Novel halogen free hydrophobic trioctylammonium

Journal of Molecular Liquids 242 (2017) 349–356

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Novel halogen free hydrophobic trioctylammonium-based protic ionic liquids with carboxylate anions: Synthesis, characterization, and thermophysical properties☆ Ghassan M.J. Al Kaisy a,⁎, M.I. Abdul Mutalib a, T.V.V.L.N. Rao b a b

Department of Chemical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Malaysia Department of Mechanical-Mechatronics Engineering, The LNM Institute of Information Technology, Jaipur 302031, Rajasthan, India

a r t i c l e

i n f o

Article history: Received 14 May 2017 Received in revised form 10 July 2017 Accepted 11 July 2017 Available online 13 July 2017 Keywords: Trioctylammonium Protic ionic liquids Carboxylate anion Thermophysical properties

a b s t r a c t A three novel hydrophobic trioctylammonium-based protic ionic liquids (PILs) with 3,4-dimethylbenzoate, salicylate, and nonanedioate anions were synthesized and their structure were confirmed using nuclear magnetic resonance, elemental analysis and Fourier transform infrared spectroscopy. The three synthesized PILs were liquids at room temperature. The thermal properties were determined using thermogravimetric analyzer and differential scanning calorimetry. Density, viscosity, surface tension, and refractive index were measured for the synthesized PILs at temperature between 293.15 and 363.15 K at atmospheric pressure and as expected, it was found to decrease with increasing temperature. The density measurements were correlated as a linear function of temperature, and the viscosities were fitted to a Vogel-Fulcher-Tamman equation. The density measured values were used to calculate the molar volume, molecular volume, standard entropy, lattice potential energy and the isobaric thermal expansion coefficients, whilst the surface excess energy and entropy were calculated using the measured surface tension data. The di-cationic nonanedioate PIL was found to have higher viscosity, surface tension, heat capacity and onset decomposition temperature. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The increasing demand globally for ionic liquids (ILs) has led to the increased attention given to protic ionic liquids (PILs) due to their ease in production method involving simple proton transfer from a Brønsted acid to a Brønsted base. The proton transfer leads to the presence of a proton-donor and acceptor sites enabling the ILs to build a hydrogen-bonded network, which is a key property distinguishing them from other ILs [1]. A subclass of PILs is ammonium-based ILs, which can be synthesized through a simple neutralization reaction between an amine and a Brønsted acid. They are relatively cheaper besides possessing lower toxicity [2]. They have a wide range of potential applications such as replacement for conventional inorganic acids in acid-catalyzed reaction media, thermal transfer fluids, and fuel cell devices [3]. Ammoniumbased PILs has received significant attention in the application as electrolyte for electrochemical capacitors for energy storage devices based ☆ Notes; The authors declare no competing financial interest. ⁎ Corresponding author. E-mail address: [email protected] (G.M.J. Al Kaisy).

http://dx.doi.org/10.1016/j.molliq.2017.07.037 0167-7322/© 2017 Elsevier B.V. All rights reserved.

on metal oxide faradaic processes [4]. Recently, ammonium-based PILs have shown promising applications in biochemical process where they have demonstrated good biocompatible co-solvents stabilizer for the native structure of proteins [5–7] and replacement for commonly used organic solvent modifiers for the reversed phase HPLC separation of proteins [8]. Ammonium and hydroxylammonium PILs have also attracted attention towards CO2 absorption from natural and flue gas [9,10]. The ammonium-based PILs has also been evaluated as lubricants and lubricant additives for different metals contacts by several researchers and found to have superior tribological performance compared to the mineral lubricating oils and conventional ILs [11,12]. In this work, a novel group of halogen-free hydrophobic trioctylammonium [TOA] 3,4-dimethylbenzoate, [TOA] salicylate, and di[TOA] nonanedioate PILs were successfully synthesized. The structures of the three new PILs were confirmed using NMR, FTIR, and CHNS analysis. Their thermophysical properties such as density, viscosity, surface tension, refractive index and heat capacity were measured over a temperature range of 298.15 to 363.15 K at atmospheric pressure, for three different anions to explore the effects of the two parameters on its properties. The measured density values were then used to estimate the molar volume, molecular volume, thermal expansion coefficients,

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Bruker Avance 500 spectrometer. FTIR spectra were collected using Thermo Scientific Nicolet™ iS™10 spectrometer. Carbon, hydrogen, nitrogen, and sulfur content were determined using an elemental analyzer (elementar vario MICRO). The thermal decomposition profile for each PIL was collected using a Perkin-Elmer TGA, Pyris V-3.81 thermogravimetric analyzer. The analyzer heating profile was set at the rate of 10 °C min−1 under nitrogen atmosphere. The glass transition temperature (Tg) of the PILs was measured using differential scanning calorimetry (Perkin-Elmer, model pyris 1). The samples were heated in a nitrogen atmosphere at heating rate of 5 °C min−1 from 0 °C to 130 °C, followed by cooling from 130 °C to −150 °C and finally reheating again to 130 °C, at the rate of 5 °C min−1. The water content was measured using the coulometric Karl Fischer titrator (DL 39, Mettler Toledo) using Hydranal coulomat AG reagent. Fig. 1. Chemical structure of trioctylammonium (TOA) cation with 3,4-dimethylbenzoate, salicylate, and nonanedioate anion.

