Synthesis, characterization and the effect of temperature on different physicochemical properties of protic ionic liquids

Abstract

In this work, eleven protic ionic liquids (PILs) containing different cations and anions were prepared and their physicochemical properties were measured. The structures of all the PILs were confirmed using NMR, and elemental analysis (CHNS) was carried out. The physicochemical properties such as density, surface tension, viscosity and thermal degradation behaviour were measured, and the effect of the cations/anions was investigated. The density and viscosity were measured within the temperature range of 293.15–373.15 K at atmospheric pressure. The thermal expansion coefficient values were calculated from the density data. Surface tension was measured in the temperature range of 293.15 to 353.15 K and the values were used to estimate the surface entropy and enthalpy of the ionic liquids at 303.15 K. The boiling and critical temperature are also estimated according to the Eötvos and Rebelo methods. The refractive indices were measured within the temperature range of 293.15 to 323.15 K. The thermal gravimetric analysis was performed in the temperature range of 373.15–773.15 K.

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Introduction

Ionic liquids are organic salts with low melting points (<100 °C), some even at room temperature and sometimes measured as low as −96 °C. Therefore, under traditional organic liquid phase reaction conditions they can be used as solvents.¹ As reaction media, the application of ILs for a wide variety of synthetic processes is an area of intensive research. Ionic liquids provide good thermal stability, designable structures, a recyclable green-solvent for many chemical reactions and they are vapourless.^2,3 ILs are also being used in a variety of applications viz. in the food industry,⁴ extraction processes,⁵ nuclear science,⁶ biotechnology,⁷ material engineering,⁸ and as electrolytes in batteries,⁹ fuel cells,¹⁰ solar cells,¹¹ biosensors,¹² light emitting electrochemical cells,¹³ etc. Apart from these important applications, ILs in confined geometries^14,15 have recently attracted attention along with their use in synthesis,¹⁶ as reaction media,¹⁷ and the effect of salts on ILs.¹⁸ There have been research papers and reviews about almost all parts of aprotic ILs (AILs), including their role in the synthesis of nanomaterial structures, electrochemistry, catalysis, and as reaction media.^19–21 Currently protic ionic liquids (PILs) have popularity due to their extensive liquid temperature range, ionic conductivity, and high thermal stability, which is ample for practical applications.²² PILs, usually synthesized via the neutralization reaction of a Brønsted acid and base, establish one of the most vital sub-classes of ILs. Although the 1^st IL (ethylammonium nitrate) ever reported (ca. 100 years ago) is a protic ionic liquid, the study of the structure–property relationships of these materials has newly been initiated and remains in the developmental stage.²³ The important characteristic that differentiates PILs from other ILs is the transfer of a proton from the acid to the base, to make proton donor and proton acceptor sites.²⁴ Nowadays, proton conducting electrolytes are developing as beneficial materials owing to various possible applications such as in fuel cells, as an electrolyte in aqueous batteries, in double layer capacitors, in actuators or in dye-sensitized solar cells. There are numerous potential new fields of application for PILs.²⁵ To date, based on IL studies there is limited literature available on PILs. However, this family of ILs have several suitable properties and potential applications due to their protic nature.²⁶ Industrial process design and IL-based new products are only possible when the physicochemical properties of the ILs such as density, surface tension and viscosity are acceptably known. Unfortunately, there is limited literature available on the physicochemical properties and characterization of ILs, but it is required to accumulate a large data bank of these important properties, not only for product design and processes but for the adequate development of their property correlations.²⁷ The diversity of the possible ILs is the reason that the experimental determination of the complete set of physicochemical properties is almost impossible, since it would require a vast investment of time and resources.²⁸ ILs depending on the cations and anions, show a broad range of physicochemical properties. Many studies have investigated their different physicochemical properties.²⁹ The physicochemical properties of PILs are determined and are discussed according to the nature of the cation and anion. The physicochemical properties analysis of PILs depends on the temperature having been measured and examined in detail. This study of the physicochemical properties of PILs set out to present reliable data, which include viscosity, density, diffusion coefficient, surface tension, refractive index and thermal behaviour, over a wide temperature range.

Experimental section

All the chemicals such as 1-methylpiperidine, 4-methylpyridine, ethylamine, 1-methylimidazole, 1,4-sultone, pyridine, trifluoromethanesulfonic acid (Merck), pyrazole, sulphuric acid (Sigma-Aldrich), piperazine, and methylpyrrolidine, (Acros) were purchased and were used as received.

