Influence of temperature on thermophysical properties of ammonium ionic liquids with N-methyl-2-pyrrolidone

Thermochimica Acta 545 (2012) 131–140

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Influence of temperature on thermophysical properties of ammonium ionic liquids with N-methyl-2-pyrrolidone T. Kavitha a , Pankaj Attri b , P. Venkatesu b,∗ , R.S. Rama Devi a , T. Hofman c a b c

Department of Chemistry, Sri Padmavathi Womens Degree and Post Graduate College, Tirupati 517502, India Department of Chemistry, University of Delhi, Delhi 110007, India Division of Physical Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland

a r t i c l e

i n f o

Article history: Received 12 April 2012 Received in revised form 4 July 2012 Accepted 8 July 2012 Available online 16 July 2012 Keywords: N-methyl-2-pyrrolidone Ammonium ionic liquid Thermophysical properties Molecular interactions Anion effect

a b s t r a c t To address the molecular interactions between ionic liquids (ILs) such as trimethylammonium acetate [(CH3 )3 NH][CH3 COO] (TMAA), trimethylammonium hydrogen sulphate [(CH3 )3 NH][HSO4 ] (TMAS), trimethylammonium dihydrogen phosphate [(CH3 )3 NH][H2 PO4 ] (TMAP) and triethylammonium dihydrogen phosphate [(CH3 CH2 )3 NH][H2 PO4 ] (TEAP) with N-methyl-2-pyrrolidone (NMP), density (), ultrasonic sound velocity (u) and viscosity () measurements have been performed across the entire concentration range and the wide temperature range from 298.15 to 313.15 K in steps of 5 K under ambient pressure. Further, on the basis of the measured properties, excess volumes (VE ), isentropic compressibility deviations (s ) and viscosity deviations () were obtained for all these systems under the same experimental conditions. The results are correlated by the Redlich–Kister type equation. The results are also analysed on anion dependent phenomena and temperature influence on measured and derived properties of ILs with NMP mixtures. A qualitative analysis of molecular interactions between ILs and NMP is discussed in terms of the ion-dipole, ion-pair interactions, and hydrogen bonding. © 2012 Elsevier B.V. All rights reserved.

1. Introduction One of the most popular and widely used solvent in the industries and academics is N-methyl-2-pyrrolidone (NMP). Moreover, NMP is a dipolar aprotic basic solvent that is frequently used to recover hydrocarbons from petrochemical processes by extractive distillation. It is a hygroscopic colourless solvent with a mild amine odour. It has large chemical and thermal resistance, low toxicity and high selectivity [1]. NMP is also used as an extractive agent for paraffin and aromatic separations. Moreover, NMP is a strong polar liquid and has the potential for use in the solvent extraction process for separating polar substances from non-polar substances [1–5], as it has a large dipole moment ( = 4.09 D) and a high dielectric constant (ε = 32.2 at T = 298.15 K) [2]. Recently, ionic liquids (ILs) are being realized as a best solvent media for the chemical processes. The physical properties of ILs, such as density, viscosity, melting point, polarity, and miscibility with water or molecular solvents, can be finely tuned by changing either the anion or its cations [6–13]. The non-volatile nature of ILs provides environmental advantages [7]. They are entirely composed of ions that have designable properties [14].

∗ Corresponding author. Tel.: +91 11 2766 6646 142; fax: +91 11 2766 6605. E-mail addresses: [email protected], [email protected], [email protected] (P. Venkatesu). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.07.004

The development of neoteric solvents, i.e., ILs, for chemical synthesis holds great promise for green chemistry applications [15–17]. The physicochemical properties of ILs are quite sensitive towards the structure and nature of its cations and anions. ILs are also chemically diverse owing to the huge number of possible ion combinations that can be synthesized in laboratory level [18–21]. Besides, ILs have many other popular properties such as the good ionic conductivities, good thermal stability and wide electrochemical window, also used as green solvents for biochemistry, catalysis reactions, chemical synthesis, separation science and polymer chemistry [8,13,19–24]. The variations in thermophysical properties of ILs, such as density (), ultrasonic sound velocity (u) and viscosity () are observed to be very sensitive to the change in either cation or anion [25]. Thermophysical properties of many ILs are limited as compared with conventional organic solvents although it is growing at a phenomenal rate [26]. Obviously, an interesting aspect is that ILs being able to influence on the physicochemical properties of mixtures and even a slight variation in their ions that can lead to substantial differences in their properties. Inevitably, knowledge for much of the physiochemical properties will be required on the other families of ILs. However, very little work is known about the appropriate variation in physicochemical properties upon varying either an anion or the cation in ILs. Understanding of the thermophysical properties of IL with polar solvent is of great importance for chemists and chemical engineers. To improve the applications of ammonium based

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Table 1 Solvent molecular weight (MW), purity, density (), ultrasonic sound velocity (u) and viscosity () for the solvents such as ILs and NMP at 298.15 K. MW (g mol−1 )

Solvent NMP

TMAA TMAP TMAS TEAP a b c d e f g h i j k l m

 (g cm−3 )

Purity

a,b,c,d,e,f,g

99.13

99%

1.02374 1.02590h,i

119.05 157.02 157.01 199.19

99% 99% 99% 99%

1.05385 1.35361 1.46758 1.12570

u (m s−1 )

 (mPa s)

1552b,j

1.66k 1.49b,l 1.68m 2.76 7.64 5.10 64.31

1544 1672 1564 1794

At 303.15 K. 1.02395 at 303.15 K from Ref. [1]. 1.02370 at 303.15 K from Ref. [2]. 1.02342 at 303.15 K from Ref. [33]. 1.02347 at 303.15 K from Ref. [34]. 1.02340 at 303.15 K from Ref. [35]. 1.02376 at 303.15 K from Ref. [36]. 1.02590 from Ref. [2]. 1.025590 from Ref. [31]. 1527 at 303.15 K, from Ref. [1]. 1.66 from Ref. [2]. 1.55 at 303.15 K, from Ref. [1]. 1.68 from Ref. [37].