standard entropy, and lattice energy. Thermal decomposition (onset) temperature (Tonset) and glass transition temperature (Tg) of the PILs were analyzed using TGA and DSC, respectively. 1.1. Experimental 1.1.1. Materials Trioctylamine ≥ 93%, and nonanedioic acid ≥ 90% were purchased from Merck, whilst 3,4-dimethylbenzoic acid 98% and salicylic acid ≥ 99% were purchased from Aldrich. All chemicals were used as received without any further treatment. 1.1.2. Synthesis of ionic liquids The new PILs group was synthesized through neutralization reaction between trioctylamine (TOA) and selected carboxylic acids. The corresponding PILs were synthesized by mixing equimolar amounts of (TOA) with 3,4-dimethylbezoic acid and salicylic acid, whilst (2:1) molar ratio was used for the TOA and nonanedioic acid [13]. The two necked flask equipped with a reflux condenser and a dropping funnel was used to conduct the neutralization reaction. The reaction flask was placed in an oil bath. The TOA was placed first in the reaction flask before carboxylic acid was added gradually under continuous stirring. After completing the acid addition, the temperature of the mixture was slowly increased and maintained at 60 °C using oil bath. The reactions of 3,4-dimethylbezoic acid with salicylic acid were left for 24 h and with the nonanedioic acid for 96 h, under continuous stirring. All obtained PILs were found to be liquid at room temperature and showed a slight yellowish color. Each synthesized PIL was dissolved in methanol, stirred with activated carbon for 15 min prior to subjecting them to filtration to remove the activated carbon. The methanol was then removed using rotary evaporator, and the PILs were dried using vacuum line at 50 mbar with temperature kept at 60 °C for 24 h. The chemical structures demonstrating the cation and anions of the new PILs are presented in Fig. 1. 1.1.3. Characterization of protic ionic liquids The structure of each of the three PILs was confirmed using NMR, CHNS and FTIR analysis. The NMR analysis was performed using a

1.1.3.1. Density ρ and viscosity η measurements. Density and viscosity were measured using an Anton Paar Stabinger viscometer (SVM3000). Measurements were conducted over the temperature range of 293.15–363.15 K. The viscometer was calibrated with Millipore-grade water according to the equipment manual prior to conducting the data measurements. 1.1.3.2. Refractive index nD measurements. Refractive indexes were measured using an ATAGO digital refractometer (RX-5000α). Measurements were conducted over the temperature range of 293.15–333.15 K. 1.1.3.3. Surface tension γ measurements. Surface tensions were determined using the pendant drop method; the drop was generated by a syringe and photographed using a camera (OCA 20). Software (SCA 22) was used to evaluate the shape of the generated drops. Surface tension measurements were recorded over a temperature range of 293.15– 333.15 K. 1.1.3.4. Heat capacity Cp measurements. The heat capacities of the synthesized PILs were measured using differential scanning calorimetry (Perkin-Elmer, model pyris 1). The samples were sealed in 100 μL aluminum pans with a pinhole. The heating program comprised of an isothermal segment of 15 min at 258.15 K, followed by a constant heating rate of 20 K·min−1 to 353.15 K, before maintaining it constant for 15 min. The heat capacities of the PILs were determined relative to a sapphire sample which produces the highest accuracy. 2. Results and discussion The structures of the 3 synthesized PILs were confirmed by NMR, FTIR, and elemental analysis. Their molar mass and water content are listed in Table 1. Trioctylammonium 3,4-dimethylbenzoate (C33H61NO2) 1 H NMR (500 MHz, δ ppm CD3OD) δ 7.75 (s, 1H), 7.69 (s, 1H), 7.12 (s, 1H), 3.08–3.00 (m, 5H), 2.30 (d, J = 4.7 Hz, 6H), 1.75–1.63 (m, 6H), 1.45–1.25 (m, 30H), 0.91 (t, J = 6.9 Hz, 9H). 13 C NMR (126 MHz, δ ppm CD3OD) δ 174.00, 138.94, 135.42, 130.20, 128.69, 126.63, 52.45, 31.53, 28.85, 26.43, 23.53, 22.32, 18.55, 13.08 . CHNS elemental analysis: C (77.883%), H (10.600%), N (3.114%). Theoretical: C (78.67%), H (12.20%), N (2.78%).

Table 1 Molar mass, purity, glass transition, decomposition temperatures, and water content of the 3 PILs. PILs

Molar mass g mol−1

Purity %

Tg °C

Tonset °C

Td (5% mass loss) °C

Water content ppm

[TOA][dimethylbenzoate] [TOA][salicylate] [TOA]2[nonanedioate]

503.843 491.789 895.558

≥98 ≥99 ≥97

−79.75 −71.28 ND

189.10 212.50 227.04

157.80 186.04 166.05

352 399 48

ND: not detected.

