Effect of Structural Variations on the ... - ACS Publications

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Effect of Structural Variations on the Thermophysical Properties of Protic Ionic Liquids: Insights from Experimental and Computational Studies Amir Sada Khan,*,†,‡ Zakaria Man,† Mohamad Azmi Bustam,† Girma Gonfa,†,§ Fai Kait Chong,⊥ Zahoor Ullah,†,∥ Asma Nasrullah,⊥ Ariyanti Sarwono,† Pervaiz Ahmad,# and Nawshad Muhammad*,∇ †

Centr of Research in Ionic Liquids (CORIL), Department of Chemical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia ‡ Department of Chemistry, University of Science and Technology, Bannu 28100, Khyber Pakhtunkhwa, Pakistan § College of Biological and Chemical Engineering, Addis Ababa Science and Technology University, 16417 Addis Ababa, Ethiopia ∥ Department of Chemistry, The Balochistan University of IT, Engineering and Management Sciences (BUITEMS), Takatu Campus, Quetta 87100, Pakistan ⊥ Fundamental and Applied Science Department, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia # Department of Physics, Abbottabad University of Science and Technology, Havelian, KP, Pakistan ∇ Interdisciplinary Research Centre in Biomedical Materials, COMSAT Institute of Information Technology, Lahore, Pakistan S Supporting Information *

ABSTRACT: In this work, new protic ionic liquids (PILs) based on 1-methylimidazolium/1-propanenitrileimidazolium cations with hydrogen sulfate, methanesulfonate, trifluoromethanesulfonate, para-toluenesulfonate, trifluoroacetate, and acetate anions were synthesized. The structures and purity of the products were confirmed by using 1H and 13C NMR and CHNS elemental analysis. The effect of structural variations on the thermophysical properties, namely, refractive index, density, and viscosity, was evaluated in a wide temperature range. The viscosity and density values were measured within the temperature range of 293.15−373.15 K. The density values were used further to calculate more properties like thermal expansion coefficient, molecular volume, standard entropy, and the lattice energy. Moreover, the experimental values of viscosity were used for the calculation of activation energy. Refractive indices were measured within the temperature range of 293.15−323.15 K, and these values were also used in the calculation of electronic polarizability. Acid numbers of the prepared PILs were measured and correlated with their structure moiety. In addition, the density functional theory (DFT) calculations were performed to get a deeper insight into the effect of structural variations of the ion pairs on their physical properties.

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INTRODUCTION Ionic liquids (ILs) are organic molten salts having melting point below 100 °C at atmospheric pressure. Unlike common organic solvents, they have unique and superior physical properties such as very low vapor pressure, high thermal and chemical stabilities, nonflammability, distinctive solvation potential, low melting point, high ionic conductivities, and wide liquid temperature range making them distinct candidates for various applications.1 Owing to large possibility of combing the various cations and anions, ILs have received great attention in science and technology.2−5 Recently, protic ILs have been the principle focus of numerous studies because of their simple synthesis methods, low cost of synthesis and purification, and their biodegradable nature. The most important property of protic ILs which differentiate them from other ILs is the easy transfer of a proton from the acid to base, to make proton donor and acceptor sites.1,6 PILs have extensive applications and been applied in organic synthesis, chromatography © 2017 American Chemical Society

techniques, electrolytes, catalyst, and solvent. In this scenario, PILs is currently extensively known as a new class of advanced acidic catalysts for many organic transformations.7−9 The properties of PILs can be further enhanced by incorporating functional group to a cation, anion, or on both. There are many reports available on synthesis and applications of functionalized ILs. Among functionalized ILs, cyano-based ILs are attracting considerable attention for industrial applications because of their higher thermal stability and lower melting points and viscosities compared to most of other reported ILs.10,11 Cyano-based ILs contain one or more cyano (CN) groups in their structural moieties. The high electronegativity, triple bond, and delocalized charges of the cyano group offered them unique properties and applications.12 Received: June 2, 2016 Accepted: August 10, 2017 Published: August 24, 2017 2993

DOI: 10.1021/acs.jced.6b00450 J. Chem. Eng. Data 2017, 62, 2993−3003

Journal of Chemical & Engineering Data

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Dayson et al.13 for the first time reported nitrile functionalized ILs and found to be good solvents and ligands for the catalysis reactions. ILs having a CN group also have better cellulose dissolution ability because of the electron-withdrawing nature and small size of the CN group. Multiple bonds of CN group induce strong polarity in ILs, which needed to create polarization interface with cellulose molecules for effective dissolution.14 Nitrile-functionalized ILs have the ability to establish superior media for generation of stable and monodispersed nanoparticles in comparison to nonfunctionalized ILs.15 Industrial process design and the yield of ILs-based products depend on the thermophysical properties, namely, density, viscosity, refractive index, thermal behavior, and surface tension of ILs. Therefore, an accurate and systematic thermophysical properties measurement of ILs is important in both fundamental and applied research as it can help us in screening to find the most suitable ILs for targeted applications. Therefore, the knowledge of thermophysical properties is significant to decide the potential applications of the synthesized PILs.16 In this study, we prepared a novel series of PILs using 1propyronitrile imidazolium/1-methyl imidazolium cations, with acids of widely varying strength such as sulfuric acid (H2SO4), methanesulfonic acid (CH3SO3H), trifluoromethanesulfonic acid (CF3SO3H), para-toluenesulfonic acid (p-TSA), trifluoroacetic acid (CF3CO2H), and acetic acid (CH3CO2H). The structures and purity of the synthesized PILs were confirmed using NMR and elemental analysis (CHNS). To understand the nature of these new synthesized PILs, physicochemical properties such as density, viscosity, refractive index, and thermal behavior were studied over a wide temperature range. The acidity (acid number) was measured and correlated with structure moiety especially anions structure. Furthermore, density functional theory (DFT) calculations were carried out to investigate the effect of structural variations of the cations and anions on the properties of the synthesized PILs.

