Experimental and theoretical excess molar properties of imidazolium based ionic liquids with isomers of butanol

Thermochimica Acta 634 (2016) 38–47

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Experimental and theoretical excess molar properties of imidazolium based ionic liquids with isomers of butanol Zuber S. Vaid a , Utkarsh U. More a , Shantilal B. Oswal b , Naved I. Malek a,∗ a b

Applied Chemistry Department, S. V. National Institute of Technology, Surat 395 007, India Department of Chemistry, V.N. South Gujarat University, Udhna-Magdalla Road, Surat 395 007, India

a r t i c l e

i n f o

Article history: Received 3 October 2015 Received in revised form 15 March 2016 Accepted 19 March 2016 Available online 21 March 2016 Keywords: 1-Octyl-3-methylimidazolium tetrafluoroborate Isomers of butanol Excess molar volume Excess molar isentropic compressiblility Deviation in refractive index PFP theory ERAS model

a b s t r a c t The experimental densities (), speeds of sound (u), and refractive indices (nD ) of binary mixtures of 1-octyl-3-methylimidazolium tetrafluoroborate ([C8 mim][BF4 ]) with isomers of butanol (1-butanol, 2methyl-1-propanol, and 2-methyl-2-propanol) were measured at 0.1 MPa and at 298.15,  E  E  308.15 and 318.15 K. Excess molar volumes Vm , excess molar isentropic compressibilities Ks,m , and deviations in refractive index ( nD ) were calculated from the experimental data and were fitted to the Redlich–Kister polynomial equation. The results have been interpreted in terms of interstitial accommodation, ion–dipole interactions, formation of the hydrogen bonds, and structural factors involved in the mixture of ionic liquid and molecular organic solvents. Various mixing rules were used to predict the refractive indices and the data have been compared with the experimental results. In addition, analysis E of the present Vm data were done through the Prigogine–Flory–Patterson (PFP) theory and the extended real associated solution (ERAS) model. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Among the various classes of the ILs [1,2], imidazolium based ILs are the most extensively studied and used. The ILs are used in various fields, including in catalysis, in electrochemistry, in preparation of highly efficient fuel and solar cells, in the column as the stationary phase in chromatography, in the synthesis of nano-materials and also in various industrial applications [1–5]. Several technological process used in the industries are designed and implemented successfully from the knowledge of the thermophysical properties of ILs solutions in organic solvents. Reliable and accessible reference data on the physical (density, speeds of sound and viscosity) and chemical properties of pure components (ILs) and their mixtures are very crucial for: several industrial process engineering, transportation and storage of fluids, equipment designing, to develop the models for process design and energy efficient processes, to qualify and quantify the energy efficiency. In the literature, the data pertaining to ILs (pure and mixture with organic solvents) are very limited as well as scattered, which makes them inaccessible to the industry. Furthermore, along with the physical properties,

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N.I. Malek). http://dx.doi.org/10.1016/j.tca.2016.03.026 0040-6031/© 2016 Elsevier B.V. All rights reserved.

several derived properties are also important to characterize the compounds in their pure form as well as in the mixtures and provide the end user the best information about the behavioral changes occurred when the pure ILs is mixed with the organic solvents through the knowledge of molecular interaction. Our laboratory is engaged in the systematic experimental and theoretical investigations of the thermodynamic, transport, acoustic, and optical properties of the binary mixtures involving ILs and molecular organic solvents as the functions of composition and temperature [6–12]. In the previous papers [12,13], the thermophysical properties of the binary mixtures of imidazolium based ILs: [C6 mim][BF4 ] and [C8 mim][BF4 ] with the cyclic ethers and alkyl amines were reported. Alaknol as being highly polar H-bonding molecular solvents and used in many organic synthesis has been selected in the present work. Here we report the volumetric, acoustic and optical properties of binary mixtures; [C8 mim][BF4 ] + isomers of butanol (1-butanol, 2-methyl1-propanol and 2-methyl-2-propanol) at (298.15, 308.15 & 318.15) K and at 0.1 MPa pressure. The isomers of butanol were selected as one of the components in order to see the effect of different position of OH and CH3 groups in butanol molecule on the investigated  thermophysical properties. Only the excess molar volumes E of [C mim][BF ] + 1-butanol at 298.15 K have been reported Vm 8 4 in literature [14]. The results have been interpreted in terms of interstitial accommodation, ion–dipole interactions, H-bond for-

Z.S. Vaid et al. / Thermochimica Acta 634 (2016) 38–47

39

Table 1 CAS number, supplier and purity of components. Component

CAS Number

Supplier

Initial Final Purity on mass fraction

Purification Method

Purity Analysis

1-butanol 2-methyl-1-propanol 2-methyl-2-propanol [C8 mim][BF4 ]c

71-36-3 78-83-1 75-65-0 244193-52-0

Merck Merck Merck Synthesized

0.995 0.990 0.990

F.D.b F.D.b F.D.b None#

GCa GCa GCa 1 H NMR$

0.996 0.993 0.993 >0.98

None# = Synthesized ILs were kept in bottles under an inert gas. To reduce the water content to negligible values, ILs were kept at 0.2 Pa pressure in vacuum and at 343.15 K temperature for several days, prior to their use. 1 H NMR$ = Purity was determined through the corresponding NMR spectra with cation and anion specific peak integrals. a GC = Gas Chromatography. b F.D.= Fractional Distillation. c water content was 89 ppm. Table 2 Comparison of experimental values of densities (), speeds of sound (u), and refractive indices (nD ) of pure liquids with literature at different temperatures literature. Liquid

T/K

/kg m−3

u/m s−1

nD

Expt.

Lit

Expt.

Lit.

Expt.

Lit.