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FT-IR (cm− 1): 2956.67, 2923.31, 2854.10, 1702.72, 1611.55, 1548.79, 1466.12, 1375.53, 1251.47, 1218.22, 1181.18, 1127.04, 907.74, 840.35, 723.01. Trioctylammonium salicylate (C31H57NO3) 1 H NMR (500 MHz, δ ppm CD3OD) δ 7.86 (s, 1H), 7.28 (s, 1H), 6.79 (d, J = 7.6 Hz, 2H), 3.14–3.03 (m, 6H), 1.77–1.60 (m, 6H), 1.45–1.22 (m, 31H), 0.91 (t, J = 7.0 Hz, 9H). 13 C NMR (126 MHz, δ ppm CD3OD) δ 174.67, 161.28, 132.36, 130.29, 118.86, 117.56, 115.81, 52.62, 31.50, 28.78, 26.29, 23.45, 22.30, 13.08 . CHNS elemental analysis: C (74.951%), H (11.113%), N (3.171%). Theoretical: C (75.71%), H (11.68%), N (2.85%). FT-IR (cm−1): 2954.053, 2923.89, 2854.94, 1631.94, 1485.00, 1455.32, 1376.93, 1301.77, 1259.37, 1220.31, 1138.06, 806.80, 757.35, 723.61, 704.14. Di-trioctylammonium nonanedioate (C57H118N2O4) 1 H NMR (500 MHz, δ ppm CD3OD) δ 3.13–3.02 (m, 12H), 2.23 (s, 4H), 1.71–1.61 (m, 16H), 1.41–1.32 (m, 66H), 0.93 (t, J = 6.9 Hz, 18H). 13 C NMR (151 MHz, CD3OD) δ 183.0, 54.02, 36.92, 32.87, 30.31 (d, J = 40.1 Hz), 27.66, 26.85, 24.85, 23.67, 14.40. CHNS elemental analysis: C (77.020%), H (13.520%), N (3.410%). Theoretical: C (76.450%), H (13.280%), N (3.130%). FT-IR (cm− 1): 2952.62, 2923.68, 2854.24, 1715.73, 1465.81, 1378.13, 1196.99, 1091.51, 723.17. Fig. 2 shows the FTIR spectra of the synthesized PILs. The asymmetric and symmetric C\\H stretches in the alkyl chains of the PILs are represented by the broad, strong vibrational peaks within the range of 2956–2854 cm−1. The characteristic peak of carbonyl group C_O for the studied ILs appeared in the range of 1715–1702, and 1631 cm−1. For the two PILs with the aromatic ring; the ring C_C stretch appeared at 1548.79 and 1485.00 cm−1, and the C\\H bend (in benzene rings) is observed to be within the ranges of 741–840 and 757–806 cm−1. The C\\N stretch of the TOA cation appeared at 1181, 1138, and 1196 cm−1. The\\(CH2)n bend appeared at 723 cm−1. These vibrations results confirmed the synthesized PILs. Fig. 3 shows the TGA profiles at a heating rate of 10 °C/min of the PILs. The subsequent onset decomposition temperatures (Tonset) and the 5% mass loss decomposition temperature (Td) are listed in Table 1. The decomposition temperature of the di-trioctylammonium PIL was found to be the highest within the PILs group which is most likely due to the increased in the van der Waals interactions between methylene units of the long alkyl chain anion. This creates higher interactive forces leading to stronger attraction. The studied PILs are more thermally stable than the tetrabutylammonium-amino acids AAILs group [10] and

351

Fig. 3. TGA thermal decomposition profiles of PILs: black, [TOA][dimethylbenzoate]; red, [TOA][salicylate]; blue, [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

within the same range of the PILs group of the same cation but different anions reported in our earlier work [13]. 2.1. Density The density ρ of the three PILs was measured over the temperature range of 293.15 K–363.15 K at atmospheric pressure and the results are listed in Table 2 with the corresponding plots shown in Fig. 4. The density trend was found to decrease in the order of [TOA][salicylate] N [TOA]2[nonanedioate] N [TOA][dimethylbenzoate] with 0.9352, 0.9262, and 0.9009 g cm−3 respectively at 298.15 K. These results are slightly higher than the measured values reported in our earlier work for PILs group of the same cation but with 4-tert-butylbenzoate, 4-phenylbutanoate and 2-hexyldecanoate anions which are 0.8904, 0.8864, and 0.8456 g cm−3 respectively at 298.15 K [12]. Likewise, low density values for ammonium-based PILs within the range of 0.875–0.889 g cm−3 were reported for tetraoctylammonium PILs with oleate and linoleate anions [14]. Conversely, these results are far below the density values reported for ammonium-based PILs with shorter alkyl chains such as: triethylammonium acetate [15], and trimethylammonium acetate [16] which are 1.01586, and 1.05385 g cm−3 respectively at 298.15 K. Accordingly, it can be concluded that the density of ammonium-based PILs decreases with an increase in the multiple alkyl chain length of the cation. The increment in carbon atoms number in the alkyl chain of the ammonium cation causes the PILs to be bulkier and hence longer average distance between the ions. Hence, the formation of hydrogen bond becomes more difficult.

Table 2 Experimental density ρ (g cm−3) values as a function of temperature at atmospheric pressure (1006 mbar).