ILs, it is essentially to explore their thermophysical properties with other molecular solvents. Therefore, a deep knowledge of thermophysical properties of liquid mixtures containing ILs has essentially required for scientific community. The present work is a part of our comprehensive investigation on the thermophysical properties of ammonium based ILs with polar solvents [19,20,27–30]. Here, we now uncover the characterization

a

for the structural variation of ions of ammonium based ILs as well as molecular interactions with a molecular solvent, NMP, through temperature variable properties. A survey of literature reveals that thermophysical properties of pure ILs have been reported extensively implying widespread from both fundamental and industrial points of view. However, the thermophysical properties of ILs in molecular solvents,

1.060

b 1.40

1.055

1.35

1.050 1.30

-3

ρ / (g.cm )

-3

ρ / (g.cm )

1.045 1.040 1.035 1.030

1.25 1.20 1.15

1.025 1.10

1.020 1.05

1.015 1.010 0.0

c

0.2

0.4

x1 0.6

0.8

1.00 0.0

1.0

d

1.50

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

x1 1.14

1.45 1.12

1.40 1.10

1.30

-3

ρ / (g.cm )

-3

ρ / (g.cm )

1.35

1.25 1.20 1.15

1.08 1.06 1.04

1.10 1.02

1.05 1.00 0.0

0.2

0.4

x1

0.6

0.8

1.0

1.00 0.0

0.2

0.4

x1

Fig. 1. Densities () for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid line represents the smoothness of these data.

T. Kavitha et al. / Thermochimica Acta 545 (2012) 131–140

particularly polar solvents, have not been explored in a systematic way. In this context, our aim is to explore the thermophysical properties such as , u and  for the mixed solvents of ILs and polar solvent to expand the basic needs for scientific research. Further, to gain some insight into the several aggregations of molecular interactions of ILs with polar compounds, we have synthesized the ILs of different combinations of ions, for our study. We report here the temperature dependence properties of , u and  as well as derived properties of excess molar volume (VE ), deviation of isentropic compressibility (s ), and deviation in viscosity () for ammonium based ILs, such as trimethylammonium acetate [(CH3 )3 NH][CH3 COO] (TMAA), trimethylammonium dihydrogen phosphate [(CH3 )3 NH][H2 PO4 ] (TMAP), trimethylammonium hydrogen sulphate [(CH3 )3 NH][HSO4 ] (TMAS) and triethylammonium dihydrogen phosphate [(CH3 CH2 )3 NH][H2 PO4 ] (TEAP) with NMP over a complete mole fraction range at 5 K intervals over a wide temperature range of 298.15–313.15 K. The VE , s , and  values were fitted to Redlich–Kister equation. A survey of the literature shows that no studies of VE , s , and  are reported for ammonium ILs with NMP. This study intends to draw molecular level information from the macroscopic properties on the molecular interaction between ILs and NMP.

2. Experimental 2.1. Materials NMP (>99% of purity), triethyl amine, phosphoric acid, sulphuric acid and acetic acid were purchased from Merck (>99% of purity) chemical company. Trimethyl amine (40%) was purchased from

a

Merck chemical company. NMP was stored over freshly activated 3A molecular sieves and was purified by the standard method described by Riddick et al. [2]. The fluids were degassed with ultrasound and kept in dark over Fluka 0.3 nm molecular sieves for several days and purified by fractional distillation. The purity of the NMP was verified by measuring the , u and  which were in good agreement with the literature values [1,2,31–37]. The purity of the sample was further confirmed by GLC single sharp peaks. To our knowledge, the values of , u and  for studied ammonium ILs are not available in the open literature, therefore, it is not possible to compare our data with any literature values in the present study. 2.2. Preparation of ammonium ILs All ammonium ILs used in this work, namely, TMAA, TMAP, TMAS and TEAP of general type [amine][anion] were synthesized, in our laboratory and the detailed procedure have been delineated in elsewhere [12,13,19,30]. 2.2.1. Synthesis of trimethylammonium acetate (TMAA) The synthesis of TMAA was carried out in a 250 mL roundbottomed flask, which was immersed in a water-bath and fitted with a reflux condenser. Acetic acid (1 mole) was added drop wise to a trimethyl amine (1 mole) at 343.15 K for 1 h. The reaction mixture was heated at 353.15 K under vigorous stirring for 2 h to ensure that the reaction had proceeded to completion. The reaction mixture was then dried at 353.15 K until in high vacuum (5 mm Hg) the weight of the residue remained constant. The sample was analysed by Karl Fisher titration revealed very low levels of water (below 70 ppm). The yield of TMAA was 98%. 1 H NMR (DMSOd6 ): ı (ppm)

b

1600

1710 1680

1580

1650 -1 u / (m.s )

1560

-1 u / (m.s )

133

1540

1620 1590

1520

1560 1500

1530 1480 0.0

c

0.2

0.4

0.6

0.8

0.0

1.0

x1

d

1600 1590

0.2

0.4

0.2

0.4

x1 0.6

0.8

1.0

1850 1800

1580

u / (m.s-1)

-1 u / (m.s )

1750

1570 1560 1550 1540

1700 1650 1600

1530

1550

1520

0.0

0.2

0.4

x1

0.6

0.8

1.0

1500 0.0

x1 0.6

0.8

1.0

Fig. 2. Ultrasonic sound velocity (u) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid line represents the smoothness of these data.