298.15

1105.02

1485.11

1491 [23] 1482.2 [25]

1.43253

1.43422 [23] 1.4330 [26]

308.15

1098.32

1458.54

1454.4 [25]

1.42984

318.15

1091.54

1105.30 [23] 1104.31 [24] 1114.4 [27] 1091.2 [28] 1103.68 [29] 1114.4 [30] 1085.5 [31] 1107.5 [27] 1090.78 [24]

1433.21

1428.3 [25]

1.42715

298.15

805.758

1242.41

1242.6 [32] 1238.99 [34,35]

1.39728

1.3973 [33]

308.15

798.046

1207.54

1207.6 [44] 1205.79 [34,35]

1.39324

1.39318 [33]

318.15

790.212

805.74 [14] 805.76 [33] 805.62 [34,35] 805.75 [36,37] 805.752 [38] 805.64 [39] 805.60 [40] 806.07 [41] 806.282 [36] 806.0 [42] 798.04 [43] 798.053 [33] 797.93 [34,35] 790.18 [45] 790.231 [33]

1.38911

1.38901 [33]

2-methyl-1-propanol 298.15

797.824

1189.02

1189.6 [47] 1186.72 [34,35] 1188.0 [50]

1.39408

1.3939 [46]

308.15

790.629

1155.94

1156.2 [48] 1153.86 [34,35]

1.38925

318.15

781.982

797.84 [46] 798.52 [48] 797.77 [34,35] 798.03 [49] 797.8 [50] 797.72 [43] 790.04 [44] 790.63 [48] 798.90 [34,35] 781.98 [43]

2-methyl-2-propanol 298.15

780.720

1121.41

1121.2 [51]

1.38488

1.3938 [47]

308.15

770.212

1081.97

1188.0 [49] 1082.8 [51]

1.37985

1.37947 [33]

318.15

759.502

797.90 [47] 780.80 [51] 780.99 [52] 781.2 [36] 780.68 [42] 770.19 [33] 770.10 [51] 759.49 [51] 759.87 [53] 759.45 [54]

1.37402

1.37384 [33]

[C8 mim][BF4 ]

1-butanol

1171.51

1118.62

1038.22

1.38444

Standard uncertainties u are: u() = 0.08 kg m−3 , u(u) = 0.5 m s−1 , u(nD ) = 0.0008, u(T) = 0.01 K and u(P) = 0.005 MPa. All the experiments were carried out at 0.1 MPa.

mation and structural factors involved in the liquid components. Further, the refractive indices of the binary mixtures were correlated with Lorentz–Lorenz, Dale–Gladstone, Eykman, Arago–Boit,

Newton, Oster, Heller, and Wiener mixing rules [15]. The PFP theory [16–18] and the ERAS model [17,19,20] were also used to analyse E data. the Vm

40

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Table 3 Densities (), speeds of sound (u), molar isentropic compressibilities (Ks,m ) and refractive index (nD ) for [C8 mim][BF4 ] + isomers of butanol at (298.15, 308.15 and 318.15) K. x1 is the mole fraction of [C8 mim][BF4 ].

x1

/kg m−3

u/m s−1

Ks,m **

nD

0.8062 0.8998 1.0000

1067.26 1079.78 1091.54

1394.28 1411.32 1433.21

109.32 112.59 115.32

1.42474 1.42604 1.42715

[C8 mim][BF4 ] + 2-methyl-2-propanol 298.15 K 780.72 1121.41 0.0000 0.1017 858.69 1188.32 914.56 1254.23 0.1988 959.49 1309.41 0.3011 0.4052 994.81 1351.45 1382.51 0.5002 1020.68 0.6040 1044.05 1410.10 0.7024 1062.66 1433.38 0.7992 1078.39 1451.28 1092.82 1467.56 0.9014 1105.02 1485.11 1.0000

96.70 91.75 88.16 87.08 88.04 89.81 92.42 95.11 98.26 101.79 104.79

1.38488 1.40022 1.40992 1.41591 1.41962 1.42177 1.42411 1.42628 1.42805 1.43025 1.43253

u/m s−1

Ks,m **

nD

[C8 mim][BF4 ] + 1-butanol 298.15 K 805.76 0.0000 878.23 0.1027 929.75 0.2022 0.3056 970.63 0.3989 999.88 0.5062 1027.17 0.6018 1047.22 1064.35 0.6978 1080.20 0.8014 0.9062 1094.06 1105.02 1.0000

1242.41 1273.22 1306.52 1339.69 1366.06 1392.84 1413.66 1431.92 1450.18 1468.28 1485.11

73.96 77.13 80.20 83.38 86.26 89.57 92.52 95.48 98.67 101.90 104.79

1.39728 1.40763 1.41502 1.41903 1.42183 1.42405 1.42594 1.42791 1.42974 1.43151 1.43253

308.15 K 0.0000 0.1027 0.2022 0.3056 0.3989 0.5062 0.6018 0.6978 0.8014 0.9062 1.0000

798.05 870.62 922.28 963.30 992.71 1020.16 1040.32 1057.54 1073.47 1087.36 1098.32

1207.54 1240.81 1274.68 1308.68 1336.03 1363.17 1384.75 1403.82 1422.98 1441.58 1458.54

79.81 81.86 84.12 86.69 89.35 92.81 96.06 99.50 103.22 106.90 109.97

1.39324 1.40492 1.41254 1.41692 1.41956 1.42175 1.42362 1.42542 1.42732 1.42861 1.42984

308.15 K 0.0000 0.1017 0.1988 0.3011 0.4052 0.5002 0.6040 0.7024 0.7992 0.9014 1.0000

770.21 849.64 906.00 951.41 987.25 1013.40 1037.31 1056.01 1071.80 1086.23 1098.32

1081.97 1157.22 1221.58 1275.56 1322.25 1353.61 1383.32 1408.96 1428.20 1443.32 1458.54

106.73 98.94 94.81 93.43 93.48 95.12 97.39 99.76 102.77 106.55 109.97

1.37985 1.39681 1.40684 1.41342 1.41719 1.41911 1.42168 1.42348 1.42562 1.42769 1.42984

318.15 K 0.0000 0.1027 0.2022 0.3056 0.3989 0.5062 0.6018 0.6978 0.8014 0.9062 1.0000

790.21 862.90 914.69 955.89 985.42 1013.00 1033.31 1050.61 1066.62 1080.57 1091.54