Elemental analyses were performed using CHNS-932 (LECO instruments). Water content in synthesized ILs was determined using Karl Fisher Titration (Mettler Toledo DL39). 1-Propanenitrileimidazolium Hydrogen Sulfate [C2CNIM][HSO4]. Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 3.221−3.247 (t), 3.715 (s), 4.663−4.689 (t), 7.656 (s), 7.803 (s), 9.115 (s). 13C NMR (500 MHz, CH3OH): 16.907, 44.707, 54.438, 117.259, 120.548, 122.033, 135.525. CHNS elemental analysis: Calculated (%): C: 29.27, H: 3.44, N: 20.48, S: 15.63. Found (%): C: 29.11, H: 3.47, N: 20.35, S: 15.59. Elemental analysis standard uncertainty μ(E) = 0.075%. 1-Propanenitrileimidazolium Methanesulfonate [C2CNIM][CH3SO3]. Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 2.768 (s), 3.235 (s), 3.325−3.332 (t), 4.656− 4.682 (t), 7.678 (s), 7.819 (s), 9.134 (s). 13C NMR (500 MHz, CH3OH): 18.822, 38.493, 44.698, 117.069, 120.483, 122.053. CHNS elemental analysis: Calculated (%): C: 35.46, H: 4.46, N: 20.68, S: 15.78. Found (%): C: 35.50, H: 4.50, N: 20.56, S: 15.62. Elemental analysis standard uncertainty μ(E) = 0.050%/ 1-Propanenitrileimidazolium p-Toluenesulfonate [C2CNIM][p-TSA]. Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 2.365 (s), 3.135−3.161 (t), 4.560−4.587 (t), 5.161 (s), 7.245−7.260 (d), 7.554 (t), 7.709−7.713 (t), 7.740 (s), 7.756 (s), 8.995 (s). 13C NMR (500 MHz, CH3OH): 18.864, 20.248, 44.519, 117.080, 120.889, 120.932, 121.779, 121.824, 125.550, 128.875, 135.632, 140.901, 141.755. CHNS elemental analysis: Calculated (%): C: 51.60, H: 4.69, N: 15.04, S: 11.48. Found (%): C: 51.40, H: 5.10, N: 15.10, S: 11.42. Elemental analysis standard uncertainty μ(E) = 0.052%. 1-Propanenitrileimidazolium Trifluoromethanesulfonate [C2CNIM][CF3SO3]. Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 3.164−3.190 (t), 4.591−4.617 (t), 4.950 (s), 7.550 (t), 7.642 (s), 7.760 (s), 9.055 (s). 13C NMR (500 MHz, CH3OH):18.694, 44.671, 116.747, 119.04, 120.449, 121.950, 135.501. CHNS elemental analysis: Calculated (%): C: 29.20, H: 2.75, N: 16.34, S: 12.47. Found (%): C: 29.10, H: 2.87, N: 16.20, S:12.30. Elemental analysis standard uncertainty μ(E) = 0.072%. 1-Propanenitrileimidazolium Acetate [C2CNIM][CH3COO]. Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 1.665 (s), 2.843−2.868 (t), 4.243−4.268 (t), 7.119 (s), 7.242 (s), 8.303 (s). 13C NMR (500 MHz, D2O): 19.261, 23.014, 43.668, 118.142, 121.090, 122.689, 135.826, 180.356. CHNS elemental analysis: Calculated (%): C; 50.29, H: 5.43, N: 25.14, O: 19.14. Found (%): C: 50.10, H: 5.35, N: 25.20, O: 18.90. Elemental analysis standard uncertainty μ(E) = 0.112%. 1-Propanenitrileimidazolium Trifluoroacetate [C2CNIM][CF 3 COO]. Spectroscopic data: 1 H NMR (500 MHz, CH3OH): δ = 3.164−3.190 (t), 4.591−4.617 (t), 7.611 (s), 7.735 (s), 9.016 (s). 13C NMR (500 MHz, CH3OH): 18.774, 44.639, 115.613, 116.872, 117.941, 120.605, 121.902, 135.525. CHNS elemental analysis: Calculated (%): C: 38.02, H: 2.73, N: 19.00, O: 14.47. Found (%): C, 38.17, H, 2.84; N, 18.96; O, 14.20. Elemental analysis standard uncertainty μ(E) = 0.012%. 1-Methylimidazolium Hydrogen Sulfate [MIM][HSO4]. Spectroscopic data: 1H NMR (500 MHz, DMSO): δ = 3.86 (s), 5.72 (s), 7.59 (s), 7.66 (s), 8.98 (s). 13C NMR (500 MHz, DMSO): 35.234, 119.270, 123.057, 135.809. CHNS elemental analysis for C4H8N2SO4. Found (%): C: 26.68, H: 4.45, N: 15.56, S: 17.86. Calculated (%): C: 26.66, H: 4.48, N: 15.55, S: 17.80. Elemental analysis standard uncertainty μ(E) = 0.015%. Physical Characterization. Thermophysical properties such as refractive index, density, viscosity, and thermal stability