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a

b

5.0 4.5

35 30

4.0

η / (mPa.s)

η / (mPa.s)

25

3.5 3.0

20 15

2.5 10

2.0 5

1.5 1.0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

0.4

e

0.6

0.8

1.0

0.6

0.8

1.0

x1

x1

d

13

70

12 60

11 10

50

η (mPa.s)

η (mPa.s)

9 8 7 6 5

40 30 20

4 3

10

2 1 0.0

0.2

0.4

0.6

0.8

1.0

0 0.0

0.2

0.4

x1

x1

Fig. 3. Viscosities () for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid line represents the smoothness of these data.

2.51 (s, 1H), 2.10 (s, 3H). HRMS calculated for C5 H13 NO2 119.09, found 119.05.

3. Methods 3.1. Sample preparation

2.2.2. Synthesis of trimethylammonium dihydrogen phosphate (TMAP) The similar procedure as that described above for TMAA was followed with the exception of the use of phosphoric acid [anion] instead of acetic acid. The yield of TMAP was 97%.1 H NMR (DMSOd6 ): ı (ppm) 2.51 (s, 9H), 2.74 (s, 1H). HRMS calculated for C3 H12 NO4 P 157.05, found 157.02. 2.2.3. Synthesis of trimethylammonium hydrogen sulphate (TMAS) The similar procedure as that delineated above for TMAA was followed with the exception of the use of sulphuric acid [anion] instead of acetic acid. The yield of TMAS was 98%.1 H NMR (DMSOd6 ): ı (ppm) 2.52 (s, 9H), 2.75 (s, 1H). HRMS calculated for C3 H11 NO4 S 157.04, found 157.01. 2.2.4. Synthesis of triethylammonium dihydrogen phosphate (TEAP) The similar procedure as that described above for TMAA was followed with the exception of the use of phosphoric acid [anion] instead of acetic acid and triethylamine instead of trimethylamine. The yield of TEAP was 97%. 1 H NMR (DMSOd6 ): ı (ppm) 1.18 (t, 9H), 3.06 (m, 6H), 6.37 (s, 1H). HRMS calculated for C6 H18 NO4 P 157.10, found 157.08. The molecular weight, normal purity, , u and  of ILs and NMP, which are studied in the present work, are summarized in Table 1.

All solutions were prepared by mass using a Mettler Toledo analytical balance with an accuracy of ±1 × 10−4 g for all measurements. The estimated uncertainty in the mole fraction composition was found to be less than 5 × 10−4 . Mixing of the two components was promoted by the movement of a small glass sphere (inserted in the vial prior to the addition of the ILs) as the flask was slowly and repeatedly inverted. After mixing the sample, the bubble-free homogeneous sample was transferred into the U-tube of the densimeter or the sample cell of ultrasonic interferometer through a medical syringe. 3.2. Density measurements of ammonium ILs with NMP The  measurements of pure materials and the binary mixtures of ILs + NMP were carried out using an Anton-Paar DMA 4500 M vibrating-tube densimeter, equipped with a built-in solid-state thermostat and a resident program with accuracy of temperature of ±0.03 ◦ C. The uncertainty in the  experimental measurements is ±5 × 10−5 g cm−3 . The instrument was calibrated once a day with double distilled, deionized water and with air at 293.15 K as standards. The excess molar volumes (VE ) of ILs with NMP systems over the IL concentration range at different temperatures have been deduced from the  of the pure compounds and mixture (m ) using the standard equations [27,28].

T. Kavitha et al. / Thermochimica Acta 545 (2012) 131–140

135

Table 2 Estimated parameters of Eq. (1) and standard deviation,  for the systems of ILs with NMP at various temperatures. Systems

T (K)

VE /cm3 mol−1

TMAA + NMP

298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

−3.722 −3.085 −2.767 −2.355 −15.925 12.272 −10.646 −8.013 −19.593 21.329 −23.952 26.340 −8.318 −7.282 −6.413 −5.785

298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

−67.32 −80.15 −87.66 −100.175 −240.05 −253.72 −279.26 −302.15 −27.78 −20.28 −12.27 1.16 −244.60 −261.38 −289.58 −305.25

58.84 70.10 77.46 71.88 130.61 129.62 121.78 125.07 92.12 106.65 106.64 118.80 82.25 61.57 74.95 68.15

14.20 −6.01 −22.77 −35.81 −47.07 −53.83 49.52 4.12 −41.63 −18.77 −1.80 −21.23 15.56 −53.84 −37.72 −87.80

– 14.01 – – −11.01 – 13.14 −292.02 26.15 20.67 47.14 51.44 −87.95 – – –

– – – – 106.60 – −216.69 – – – – 54.48 – – – –

0.6 0.2 0.4 0.9 0.5 1.4 1.2 1.4 0.3 0.3 0.4 0.4 0.8 1.2 0.7 2.5

298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15 298.15 303.15 308.15 313.15

9.94 10.07 9.43 8.39 102.18 88.55 66.04 52.55 31.48 23.86 20.27 16.96 −86.42 −81.22 −73.79 −58.95

−5.07 −5.75 −7.14 −6.91 −66.17 −60.74 −42.73 −29.83 −31.59 −27.86 −22.09 −21.15 −47.19 −55.36 −47.18 −34.63

2.41 – −3.25 −4.18 11.32 24.58 28.73 19.12 1.85 11.44 7.77 12.42 14.60 12.28 15.65 −11.65

– – 3.97 3.89 16.03 20.06 11.05 −7.00 6.01 11.21 7.14 −20.21 −22.87 3.50 3.75 6.00