1171.51 1206.70 1241.85 1276.28 1304.15 1333.27 1356.04 1377.02 1397.18 1416.32 1433.21

86.49 88.12 90.11 92.58 95.18 98.40 101.55 104.79 108.46 112.15 115.32

1.38911 1.40232 1.40964 1.41438 1.41736 1.41965 1.42126 1.42301 1.42444 1.42615 1.42715

318.15 K 0.0000 0.1017 0.1988 0.3011 0.4052 0.5002 0.6040 0.7024 0.7992 0.9014 1.0000

759.50 840.32 897.39 943.34 979.38 1005.90 1029.92 1049.10 1064.98 1079.48 1091.54

1038.22 1119.11 1190.22 1246.52 1293.34 1324.88 1354.52 1380.25 1402.23 1417.19 1433.21

119.21 108.28 101.94 99.64 99.38 100.87 103.10 105.40 108.04 111.94 115.32

1.37402 1.39317 1.40405 1.41058 1.41452 1.41635 1.41871 1.42104 1.42272 1.42513 1.42715

[C8 mim][BF4 ] + 2-methyl-1-propanol 298.15 K 797.82 1189.02 0.0000 1230.34 0.1084 876.21 0.2056 927.81 1274.12 966.65 1314.42 0.3002 0.4082 1001.41 1354.08 1024.88 1381.12 0.4989 1047.08 1407.23 0.6024 1065.38 1429.11 0.7044 1080.91 1447.22 0.8062 1093.30 1463.75 0.8998 1105.02 1485.11 1.0000

82.37 83.32 83.86 84.84 86.72 88.99 92.01 95.30 98.90 102.08 104.79

1.39408 1.40643 1.41442 1.41922 1.42292 1.42482 1.42664 1.42845 1.43004 1.43121 1.43253

308.15 K 0.0000 0.1084 0.2056 0.3002 0.4082 0.4989 0.6024 0.7044 0.8062 0.8998 1.0000

790.63 869.11 920.72 959.59 994.40 1017.95 1040.26 1058.63 1074.22 1086.63 1098.32

1155.94 1198.28 1242.42 1282.12 1322.40 1351.26 1378.24 1402.12 1421.08 1437.08 1458.54

88.74 89.30 89.57 90.49 92.22 94.25 97.20 100.28 103.87 107.22 109.97

1.38925 1.40292 1.41142 1.41636 1.41998 1.42182 1.42372 1.42562 1.42748 1.42866 1.42984

318.15 K 0.0000 0.1084 0.2056 0.3002 0.4082 0.4989 0.6024 0.7044

781.98 860.80 912.57 951.64 986.65 1010.44 1032.97 1051.52

1118.62 1162.24 1206.42 1246.23 1287.52 1317.82 1347.40 1373.28

96.87 96.78 96.71 97.40 98.82 100.58 103.15 105.97

1.38444 1.40008 1.40854 1.41407 1.41801 1.41925 1.42108 1.42306

x1

/kg m−3

Table 3 (Continued)

E * Unit of Ks,m andKs,m is dm3 TPa−1 mol−1 . Standard uncertainties u are: u(x) = 0.01, u() = 0.08 kg m−3 , u(u) = 0.5 m s−1 , u (Ks,m ) = 0.05 dm3 TPa−1 ·mol−1 , u(nD ) = 0.0008, u(T) = 0.01 K and u(P) = 0.005 MPa. All the measurements were carried out at 0.1 MPa.

2. Experimental 2.1. Materials [C8 mim][BF4 ] was synthesized in our laboratory following the two-step synthetic procedure [12,13,21], the formation of the halide intermediate followed by the anion exchange of the tetrafluoroborate anion. The purification procedure has been described in our previous publications [12,13] and the purity was determined through the corresponding 1 H NMR spectra with cation and anion specific peak integrals. 1 H NMR of the synthesized IL (200 MHz, TMS, DMSO-d6, Fig. S1) 0.86 (3H, t, N (CH2 )5 CH3 ), 1.25 (br. S, 10H, N CH2 CH2 (CH2 )5 CH3 ),1.78 (2H, quintet, N CH2 CH2 (CH2 )5 CH3 ), 3.85 (3H,singlet, N CH3 ), 4.15 (t, 2H, N CH2 CH2 (CH2 )5 CH3 ), 7.70(s, 1H), 7.77 (s, 1H), 9.10 (s, 1H). Water content in the IL was determined by Karl Fisher titration (Metrohm, 890 Titrando) and was found to be 89 ppm. Argentometric titration method was used to estimate the purity of IL. Briefly, initially AgNO3 solution was standardized against a 0.015 mol dm−3 of NaCl using AgCrO4 as indicator. Using this AgNO3 as titrant, several aliquots (10 cm3 ) of IL solution was tested. 1-Butanol, 2-methyl-1-propanol and 2-methyl-2-propanol purchased from Merck were stored over KOH and fractionally distilled prior to use [22]. The purities of the butanol isomers were checked using gas

Z.S. Vaid et al. / Thermochimica Acta 634 (2016) 38–47

chromatograph, which is equipped with semi-capillary methyl silicone column (OD: 530 ␮m) and a flame-ionization detector. The initial and final purities of the chemicals used in the present work with their purification methods are reported in Table 1. The purity of the 1-butanol, 2-methyl-1-propanol, and 2-methyl-2-propanol were better than 0.996, 0.993 and 0.993 mol%, respectively. Estimated purity of our synthesized IL is >0.98 (on mass% basis). In Table 2, we had compared the experimental data of densities, speeds of sound and refractive indices with the literature values for the synthesized [C8 mim][BF4 ] and butanol isomers [14,23–54]. The experimental data for the butanols are in reasonable agreement with the literature values. The density values measured here for [C8 mim][BF4 ] and those compared in Table 2 with literature data at all the three temperatures, show that our densities and literature values are within experimental uncertainties. The difficulty in comparing the experimental and literature data are associated with: (i) different experimental pressure and temperature (ii) the high uncertainty in the results and (iii) different moisture content. We have reported the most comparable pressure and temperature intervals in the present investigation. Considering all the literature values at 298.15 K [14,23–55], the average value of density for [C8 mim][BF4 ] is 1105.55 kg m−3 , which is 0.048% higher than our measured values. This small discrepancy can be attributed to the different moisture contents in the [C8 mim][BF4 ] samples. Analytical balance (B 204-S, Mettler Toledo, Switzerland) having an uncertainty of ±1 × 10−7 kg was used for the preparation of the mixture by mass. The samples were protected from the atmospheric moisture by placing the balance in the dry box. The uncertainty for the mole fraction was estimated to be less than ±1 × 10−2 .