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EXPERIMENTAL SECTION Materials. All of the chemicals such as imidazole, 1methylimidazole, acrylonitrile, sulfuric acid, trifluoromethanesulfonic acid, para-toluenesulfonic acid, trifluoroacetic acid, acetic acid, methanol, diethyl ether, and ethyl acetate of analytical grade were purchased from Merck and used without any further purification. Synthesis and Characterization. The present PILs were synthesized in two steps; in the first step, imidazole (0.2 mol) and methanol were added into the three-necked flask and stirred until imidazole completely dissolved. Acrylonitrile (0.23 mol) was added dropwise, and the reaction mixture was stirred at 55 °C for 24 h. The unreacted material was removed by vacuum rotary at 70 °C. In the second step, 1-propyronitrile imidazolium was dissolved in acetonitrile, and respective acids were added dropwise followed by a continuous stirring for 12 h. The resultant final product was washed with diethyl ether and ethyl acetate to remove the unreacted materials. The synthesized PILs were subjected to drying using vacuum oven at 60 °C for at least 24 h. 1 H NMR and 13C NMR spectra of PILs were recorded on a Bruker Avance 500 MHz using tetramethylsilane (TMS) as an internal standard solvent. NMR spectra (1H and 13C) is provided as Supporting Information (SI, Figures S1−S14). 2994



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Figure 1. Scheme of the synthesis of protic ionic liquids.

Table 1. Name, Abbreviation, and Chemical Structure of the Synthesized PILs

Millipore quality water followed by ILs for which the data already reported in the literature.1,6 The measurement was performed in triplicate to obtain the average value. An acid−base titration method was used for the determination of the acid value of PILs.17 Approximately 100 mg of PIL sample was dissolved in 10 mL of 2-propanol, and some drop of phenolphthalein was added as an indicator. The mixture for each PILs were titrated against 0.1 N NaOH solution, and the end point was noted. The same procedure was used for a blank titration without addition of IL. The acid values for the PILs were determined using the following equation.

of the synthesized PILs were discussed subsequently below. The ILs that were obtained as solids are ([C2CNIM][CH3SO3], [C2CNIM][CF3SO3], and [C2CNIM][CF3COO]) at room temperature and are not evaluated for their properties such as viscosity and density. However, the PILs, which are liquid at room temperature, were subjected to measure their detail physiochemical properties. The refractive index values of synthesized PILs were measured using an ATAGO digital refractometer (RX-5000α) with the accuracy of ±4.5 × 10−5 in temperature ranging from 293.15 to 323.15 K with five-degree intervals. Densities and viscosities of synthesized PILs were measured using Anton-Parr viscometer (model SVM3000) at atmospheric pressure in temperature ranging from 293.15 to 373.15 K with ten-degree intervals. Before measurement of density and viscosity, the instrument was calibrated using ultrapure

acid value = 2995

(B − A)M × 40 W

(1) DOI: 10.1021/acs.jced.6b00450 J. Chem. Eng. Data 2017, 62, 2993−3003


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temperature, Ea is the energy of activation (J/mol), and A0, A1, A2, and A3 are adjustable parameters. The standard deviation (SD) was determined using the following expression:

where A is the volume of NaOH solution needed for the titration of blank (mL), B is the volume of NaOH solution needed for the titration of the sample (mL), M is the molarity of NaOH solution, and W is the mass of sample in gram. The thermal degradation temperature of PILs in the nitrogen atmosphere was measured using PerkinElmer TGA, Pyris-1, V3.81 in the temperature range of 323−773 K with the heating rate of 10 K/min. A Mettler-Toledo differential scanning calorimeter (DSC822e) with Mettler-Toledo STARe software was used for the measurement of glass transition temperature (Tg) and melting point (Tm). The glass transition temperature is the midpoint of a small heat capacity change on heating from the amorphous glass state to a liquid state. The melting point is the onset of an endothermic peak on heating.18 The instrument was calibrated with indium (calibration standard, purity >99.999%) by heating from 120 to 180 °C at 10 °C/min with atmosphere nitrogen, 50 cm3/min. The sample was weighted in aluminum pans and subjected to thermocycles in which initially the sample was heated in a nitrogen atmosphere with the heating rate of 10 °C·min−1 from 0 to 110 °C, cooled from 110 °C to −150 °C, and then heated again to 110 °C. The glass transition temperatures were determined with a temperature accuracy of 1 °C. Computational Methods. Density functional theory (DFT) calculations were performed to understand the effect of the structural variations of the cations and anions on the molecular properties of the ILs. Turbomole 6.2 software package was used for the structural optimizations and quantum calculations.The geometry optimizations were performed using the dispersion corrected density functional theory (DFT-dsp) method, utilizing the Becke-Perdew-86 (BP86) functional19,20 with the resolution of identity (RI) approximation and a tripleξ valence polarized basis set (TZVP).21,22 Natural bond orbital (NBO) population analyses for the species were performed at the same level and basis set. The ideal screening charges on the molecular surface of the species were calculated using COSMOthermX program (version C2.1) with BP_TZVP_C21_0111.ctd parametrization.23

N

SD =

nD = A 2 + A3T

(3)

ln η = ln η∞ +

Ea RT

(5)

Figure 2. Refractive indices (nD) as a function of temperature for the protic ionic liquids.