Fig. 2. FTIR spectrum of PILs: black, [TOA][dimethylbenzoate]; red, [TOA][salicylate]; blue, [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

T/K

ρ (g cm−3) [TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[nonanedioate]

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

0.9009 0.8937 0.8867 0.8797 0.8726 0.8654 0.8581 0.8510

0.9352 0.9283 0.9216 0.9148 0.9080 0.9010 0.8939 0.8872

0.9262 0.9194 0.9126 0.9057 0.8989 0.8919 0.8849 0.8778

Standard uncertainties u, are u(ρ) = ±0.0005 g·cm−3, u(T) = ±0.02 K, and u(P) = ±1.75 mbar.

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The measured density was then used to calculate other important properties, such as standard molar volume (Vm), molecular volume (V), standard entropy (S°), lattice potential energy (UPOT), and the isobaric thermal expansion coefficients αP for the three new PILs. 2.1.1. Standard molar volume The standard molar volume (Vm), can be defined as the volume occupied by 1 mol of a substance at a given temperature and pressure. It is equal to the molar mass (M) divided by the mass density (ρ) [17] as shown in the equation (Eq. (3)) below:

Vm ¼ M

Fig. 4. Density as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate]; blue , [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Similarly, the density follows similar trend when the alkyl chain length of the anion increases, and it is also affected by the type of the anion. The salicylate PILs with hydroxyl group demonstrated the highest density in the group which could be mostly due to the additional hydrogen bonding. Generally, all the synthesized ammonium-based PILs in this work exhibit lower density values compared to water. The effect of temperature on PILs' density shows a linear reduction with increasing temperature, similar to the general trend reported in literature. The linear relation between density and temperature can be represented by the following equation (Eq. (1)) below: ρ ¼ A ο þ A1 T



ð3Þ

ρ

where Vm is molar volume (cm3·mol− 1), M the molecular weight (g·mol−1) and ρ the density (g·cm− 3) at 298.15 K. The calculated Vm values are listed in Table 4 and its sequence in increasing order follows the trend of [TOA][salicylate] b [TOA][dimethylbenzoate] b [TOA] 2 [nonanedioate] with respective values of 527.7605, 561.4606, and 970.3805 cm3·mol− 1 at 298.15 K accordingly. The di cationic PIL has the largest Vm due to weaker molecular forces resulting from looser long chain anion structure. The longer alkyl chain anion tends to occupy more space and results in larger PIL molecule. The other PILs in the group gave much smaller Vm due to the stronger molecular forces resulting from the tighter molecular structure of the anions. The anions contain aromatic ring in their structure and display a planar geometry which subsequently occupies less space resulting in smaller PIL molecule. The respective molecular volumes (V) of the PILs were calculated from the molar volume and Avogadro's constant (NA) using the equation (Eq. (4)) below: V ¼ Vm



ð4Þ

NA

ð1Þ

where ρ (g·cm−3) is the density of the PILs, T is the temperature in K, and Aο, A1 are the correlation coefficients. The respective values of the correlation coefficients for the synthesized PILs are listed in Table 3, together with the coefficient of determination R2 and its standard deviations (SD) which was calculated using equation (Eq. (2)) below: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 ðZexp−ZcalÞ SD ¼ n

ð2Þ

where SD is the standard deviation, Zexp is the experimental value, Zcal is the calculated value, and n is the number of experimental points. Table 3 Standard deviation SD values, R2, and the fitting parameters of Eq. (1). PILs

SD

R2

Aο

A1

[TOA][dimethylbenzoate] [TOA][salicylate] [TOA]2[nonanedioate]

0.0174470 0.0168144 0.0169194

0.99998 0.99997 0.99997

1.10974 1.13650 1.12883

−7.12262 × 10−4 −6.86429 × 10−4 −6.90714 × 10−4

where NA is the Avogadro's constant. The molecular volumes of the PILs are listed in Table 4, which follow similar sequence as expected for the standard molar volume Vm where [TOA][salicylate] b [TOA][dimethylbenzoate] b [TOA]2[nonanedioate]. 2.1.2. Standard entropy The following equation (Eq. (5)) developed by Glasser [18] was used to calculate the standard entropy S° of the three synthesized PILs:     −1 So J  K−1 mol ¼ 1246:5  V nm3 þ 29:5

ð5Þ

The calculated values are listed in Table 4 and the trend follows the order of [TOA][salicylate] b [TOA][dimethylbenzoate] b [TOA]2[nonanedioate] with respective values of 1092.39, 1162.14, and 2008.55 J·K− 1·mol− 1. The di trioctylammonium PIL showed the highest S° value which is due to its largest size, causing the least interaction between the cation and anion. This subsequently leads to higher disorder state hence higher standard entropy.

Table 4 Values of volume properties and surface properties of the studied PILs at 298.15 K and atmospheric pressure (1006 mbar). PILs

V nm3

Vm cm3·mol−1

S° × 104 J·K−1·mol−1

SS mJ·K−1·m−2

ES mJ·m−2

UPOT kJ·mol−1

[TOA][dimethylbenzoate] [TOA][salicylate] [TOA]2[nonanedioate]

9323.27 8763.66 16,113.5

561.4606 527.7605 970.3805

1162.1488 1092.3943 2008.5566

0.06483 0.03353 0.12647

43.6780 28.9969 69.2617

104.976 105.051 104.480

Standard uncertainty u, is u(P) = ±1.75 mbar.