where Vm , Mm , , and u are the molar volume, molar mass, density, and speed of sound of the mixture, respectiely. To have better agreement with the other thermodynamic quantities, here we have calculated the mole-intensive quantity Ks,m inplace of the volume-intensive S [57].Measured densities, speeds of sound, and refractive indices and derived molar isentropic compressibility, for [C8 mim][BF4 ] + butanol mixtures at 298.15, 308.15 and 318.15 K and at 0.1 MPa have been summarised in Table 3.  E The excess molar isentropic compressibility Ks,m was calcu-



id lated using ideal solution molar isentropic compressiblity Ks,m by: E id KS,m = KS,m − KS,m



(3)

id of the ideal solution here, the molar isentropic compressibility Ks,m was calculated by the equation proposed by Benson and Kiyohara [58] 2

o 2 EP,i

i=1

o CP,i

id o Ks,m = ˙ xi [KS,i +T

] − T[

o ) (xi EP,i o xi CP,i

2

]

(4)

o ) term is the product of the molar volume where expansibility (EP,i o o = V o ˛o ),C ∗ and (Vi ) and the isobaric expansivity (˛oP,i ) (i.e.EP,i i P,i P,i o KS,m stand for isobaric molar heat capacity and molar isentropic compressibility of the pure liquid component i, respectively. The o and ˛o needed are listed in Table 4. The uncertainty values of CP,i P,i

for this excess property is 0.005 dm3 TPa−1 mol−1 . E ) for the binary mixture was calculated Excess molar volume (Vm by: E Vm =

2.2. Apparatus and procedure

41

2 

xi Mi (−1 − i−1 )

(5)

i=1

2.2.1. Densities and speeds of sound Anton-Paar DSA 5000 digital vibrating U-tube densimeter was used to measure the densities and speeds of sound of the pure and the binary mixtures. The instrument measures density and speeds of sound at 3 MHz, automatically and simultaneously with controlling the temperature by ±0.001 K through built in solid-state thermostat. Calibration of the instrument was performed daily at all studied temperatures by dry air and air free double distilled water. The details of the experimental procedures have been described in previous publication [56]. The estimated uncertainty for densities, speeds of sound, excess molar volumes and excess molar isentropic compressibility were within ±0.08 kg m−3 , ±0.5 m s−1 , ± 0.06 × 10−6 m3 mol−1 and ±0.005 dm3 TPa−1 mol−1 , respectively. 2.2.2. Refracive indices Refractive indices were measured using an automatic refractometer (Abbemat 300, Anton Paar) with the temperature accuracy of ±0.001 K. Calibration of the apparatus was carried out by measuring the refractive index of Millipore water (Elix-3) at T = 298.15 K prior to the experiments. The estimated standard uncertainties of the refractive index and deviation in refractive index measurements were ±0.0008 and ±0.004. 3. Results and discussion Isentropic compressibilities (kS ) and molar isentropic compressibilities (Ks,m ) were obtained from Eqs. (1) and (2), respectively [48].



−1 s = −Vm ∂Vm /∂P



KS,m = − ∂Vm /∂P





S



S

= u2

−1



= Vm Mm u2

= Vm S = Mm /(u)2

−1

(1) (2)

where xi , Mi , i , and  are the mole fraction, molar mass and density of the pure component i and mixtures, respectively. Deviations in refractive index on volume fraction average basis were calculated as [59]:  nD = nD − ˙i nD,i

(6)

where  is the volume fraction of the component i, nD and nDi are the refractive index of the mixture and the pure component, respectively. The excess molar volumes, excess molar isentropic compressibilities, and deviations in refractive indices derived from the experimental data were summarised in the Supplimentray information as Tables S1. Excess or mixing functions were correlated by Redlich–Kister polynomial equation [60]: E E Vm , KS,m , and  nD

n 

= x1 (1 − x1 )

i−1

Ai (1 − 2x1 )

(7)

i=1

where x1 is the mole fraction of IL. The coefficients Ai of Eq (7) were obtained by employing the least-squares fit method with equal weights assigned to each point. The corrsoponding standard deviations were also reported in Table 5. E for [C mim][BF ] with butanol isomers The dependence of Vm 8 4 is presented in Fig. 1 and as supplementary materials Figs. SI-2–4 at all the three investigated temperatures alongwith the literature E was values at 298.15 K [14]. In all the systems, except 1-butanol, Vm found to be negative and becomes more negative with increasing E the temperature from 298.15 to 318.15 K, i.e.∂Vm ⁄∂T is negative. For E was negative for the alkanol the system containing 1-butanol, Vm rich region, whereas in the IL rich region it was positive at all the E values for [C mim][BF ] +1studied temperatures. Our present Vm 8 4 butanol at 298.15 K are in reasonable agreement with the published

42

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Table 4 Isobaric thermal expansion coefficient (␣P ), isochoric molar heat capacity(CP ), Flory theory parameters: characteristic volume(V*), reduce volume (V˜ ), characteristic pressure (P*), and molecular surface to volume ratio (S) of pure components. Component

T (K)

104 ␣ K−1

CP J mol−1 K−1

106 V* m3 mol−1

V˜

10−6 P* J m−3

S n m−1

[C8 mim][BF4 ]