Synthetic procedure of protic ionic liquids (general procedure)

The proposed cationic moiety (1-methylpiperidinium, 4-methylpyridinium, piperazinium, methylpyrrolidinium, ethylammonium, pyrazolium, 1-methylimidazolium (0.1 mol)) for each ionic liquid was dissolved in 5 mL of acetonitrile and then concentrated H₂SO₄ (0.1 mol) was added dropwise under cooling conditions. The reaction mixture was stirred for 24 h and refluxed in the presence of a flow of nitrogen gas. The obtained 1-methylpiperidinium hydrogensulfate [MPip][HSO₄], 4-methylpyridinium hydrogensulfate [MPy][HSO₄], piperazinium hydrogensulfate [Pi][HSO₄], methylpyrrolidinium hydrogensulfate [MPyr][HSO₄], ethylammonium hydrogensulfate [EAm][HSO₄], pyrazolium hydrogensulfate [P][HSO₄], and 1-methylimidazolium hydrogensulfate [MIM][HSO₄] ionic liquids were washed with diethyl ether three times and dried in a vacuum oven overnight.

Synthesis of protic ionic liquids with SO₃H-functionalized side chains

To obtain the SO₃H-functionalized PILs, 1-methylimidazole/pyridine and 1,4-sultone were mixed in equal molar concentrations and stirred for 36 h at 40 °C without using any reaction solvent. A white solid product (zwitterion) was obtained which was washed three times with toluene to remove any unreacted reactants and further dried in a vacuum oven. After drying, a stoichiometric amount of concentrated sulfuric acid or trifluoromethanesulfonic acid was added dropwise to the prepared zwitterion using continued stirring at room temperature for 30 minutes. The reaction temperature was then increased to 50 °C and the stirring was continued for another 10 h which resulted in the formation of the desired PILs i.e. 1,4-sultone-methylimidazolium hydrogensulfate [BSMIM][HSO₄], 1,4-sultone-methylimidazolium trifluoromethanesulfonate [BSMIM][CF₃SO₃], 1,4-sultone-pyridinium hydrogensulfate [BSPy][HSO₄], and 1,4-sultone-pyridinium trifluoromethanesulfonate [BSPy][CF₃SO₃]. The synthesized PILs were washed with toluene and diethyl ether repeatedly to remove the unreacted materials and further subjected to drying under vacuum. The synthesized ILs structures are listed in Table 1.

Table 1 Name, abbreviation and chemical structure of the synthesized ILs

Description	Abbreviation	Chemical structure
1-Methylpiperidinium hydrogensulfate	[MPip][HSO4]
4-Methylpyridinium hydrogensulfate	[MPy][HSO4]
Piperazinium hydrogensulfate	[Pi][HSO4]2
Methylpyrrolidinium hydrogensulfate	[MPyr][HSO4]
Ethylammonium hydrogensulfate	[EAm][HSO4]
Pyrazolium hydrogensulfate	[P][HSO4]2
1-Methylimidazolium hydrogensulfate	[MIM][HSO4]
1,4-Sultone-methylimidazolium hydrogensulfate	[BSMIM][HSO4]
1,4-Sultone-methylimidazolium trifluoromethanesulfonate	[BSMIM][CF3SO3]
1,4-Sultone-pyridinium hydrogensulfate	[BSPy][HSO4]
1,4-Sultone-pyridinium trifluoromethanesulfonate	[BSPy][CF3SO3]

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Characterization

¹H and ¹³C NMR spectra were recorded in a (DMSO and D₂O) solvent on a Bruker Avance 500 spectrometer. Carbon, hydrogen, nitrogen, and sulphur content was analyzed using an elemental analyzer (CE Instruments EA-1110). Additionally, their structure was also characterized using FTIR (Shimadzu) and their spectra were recorded from 500–4000 cm⁻¹. A coulometric Karl Fisher titrator, DL 39 (Mettler Toledo), was used for the water determination of the synthesized PILs using the Hydranal coulomat AG reagent (Riedel-de Haen), the method adopted by our research group.³⁰ The measurement was made in triplicate for each IL, and the average values are reported.

Density and viscosity. An Anton Paar viscometer (model SVM3000) was used to measure the viscosity of the PILs. Density measurements were carried out using an Anton Paar densitometer (DMA 5000). Standard uncertainties are u(ρ) = ±0.00001 g cm⁻³, u(η) = ±0.32% mPa s, and u(T) = ±0.01 K.