– – – – −73.11 −101.29 −105.35 −85.63 – −18.55 −15.91 – −57.15 −37.59 −38.09 12.23

0.05 0.04 0.05 0.06 0.32 0.35 0.42 0.25 0.12 0.09 0.06 0.13 0.28 0.26 0.18 0.11

TMAP + NMP

TMAS + NMP

TEAP + NMP

s /TPa−1

TMAA + NMP

TMAP + NMP

TMAS + NMP

TEAP + NMP

/mPa s

TMAA + NMP

TMAP + NMP

TMAS + NMP

TEAP + NMP

a0

3.3. Ultrasonic sound velocity measurements of ammonium ILs with NMP A single crystal ultrasonic interferometer (model F-05) from Mittal Enterprises, New Delhi, India, at 2 MHz frequency was used for u measurements for pure solvents and mixtures of ILs with NMP at various temperatures as a function of IL concentration. Prior to measurements, the interferometer was calibrated with double distilled water and purified methanol. A thermostatically controlled, well-stirred circulated water bath with a temperature controlled to ±0.01 K was used for all the ultrasonic sound velocity measurements for pure components and mixtures. The uncertainty in experimental u values has been found to be lower than ±1 m s−1 . 3.4. Viscosity measurements of ammonium ILs with NMP Vibro viscometer (Model: SV-10 A&D Company Limited, Japan) was used for  measurements of pure materials and binary

a1 4.783 4.563 4.285 4.306 10.708 9.007 8.094 8.459 36.120 35.528 37.023 37.701 3.276 3.727 3.946 4.503

a2 −1.060 −0.630 – – – – 2.991 −3.040 −5.885 0.801 −9.548 −9.314 −1.867 −0.353 −0.606 −0.038

a3



Y

a4 – – – – – – – −4.270 – 8.180 10.130 16.390 3.601 2.327 2.525 1.444

– – – – – – – 10.270 – – 25.570 33.403 – – – –

0.014 0.009 0.008 0.011 0.085 0.087 0.096 0.054 0.131 0.144 0.150 0.114 0.075 0.030 0.025 0.033

mixtures of IL + NMP at various temperatures. The instrument has been provided with two sensor plates of gold coating. The sample cell was placed under these sensor plates. The measurements of  were taken from the digital display device attached to the vibro viscometer. Viscosity measurements of the sample were taken at heating rate of 1 ◦ C/15 m to attain the thermodynamic equilibrium. Viscometer was calibrated with pure water and purified methanol at various temperatures. A thermostatically controlled, well-stirred circulated water bath with a temperature controlled to ±0.01 ◦ C was used for all the  measurements. Typically, the viscosities uncertainty is to be ±0.01 mPa s. 4. Results and discussion This paper reports a collection of new experimental , u and  values for a series of ammonium ILs with NMP at a wide range of temperature from 298.15 to 313.15 K in steps of 5 K and atmospheric pressure. The experimental values of , u and  are summarized in Table 1S and graphically presented in Figs. 1–3 for

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a

b 0.2

0.5 0.0

0.0

-0.5 -1.0

VE/ cm3. mol-1

VE/ cm3. mol-1

-0.2 -0.4 -0.6 -0.8

-1.5 -2.0 -2.5 -3.0 -3.5

-1.0

-4.0 -1.2 -1.4 0.0

c

-4.5 0.2

0.4

0.6

0.8

-5.0 0.0

1.0

x1

d

10

8

0.2

0.4

0.2

0.4

x 1

0.6

0.8

1.0

0.6

0.8

1.0

0.0

-1

V / (cm .mol )

6

-1.0

3

4

E

E

3

V /cm .mol

-1

-0.5

-1.5

2

-2.0

0

-2

0.0

0.2

0.4

0.6

0.8

1.0

x1

-2.5 0.0

x1

Fig. 4. Excess molar volumes (VE ) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid (–) lines correlated by Redlich–Kister equation.

ILs, NMP and their mixtures as function of IL concentration. Practically, ILs are miscible with molecular solvents with medium- to high-dielectric constants i.e. solvents containing of polar molecules and immiscible with low dielectric liquids [26]. In the present study, all of ILs are completely miscible in NMP (ε = 32.2 at 298.15 K), since NMP is a high dielectric and polar solvent. The effect of ILs on the , u and  in the NMP has been analysed as a function of temperature. Fig. 1a reveals significant increase in  for the mixtures of TMAA + NMP up to ≈0.4000 at all studied temperatures. Afterwards, a slightly increase in  was found in this mixture as the mole fraction of IL increases at all temperatures. On the other hand, the  values for the mixture of TMAP, TMAS or TEAP + NMP increase sharply (Fig. 1b–d) at all studied temperatures. The results in Fig. 1 explicitly elucidate that the  values significantly decrease as the temperature increases in all four systems. Careful examination of Fig. 1 reveals that IL system containing acetate anion significantly shows lower  values than sulphate and phosphate ions of IL systems. The TMAP or TMAS systems display the highest  values due to increased the size of the phosphate or sulphate ions respectively. The lower densities of TMAA + NMP are due to the weaker intermolecular interactions between of TMAA with NMP than the interactions between TMAP or TMAS with NMP. While comparing the densities of TEAP + NMP and TMAP + NMP, we observed that TEAP + NMP have less  values as compare with TMAP + NMP at all studied temperatures (Fig. 1). Obviously, the