298.15 308.15 318.15

6.099 [12] 6.137 [12] 6.175 [12]

500.3 [23] 506.6 [23] 512.0 [23]

219.81 220.07 220.36

1.1541 1.1665 1.1752

526 530 533

10.45 [72]

1butanol

298.15 308.15 318.15

9.648a 9.741a 9.838a

177.2 [22] 181.6b 186.0b

74.16 74.36 74.57

1.2410 1.2494 1.2583

467 464 460

14.56c

74.55 74.70 74.98

1.2470 1.2553 1.2649

443 441 435

14.54c

72.09 72.43 72.81

1.3148 1.3253 1.3370

557 549 535

14.92c

9.929a 181.0 [22] 298.15 2308.15 10.020a 185.6b methyl10.130a 190.0b 318.15 1propanol 215.4 [22] 298.15 13.590a 2308.15 13.776a 219.2b methyl13.970a 223.0b 318.15 2propanol a Derived from density data at different temperatures. b estimated from group contribution method of Chueh-Swanson and Missenard [73]. c Calculated as per Bondi [74]. E is reached near 0.3 mol fracliterature data [14]. A minimum in Vm tion of IL in all the three mixtures. Similar minima around 0.2 mol fraction of IL were reported for [C8 mim][BF4 ] with methanol, ethanol, 1-propanol, 2-propanol and 1-butanol [14,55]. Holbrey and Sedon [21] reported the formation of liquid clathrate in the ILs ([C4 mim][PF6 ] and [C4 mim][BF4 ]) and trichloromethane system at the specific concentration of IL (0.3 mol fraction of IL) which is E . On comparing V E for common IL responsible for the minima in Vm m E decreases in with different butanol isomers, it is observed that Vm following sequence: 1-butanol > 2-methyl-1-propanol > 2-methyl2-propanol. Thus, on branching alkyl group and shifting OH from E became more negative. Thus, the alkyl group strucposition 1–2, Vm ture and the position of OH in butanol play a significant role in volumetric behaviour of mixture formation.

E as a function of composition and temperaThe behaviour of Ks,m E behaviour, tures is shown in Fig. 2 and in Figs. SI-5–7. Similar to Vm E is negative and becomes more negative on increasing the temKs,m E /∂T is negative for all perature from 298.15 to 318.15 K, i.e.∂KS,m

E , for the three mixtures, except for IL + 1-butanol. Similar to the Vm E the system containing 1-butanol, Ks,m is negative in the alkanol rich region, whereas in the dilute alkanol region it is slightly positive. E become more negative in the order 1-butanol, 2The values of Ks,m E and methyl-1-propanol and 2-methyl-2-propanol, similar to the Vm lead to a minima in the composition range around 0.4–0.5 mol fraction of IL (Fig. 2). The minima varies from −2.10 dm3 TPa−1 mol−1 for 1-butanol to −12.06 dm3 TPa−1 mol−1 for 2-methyl-2-propanol at 298.15 K

Table 5 Coefficients Ai of Eq. (7) along with standard deviations ␴ of binary mixture properties. T/K

A1

A2

A3

A4



[C8 mim][BF4 ] + 1-butanol E 106 Vm /m3 mol−1 298.15 308.15 318.15 E −1 /dm3 TPa−1 mol298.15 Ks,m 308.15 318.15 298.15  nD 308.15 318.15

0.021 −0.075 −0.166 −7.73 −9.25 −10.80 0.0368 0.0400 0.0490

−0.604 −0.521 −0.463 −5.14 −5.80 −5.32 0.0343 0.0256 0.0360

0.046 −0.020 −0.081 4.73 3.28 1.18 0.0261 0.0338 0.0332

0.036 0.010 −0.003 1.79 0.94 −0.45 −0.0027 0.0187 0.0212

0.0013 0.0016 0.0010 0.020 0.024 0.022 0.0003 0.0002 0.0002

[C8 mim][BF4 ] + 2-methyl-1-propanol E 106 Vm /m3 mol−1 298.15 308.15 318.15 E −1 /dm3 TPa−1 mol298.15 Ks,m 308.15 318.15 298.15  nD 308.15 318.15

−1.197 −1.349 −1.472 −18.80 −20.81 −22.52 0.0470 0.0502 0.0551

−1.350 −1.150 −0.970 −8.90 −7.41 −6.39 0.0383 0.0428 0.0482

−0.051 −0.410 −0.737 13.15 12.23 9.82 0.0167 0.0240 0.0317

0.660 −0.247 −1.134 4.42 1.031 −1.42 −0.0036 −0.0033 0.0026

0.0513 0.0428 0.0541 0.1045 0.1469 0.1506 0.0002 0.0002 0.0003

[C8 mim][BF4 ] + 2-methyl-2-propanol E /m3 mol−1 298.15 106 Vm 308.15 318.15 E −1 /dm3 TPa−1 mol298.15 Ks,m 308.15 318.15 298.15  nD 308.15 318.15

−2.496 −3.210 −3.849 −45.14 −53.76 −66.80 0.0535 0.0585 0.0642

−2.341 −2.080 −2.353 −30.05 −29.30 −43.00 0.0520 0.0567 0.0639

0.388 −0.326 −1.117 −0.21 −13.85 −21.14 0.0287 0.0353 0.0459

1.662 0.608 0.310 −9.14 −6.94 −4.15 0.0050 0.0084 0.0125

0.0180 0.0158 0.0194 0.174 0.194 0.117 0.0002 0.0002 0.0002

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43

Table 6 Average percentage deviations of the experimental refractive index results from predicted by Lorentz −Lorenz (LL), Gladstone–Dale (GD), Eykman (EYK), Arago-Biot (AB), Newton (NW), Oster (OST), Heller (HE) and Weiner (WI) relations for [C8 mim][BF4 ] + 1-butanol, + 2-methyl-1-propanol, and + 2-methyl-2-propanol at 298.15, 308.15 and 318.15 K and at 0.1 MPa. LL