Surface tension. Surface tension was measured using a pendant drop method, and a syringe was used to generate the drop. A camera (OCA 20) was used to take photographs. To evaluate the shape of the drop we used software (SCA22). The measurement was recorded in the temperature range of 293.15–353.15 K. The measurements were performed with an accuracy of ±0.04 K and uncertainties of ±1.2%. All measurements were performed in triplicate and the average values are reported.

Refractive index. The refractive index was measured for all ionic liquids using an ATAGO digital refractometer (RX-5000α). The refractive index of the samples was measured within the temperature range of 293.15 to 323.15 K with an accuracy of ±0.05 K and uncertainties of 3.5 × 10⁻⁵. Likewise, triplicate measurements were noted and the values were reported as an average.

Thermal decomposition. The thermal decomposition temperature of the PILs was measured using a thermogravimetric analyzer (PerkinElmer, Pyris V-3.81). Samples were heated from 50 to 500 °C in a crucible under a nitrogen atmosphere and the heating rate was 10 K min⁻¹. The accuracy of the measurement is better than ±1 K.

Melting point and glass transition temperature determination. The melting point and glass transition temperature were determined using DSC (differential scanning calorimetry; PerkinElmer, model pyris 1). The samples were weighed in aluminium pans and these sealed pans were heated in a nitrogen atmosphere from 0 to 130 °C before cooling to −150 °C and heating again to 130 °C. The heating and cooling rate was 10 °C min⁻¹.

Results and discussion

The prepared PILs were obtained in a good yield with a high purity of more than 98%. The purity of the ionic liquids was assessed from the mole fraction of water (using a coulometric Karl Fischer titrator) and the mole fraction of unreacted impurities was determined using NMR and elemental analysis. The NMR, elemental analysis, and water content data for each ionic liquid are given below.

1-Methylpiperidinium hydrogensulfate [MPip][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.479 (s, 1H), 1.695–1.741 (m, 4H), 2.636 (s, 3H), 2.977 (s, 4H).

¹³C NMR (500 MHz, DMSO): 21.692, 23.253, 43.801, 54.392.

CHNS elemental analysis for C₆H₁₅NSO₄, found (%): C: 36.58, H: 8.01, N: 7.20, S: 17.08. Calculated (%): C: 36.53, H: 7.66, N: 7.10, S: 16.26.

FT-IR (cm⁻¹): 3456.07, 2947.29, 2712.00, 2541.65, 1648.61, 1453.04, 1193.30, 1037.46, 852.59, 595.14.

4-Methylpyridinium hydrogensulfate [MPy][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.898 (s, 3H), 7.789–7.801 (d, 2H), 8.703–8.710 (d, 2H).

¹³C NMR (500 MHz, DMSO): 21.957, 127.602, 143.086, 157.762.

CHNS elemental analysis for C₆H₉NSO₄, found (%): C: 38.22, H: 4.51, N: 7.09, S: 15.26. Calculated (%): C: 37.69, H: 4.74, N: 7.33, S: 16.77.

FT-IR (cm⁻¹): 3439.27, 2941.18, 2712.00, 2541.65, 1644.02, 1453.80, 1194.83, 1022.95, 857.17, 589.03.

Piperazinium hydrogensulfate [Pi][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.510 (s, 4H), 2.712 (s, 4H).

¹³C NMR (500 MHz, DMSO): 47.29.

CHNS elemental analysis for C₄H₁₃N₂SO₄, found (%): C: 26.04, H: 7.45, N: 16.72, S: 18.18. Calculated (%): C: 25.94, H: 7.07, N: 15.12, S: 17.31.

FT-IR (cm⁻¹): 3447.67, 2937.36, 2686.03, 2541.65, 1631.04, 1469.85, 1231.50, 1112.33, 1031.35, 793.77, 618.82.

Methylpyrrolidinium hydrogensulfate [MPyr][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.840–1.897 (m, 2H), 1.969–2.040 (m, 2H), 2.819 (s, 3H), 2.942–3.012 (m, 2H), 3.472–3.526 (m, 2H), 7.078 (s, 1H).

¹³C NMR (500 MHz, DMSO): 33.173, 120.913, 128.867, 138.