lower alkyl chain length of cation of ILs are much dense than higher alkyl chain length of ILs. These observed results are quite consistent with the existing results [38–40], in which  values decrease with increasing the cation alkyl chain length of ILs. With increasing the number of carbon atoms in the alkyl chain length of the cation, the change in investigated properties is very high from methyl to ethyl group of ILs. The u values of ILs with NMP were measured starting from 298.15 K and increasing the temperature in steps of 5 K up to 313.15 K and the data is displayed in Fig. 2 as a function of IL concentration under atmospheric pressure. As it can be seen from Fig. 2a, the u approximately increases as the concentration of TMAA increases in NMP system up to ≈0.1700. Later, the values of u drastically decrease with increasing the mole fraction of TMAA at all studied temperatures. Whereas, in the case of TMAP + NMP mixture, the u values increase sharply as the concentration of TMAP increases in NMP up to ≈0.5900, further decreases with increase in mole fraction of TMAP as shown in Fig. 2b. The values of u roughly increase as the concentration of TMAS increases in NMP system up to ≈0.6000, afterwards, the values of u decrease with increasing the mole fraction of TMAS at all studied temperatures (Fig. 2c). In the case of TEAP + NMP, u values increase up to ≈0.6200 mole fraction of IL and later a slight decrease is observed in the u values up to 0.9900 mole fraction of IL at all investigated temperatures (Fig. 2d). As it can be seen from Fig. 2, the u sharply decreases as the

T. Kavitha et al. / Thermochimica Acta 545 (2012) 131–140

a

b

137

0

0 -10

-5 -20 Δκ s / (TPa-1)

Δκ s / (TPa-1)

-10 -15 -20

-60

-30

c

-40 -50

-25

-35 0.0

-30

0.2

0.4

x1

0.6

0.8

-70 0.0

1.0

d

15

0.2

0.4

0.2

0.4

x1

0.6

0.8

1.0

0.6

0.8

1.0

0 -10

10

-20 Δκs / (TPa )

-30

-1

-1

Δκs / (TPa )

5 0 -5

-40 -50 -60

-10

-70 -15 -20 0.0

-80 0.2

0.4

0.6

0.8

1.0

-90

0.0

x1

x1

Fig. 5. Deviation in isentropic compressibility (s ) for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid (–) lines correlated by Redlich–Kister equation.

temperature increases in all the systems, except in TMAP + NMP system, in which we observed higher values at 313.15 K. To obtain the mechanism events of the ILs role in the molecular interactions with NMP, we further have measured the  values for ILs with NMP under the same experimental conditions. The experimental data are presented in Fig. 3 as a function of IL concentrations at different temperatures. Fig. 3 shows that the increase of the  values of the IL caused by the addition of NMP and also intermolecular interactions as well as structural arrangement of solvents. It is clear that the  values increase with increasing the mole fraction of TMAA up to ≈0.4200, later it decreases at all studied temperatures for the systems of TMAA + NMP (Fig. 3a). The  values for the mixture of TMAP + NMP increases rapidly up to ≈0.4400 mole fraction of IL, later an abruptly decreases up to the mole fraction ≈0.9990, as displayed in Fig. 3b. We observed the similar trend in TMAS + NMP that the  increases rapidly up to ≈0.4000 mole fraction of IL (Fig. 3c). While the trend is reversed in TEAP + NMP mixture the  values increase significantly upon increasing the mole fraction of TEAP at all studied temperatures, as shown in Fig. 3d. This reversed trend is due to the increase in alkyl chain length in TEAP. It is evident from the results in Fig. 3 that, as expected, a monotonic decrease in  is observed as the temperature is increased from 298.15 to 313.15 K for IL + NMP mixture of a fixed composition. At a given temperature,  of the TMAP + NMP mixture is found to be lower than that of TEAP + NMP system. Clearly, the  values increased with increasing the number of carbon atoms in the alkyl chain and are likely to be influenced by the cation size of IL. However, in the case of TEAP with NMP the columbic interactions strengthen as increasing the concentration of IL thereby the  is

high at all investigated concentrations. We have anticipated for the high  in the case of TEAP that the size of cation is large enough to facilitate the interactions between anion and cation instead of between IL and a molecular liquid. On the other hand, due to the large gap in the size of anion and cation of TMAP the interactions are not enough to enhance the  values. However, interestingly the temperature shows similar effect on all binary mixtures. The highest  of the IL containing the [TEA]+ cation can be explained by strong molecular interactions due to hydrogen bonding. The lower  of the [TMA]+ IL can be explained by the charge delocalization in the anion, a lower molecular weight and size of the IL. The highest  found for TEAP having the highest molecular weight of IL. Usually, a low molecular weight of IL is having lower  values, in which the cation has sufficient side chain mobility [41]. Excess molar volume (VE ), deviation in isentropic compressibility (s ) and the deviation in viscosities () were evaluated from the experimental data according to well-known thermodynamic expressions [19,27,28]. Isentropic compressibility (s ) was calculated from Laplace equation. The composition dependence of the VE , s and  properties represents the deviation from ideal behaviour of the mixtures and provides an indication of the interactions between IL and NMP. The following Redlich–Kister expression was used to correlate these properties: Y = x1 x2

 n 



ai (x1 − x2 )

i

(1)

i=0

where Y refers to VE or s or . ai are adjustable parameters and can be obtained by least-squares analysis. Values of the fitted

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a

b

3.0

30 25

2.5

20

Δη / (mPa.s)

Δη / (mPa.s)

2.0

1.5

1.0

15 10 5

0.5

0.0 0.0

0.2

0.4

0.6

0.8

-5 0.0

1.0

x1

d

10

-5

6

-10

2

0 0.0

0.4

0.6

0.8

1.0

0

8

4

0.2

x1

Δη (mPa.s)