GD

EYK

AB

NW

OST

HE

WI

[C8 mim][BF4 ] + 1-butanol 298.15 −0.12 308.15 −0.09 318.15 −0.06

−0.13 −0.10 −0.07

0.06 0.10 0.14

−0.13 −0.10 −0.07

−0.13 −0.10 −0.07

−0.13 −0.10 −0.07

−0.12 −0.09 −0.06

−0.13 −0.10 −0.07

[C8 mim][BF4 ] + 2-methyl 1-propanol 298.15 −0.08 308.15 −0.07 −0.03 318.15

−0.08 −0.07 −0.04

0.10 0.12 0.17

−0.08 −0.07 −0.04

−0.09 −0.08 −0.04

−0.09 −0.07 −0.04

−0.08 −0.07 −0.03

−0.08 −0.07 −0.04

[C8 mim][BF4 ]+ 2-methyl 2-propanol −0.14 298.15 308.15 −0.11 318.15 −0.09 0.09

−0.15 −0.12 −0.09 0.09

0.07 0.10 0.13 0.11

−0.15 −0.12 −0.09 0.09

−0.15 −0.13 −0.10 0.11

−0.15 −0.12 −0.10 0.11

−0.14 −0.12 −0.09 0.09

−0.15 −0.12 −0.09 0.11

Table 7 E E E Values of Flory interaction parameter X12 , three contributions: Vm (int), Vm (ip), and Vm (f v) at maximum deviation at298.15, 313.15 and 318.15 K and standard deviation E E and VPFP . ␴between Vexp

a

Temp

10−6 X12

K

J m−3

[C8 mim][BF4 ]+ 1-butanol 298.15 308.15 318.15

27.20 29.20 31.00

E 106 ␴ (Vm)

E 106 Vm (int)

m3 mol−1

E 106 Vm (ip) m3 mol−1

E 106 Vm (f v) m3 mol−1

m3 mol−1

0.453 0.507 0.561

−0.251 −0.288 −0.336

−0.246 −0.263 −0.280

0.078 0.058 0.048

[C8 mim][BF4 ]+ 2-methyl-1-propanol 20.81 298.15 308.15 21.46 318.15 23.64

0.353 0.380 0.437

−0.384 −0.426 −0.492

−0.277 −0.295 −0.314

0.047 0.032 0.017

[C8 mim][BF4 ]+ 2-methyl-2-propanol 298.15 12.19 308.15 8.46 8.61 318.15

0.225 0.164 0.174

0.248 0.157 0.014

−1.019 −1.087 −1.154

0.131 0.071 0.056

Standard deviation was calculated considering whole composition range.

Table 8 ERAS parameters of pure components used in the model at 298.15 K. Component

104 ␬T MPa−1

[C8 mim][BF4 ] 1-butanol 2-methyl-1-propanol 2-methyl-2-propanol

4.715 9.532 10.411 12.577

KB

148 81 105

106 V* m3 mol−1

10−6 P* J m−3 kJ mol−1

∗h

106 ∗v m3 mol−1

221.49 75.34 76.08 73.46

516 407 373 486

0 −25.1 −25.1 −25.1

0 −5.6 −5.6 −5.6

Table 9 Cross-Parameter XAB , KAB and VAB for binary mixtures and comparison of equimolar experimental and ERAS model results at 298.15 K. Mixture

106 XAB

KAB

J m−3 IL + 1-butanol IL + 2-methyl-1-propanol IL + 2-methyl-2-propanol

61.9 61.9 61.8

110 20 60

The over all behaviour of both, the large negative excess molar E ) and large negative excess molar isentropic compressvolume (Vm E ) can be predicted through several phenomenons: ibilities (Ks,m i) Free volume effect/interstitial accommodation: In the present case, the notable difference in the molar volumes of [C8 mim][BF4 ] and butanol isomers makes it possible for the relatively small butanol molecules to enter into the interstices of the [C8 mim][BF4 ] upon mixing. This is very well supported

106 VAB

E 106 Vm

m3 mol−1

Expt m3 mol−1

ERAS m3 mol−1

␴ m3 mol−1

−0.7 −4.2 −6.5

−0.0340 −0.3499 −0.6000

−0.0380 −0.3620 −0.5803

0.108 0.068 0.046

by the molecular dynamics simulation study [61]. A similar phenomenon was observed for [C8 mim][BF4 ] with methanol, ethanol, 1-propanol, and 2-propanol [14], [C4 mim][BF4 ] and [C6 mim][BF4 ] with ethanol [62]. Further, the kinetic energy of molecules increases on increaseing the temperature, that leads to depolymerisation of butanol molecules, as a result the depolymerised molecules of smaller size fit more effectively into the empty spaces available in the voids of the bigger [C8 mim][BF4 ] molecule. The overall effect is the more subtrac-

44

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0.2

106 /VmE m3 .mol-1

0.0

-0.2

-0.4

-0.6

-0.8 0.0

0.2

0.4

x1

0.6

0.8

1.0

Fig. 1. Excess molar volumes for [C8 mim][BF4 ] with isomers of butanol at 298.15 K. (Heintz et al. [16] (䊐), 1-butanol ), 2-methyl-1-propanol ( ) and 2-methyl-2propanol ( )). Solid lines have been drawn from Eq. (7) using the coefficients given in Table SI-4.

tion in the volume of the mixture on raising the temperature E that finally results into negative ∂Vm ⁄∂T . ii) The H-bond formation between [C8 mim][BF4 ] and butanols (cross-association): The hydrogen atom at C2 position of the [C8 mim][BF4 ], is acidic and can form H-bond with the oxygen atom of butanols [63]. Similarly, anion [BF4 ]− forms H-bond with hydrogen atom of butanol. Both factors lead to the subtraction in volume on the mixture formation. iii) Interaction between IL cation and dipolar butanol: Cation [C8 mim]+ of the IL interact with the dipole of butanol, that results in the contraction of the volume, contributing to negE [20]. ative Vm iv) Disruption of H-bonding self-association in butanols: On mixing IL with butanol, butanol molecules depolymerise resulting into E. increase in volume i.e. contributing to positive Vm E indicate that the filling effect The overall negative values of Vm of butanols in the interstices of IL (i), H-bond formation between butanol and ions of IL (ii), and the ion–dipole interactions between butanols and the imidazolium ring of the IL (iii), all contributing to the negative values of the excess molar volumes. These three contri-