G.M.J. Al Kaisy et al. / Journal of Molecular Liquids 242 (2017) 349–356

2.1.3. Lattice potential energy (or crystal energy) The following equation (Eq. (6)) was used to calculate the crystal energy UPOT of the synthesized PILs according to Glasser's theory [18]: 

U POT kJ  mol

−1



ρ

¼ 1981:2 =M

1 =

3

η (mPa·s)

ð6Þ

The calculated values are listed in Table 4, and the trend follows ascending order of [TOA]2[nonanedioate] b [TOA][dimethylbenzoate] b [TOA][salicylate], with values of 104.480, 104.976, and 105.051 kJ·mol−1 respectively. The calculated UPOT for the three PILs show much lower values than the CsI which has the lowest lattice energy (613 kJ·mol−1) among all the alkali-halide fused salts [19]. The findings agree with all the earlier reports on ILs where it is expected to exhibit low lattice energy values which is attributed to their liquid nature at room temperature. 2.1.4. Isobaric thermal expansion coefficient The density measured values for the synthesized PILs were used further to calculate the isobaric thermal expansion coefficient or isobaric cubic expansion coefficient αP using the following equation (Eq. (7)) [20]: αP ¼ −

Table 6 Measured viscosity η (mPa·s) values as a function of temperature at atmospheric pressure (1006 mbar). T/K

þ 103:8

 1 ∂ρ A1 ¼− Aο þ A1 T ρ ∂T P

353

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

[TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[nonanedioate]

126.6 68.8 40.4 25.5 16.9 11.7 8.4 6.3

348.2 177.3 97.6 57.7 36.3 24.0 16.6 10.4

3202.1 1492.8 749.4 403.6 232.7 140.9 88.3 56.8

Standard uncertainties u, are u(η) = ±0.35%, u(T) = ±0.02 K, and u(P) = ±1.75 mbar.

generally described in previous literature, the IL asymmetry and the size of its alkyl chains led to further entanglement, which in turn causes higher resistance to shear stress and consequently higher viscosity as demonstrated by [TOA]2[nonanedioate] PIL. The [TOA][salicylate] PIL shows higher viscosity than [TOA][dimethylbenzoate] PIL, most likely due to the additional

ð7Þ

where Aο and A1 are the correlation coefficients, obtained from Eq. (1). The calculated αP values as a function of temperature for the different anion are presented in Table 5. It is found that the changes in the thermal expansion coefficient with temperature are extremely small and sufficiently negligible to assume that the thermal expansion coefficients of the synthesized PILs are temperature independent within the studied temperature range. In addition, the calculated αP values are much lower as compared to the common organic solvents [19] due to higher intermolecular interactions [13]. 2.2. Viscosity The viscosity η(mPa·s) of the three PILs was measured over the temperature range of 293.15 K–363.15 K at atmospheric pressure and the results are listed in Table 6 with the corresponding plots shown in Figs. 5 and 6, and log η as a function of temperature is shown in Fig. 7. Consistently, the measured viscosity shows an inverse relationship with temperature, and follows the descending order sequence of [TOA]2[nonanedioate] N [TOA][salicylate] N [TOA][dimethylbenzoate], with values of 3202.1, 348.24, and 126.56 mPa·s respectively at 293.15 K. The nonanedioate PIL shows highest viscosity compared to the other two in this group. The increase in viscosity could be attributed to the increased in van der Waals interactions between the methylene units of the long alkyl chain of the nonanedioate anion, initiating higher interactive forces leading to stronger attraction as discussed earlier. As

Fig. 5. Viscosity as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate] PILs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 5 Estimated thermal expansion coefficients αP·10−4/K−1 as a function of temperature at atmospheric pressure (1006 mbar). T/K

293.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15

αP·10−4 (K−1) [TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[nonanedioate]

7.90576 7.96876 8.03277 8.09782 8.16393 8.23113 8.29944 8.36890

7.33934 7.39361 7.44868 7.50458 7.56132 7.61893 7.67742 7.73682

7.45632 7.51233 7.56920 7.62693 7.68554 7.74507 7.80552 7.86693

Standard uncertainty u, is u(P) = ±1.75 mbar.

Fig. 6. Viscosity as a function of temperature for [TOA]2[nonanedioate] PIL.

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G.M.J. Al Kaisy et al. / Journal of Molecular Liquids 242 (2017) 349–356 Table 8 Experimental surface tension γ (mJ·m−2) values as a function of temperature at atmospheric pressure (1006 mbar). T (K)

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

γ(mJ · m−2) [TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[nonanedioate]

24.7 24.4 24.0 23.7 23.4 22.9 22.7 22.4 22.1

19.1 19.0 18.8 18.6 18.5 18.3 18.1 17.9 17.8

32.3 31.7 30.9 30.1 29.5 28.9 28.4 27.8 27.1

Standard uncertainties u, are u(γ) = ±1.2%, u(T) = ±0.04 K, and u(P) = ±1.75 mbar.