Table S1 in the SI for which the standard uncertainty values are u(nD) = 3.5 × 10−3. As the experimental results shows that the refractive indices decrease linearly with the increase of temperature, as reported elsewhere.24 The values of refractive indices are in the range of 1.47508−1.54509. The decreasing order of refractive indices of the investigated PILs is [C2CNIM][p-TSA] > [C2CNIM][CH3SO4] > [C2CNIM][HSO4] > [MIM][HSO4] > [C2CNIM][CH3COO]. The result indicates that there is a strong effect of anion structure on the refractive index value. The lower values are noted for the [C2CNIM][CH3COO], while the higher values are observed for the PIL [C2CNIM][p-TSA] owing to more aromaticity (electron mobility) in its structure.24 Furthermore, for the PIL with the same anion, that is, [HSO4], the refractive index value of the nitrile functionalized ionic liquid, [C2CNIM][HSO4] is higher than the methyl functionalized PIL, [MIM][HSO4].1,6 The high refractive index value of the nitrile functionalized ILs may be due to the additional electron mobility in the nitrile functionalized side chain compared to nonfunctionalized imidazolium alkyl chain.12,25 Generally, the refractive indices increase with the increase of alkyl chain length of functional group attached to the cation as the mobility of the electrons was suggested to increase.26 Zahoor et al.4 reported refractive index values for [MIM][HSO4] which have differences with the current work are about 0.006. The fitting parameters and standard deviation of refractive indices for PILs are summarized in Table S2 in the SI. Density. The experimental measured values of density for the PILs at nine selected temperatures between 293.15 and 373.15 K are shown in Figure 3, and the values are given in Table S3 in the SI. The decreasing order of the density for synthesized PILs are [MIM][HSO4] > [C2CNIM][HSO3] >

RESULTS AND DISCUSSION Synthesis and Characterization. The PILs were prepared in two steps. In the first step, acrylonitrile was added dropwise to an imidazole solution in methanol. The reaction was stirred at 55 °C for 24 h to get the product. In the second step, 1propyronitrile imidazolium was dissolved in acetonitrile, and the respective acid was added dropwise followed by continuous stirring for 12 h. The synthesized PILs were dried in a vacuum oven at 60 °C for at least 24 h. The scheme of synthesis of PILs is shown in Figure 1. The names, abbreviations, and chemical structures of the synthesized PILs are given in Table 1. The dependence of the values of refractive index, density, and viscosity on temperature were fitted by the least-squares method using the following equations: (2)

N

Refractive Index. The measured refractive index (nD) data for the synthesized PILs as a function of temperature (293.15− 323.15 K) are shown in Figure 2, and the values are listed in

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ρ = A 0 + A1T

∑i (Zexp − Zcalc)2

(4)

where T is the absolute temperature in Kelvin, R is the universal gas constant (8.314 J/K·mol), η∞ is the viscosity at infinite 2996



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ranging from 4.21 × 10−4 to 7.53 × 10−4 K−1, are typical values of thermal expansion coefficient for ILs. Among these synthesized PILs, [C2CNIM][CH3COO] has the highest α value, while [MIM][HSO4] has the lowest value. PILs containing a methyl group on the imidazole ring such as [MIM][HSO4] have small thermal expansion coefficients as compared to those having a propyronitrile group on the imidazole ring, [C2CNIM][HSO4]. This increase in thermal expansion could be due to increase in steric hindrance, which causes the decrease in the interactions between cation and anion of ionic liquid and thereby results in higher values are measured for propyronitrile containing side chain ionic liquids. The effect of alkyl chain length on thermal expansion coefficient for amino acid based ILs was also observed by Santis et al.18,24 Our values are slightly lower than those measured by De Santis et al. for sulfate based ILs.24 The lower values of α and less expansion with temperature for [MIM][HSO4] and [C2CNIM][HSO4] is due to the additional capability to form hydrogen bonds.31 The α values of the PILs are considerably lower than that of most molecular organic liquids (10−3 K−1) while greater than the α value of high temperature classical molten salt (1−2 × 10−4 K−1). In this study, a considerable change was noted in thermal expansion coefficient value with respect to temperature. This increase in thermal expansion coefficient values with increase of temperature is due to decrease in ordering of PILs.28 Molar Volume. Molar volume (Vm) is the volume occupied by one mole of a substance at standard temperature and pressure. The Vm of PILs at various temperature and atmospheric pressure can be calculated using the following equation:

Figure 3. Density (ρ) as a function of temperature for the protic ionic liquids.

[C2CNIM][p-TSA] > [C2CNIM][CH3COO]. The [MIM][HSO 4 ] has higher density values, while [C 2 CNIM][CH3COO] have lower values under same experimental conditions. The higher values of density for PILs containing the sulfate anion is due to the high molecular weight of anion compared to lower molecular weight anion such as acetate.3 Likewise, the PILs containing 1-ethyl-3-methylimidazolium cation with the different anions such as acetate and tosylate showed the same trend as observed in this study.3 In general, the density of ILs increasing with the increase of anion molecular weight; however, sometimes density decreases with increasing the anion molecular weight.24,27,28 To understand the relationship between the densities of ILs with structure of anion is unclear, probably due to the large number of factors, namely, ion shape, size, geometry, charge density, and molecular weight. A decrease in the values of densities was observed for the PILs by incorporating propyronitrile functional group on the cation, referring to density values of [MIM][HSO4] and [C2CNIM][HSO4] containing same anion but different cations. This decrease in density of PILs is due to an increase of volume occupied by the propyronitrile functional group as compared to the methyl group. Gardas et al. reported the decrease in density of sulfate based ILs with the increase of alkyl chain length on imidazole.29 With an increase of temperature, the decrease in density of ILs were observed; this decrease in density values with temperature is due to the decrease in the van der Waal forces of interactions.30 The fitting parameter values with R2 and standard deviation (SD) for empirical correlation of density of the measured ILs are reported in Table S4 in SI. Thermal Expansion Coefficient. The values of thermal expansion coefficient (α) or cubical expansion can be calculated using the relation between the temperature and density by the following expression: A1 1 ⎛ ∂ρ ⎞ αp = − ⎜ ⎟ = − ⎝ ⎠ ρ ∂T p A 0 + A1T