Δη (mPa.s)

c

0

-15

-20

0.2

0.4

0.6

0.8

1.0

x1

-25 0.0

0.2

0.4

x1

0.6

0.8

1.0

Fig. 6. Deviation in viscosity () for the mixtures of ILs + NMP as function of the composition expressed in the mole fraction (x1 ) of IL for (a) TMAA (1) + NMP (2); (b) TMAP (1) + NMP (2); (c) TMAS (1) + NMP (2); and (d) TEAP (1) + NMP (2) at (), 298.15 K; (), 303.15 K; (), 308.15 K and (䊉), 313.15 K at atmospheric pressure. The solid (–) lines correlated by Redlich–Kister equation.

parameters are listed in Table 2 along with the standard deviations of the fit. The values of VE , s and  for the binary mixtures at various temperatures as function of ILs concentrations are included in Table 1S. Figs. 4–6 display the values of VE , s and  along with the fitted curves, respectively, for the NMP with ILs as function of IL concentrations at different temperatures. Fig. 4a illustrates that the VE values for TMAA with NMP are negative in the entire range of composition, except at ILrich concentration at all temperatures, in which we observed slightly positive values at IL-rich compositions. It approaches the minimum values in TMAA + NMP system at mole fractions of TMAA about x1 ≈ 0.3100 at all investigated temperatures. This behaviour can be explained in terms of hydrogen bonding that is certainly more-dependent than columbic interactions. The minimum decreases in VE as temperature increases for TMAA + NMP system and this can be due to hydrogen bonds between molecules and IL. The negative excess molar volumes reveal that a more efficient packing or attractive interactions occurred when TMAA mixed with NMP. The interactions between the NMP molecules and the ions of TMAA are due to ion-dipole interactions. This will reduce the hydrogen bond between the cation and anion in the IL, which contribute to the negative VE values. The results observed for TMAP + NMP mixtures that the VE values are negative at all investigated temperatures, except at very IL-rich compositions at 313.15 K, in which we observed positive value (Fig. 4b). On the other hand, the TMAS + NMP mixture the VE values reveals that an inversion in the sign from negative to positive deviation, which imply that interactions between IL and NMP decrease as the concentration of the IL increases. The VE values

are positive for IL-rich compositions and negative for NMP rich compositions for the mixture of TMAS + NMP at all investigated temperatures (Fig. 4c). Decrease in the magnitude of the negative VE values with an increase in IL composition can attribute to the decrease of hydrogen bonding. Although it can also be understood as an increase in the concentration of the IL, a decrease of packing efficiency between NMP and IL, contributes to positive deviation. The minimum value occurs at the mole fraction of about ≈0.1100, and maximum at ≈0.6800 for the system of TMAS + NMP at all investigated temperature (Fig. 4c). The absolute values of VE are found to increase with an increase of temperature in the system. The values of VE for TEAP with NMP are negative in the entire range of composition, at various temperatures indicating negative deviations from ideal behaviour. It approaches the minimum values in TEAP + NMP system at x1 ≈ 0.3900 at all investigated temperatures, as seen in Fig. 4d. This behaviour can be explained in terms of hydrogen bonding that is certainly more T-dependent than columbic interactions. The experimental results in Fig. 4 depict that the values of VE for NMP with TMAA or TMAP or TEAP are negative over the whole concentration range at all investigated temperatures, except IL-rich composition of TMAA or TMAP system. This might be due to the large difference between molar volumes of the NMP and TMAA or TMAP or TEAP. It is possible that the relatively small organic molecules fit into the interstices upon mixing at all studied temperatures. Therefore, the filling effect of organic molecular liquids in the interstices of ILs, and the ion-dipole interactions between organic molecular liquid and ILs, all contribute to the negative values of VE . The negative VE values reveal that a more efficient packing or attractive interaction occurred when these IL and NMP were

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mixed at all cases. The interactions between the NMP molecules and the ions of IL, are due to ion-dipole interactions. Fig. 5 demonstrates the s values of ILs + NMP over the whole composition range at various temperatures as a function of ILs concentration. It is clear that different phenomena of s were observed for the various ILs with NMP. For the TMAA or TMAP + NMP mixture, the graphical representations in Fig. 5a and b exhibit negative s values over the entire composition range and approach the minima at x1 ≈ 0.3800 or ≈0.4000 respectively, at all investigated temperatures. The negative s values of TMAA or TMAP + NMP are attributed to the strong attractive interactions due to the solvation of the ions in these solvents. The negative values of s of the IL with NMP imply that solvent molecules around solute are less compressible than the solvent molecules in the bulk solutions. Further addition of IL there is a decrease in the compressibility graph at all temperature ranges. This might be due to decrease in attraction of NMP and IL molecules in IL rich concentration region. These discrepancies vary from IL to IL and solvent to solvent and also depend on the nature as well as structural arrangement of IL and solvent. The s values for the mixture of TMAP + NMP are higher as compared with the mixture of TMAA + NMP under same experimental conditions. In contrast, Fig. 5c exhibits inversion sign from negative to positive deviation, means the interaction between TMAS and NMP decreases as the concentration of the IL increases. As the concentration of the IL increases and the large portion of the NMP molecules are solvated, the amount of bulk solvent decreases causing a decrease in the compressibility. As the mole fraction increases the negative deviation increases up to ≈0.2000, while on further addition of the IL there is decrease in the compressibility and enters into positive deviation, and has a maximum value at the mole fraction of ≈0.7900 at all temperatures. Further, the negative s values of TEAP + NMP over the whole composition range at various temperatures as a function of ILs concentration (Fig. 5d). The negative values of the s of the TEAP with NMP in Fig. 5d imply that solvent molecules around the solute are less compressible than the solvent molecules in the bulk solutions. Hence, TMAA+, TMAP+, TEAP + NMP have negative s values for whole composition range at various temperatures. It can be observed in Fig. 6 that the  values for ILs + NMP are positive at a wide range of temperatures and atmospheric pressure, except for TMAP + NMP at very IL-rich composition and TEAP + NMP system. A maximum value in  for TMAA+, TMAP+ or TMAS + NMP is reached at a mole faction of IL ≈ 0.3600, ≈0.4400 or ≈0.3100, respectively. In fact, temperature influences strongly on the viscosity deviation. As the temperature increases, the deviations are reduced over the complete mole fraction range at all temperatures for all systems except TEAP + NMP. In which TEAP + NMP, the negative  values decrease with increasing the temperatures (Fig. 6d). The magnitude of the positive  values as a function of temperature can be attributed to decrease in hydrogen bonding. With rise in temperature, the interactions become very weak due to weakening of the dipolar association by the IL as well as the dissociation of the IL ion pair. The TEAP + NMP system exhibit negative deviation in viscosity, which indicates the interaction between IL and NMP decreases as the concentration of the IL increases (Fig. 6d).