0.0

butions dominate over the dispersive interactions between unlike components (iv) when IL and the butanols were mixed. Further, because of stronger interaction between components of mixtures, and efficient packing, unlike molecules come closer to each other, thereby decreasing compressibility. The values of refractive indices increases on increasing mole fraction of [C8 mim][BF4 ] in the mixture (Table 3). The deviations in refractive indices f nD on volume fraction average basis for all the three binary mixtures are positive at all studied temperatures and over the full range of compositions (Fig. 3 and Figs.SI 8–10). It is a general observation that the sign of f nD E [55,59]. Number of equations, which is opposite to that of Vm relate the refractive indices of the liquid mixture to those of the pure components have been proposed from time to time by different investigators [64]. To obtain further information about predictive ability, some published empirical equations such as Lorentz–Lorenz (L–L) [65], Gladstone–Dale (G–D) [66], Eykman (EK) [67], Arago-Biot (A–B) [68], Newton (N) [69], Oster (Os) [70], Heller (H) [64], and Weiner (W) [71] were examined for present [C8 mim][BF4 ] + butanol mixtures. The average percentage deviations (APD) calculated according to Eq. (8) are reported in Table 6. APD =

calc 1  100|ni − ni | exp n n n

exp

i=1

i

(8)

The average values of considering all mixture compositions at all the temperatures are 0.09, 0.09, 0.11, 0.09, 0.11, 0.11, 0.09, and 0.11%, for L–L, G–D, EK, A–B, N, Os, H, and W equations, respectively. All the eight mixing rules considerd here have small and almost similar values of . Therefore, the eight mixing rules considered here are almost equivalent for predictive purposes when applied to the [C8 mim][BF4 ] + butanol binary mixtures.

-2.0

KS,mE dm3 TPa-1mol -1

Fig. 3. Refractive index deviations for [C8 mim][BF4 ] with isomers of butanol at )). 298.15. (1-butanol ( ), 2-methyl-1-propanol ( ) and 2-methyl-2-propanol Solid lines have been drawn from Eq. (7) using the coefficients given in Table SI-4.

-4.0 -6.0 -8.0

3.1. Calculation with the PFP theory and ERAS model

-10.0 -12.0 -14.0 0.0

0.2

0.4

x1

0.6

0.8

1.0

Fig. 2. Excess molar isentropic compressibilities for [C8 mim][BF4 ] with isomers ), 2-methyl-1-propanol ( ) and 2-methyl-2of butanol at 298.15. (1-butanol propanol ( )). Solid lines have been drawn from Eq. (7) using the coefficients given in Table SI-4.

Both the Prigogine-Flory-Patterson (PFP) statistical theory [16–18] and the extended real associated solution (ERAS) model [17,19,20] were commonly used to predict the excess thermodynamic properties of the binary and ternary systems. A brief description is given in supplementary information. 3.1.1. The PFP theory E by three contributional terms: the This theory representsVm E E free volume (Vm (f v)), the characteristic pressure (Vm (ip)) and the E energy of interaction (Vm (int)) [18]. The pure components param-

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45

E were found to be in the range from 0.017 × 10−6 to and VPFP 0.131 × 10−6 m3 mol−1 . E The first termVm (f v) was found to be negative for all the systems 2 E studied (Table 7) as Vm (f v)is proportional to −(V˜ 1 − V˜ 2 ) [18]. The

E magnitude of negative values for Vm (f v) depends upon difference in Flory’s reduced volumes (i.e. thermal expansion coefficients) E of involved components. Negative values of Vm (f v) increases in magnitude as the temperature increases which shows that as the temperature increases, more free volume in the [C8 mim][BF4 ] becomes available to accommodate the smaller butanol molecules E . Thus (V E which resulted in more negative Vm m (f v)) contribution explains the free volume/interstitial accommodation effect discussed in the earlier section. The second term, i.e. characteristic E pressure (Vm (ip)) term, which is dependent on the structurebreaking effect is proportional to (P1∗ − P2∗ )(V˜ 1 − V˜ 2 ) and can have both the negative and positive sign depending upon the magnitude of P* and V˜ of unlike components [18]. For the systems E [C8 mim][BF4 ] + 1-butanol and 2-methyl-1-propanol, Vm (ip) is negative while it is positive for [C8 mim][BF4 ] +2-methyl-2-propanol. E The Vm (ip) term is related to the structure-braking effect of the [C8 mim][BF4 ] on the H-bonds between the butanol molecules and so the butanol molecules can be placed around the [C8 mim][BF4 ] E [75]. Different values of Vm (ip) correspond to different hydrophobic forces, depending upon structure of butanol isomer. The third E contribution term Vm (int), representing the energy of interacE for tion is positive for all the three systems.Though, direct Hm E [C8 mim][BF4 ] +butanol is not available, but observed values of Hm for [C4 mim][BF4 ] + ethanol, [C6 mim][BF4 ] + ethanol and 1-butyl3-methylpyridinium tetrafluoroborate [C4 3mpy][BF4 ] + ethanol at 303.15 K [61,76] and 1-butyl-2- methylpyridinium tetrafluoroborate [C4 2mpy][BF4 ] + 1-butanol at 318.15 K [77] are in the range E 2450–2600 J mol−1 at x1 = 0.5. We may approximately assume Hm values of similar order for the present [C8 mim][BF4 ] + butanol systems, though may be of different magnitude. Such positive valE definitely result into positive contribution to V E ues of Hm m (int). E E E Analysis of three contribution terms Vm (f v) , Vm (ip) , andVm (int) E reveals that free volume contribution Vm (f v) plays a major E for the role in the overall large negative values of the Vm present [C8 mim][BF4 ] + butanol mixtures. We can conclude that, it is possible to describe the volumetric behaviour of present [C8 mim][BF4 ] + butanol mixtures by the application of the PFP theory quite successfully.