Vm = M/ρ

(7) −1

where Vm is the molar volume in cm ·mol , M is the molecular weight in g·mol−1, and ρ is the density in g·cm−3 at 298.15 K. The values of Vm calculated for the PILs at various temperature are listed in Table S6 in SI. The Vm of the studied PILs follows the order of [C2CNIM][p-TSA] > [C2CNIM][CH3COO] > [C2CNIM][HSO4] > [MIM][HSO4]. The molar volumes of the studied PILs are higher than their corresponding chloride based ILs reported by Muhammad.32 Among all PILs, [C2CNIM][p-TSA] shows a higher molar volume (232.96 cm3·mol−1), and [MIM][HSO4] shows the lower molar volume (123.10 cm3·mol−1) at 303.15 K. The higher molar volume of the [C2CNIM][p-TSA] is due to the large size of the anion which leads to weak interaction with the cation.33 The lower Vm value of [MIM][HSO4] is due to the small size of the ions which leads to the strong interaction between cation and anion. The molar volume of ionic liquid increases with the increase in chain length of the functional group attached to the cation or anion.24 Standard Entropy. The values of standard entropy (So) for the prepared PILs were calculated from their molar volume using the relationship established by Glasser:34 3

S o(J ·K−1·mol−1) = 1246.5 V (nm 3) + 29.5

(8)

where V is the molar volume of the PILs. The values of the standard entropy calculated for the PILs are listed in Table S6 in SI. The standard entropies measured for [C2CNIM][pTSA], [C 2 CNIM][CH 3 COO], [C 2 CNIM][HSO 4 ], and [MIM][HSO4] are 511.89, 462.03, 347.35, and 283.78 J·K−1· mol−1, respectively. The lower value of standard entropy for [MIM][HSO4] is due to more symmetry and fewer resonance

(6)

where ρ and T are the density and temperature, respectively. The values of thermal expansion coefficients as a function of temperature are summarized in Table S5 in the SI. The variations of the thermal expansion coefficient value with temperature are significant for the present PILs. The values, 2997



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TSA] is due to the large size of an anion. The free volumes of the synthesized PILs, that is, C2CNIM][HSO4], [C2CNIM][CH3COO], and [MIM][HSO4], are quite similar to the protic ionic liquids reported by Gardas et al.39 Electronic Polarizability. The electronic polarizability (Rm) for a substance can be calculated from the refractive index value and can provide significant information concerning the force between molecules or their behaviors in solution.41,42 The polarizabilities blur the permanent partial charge distribution and thus act as lubricant between the ions.18 The electronic polarizability has been calculated using the experimental values of refractive index by the Lorenz−Lorentz equation given as below:43

structures for its cation and anion. Therefore, a fewer number of conformational structures will be predicated, and ultimately the standard entropy will be lower. In this regard, the higher standard entropy values for [C2CNIM][p-TSA] are attributed to its more possible conformational structures. The values of standard entropy of these PILs were measured in the same range of other ILs like [CnMim]alaninate and [CnMim]glycinate (where n = 2−6), ranging from 421−456 and 429− 469 kJ·mol−1, respectively.18,35,36 Lattice Energy. The lattice energy (UPOT) express information about the interaction of the cation and anion. The lattice energy at 298.15 K were calculated using Glasser’s theory34 as follows: UPOT (kJ·mol−1) = 1981.2(ρ /M )1/3 + 103.8

⎡ n2 − R m = ⎢ D2 ⎣ nD +

(9)

−3

where, ρ and M are the density (g·cm ) and molecular mass (g·mol−1), respectively. The estimated lattice energies are shown in Table S6 in SI. As expected the UPOT of the studied ILs is much lower than those of inorganic fused salts. The UPOT for cesium iodide is 613 kJ·mol−1,37 which has the lowest UPOT among alkali halides. The UPOT of the present PILs are in the order as [MIM][HSO4] > [C2CNIM][HSO4] > [C2CNIM][CH3COO] > [C2CNIM][p-TSA]. The [MIM][HSO4] has the highest UPOT energy value, while [C2CNIM][p-TSA] has the lowest value. Moreover, the UPOT of these prepared PILs is in similar range as reported for chloride based ILs. The calculated U POT for 3-(3-butyl-1H-imidazol-3-ium-1-yl)propanenitrile chloride ([C2CNBIM]Cl), 3-(3-allyl-1H-imidazol-3-ium-1-yl)propanenitrile chloride [C2CNAIM]Cl, 3-(3benzyl-1H-imidazol-3-ium-1-yl)propanenitrile chloride ([C2CNBzIM]Cl), and 3-[3-(2-hydroxyethyl)-1H-imidazol-3ium-1-yl]propanenitrile chloride ([C2CNHeIM]Cl) are 448.9, 462.3, 439.3, and 468.2 kJ·mol−1, respectively.32 The lower value of UPOT for [C2CNIM][p-TSA] may be due to the large size of p-toluenesulfonate anion which leads to lose packing of ions than their corresponding chloride based ILs. The higher UPOT value for [MIM][HSO4] is due to the smaller size of anion and cation and strong hydrogen bonding interactions between respective ions which cause close compactness of ions.38 Free Volume (Vf). The refractive index values of PILs are useful to calculate the free volume (Vf), which is consider an important parameter, helpful in understanding the transport phenomena in ILs. The free volume of PILs can be correlated to the solubility of the low molecular weight gases such as CO2, CH4, and C2H6.30,39 With the increase in the free volume of ILs an increase in solubility of these gases was observed earlier.39 However, it is important to mention here that the solvation of the nonpolar gases is more likely controlled by electrostatic specific interaction and to small extent by free volume effect.40 The free volume of ILs is calculated by using following equation:

Vf = Vm − R

⎡ n2 − 1⎤M ⎥ = ⎢ D2 2⎦ ρ ⎣ nD +

1⎤ ⎥Vm 2⎦

(11)

where nD is the refractive index, M is the molar mass, ρ is the density, and Vm is the molar volume. The values of Rm calculated for PILs at various temperatures are listed in Table S7 in the SI. The values of Rm are found to be in the order of [C2CNIM][HSO4] < [C2CNIM][CH3COO] < [C2CNIM][pTSA] at 303.15 K are 45.281 < 50.913 < 73.426, respectively. The difference in the Rm values of ILs comes from the various anions used in their synthesis. The Rm values increase slightly with the increasing of anion size. The higher polarizability for [C2CNIM][p-TSA] is due to the large size of anion. Usually, large ions have great polarizabilities.44 This increase in the Rm values with the increase of molecular weight of anion and chain length of the cation is also reported by Ziyada and Cecilia.43 The higher value of Rm for [C2CNIM][HSO4] as compared to [MIM][HSO4] is due to the replacing of the methyl group of cation with the propyronitrile functional group. This increase in polarizability of ILs with the increase of alky chain length of alkyl group on cation is also observed in the literature.43,45 As seen from the experimental results, Rm increases linearly with the increase of temperature as reported elsewhere.30,43 Viscosity. Viscosity is an important property of ILs as it can impact on mass transport phenomena, thereby synthesis or choosing of specific ILs for particular applications; viscosity is a key and an important property. The dynamic viscosities for synthesized PILs are measured at temperatures ranging of 293.15−373.15 K with atmospheric pressure are reported in Table S8 in the SI. The dynamic viscosity of PILs decreases with the increase of temperature (Figure 4). The decreasing order in dynamic viscosities of prepared PILs is as follows: [C2CNIM][p-TSA] > [C2CNIM][HSO4] > [MIM][HSO4] > [C2CNIM][CH3COO]. The result demonstrates that the viscosities of ILs depend on the molecular weight, structure, and symmetry of cation and, particularly, anions. The viscosity of [C2CNIM][p-TSA] is significantly higher than the viscosities of these synthesized PILs and other common ILs. The high viscosity for [C2CNIM][p-TSA] is due to a bulky anion which has higher molecular weight and great steric hindrance and offers more resistance to flow.24 The ILs with the HSO4− anion shows higher viscosities compared to the CH3COO−, likewise in this study, [C2CNIM][HSO4] has a higher viscosity than [C2CNIM][CH3COO]. This high viscosity for sulfate based PILs is due to the higher molecular weight and hydrogen bonding between anion and cation which leads to strong interaction between ions. The protic ionic liquid containing the propyronitrile ([C2CNIM]) cation with the HSO4− anion shows a higher viscosity value as compared to PILs containing

(10)

where R is the molar refraction and Vm is the molar volume. The free volumes determined for the PILs are listed in Table S7 in the SI. The above formula can be used to calculate the free volume of spherical molecules as well as nonspherical ions. The order of decreasing the free volume of the synthesized PILs are [C2CNIM][p-TSA] > [C2CNIM][CH3COO] > [C2CNIM][HSO4] > [MIM][HSO4]. It has been observed that the free volumes are increasing with the increase of the anion size. Thus, the higher value of the free volume for [C2CNIM][p2998



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from the result, the magnitude of activation energy increases with increasing ion size. The higher value of Ea for [C2CNIM][p-TSA] is attributed to their high molecular weight and larger size of anion which results in the decrease of the mobility of ions. The value of Ea for [C2CNIM][CH3COO] is comparable to the value of caprolactam acetate reported in the literature. The change in Ea value with anion is assigned to its geometry and interaction with cation.49,50 Acidic Properties. The acidic properties for PILs determined by using acid−base titration method are listed in Table S10 in the SI. The acid value for [C2CNIM][CF3SO3], [C 2 CNIM][p-TSA], [C 2 CNIM][CH 3 SO 3 ], [C 2 CNIM][HSO4], [C2CNIM][CH3COO], and [C2CNIM][CF3COO] are 0.104, 0.144, 0.152, 0.16, 0.188, and 0.204 g/g, respectively. The result indicates that the acid value is affected by the nature of anions. Among the synthesize PILs, [C2CNIM][CF3COO] has a higher acidic value in comparison to other PILs. The higher acid number for PILs [C2CNIM][CH3COO] and [C2CNIM][CF3COO] can be assigned to acetate ion. Based on the electronegativity, the higher the electronegativity of conjugate base of acid more it is acidic. Here in the PILs the anions of ionic liquids are considered as conjugate bases of the respective acids. The [CF3COO] being more electronegative renders the [C2CNIM][CF3COO] more acidic, while the HSO4 anion is less electronegative which import less acidity to [C2CNIM][HSO4].17 Thermal Stability. The thermal properties of PILs were measured by the thermogravimetric analysis (TGA) are summarized in Table S11 in the SI. The change in weight of PILs with the increase of temperature at a heating rate of 10 °C min−1 is shown in Figure 5. The results indicated that PILs