Fig. 7. log η as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate]; blue , [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

hydrogen bonding possible for this PIL. In addition, this is also contributed by another inter-ionic interactions exists between the protonated cation and the salicylate anion. The experimental viscosity data were correlated using the following Vogel–Fulcher–Tammann equation (VFT) [21] shown in the (Eq. (8)) below: B logη ¼ A þ ðT−T ο Þ

ð8Þ

where A, Β, and Tο are fitting parameters. The best-fit parameters are listed in Table 7, together with the coefficient of determination R2 of the fitting equation. The VFT equation fits are represented by the solid red lines through the viscosity data in Figs. 5 and 6, which are in good agreement with the experimental data. 2.3. Surface tension Surface tensions γ(mJ · m−2) of the three PILs were measured within a temperature range of 293.15 K–333.15 K at atmospheric pressure which are given in Table 8 and plotted in Fig. 8. It shows an inverse linear relationship with increasing temperature, and decreases in the order of [TOA]2[nonanedioate] N [TOA][dimethylbenzoate] N [TOA][salicylate], with the values of 32.27836, 24.67258, and 19.10292 mJ · m−2 respectively at 298.15 K. The bigger di-cation [TOA]2[nonanedioate] PIL molecule exhibited higher surface tension than the smaller PILs within the group, indicating a superior structural organization at the air–liquid interface, stronger intermolecular interactions and higher surface free energy. In comparison to the triglycerides compounds, the surface tension value for the nonanedioate PIL is less than glyceroltrioleate (34.63 mJ · m− 2) but more than glyceroltributyrate (30.85 mJ · m−2). The dimethylbenzoate PIL has a γ value less than dodecane (25.35 mJ · m− 2) but more than decane (23.83 mJ · m−2 which belongs to linear alkane group. The salicylate PIL has a γ value in between heptane (20.14 mJ · m− 2) and hexane

(18.40 mJ · m− 2 and within the range of methylamine (19.89 mJ · m−2) and ethylamine (19.84 mJ · m−2) which belongs to the linear aliphatic n-amine group. Nevertheless, these values are generally much lower than water (71.99 mJ· m−2) [19]. The surface tensions showed a linear relation with temperature which can be described by the following equation (Eq. (9)) below:   γ mJ  m−2 ¼ a−bT

ð9Þ

where the intercept a of the equation can be identified as the surface excess energy ES (mJ · m−2) and the slope b as the surface excess entropy SS (mJ · m−2 · K−1) [22], as represented below:   γ mJ  m−2 ¼ ES −SS T

ð10Þ

The respective estimated values of ES and SS for the PILs at 298.15 K are given in Table 4. The values of the surface excess energy and entropy for the synthesized PILs are compared in Fig. 9 against some liquid organic compounds and water [23]. The surface excess energy and entropy of [TOA]2[nonanedioate] PIL is found to be in between benzylalcohol and chlorobenzene, whilst the value for [TOA][3,4-dimethylbenzoate] is found to be close to acetone but reasonably below methanol. This refers to the nonanedioate PIL without aromatic ring, showing a more organized structure at the air–liquid interface. The [TOA][salicylate] PIL comprising of hydroxyl group, exhibited surface excess energy and entropy well below all the above ranges and even below the acetone.

Table 7 VFT fit parameters of Eq. (8) for the studied PILs. PILs

R2

ηο

Β

Tο

[TOA][dimethylbenzoate] [TOA][salicylate] [TOA]2[nonanedioate]

1.0 0.99998 1.0

0.02117 0.00892 0.00382

1153.62101 1549.46552 2292.96957

160.48945 146.59365 125.03875

Fig. 8. Surface tension as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate]; blue , [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

G.M.J. Al Kaisy et al. / Journal of Molecular Liquids 242 (2017) 349–356

Fig. 9. Comparison plot of the relationship of SS and ES for the studied PILs and values for neutral organic liquids from literature.

355

Fig. 10. Refractive index as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate]; blue , [TOA]2[nonanedioate]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.4. Refractive index The refractive index nD was measured at atmospheric pressure within a temperature range of 293.15 K–333.15 K, and are given in Table 9 and plotted in Fig. 10. The trend follows a decreasing order of [TOA][salicylate] N [TOA][dimethylbenzoate] N [TOA]2[nonanedioate], with values of 1.48645, 1.48030, and 1.44918 respectively at 298.15 K. The salicylate-based PIL which contains aromatic ring and OH group within its structure displays the highest refractive index value whilst the nonanedioate-based PIL which does not possess any aromatic ring shows the lowest. It was found that the results fall within the range of the values reported in our previous work for PILs containing the same cation but with different anions i.e., [TOA][4-tert-butylbenzoate] of 1.47671, [TOA][2-hexyldecanoate] of 1.45143, and [TOA][4phenylbutanoate] of 1.47105 at 298.15 K [13]. The results are lower than the values reported [16] for triethylammonium acetate and triethylammonium hydrogen sulfate i.e., 1.501 and 1.516 respectively at 298.15 K, and higher than the values of trimethylammonium acetate and trimethylammonium hydrogen sulfate i.e., 1.392 and 1.406 respectively at 298.15 K. Typically, the refractive index value decreases with the increase in the cation and anion's side alkyl chain length. The refractive index measured values of the synthesized PILs exhibited an inverse linear relation with temperature which can be represented by the following equation (Eq. (11)): nD ¼ A2 þ A3 T