E at 298.15 K. Experimental ( ), PFP theory ( ), ERAS Fig. 4. Comparison of Vm model ( ). (A) [C8 mim][BF4 ] + 1-butanol, (B) [C8 mim][BF4 ] + 2-methyl-1-propanol, and (C) [C8 mim][BF4 ] + 2-methyl-2-propanol.

eters for the PFP theory are included in Table 4. The Flory contact interaction parameter X12 , the only adjustable parameter, needed E values in in the PFP theory was obtained by experimental Vm E absence of the experimental excess molar enthalpy (Hm ). The Flory contact interacrion parameter X12 was found to be positive for all the investigated mixtures, and have values in the range from 8.46 E and V E to 31.54 × 10−6 J m−3 . A comparison between the Vexp at PFP 298.15 K is shown graphically in Fig. 4. The values of three contribuE at equimolar compositon E E E tions: Vm (f v) , Vm (ip) , andVm (int) to VPFP E and V E are and corresponding standard deviations between Vexp PFP E betweenV E summarised in Table 7. The standard deviationsVm exp

3.1.2. The ERAS model Thermodynamic properties of the mixtures having one or both the components associating consequtively can be interpreted using ERAS model [19,20]. In this model, the reaction is proposed in which the liquid molecules which can associate are passing through the monomeric to multimeric state, and the reactions are charecterized with reaction enthalpy denoted by h* and reaction volume denoted by v*. As per the theory, the excess volume is represented by the physical and chemical contributions. The physical properties and characteristic parameters used in the ERAS model for pure liquids are listed in Table 8. The values of required cross parameE and V E ters XAB , KAB and v∗AB along with the Vexp at equimolar ERAS composition at 298.15 K are given in Table 9. E at 298.15 K are compared The theoretical results of VERAS with experimental results and the data obtained by PFP theory E ) between experimental in Fig. 4. The standard deviations (Vm and ERAS results considering all compositions are from 0.046 to 0.108 × 10−6 m3 mol−1 . Thus, it is possible to predict the excess molar volume of [C8 mim][BF4 ] + butanol mixtures through ERAS model with reasonable success.

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4. Conclusions Densities, speeds of sound, molar isentropic compressibilities, refractive indices, excess molar volume, excess molar isentropic compressibilities, and deviation in refractive indces for [C8 mim][BF4 ] + 1-butanol, + 2-methyl-1-propanol, and + 2-methyl2-propanol mixtures at 298.15, 308.15 and 318.15 K are reported. E andK E for all the mixtures are negative where as  n is TheVm  D s,m positive at the most of compositions and at all the temperatures. Interstitial accommodation, formation of H-bonds and ion-dipole interaction between the [C8 mim][BF4 ] and butanol molecules were E and K E . The considered to be main contributors for negative Vm s,m E results from Vm suggests that the alkyl group structure and position of OH group in butanol make significant contribution towards the volumetric behaviour of the mixture. Further, the negative value of E also suggest the stronger interaction between components the Ks,m of mixtures, and efficient packing. The decrease in compressibility also suggests that upon mixing, unlike molecules come closer to each other. Eight mixing rules considered to correlate the refractive index of the mixtures in terms of pure component properties were able to predict refractive index with maximum value of 0.11%. The PFP theory and ERAS model have been quite successful in describing volumetric behaviour of [C8 mim][BF4 ] + butanol mixtures. Acknowledgements Finacial assistance by SVNIT through Faculty Research Grant is acknowledged here. N.I.M. acknowledge Department of Science and Technology, New Delhi for providing the financial assistance through grants No. SR/FT/CS-014/2010 to carry out this work. Financial assistance by Council of Scientific and Industrial Research (CSIR), New Delhi through grant No. 01(2545)/11/EMR-II is also acknowledged. Maulana Azad National Fellowship, (MANF-201213-MUS-GUJ-10818) for a research fellowship to Z. Vaid and TEQIP fellowship to U. More is kindly acknowledged here. 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.tca.2016.03.026. References [1] T. Welton, Room-Temperature ionic liquids. solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2083. [2] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation, Green Chem. 3 (2001) 156–164. [3] J.S. Wilkes, Properties of ionic liquid solvents for catalysis, J. Mol. Catal. A 214 (2004) 11–17. [4] J.F. Brennecke, E.J. Maginn, Ionic liquids: innovative fluids for chemical processing, AIChE J. 47 (2001) 2384–2389. [5] S. Trivedi, N.I. Malek, K. Behera, S. Pandey, Temperature-dependent solvatochromic probe behavior within ionic liquids and (ionic liquid + water) mixtures, J. Phys. Chem. B 114 (2010) 8118–8125. [6] S.L. Oswal, J.S. Desai, S.P. Ijardar, N.I. Malek, Studies of viscosities of dilute solutions of alkylamine in non-electrolyte solvents: II. Haloalkanes and other polar solvents, Thermochim. Acta 427 (2005) 51–60. [7] V. Pandiyan, S.L. Oswal, N.I. Malek, P. Vasantharani, Thermodynamic and acoustic properties of binary mixtures of ethers V. Diisopropyl ether or oxolane with 2- or 3-chloroanilines at 303.15, 313.15 and 323.15 K, Thermochim. Acta 524 (2011) 140–150. [8] S.P. Ijardar, N.I. Malek, S.L. Oswal, Studies on volumetric properties of triethylamine in organic solvents with varying polarity, Indian J. Chem. 50A (2011) 1709–1718. [9] N.I. Malek, S.P. Ijardar, Z.R. Master, S.L. Oswal, Temperature dependence of densities, speeds of sound, and derived properties of cyclohexylamine + cyclohexane or benzene in the temperature range 293.15-323.15 K, Thermochim. Acta 547 (2012) 106–119.

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