CHNS elemental analysis for C₅H₁₃NSO₄, found (%): C: 13.10, H: 7.12, N: 7.92, S: 16.69. Calculated (%): C: 32.87; H, 7.15, N, 7.64; S, 17.50.

FT-IR (cm⁻¹): 3443.09, 2937.36, 2702.84, 2545.47, 1632.56, 1461.44, 1197.12, 1035.93, 848.77, 587.43.

Ethylammonium hydrogensulfate [EAm][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.161–1.190 (t, 3H), 3.042–3.086 (q, 2H), 3.860 (s, N⁺H₃).

¹³C NMR (500 MHz, DMSO): 8.987, 46.265.

CHNS elemental analysis for C₂H₉NSO₄, found (%): C: 16.86, H: 7.03, N: 8.51, S: 19.20. Calculated (%): C: 16.78, H: 6.34, N: 9.78, S: 22.40.

FT-IR (cm⁻¹): 3451.49, 2942.71, 2715.82, 2541.65, 1631.04, 1456.86, 1180.32, 1035.93, 857.17, 593.61.

Pyrazolium hydrogensulfate [P][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 3.381 (s, 2H), 6.260–6.267 (t, 1H), 7.505 and 7.712 (d, 2N–H).

¹³C NMR (500 MHz, DMSO): 107.441, 134.528.

CHNS elemental analysis for C₃H₇N₂SO₄, found (%): C: 21.30, H: 4.48, N: 17.14, S: 18.98. Calculated (%): C: 21.56, H: 4.22; N: 16.76, S: 19.18.

FT-IR (cm⁻¹): 3434.66, 2937.36, 2699.09, 2541.65, 1631.04, 1462.97, 1188.72, 1041.28, 844.18, 602.02.

1-Methylimidazolium hydrogensulfate [MIM][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 3.86 (s, 3H), 5.72 (s, 1H), 7.59 (s, 1H), 7.66 (s, 1H), 8.98 (s, 1H).

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, 17.86. Calculated (%): C: 26.66, H: 4.48, N: 15.55, S: 17.80.

FT-IR (cm⁻¹): 3434.68, 2941.18, 2705.89, 2541.65, 1648.61, 1461.44, 1188.72, 1042.81, 850.30, 593.61.

1,4-Sultone-methylimidazolium hydrogensulfate [BSMIM][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, D₂O): δ = 1.570–1.632 (m, 2H), 1.850–1.910 (m, 2H), 2.786–2.817 (t, 2H), 3.750 (s, 3H), 4.088–4.117 (t, 2H), 7.297 (s, 1H), 7.353 (s, 1H), 8.584 (s, 1H).

¹³C NMR (500 MHz, D₂O): 20.949, 28.099, 35.695, 48.932, 50.092, 122.178, 123.680, 135.942.

CHNS elemental analysis for C₈H₁₆N₂S₂O₇, found (%): C: 30.18, H: 5.25, N: 8.20, S: 19.11. Calculated (%): C: 30.37, H: 5.10, N: 8.86: S: 20.27.

FT-IR (cm⁻¹): 3451.08, 2945.73, 2716.49, 2541.31, 1640.91, 1452.04, 1192.52, 1048.34, 854.42.

1,4-Sultone-pyridinium hydrogensulfate [BSPy][HSO₄]

Spectroscopic data: ¹H NMR (500 MHz, D₂O): δ = 1.376–1.461 (m, 2H), 1.731–1.807 (m, 2H), 2.542–2.573 (t, 2H), 2.894–2.919 (t, 2H), 4.230–4.260 (t, 2H), 7.662–7.690 (t, 1H), 8.132–8.163 (t, 2H), 8.435–8.447 (d, 2H).

¹³C NMR (500 MHz, D₂O): 20.679, 22.564, 29.088, 49.806, 60.967, 128.142, 143.975, 145.530.

CHNS elemental analysis for C₉H₁₅NS₂O₇, found (%): C: 33.70, H: 5.08, N: 4.21, S: 20.12. Calculated found (%): C, 34.50; H, 4.83; N, 4.47; S, 20.47.

FT-IR (cm⁻¹): 3330.69, 2947.18, 2708.56, 2544.20, 1656.77, 1453.48, 1191.80, 1044.01, 858.02, 597.78.

1,4-Sultone-methylimidazolium trifluoromethanesulfonate [BSMIM][CF₃SO₃]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.539–1.600 (m, 2H), 1.858–1.917 (m, 2H), 2.580–2.610 (t, 2H), 3.853 (s, 3H), 4.173–4.201 (t, 2H), 7.701 (s, 1H), 7.763 (s, 1H), 9.130 (s, 1H).