Figure 4. Viscosity (η) as a function of temperature for protic ionic liquids.

the 1-methyl imidazolium cation ([MIM]). This higher viscosity is due to the large size of the ([C2CNIM]) cation as compared to the ([MIM]) cation.27 Another possible reason for the higher viscosity of [C2CNIM][HSO4] as compared to [MIM][HSO4] might be due to the propyronitrile functional group attached to imidazole which leads to a higher van der Waals interaction. The result of the experimental values of viscosity has confirmed that the incorporation of functionalized group on the cation and increase in the molecular weight of the anion cause the increase in viscosities of PILs.24 Generally, the variation of viscosity of ILs with the structure of the anion is difficult to predict due to large number of parameters such as ion size, shape, charge density, polarizability and flexibility of ions and planarity of molecular geometry that strongly affecting the viscosity of ILs.1,46,47 The dependence of viscosity values of PILs at nine selected temperatures in the range 293.15−373.15 K is shown in Figure 4. It has been confirmed from the experimental results that the viscosity decreases rapidly with the increase of temperature. Many researchers have already reported this rapid decrease in viscosities of many ILs with temperature.1,24,48 The viscosity of [C2CNIM][HSO4] (7937 mPa·s) is lower compared with 1,4-sultone-methylimidazolium hydrogen sulfate [BSMIM][HSO4] (49850 mPa·s) having different cations and the same anion. This higher viscosity of [BSMIM][HSO4] is due to the sultone group.1 The viscosity of [C2CNIM][p-TSA] is in the same range of 1-butyl-imidazolium hydrogen sulfate [C4C0IM][HSO4]6. Similarly, the viscosity of [C2CNIM][HSO4] (7937 mPa·s) is comparable with the viscosity of 1-octyl-3-propanenitrile imidazolium trifluoromethanesulfonate (6784.2 mPa·s), measured at 293.15 K.51 The standard deviations (SDs) and fitting parameters of viscosities for synthesized PILs are listed in Table S9 in the SI. Activation Energy. The activation energy (Ea) calculated for the PILs are listed in Table S9 in the SI. Ea is the minimum quantity of energy needed for the ions to move across each other and, therefore, can be linked with structural information on ILs. The lower value of Ea for ILs has been attributed to the lower electrostatic force acting between anions and cations; thereby ions are easily able to cross over each other’s. The Ea calculated for [C 2 CNIM][p-TSA], [C 2 CNIM][HSO 4 ], [MIM][HSO4], and [C2CNIM][CH3COO] are 75.48, 52.94, 34.12, and 36.315 J·K−1·mol−1, respectively. As can be seen

Figure 5. Thermogravimetric analysis (TG) of protic ionic liquids.

containing [C2CNIM][CF3SO3] shows higher thermal stability comparatively. The PILs contain derived from the strong acid such as H2SO4, CF3SO3H, CH3SO3H, and p-TSA show a high decomposition temperature compared to PILs derived from the weak acids such as CF3CO2H and CH3CO2H. These data are in good agreement with the reported literature.51−53 Miran et al.54 reported that the 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) based protic ILs with different anions such as acetate, trifluoroacetate, and methanesulfonate have the decomposition temperature in the range of 171−451 °C. PILs are generally believed to have lower thermal stability compared to aprotic ionic liquids. PILs due to N−H bonding in its cations structure are thermally more unstable as compared to aprotic ionic 2999



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Figure 6. DSC curves measured for protic ionic liquids.

imidazolium ring contains delocalized 3-centered-4-electron configuration across the N1−C2−N3 moiety, a double bond between C4 and C5 on the opposite side of the ring and a weak delocalization in the central region56 (Figure 7). Although the

liquids, which have an alkyl side chain instead of the proton in cations moiety such as 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2MIM][NTf2]).55 Moreover, the acetate based PILs were found to be more unstable as compared to other anions. Melting Point and Glass Transition. Glass transition temperature (Tg) is an important thermal property which gives significant information about the cohesive energy of ILs, the low Tg value representing low cohesive energies for ILs. The cohesive energy increased for ILs with the increase of attractive forces such as hydrogen bonding, van der Waals, and Coulombic interaction, while it is decreased by their pulsive Pauli interaction due to overlapping of closed electron shells. It is worth to mention that low Tg values indicate that the ionic liquid probably has desirable physicochemical properties such as low viscosity and high ionic conductivity. The DSC curves for all the synthesized PILs are shown in Figure 6. The glass transition and crystalline melting temperature for PILs are listed in Table S12 in the SI. [C2CNIM][CF3COO] and [C2CNIM][CF3SO3] PILS show only Tm, whereas [C2CNIM][p-TSA], [C2CNIM][HSO4], and [C2CNIM][CH3SO3] exhibit only Tg. [C2CNIM][CH3COO] and [MIM][HSO4] shows both Tm and Tg. The trends of DSC curve for [C 2 CNIM][p-TSA], [C 2 CNIM][HSO 4 ], [MIM][HSO 4 ], [C2CNIM][CH3SO3], and [C2CNIM][CH3COO] are similar to each other’s, but the positions of peak are different. The glass-transition temperatures determined from DSC measurements for [C2CNIM][p-TSA], [C2CNIM][HSO4], [MIM][HSO4], and [C2CNIM][CH3COO] are −75, −49, −73, and −78 K, respectively. The change in Tg was observed with the change of the anion; this effect of the change of the anion on Tg has been also previously reported by other research groups.16,54 Computational Study. DFT calculations were performed to get a deeper insight into the effect of the structural variations of the cations and anions on the structural and physical properties of the studied PILs. The electronic structure of

