22.50 17.72 12.64 8.19 4.21 2.25 1.71

80.52 56.58 43.67 36.05 26.23 20.82 15.06 9.91 5.22 2.86 2.18

Molar conductivity values are compared in figure S8 for the same mixtures at T = 298.15 K. From these figure it can be noticed that molar conductivities are much higher for ([bmpyrr][DCA] + GBL) binary mixtures, especially in the IL low concentration range, indicating better separation of the ions in case of ([bmpyrr][DCA] + GBL) mixtures comparing to those with [bmpyrr][NTf2]. Molar conductivity is related to viscosity through so-called Walden rule [59], defined as (20):

Km
ga ¼ K ¼ const;

ð20Þ

where Km is the molar conductivity, g is the viscosity. The exponent a is used to describe the decoupling of mobile ion relaxation modes

ð21Þ

Figure 8 shows the Walden plots for the investigated mixture. The solid straight line, so called ‘‘ideal Walden line”, represents 0.01 mol
dm3 KCl aqueous solution as known to be fully dissociated and to have ions of equal mobility [56]. Pure [bmpyrr][DCA] and (IL + GBL) mixtures are located in the top right-hand corner of the diagram, corresponding to the most favorable conditions for ILs [56] since the high conductivity is combined with the low

25 3 x1= 0.0987 x1= 0.1859

20

x1= 0.3029

log (Λm / S⋅cm ⋅mol )

10

5

0 0.0

0.2

0.4

0.6

0.8

1.0

x1 FIGURE 7. Electrical conductivity of mixture (j) ([bmpyrr][DCA] + GBL) and (s) ([bmpyrr][NTf2] + GBL) [45] as a function of ionic liquid mole fraction (x1), at T = 298.15 K.

x1= 0.4985 x1= 0.6954

-1

15

x1= 0.9013 x1= 1.0000

2

κ / mS⋅cm

-1

1

-1

T

-3

-5 -5

-3

-1 -1

1

3

-1

log(η / P ) FIGURE 8. Walden plot for binary systems ([bmpyrr][DCA] + GBL). The solid straight line, due to a 0.01 mol
dm3 KCl solution, fixes the position of the ideal Walden line.

N. Zec et al. / J. Chem. Thermodynamics 91 (2015) 327–335

viscosity. On the other hand, from figure 8 it can be seen that addition of GBL leads to increased ionicity showing almost an ideal behavior in GBL-rich region. 4. Conclusions Volumetric and transport properties of binary liquid mixtures of 1-butyl-1-methylpyrrolydinium dicyanamide ionic liquid with cbutyrolactone are presented in this paper at various temperatures at the pressure of 0.1 MPa in the whole composition range and compared with literature values for (1-butyl-1-methylpyrrolydinium dicyanamide + c-butyrolactone) binary system. Negative values of excess molar volumes indicate solvation of IL by GBL molecules due to a strong ion–dipole interactions weakening ion–ion interactions between [bmpyrr]+ and [DCA]. Negative viscosity deviations values are probably the consequence of formation of heteronuclear complexes due to intermolecular hydrogen bonding between hydrogen atom in the position C-2 of GBL and nitrogen atom of DCA, which is in accordance with calculated volumetric data. From electrical conductivity data it can be noticed that molar conductivities are much higher for ([bmpyrr][DCA] + GBL) binary mixtures compared to ([bmpyrr][NTf2] + GBL) system, especially in the IL low concentration range, indicating better separation of the ions in case of ([bmpyrr][DCA] + GBL) mixtures comparing to those with [bmpyrr][NTf2]. Acknowledgments N. Z. is grateful to Erasmus Mundus Basileus mobility grant enabling the research cooperation at University of Ljubljana. The work was financially supported by the Slovenian Research Agency through Grant No. P1-0201, Ministry of Education, Science and Technological Development of Serbia under project contract ON172012 and The Provincial Secretariat for Science and Technological Development of APV. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2015.08.014. References [1] R. Ge, C. Hardacre, P. Nancarrow, D.W. Rooney, J. Chem. Eng. Data 52 (2007) 1819–1823. [2] M. Hayyan, F.S. Mjalli, M.A. Hashim, I.M. Al Nashef, X.M. Tan, J. Ind. Eng. Chem. 19 (2013) 106–112. [3] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398. [4] H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. Nukuda, H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 174 (2007) 1021–1026. [5] P. Wasserscheid, R. Hal, A. Boesmann, Green Chem. 234 (2002) 400–404. [6] M. Montanino, M. Moreno, M. Carewska, G. Maresca, E. Simonetti, R. Lo Presti, F. Alessandrini, G.B. Appetecchi, J. Power Sources 269 (2014) 608–615. [7] M. Nádherná, J. Reiter, J. Moškon, R. Dominko, J. Power Sources 196 (2011) 7700–7706. [8] R.S. Kühnel, N. Böckenfeld, S. Passerini, M. Winter, A. Balducci, Electrochim. Acta 56 (2011) 4092–4099. [9] Y. An, P. Zuo, X. Cheng, L. Liao, G. Yin, Int. J. Electrochem. Sci. 6 (2011) 2398– 2410. [10] D.R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth, G.B. Deacon, Chem. Commun. 16 (2001) 1430–1431. [11] D.R. MacFarlane, S.A. Forsyth, J. Golding, G.B. Deacon, Green Chem. 4 (2002) 444–448. [12] T.J. Simons, P.M. Bayley, Z. Zhang, P.C. Howlett, D.R. MacFarlane, L.A. Madsen, M. Forsyth, J. Phys. Chem. B 118 (2014) 4895–4905. [13] Y. Yoshida, O. Baba, G. Saito, J. Phys. Chem. B 111 (2007) 4742–4749. [14] D.X. Zhuang, M.J. Deng, P.Y. Chen, I.W. Sun, J. Electrochem. Soc. 155 (2008) 575–579.

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JCT 15-230