¹³C NMR (500 MHz, DMSO): 21.940, 28.939, 36.215, 48.950, 50.865, 119.859, 122.779, 124.085, 137.036.

CHNS elemental analysis for C₉H₁₅N₂SO₆, found (%): C: 30.29, H: 5.10, N: 7.40, S: 18.90. Calculated (%): C: 29.35, H: 4.10, N: 7.61, S: 17.41.

FT-IR (cm⁻¹): 3439.55, 2945.73, 2720.81, 2547.80, 1637.31, 1454.92, 1195.40, 1044.01, 858.02, 0.38.

1,4-Sultone-pyridinium trifluoromethanesulfonate [BSPy][CF₃SO₃]

Spectroscopic data: ¹H NMR (500 MHz, DMSO): δ = 1.068–1.096 (t, 2H), 1.562–1.643 (m, 2H), 2.000–2.060 (m, 2H), 2.616–2.646 (t, 2H), 4.620–4.649 (t, 2H), 8.146–8.174 (t, 2H), 8.588–8.619 (t, 1H), 9.085–9.096 (d, 2H).

¹³C NMR (500 MHz, DMSO): 21.825, 30.225, 50.778, 60.907, 119.851, 128.583, 145.216, 145.984.

CHNS elemental analysis for C₁₀H₁₄NS₂O₆, found (%): C: 32.20, H: 4.44, N: 4.10, S: 18.25. Calculated (%): C: 32.87, H: 3.86, N: 3.83, S: 17.55.

FT-IR (cm⁻¹): 3322.76, 2947.18, 2700.63, 2539.87, 1673.35, 1451.32, 1199.73, 1051.22, 858.02, 589.13.

The viscosity, density, surface tension, refractive index and thermal study results of the synthesized PILs are discussed subsequently below. Those ILs which were obtained as solids at room temperature were not evaluated for most of their properties due to the limitation of the instruments used to measure their properties. The measured thermophysical properties were compared to the available literature. Generally for these ILs, limited literature data have been reported.

Viscosity

The dynamic viscosity data of the prepared ILs measured at the temperature range of 293.15–353.15 K and at atmospheric pressure are given in Table 2 and shown in Fig. 1. They show that with the increase in temperature the viscosities of the ILs decrease. It was observed that the viscosities of the present ILs increase as follows: [BSMIM][HSO₄] > [BSMIM][CF₃SO₃] > [BSPy][HSO₄] > [MPy][HSO₄] > [MIM][HSO₄] > [BSPy][CF₃SO₃] > [MPyr][HSO₄]. Comparatively, three SO₃H-functionalized ILs ([BSMIM][HSO₄], [BSMIM][CF₃SO₃], [BSPy][HSO₄]) show higher viscosities which might be due to the interaction of the SO₃H-functionalized side chain with the anion of the ionic liquid moiety. At 373.15 K the lowest viscosity was measured for a SO₃H-functionalized IL ([BSPy][CF₃SO₃]) and was 19.140, contrary to an IL ([MPy][HSO₄]) which does not contain the SO₃H functional group and still has higher viscosity. The higher viscosity of the [MPy][HSO₄] IL was assumed to be due to strong ion interactions, smaller size and low molecular weight as compared to the SO₃H-functionalized ILs. The higher viscosity of the [MPy][HSO₄] IL is also expected due to lower steric hindrance of the cation and anion structures and hence more interaction is observed between its ions compared to the SO₃H-functionalized ILs. This study shows that small changes in the structure of ILs can produce considerable differences in viscosity. The protic IL ([MPy][HSO₄]) containing a pyridinium cation with a HSO₄⁻ anion shows higher viscosity values compared to the imidazolium cation-containing PIL and a similar statement has been reported in the literature.³¹ Compared to conventional solvents, these ILs show high viscosities. This may be due to the van der Waals interactions, chain tangling effects and hydrogen bonding interactions. Along with the electrostatic and van der Waals interactions, which are affected by ion size, the polarizability and flexibility of the anion and the planarity of the molecular structure are also possible influences. A similar agreement, such as the higher viscosities of the room temperature ILs may be due to the additional H-bonding interactions relating the functional groups of the cations with other cations and anions, is available in literature.³² H. Ohno³³ also reported that only a few ILs have low viscosity because it strongly depends on the structure of the ionic components. The relationship between the ionic component structure and the viscosity is not completely understood, probably due to various parameters such as the ion shape, charge density, influence of other interaction forces, and a conformational change of alkyl chain, etc., involved. So our study confirmed that by incorporating a functionalized group on the cation the viscosities of the ILs increase, but the different parameters as mentioned above also strongly affect the viscosities of the ILs.