5. Conclusions The present work reports an extensive experimental data of thermophysical properties such as , u and  of four ammonium based ILs, namely TMAA, TMAP, TMAS and TEAP, with NMP as a function of temperature. From these measurements,

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we have predicted VE , s and  at each temperature as a function of IL concentration. The predicted properties were correlated by the Redlich–Kister type equation. Obviously, fascinating results are obtained for structure-based and temperature dependent properties of novel kind of ILs with NMP. The established structure-based properties of ILs enhance our understanding on molecular interactions between ILs with polar solvent and the structural arrangements of ions of ILs offer many opportunities for exploration of various scientific fields. Acknowledgments We are gratefully acknowledged the Council of Scientific Industrial Research (CSIR), New Delhi, through the Grant No. 01(2343)/09/EMR-II, Department of Science and Technology (DST), New Delhi, India (Grant No. SR/SI/PC-54/2008) and University Grants Commission (UGC), New Delhi for financial support. 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.2012.07.004. References [1] A. García-Abuín, D. Gómez-Díaz, M.D. Larubia, J.M. Navaza, Density, speed of sound, viscosity, refractive index, and excess volume of N-methyl-2pyrrolidone + ethanol (or water or ethanolamine) from T = (293.15 to 323.15) K, J. Chem. Eng. Data 56 (2011) 646–651. [2] J.A. Riddick, W.B. Bunger, T.K. Sakano, Organic Solvents, 4th ed., Wiley-Inter Science, New York, 1986. [3] P.G. Kumari, P. Venkatesu, M.V.P. Rao, M.J. Lee, H.M. Lin, Excess molar volumes and ultrasonic studies of N-methyl-2-pyrrolidone with ketones at T = 303.15 K, J. Chem. Thermodyn. 41 (2009) 586–590. ˜ E. Mwenesongole, The excess molar volumes of [4] T.M. Letcher, U. Domanska, (N-methyl-2-pyrrolidone + an alkanol or a hydrocarbon) at 298.15 K and the application of the Flory–Benson–Treszczanowicz and the Extended Real Associated Solution theories, Fluid Phase Equilib. 149 (1998) 323–337. [5] P.G. Kumari, P. Venkatesu, T. Hofman, M.V.P. Rao, Excess molar enthalpies and vapor–liquid equilibrium for N-Methyl-2-pyrrolidone with ketones, J. Chem. Eng. Data 55 (2010) 69–73. [6] S. Baj, A. Chrobok, S. Derfla, A new method for dialkyl peroxides synthesis in ionic liquids as solvents, Green Chem. 8 (2006) 292–295. [7] K.R. Seddon, Ionic liquids for clean technology, J. Chem. Technol. Biotechnol. 68 (1997) 351–356. [8] P. Wasserscheid, W. Keim, Ionic liquids – new ‘solutions’ for transition metal catalysis, Angew. Chem. Int. Ed. 39 (2000) 3772–3789. [9] E.R. Cooper, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues, Nature 430 (2004) 1012–1016. [10] H.T. Karimata, N. Sugimoto, A–T base pairs are more stable than G–C base pairs in a hydrated ionic liquid, Angew. Chem. Int. Ed. 51 (2012) 1416–1419. [11] L.C. Branco, J.G. Crespo, C.A.M. Afonso, Highly selective transport of organic compounds by using supported liquid membranes based on ionic liquids, Angew. Chem. Int. Ed. 41 (2002) 2771–2773. [12] P. Attri, P. Venkatesu, A. Kumar, Activity and stability of ␣-chymotrypsin in biocompatible ionic liquids: enzyme refolding by triethyl ammonium acetate, Phys. Chem. Chem. Phys. 13 (2011) 2788–2796. [13] P. Attri, P. Venkatesu, Thermodynamic characterization of the biocompatible ionic liquid effects on protein model compounds and their functional groups, Phys. Chem. Chem. Phys. 13 (2011) 6566–6575. [14] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2083. [15] F.M. Maia, O. Rodríguez, E.A. Macedo, Relative hydrophobicity of equilibrium phases in biphasic systems (ionic liquid + water), J. Chem. Thermodyn. 48 (2012) 221–228. [16] K.R. Seddon, Ionic liquids: a taste of the future, Nat. Mater. 2 (2003) 363–365. [17] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry, Chem. Soc. Rev. 37 (2008) 123–150. [18] M.J. Earle, J. Esperanca, M.A. Gilea, J.N.C. Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831–834. [19] P. Attri, P.M. Reddy, P. Venkatesu, A. Kumar, T. Hofman, Measurements and molecular interactions for NN-dimethylformamide with ionic liquid mixed solvents, J. Phys. Chem. B 114 (2010) 6126–6133. [20] P. Attri, P. Venkatesu, A. Kumar, Temperature effect on the molecular interactions between ammonium ionic liquids and NN-dimethylformamide, J. Phys. Chem. B 114 (2010) 13415–13425.