ð11Þ

Table 9 Experimental refractive index (nD) values as a function of temperature at atmospheric pressure (1006 mbar). T/K

nD [TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[nonanedioate]

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

1.48234 1.48030 1.47826 1.47619 1.47412 1.47209 1.47012 1.46816 1.46621

1.48824 1.48645 1.48462 1.48279 1.48091 1.47904 1.47719 1.47531 1.47344

1.45114 1.44918 1.44715 1.44534 1.44326 1.44141 1.43954 1.43758 1.43562

Standard uncertainties u, are u(nD) = ±3.5 × 10−5, u(T) = ±0.05 K, and u(P) = ±1.75 mbar.

where A2, A3 are the correlation coefficients, listed in Table 10 together with the coefficient of determination R2 and standard deviations (SD). The values of the R2 are N 0.999 and the SD are about 0.005 for the synthesized PILs. 2.5. Heat capacity The heat capacity Cp was measured at atmospheric pressure within a temperature range of 293.15 K–353.15 K, the results are given in Table 11 and plotted in Fig. 11. The values were found to follow the ascending order of [TOA][salicylate] ˂ [TOA][dimethylbenzoate] ˂ [TOA]2[nonanedioate], with values of 988.4959, 997.6091, and 1844.849 respectively at 298.15 K. The impact of temperature on PILs' heat capacity shows a linear increase with temperature, which has Table 10 Standard deviations (SDs) and the fitting parameters for Eq. (11). PILs

SD

R2

A2

A3

[TOA][dimethylbenzoate] [TOA][salicylate] [TOA]2[nonanedioate]

0.005537823 0.005077041 0.005296298

0.99994 0.99998 0.99995

1.60084 1.59699 1.56447

−4.04400 × 10−4 −3.70767 × 10−4 −3.86767 × 10−4

Table 11 Experimental heat capacity Cp (J·K−1 mol−1) values as a function of temperature at atmospheric pressure (1006 mbar). T/K

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

Cp (J·K−1 mol−1) [TOA][dimethylbenzoate]

[TOA][salicylate]

[TOA]2[C7H14 (COO)2]

992.6 997.6 1007.7 1012.7 1022.8 1032.9 1047.9 1058.1 1073.2 1083.3 1098.4 1113.5 1123.6

978.7 988.5 998.3 1008.2 1018.0 1032.7 1042.6 1057.3 1072.1 1081.9 1096.7 1106.5 1116.4

1835.9 1844.8 1862.8 1880.7 1898.6 1925.4 1943.4 1970.2 1997.1 2015.0 2041.9 2068.7 2086.6

Standard uncertainties u, are u(T) = ±1 K, u(Cp) = 5%, and u(P) = ±1.75 mbar.

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Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.07.037.

References

Fig. 11. Heat capacity as a function of temperature: black ■, [TOA][dimethylbenzoate]; red , [TOA][salicylate]; blue , [TOA]2[C7H14 (COO)2]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

been commonly reported in past studies. For the heat capacities of ionic liquids possessing similar cation but different anions, it was reported that an increase in the size of the anion leads to increase in the Cp values [24]. Within this PILs group, it was found that the heat capacity of the di cationic PIL with straight chain nonanedioate anion is considerably higher than the other PILs having aromatic ring in their anions and this concurs with findings reported in literature. The measured heat capacities of the synthesized PILs are much higher than those of conventional organic solvents, such as toluene (157.3 J K−1 mol−1) and methanol (81.1 J K−1 mol−1) [19], and higher than the values reported for ammonium-based amino acids ionic liquids [25]. 3. Conclusion The thermophysical and thermal properties of three new synthesized trioctylammonium-based PILs were studied as a function of temperature at atmospheric pressure and reported in this work. The measured thermophysical properties were found to decrease markedly with increasing temperature. The molar volume, molecular volume, standard entropy, lattice potential energy and isobaric thermal expansion coefficient were estimated from the measured density data. The variation in the volume expansion of these PILs could be considered as independent of temperature within the studied temperature range. The measured surface tension data were used to calculate the surface excess energy and entropy for the new PILs group and compared against several solvents including few organic types. The effect of long alkyl chain nonanedioate anion on the measured properties of the PILs were discussed, as it leads to higher viscosity, higher surface tension, higher heat capacity, and lower refractive index in the group of the synthesized PILs. Heat capacities of PILs were measured from 293.15 to 353.15 K. The obtained data showed that these PILs have a good heat storage capability and it could also be a good replacement for the harmful heat transfer fluids. The experimental results were matched with those found in literature, and showed a reasonable agreement. Acknowledgment The authors acknowledge support provided by center of research in ionic liquids (CORIL) Universiti Teknologi PETRONAS.