Table 2 Experimental dynamic viscosities for the protic ionic liquids at temperatures in the range of 293.15 to 373.15 K and at atmospheric pressure^a

T/K	η/(mPa)
	[MPy][HSO4]	[MPyr][HSO4]	[MIM][HSO4]	[BSMIM][HSO4]	[BSMIM][CF3SO3]	[BSPy][HSO4]	[BSPy][CF3SO3]
a Standard uncertainty μ is μ(T) = 0.01 K and the combined expanded uncertainty is uc(η) = 0.32% mPa s, (level of confidence = 0.95).
293.15	1426.0	298.19	563.01	49 850	2377.3	1567.1	537.78
303.15	683.57	185.44	312.04	16 207	1054.1	775.72	285.63
313.15	364.16	123.83	191.76	6406.1	525.43	421.40	164.68
323.15	211.07	85.950	125.36	2903.7	296.11	247.17	102.07
333.15	130.92	61.741	86.005	1449.5	176.49	154.58	67.087
343.15	85.855	45.804	61.421	784.72	112.02	101.97	46.363
353.15	58.765	34.943	45.400	455.14	75.343	70.330	33.391
363.15	41.900	26.322	34.591	280.52	52.424	50.418	24.918
373.15	30.862	22.191	27.149	182.90	37.970	37.347	19.140

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Fig. 1 Viscosity as a function of temperature for the protic ionic liquids; [MPy][HSO₄]: ■, [MPyr][HSO₄]: ●, [MIM][HSO₄]: ▲, [BSMIM][HSO₄]: ▼, [BSMIM][CF₃SO₃]: ♦, [BSPy][HSO₄]: ◄, and [BSPy][CF₃SO₃]: ►.

The standard deviation (SD)‡ and correlation coefficient values calculated for viscosity using the equations in footnotes ‡ and § are shown in Table 3a. The correlation coefficient values calculated for viscosity using the Vogel–Tamman–Fulcher equation:

η = A₀e^{A₁(T−T_g)}

are tabulated in Table 3b.

Table 3 (a) The fitting parameter values with R² and the standard deviation (SD)‡ for the empirical correlation of viscosity§ of the measured ionic liquids. (b) The fitting parameter values using the Vogel–Tamman–Fulcher equation values of the measured ionic liquids

(a)
ILs	SD × 10^−2	R^2	A0	A1
[MPy][HSO4]	1.470	0.9991	4.2351	2121.7
[MPyr][HSO4]	9.033	0.9994	2.7886	1528.7
[MIM][HSO4]	9.958	0.9994	3.1544	1699.7
[BSMIM][HSO4]	1.357	0.9997	6.3256	3169.4
[BSMIM][CF3SO3]	1.420	0.9993	4.8005	2359.5
[BSPy][HSO4]	1.688	0.999	4.6265	2273.2
[BSPy][CF3SO3]	1.292	0.9992	3.8132	1884.9

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(b)
ILs	R^2	A0	A1
[MPy][HSO4]	0.9798	51 894	−0.047
[MPyr][HSO4]	0.9859	4554.9	−0.032
[MIM][HSO4]	0.9808	14 267	−0.037
[BSMIM][HSO4]	0.9769	1 000 000	−0.069
[BSMIM][CF3SO3]	0.9777	69 739	−0.051
[BSPy][HSO4]	0.981	16 477	−0.046
[BSPy][CF3SO3]	0.9797	8793.6	−0.041