140

T. Kavitha et al. / Thermochimica Acta 545 (2012) 131–140

[21] P.J. Scammells, J.L. Scott, R.D. Singer, Ionic liquids, Aust. J. Chem. 58 (2005) 155–169. [22] K.R. Seddon, A. Stark, M.J. Torres, Influence of chloride, water, and organic solvents on the physical properties of ionic liquids, Pure Appl. Chem. 72 (2000) 2275–2287. [23] M. Blesic, J.N.C. Lopes, M.F.C. Gomes, L.P.N. Rebelo, Solubility of alkanes, alkanols and their fluorinated counterparts in tetraalkylphosphonium ionic liquids, Phys. Chem. Chem. Phys. 12 (2010) 9685–9692. [24] P.M. Reddy, P. Venkatesu, Ionic liquid modifies the lower critical solution temperature (LCST) of Poly(N-isopropylacrylamide) in aqueous solution, J. Phys. Chem. B 115 (2011) 4752–4757. [25] F. D’Anna, S. Marullo, R. Noto, Aryl Azides formation under mild conditions: a kinetic study in some ionic liquid solutions, J. Org. Chem. 75 (2010) 767–771. [26] M. Freemantle, An Introduction to Ionic Liquids, RSC Publishing, Cambridge, UK, 2010. [27] V. Govinda, P. Attri, P. Venkatesu, P. Venkateswarlu, Thermophysical properties of dimethylsulfoxide with ionic liquids at various temperatures, Fluid Phase Equilib. 304 (2011) 35–43. [28] V. Govinda, P. Attri, P. Venkatesu, P. Venkateswarlu, Temperature effect on the molecular interactions between two ammonium ionic liquids and dimethylsulfoxide, J. Mol. Liq. 164 (2011) 218–225. [29] T. Vasantha, P. Attri, P. Venkatesu, R.S. Rama Devi, Thermodynamic contributions of peptide backbone unit from water to biocompatible ionic liquids at T = 298.15 K, J. Chem. Thermodyn. 45 (2012) 122–136. [30] P. Attri, P. Venkatesu, T. Hofman, Temperature dependence measurements and structural characterization of trimethyl ammonium ionic liquids with highly polar solvent, J. Phys. Chem. B 115 (2011) 10086–10097. [31] F. Murrleta-Guevara, A.T. Rodibuez, Liquid density as a function of temperature of five organic solvents, J. Chem. Eng. Data 29 (1984) 204–206. [32] A. Pal, Y.P. Singh, Excess molar volumes and apparent molar volumes of some amide + water systems at 303.15 and 308.15 K, J. Chem. Eng. Data 40 (1995) 818–822.

[33] J. Chen, R. Shen, W. Liu, G. Yu, A study of densities and volumetric properties of binary mixtures of N-methyl-2-pyrrolidone with xylene at different temperatures and atmospheric pressure, J. Chem. Thermodyn. 39 (2007) 934–941. [34] C. Yang, Z. Liu, H. Lai, P. Ma, Thermodynamic properties of binary mixtures of N-methyl-2-pyrrolidinone with cyclohexane, benzene, toluene at (303.15 to 353.15) K and atmospheric pressure, J. Chem. Thermodyn. 39 (2007) 28–38. [35] P.G. Kumari, M. Radhamma, G.C. Sekhar, M.V.P. Rao, Excess volumes and speeds of sound of N-methyl-2-pyrrolidone with chloroethanes and chloroethenes at 303.15 K, J. Chem. Eng. Data 47 (2002) 425–427. [36] C. Yang, G. He, Y. He, P. Ma, Densities and viscosities of NNdimethylformamide + N-methyl-2-pyrrolidinone and +dimethyl Sulfoxide in the temperature range (303.15 to 353.15) K, J. Chem. Eng. Data 53 (2008) 1639–1642. [37] A. Pal, R.K. Bhardwaj, Excess molar volumes and viscosities for binary mixtures of 2-propoxyethanol and of 2-isopropoxyethanol with 2-pyrrolidinone, Nmethyl-2-pyrrolidinone, NN-dimethylformamide, and NN-dimethylacetamide at 298.15 K, J. Chem. Eng. Data 47 (2002) 1128–1134. [38] D. Xiao, L.G. Hines, S.F. Li, R.A. Bartsch, E.L. Quitevis, O. Russina, A. Triolo, Effect of cation symmetry and alkyl chain length on the structure and intermolecular dynamics of 1,3-Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide ionic liquids, J. Phys. Chem. B 113 (2009) 6426–6433. [39] K.R. Seddon, A. Stark, M.J. Torres, Viscosity and density of 1-alkyl-3methylimidazolium ionic liquids, ACS Symp. Ser. 819 (2002) 34–49. ˜ [40] S. Li, J.L. Banuelos, J. Guo, L. Anovitz, G. Rother, R.W. Shaw, P.C. Hillesheim, S. Dai, G.A. Baker, P.T. Cummings, Alkyl chain length and temperature effects on structural properties of pyrrolidinium-based ionic liquids: a combined atomistic simulation and small-angle X-ray scattering study, J. Phys. Chem. Lett. 3 (2012) 125–130. [41] P. Fröba, H. Kremer, A. Leipertz, Density, refractive index, interfacial tension, and viscosity of ionic liquids [EMIM][EtSO4 ], [EMIM][NTf2 ], [EMIM][N(CN)2 ], and [OMA][NTf2 ] in dependence on temperature at atmospheric pressure, J. Phys. Chem. B 112 (2008) 12420–12430.