Figure 7. Optimized structure and COSMO-surface charge distributions. (a) [MIM]+ structure; (b) [MIM]+ COSMO-surface charge distributions; (c) [C2CNHIM]+ COSMO-surface charge distributions.

hydrogen atoms on C2−H, C4−H, and C5−H carry positive charges, the C2−H is more acidic since it is located between two electronegative nitrogen atoms. For most 1-alkyl-3methylimidazolium based ionic liquids, the acidic C2−H is the preferential site for interaction with the anions and solutes to interact with the cation.56,57 However, if alkyl chain is not attached to the position 3 (N3−H) of the imidazolium cation, the hydrogen atom attached to the nitrogen atom could be more acid compared to C2−H. This could also be observed from the COSMO-surface charge density obtained from the quantum calculations (Figure 7b,c). Moreover, the introduction of the nitrile functional group in the imidazolium alkyl spacer increases the excess electron (basic) regions in the imidazolium alkyl chain. This could increase the cation−anion interaction and interactions between the ionic liquid molecules, which affect the physical properties of the ionic liquids. For instance, the increase in refractive index for the nitrile functionalized [C2CNIM][HSO4] compared to its counterpart [MIM]3000



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[HSO4] could be due to the excess electron mobility in the alkyl chain.57 Sigma Profile of the Cation. The effect of the structural variation of the cations on the electronic properties of the ILs can also be observed from the sigma-profile (σ-profile) of the cations. The σ-profile shows the screening charge densities, which is expressed in term of relative amount of the surface with polarity σ for a given molecule. Figure 8 shows the σ-

Figure 9. Sigma profile of the studied anions computed by COSMORS.

Figure 8. Sigma profile of the studied cations computed by COSMORS.

profile of the two cations, 1-propyronitrile imidazolium and 1methylimidazolium. The σ-profile of [MIM]+ extends from (−2.4 to 0.3) e/nm2 with a maximum peak at about −0.8 e/ nm2, while for [C2CNHIM]+ the σ-profile has wider ranges (−2.4 to 1.6). If the molecule or ionic species has σ-profile peaks in the range of σ > 1.0 or/and σ < 1.0, it will have hydrogen bond donor or/and acceptor properties.57 This shows that [C2CNHIM]+ has a more extended σ-profile indicating its strong hydrogen bond donor and acceptor capacity, while [MIM]+ acts mainly as a hydrogen bond donor. Sigma Profile of the Anion. The effect of structure variations of the anions can also be observed from their surface charge distributions and σ-profiles. As it can be seen from Figure 9, the anions have two major peaks, one in the nonpolar region (between −1 and 1 e/nm2) and the other in hydrogen bond acceptor regions (σ > 1 e/nm2). Moreover, HSO4 shows a third minor peak in the hydrogen bond donor region (σ < −1 e/nm2) due to the hydrogen attached to one of the four oxygen atoms (see Figure 10e). The peaks in the nonpolar regions (−1 and 1 e/nm2) are due to the CH3, CF3, and toluene side chain and bear nonpolar characteristics of the anions. That means that the CF3 based anions has more nonpolar characteristics compared to the CH3-based anions since the CF3-based anions have a high peak in this region. Moreover, the peaks of acetatebased anions slightly shift to the strong hydrogen bond acceptor region (to the right) compared to the sufate/sulfitebased anions, which shows the actate-based anions are stronger anions compared to the sufate/sulfite-based anions. Therefore, the acetate based anions show a higher acid number compared to other studied anions. [p-TSO3]+ has a wider peak in the nonpolar region due to the aromatic characteristic of the toluene structure in the anion.

Figure 10. COSMO-surface charge distribution of the anions.

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CONCLUSIONS Nitrile functionalized protic ionic liquids containing [CH3SO3], [HSO4], [CF3SO3], [p-TSA], [CH3COO], and [CF3COO] anions have been synthesized and characterized. The elemental analysis and NMR data confirmed the purity and structures of the synthesized PILs. Physicochemical properties such as the density, viscosity, and refractive index were demonstrated to be well-correlated with structural features of the anions. Both density and viscosity were found to decrease with increasing temperature for the range covered in the present work. The introduction of nitrile containing chain on cation cause the increase in viscosity, whereas the density was noted to decrease. The values of molar volume Vm and thermal expansion coefficient α increased linearly with temperature. The electronic polarizability Rm and activation energies Ea were calculated using the measured values of refractive index and viscosity, respectively. The present PILs were found to be thermally stable in a wide range of temperatures. The acid number analysis showed that the [CF3COO] is more electronegative which render the [C2CNIM][CF3COO] more acidic while the HSO4 anion is less electronegative which import less acidity to [C2CNIM][HSO4]. The DFT calculations showed that the incorporation of the functional groups in the imidazolium alkyl chain or change in the nature of anion creates more electrondeficient and/or electron excess regions. This could affect the cation−anion interaction, which in turn affects their physical properties. 3001



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00450. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Girma Gonfa: 0000-0002-1161-9517 Nawshad Muhammad: 0000-0001-6453-0658 Funding

The authors gratefully acknowledge the Ministry of Higher Education (MOHE) for funding the research work under the Fundamental Research Grant Scheme and the Centre of Research in Ionic Liquids (CORIL), all of the research officers and postgraduate students for helping in all aspects. Notes

The authors declare no competing financial interest.

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REFERENCES

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