[1] K. Fumino, A. Wulf, R. Ludwig, Hydrogen bonding in protic ionic liquids: reminiscent of water, Angew. Chem. Int. Ed. 48 (2009) 3184–3186. [2] B. Peric, J. Sierra, E. Martí, R. Cruañas, M.A. Garau, J. Arning, et al., (Eco)toxicity and biodegradability of selected protic and aprotic ionic liquids, J. Hazard. Mater. 261 (2013) 99–105 (2013/10/15). [3] M. Anouti, M. Caillon-Caravanier, C. Le Floch, D. Lemordant, Alkylammonium-based protic ionic liquids part I: preparation and physicochemical characterization, J. Phys. Chem. B 112 (2008) 9406–9411 (2008/08/01). [4] N.L. Nguyen, D. Rochefort, Electrochemistry of ruthenium dioxide composite electrodes in diethylmethylammonium-triflate protic ionic liquid and its mixtures with acetonitrile, Electrochim. Acta 147 (11/20/2014) 96–103. [5] P. Attri, P. Venkatesu, Exploring the thermal stability of α-chymotrypsin in protic ionic liquids, Process Biochem. 48 (3/2013) 462–470. [6] J.P. Mann, A. Mc Cluskey, R. Atkin, Activity and thermal stability of lysozyme in alkylammonium formate ionic liquids-influence of cation modification, Green Chem. 11 (2009) 785–792. [7] P. Attri, I. Jha, E.H. Choi, P. Venkatesu, Variation in the structural changes of myoglobin in the presence of several protic ionic liquid, Int. J. Biol. Macromol. 69 (8/2014) 114–123. [8] W. Wei, N.D. Danielson, Fluorescence and circular dichroism spectroscopy of cytochrome c in alkylammonium formate ionic liquids, Biomacromolecules 12 (01/06/ 2011) 290–297. [9] X. Yuan, S. Zhang, J. Liu, X. Lu, Solubilities of CO2 in hydroxyl ammonium ionic liquids at elevated pressures, Fluid Phase Equilib. 257 (8/25/2007) 195–200. [10] H. Yu, Y.-T. Wu, Y.-Y. Jiang, Z. Zhou, Z.-B. Zhang, Low viscosity amino acid ionic liquids with asymmetric tetraalkylammonium cations for fast absorption of CO2, New J. Chem. 33 (2009) 2385. [11] H. Kondo, Protic ionic liquids with ammonium salts as lubricants for magnetic thin film media, Tribol. Lett. 31 (2008/09/01) 211–218. [12] T. Espinosa, J. Sanes, A.-E. Jiménez, M.-D. Bermúdez, Protic ammonium carboxylate ionic liquid lubricants of OFHC copper, Wear 303 (6/15/2013) 495–509. [13] G.M.J. Al Kaisy, M.I.A. Mutalib, J.M. Leveque, T.V.V.L.N. Rao, Novel low viscosity ammonium-based ionic liquids with carboxylate anions: synthesis, characterization, and thermophysical properties, J. Mol. Liq. 230 (2017) 565–573. [14] D. Parmentier, S.J. Metz, M.C. Kroon, Tetraalkylammonium oleate and linoleate based ionic liquids: promising extractants for metal salts, Green Chem. 15 (2013) 205–209. [15] T. Kavitha, P. Attri, P. Venkatesu, R.S. Rama Devi, T. Hofman, Temperature dependence measurements and molecular interactions for ammonium ionic liquid with N-methyl-2-pyrrolidone, J. Chem. Thermodyn. 54 (11/2012) 223–237. [16] R. Umapathi, P. Attri, P. Venkatesu, Thermophysical properties of aqueous solution of ammonium-based ionic liquids, J. Phys. Chem. B 118 (2014) 5971–5982 (2014/ 06/05). [17] I. U. O. P. A. A. CHEMISTRY, Quantities, Units and Symbols in Physical Chemistry, 2nd ed. Blackwell Science, 1993 41. [18] L. Glasser, Lattice and phase transition thermodynamics of ionic liquids, Thermochim. Acta 421 (2004) 87–93. [19] Physical constants of organic compounds, in CRC Handbook of Chemistry and Physics, in: W.M. Haynes (Ed.), 96th Edition (Internet Version 2016), CRC Press/Taylor and Francis, Boca Raton, FL, 2016. [20] A.B. Pereiro, J.L. Legido, A. Rodrıguez, Physical properties of ionic liquids based on 1alkyl-3-methylimidazolium cation and hexafluorophosphate as anion and temperature dependence, J. Chem. Thermodyn. 39 (8/2007) 1168–1175. [21] G.S. Fulcher, Analysis of recent measurements of the viscosity of glasses, J. Am. Ceram. Soc. 8 (1925) 339–355. [22] 2 Interfacial tension: Molecular interpretation, in: J. Lyklema (Ed.), Fundamentals of Interface and Colloid Science, Volume 3, Academic Press 2000, pp. 1–78. [23] Appendix 1 - surface tensions of pure liquids and mixtures, in: J. Lyklema (Ed.), Fundamentals of Interface and Colloid Science, Volume 3, Academic Press 2000, pp. A1.1–A1.48. [24] E. Gómez, N. Calvar, Á. Domínguez, E.A. Macedo, Thermal analysis and heat capacities of 1-alkyl-3-methylimidazolium ionic liquids with NTf2–, TFO–, and DCA–anions, Ind. Eng. Chem. Res. 52 (2013) 2103–2110. [25] R.L. Gardas, R. Ge, P. Goodrich, C. Hardacre, A. Hussain, D.W. Rooney, Thermophysical properties of amino acid-based ionic liquids, J. Chem. Eng. Data 55 (2010) 1505–1515 (2010/04/08).