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Density

Fig. 2 shows the effect of the temperature on the density values of the PILs. The increasing order of density of the synthesized ILs is [BSPy][CF₃SO₃] > [BSPy][HSO₄] > [MIM][HSO₄] > [BSMIM][CF₃SO₃] > [BSMIM][HSO₄] > [MPy][HSO₄] > [MPyr][HSO₄]. In functionalised ILs, the CF₃SO₃⁻ anion-containing ILs show higher density values compared to those containing the HSO₄⁻ anion. The reason may be due to the high molecular weight of the CF₃SO₃⁻ anion. Z. B. Zhou et al.³⁴ reported different ILs with fluorinated anions and claimed that the density gradually increased as the bulkiness of the fluoro-anion increased. These obtained values were higher than those of water and traditional solvents such as ethanol, ethylacetate and methanol.³⁵ In order to know the effect of the cation structure on the density of the synthesized ILs more directly, especially when incorporating the functionalized group on the cation, we determined that all ILs except for the simple imidazole cation with a HSO₄⁻ anion had a high density value. In the functionalized ILs, the anion CF₃SO₃⁻ shows higher densities with the concerned cations. These behaviours are in good agreement with many similar reported conclusions that the density can be increased by increasing the molecular weight of the anion.³¹ Gardas et al.,²⁷ reported for imidazolium cations that when the liquid density increases it does not correspond directly to the high molecular weight of the anion. However, it can be explained that the thiocyanate anion had a stronger localized charge than dicyanamide, which gives the possibility of a strong pairing with the imidazolium and pyridinium cations, resulting in a higher density value. The density of all of these ILs showed a linear dependency with temperature (Table 4).

Fig. 2 Density (ρ) as a function of the temperature (T) for the protic ionic liquids; [MPy][HSO₄]: ■, [MPyr][HSO₄]: ●, [MIM][HSO₄]: ▲, [BSMIM][HSO₄]: ▼, [BSMIM] [CF₃SO₃]: ◄, [BSPy][HSO₄]: ►, [BSPy][CF₃SO₃]: ◆.

Table 4 Experimental values for the densities (ρ) of the ionic liquids from 293.15 to 373.15 K at atmospheric pressure^a

T/K	ρ/(g cm^−3)
	[MPy][HSO4]	[MPyr][HSO4]	[MIM][HSO4]	[BSMIM][HSO4]	[BSMIM][CF3SO3]	[BSPy][HSO4]	[BSPy][CF3SO3]
a Standard uncertainty μ is μ(T) = 0.01 K and the combined expanded uncertainty is μc(ρ) = 0.00001 g cm^−3, (level of confidence = 0.95).
293.15	1.3689	1.3633	1.4622	1.4386	1.4491	1.4775	1.4835
303.15	1.3623	1.3539	1.4555	1.4323	1.441	1.4704	1.4744
313.15	1.3556	1.3448	1.4491	1.4261	1.4327	1.4631	1.4658
323.15	1.3491	1.3372	1.4429	1.4199	1.4246	1.4562	1.4574
333.15	1.3427	1.3281	1.4367	1.4137	1.4168	1.4495	1.449
343.15	1.3363	1.3191	1.4306	1.4075	1.4091	1.4429	1.4405
353.15	1.3298	1.3101	1.4246	1.4011	1.4015	1.4363	1.4322
363.15	1.3233	1.2987	1.4187	1.395	1.394	1.4297	1.4239
373.15	1.3168	1.2894	1.4129	1.3891	1.3867	1.4234	1.4158

---

The standard deviation (SD) and the correlation coefficient values calculated for density¶ are shown in Table 5.

Table 5 The fitting parameter values with R² and the standard deviation (SD)‡ for the empirical correlation of density¶ of the measured ionic liquids

ILs	SD	R^2	A2	A3
[MPy][HSO4]	6.738 × 10^−4	1.0	7.2193	−0.0005
[MPyr][HSO4]	9.410 × 10^−4	0.998	7.2147	−0.0007
[MIM][HSO4]	1.236 × 10^−3	0.999	7.2852	−0.0004
[BSMIM][HSO4]	1.691 × 10^−3	1.0	7.2692	−0.0004
[BSMIM][CF3SO3]	2.133 × 10^−3	0.999	7.2756	−0.0006
[BSPy][HSO4]	4.730 × 10^−2	0.999	7.2493	−0.0005
[BSPy][CF3SO3]	7.927 × 10^−4	1.0	7.2989	−0.0006

---

Estimation of the volumetric properties

The apparent density values for the present ionic liquids at 298.15 K were fitted by applying the following equation:³⁶where A₀ is a constant and , where α is the thermal expansion coefficient.

The molecular volume, V_m, the standard molar entropy, S⁰, and the lattice energy of the synthesized ILs are calculated using eqn (2)–(4) respectively and the values are listed in Table 6.

S⁰(303.15)/J K mol⁻¹ = 1246.5(V_m/nm³) + 29.5 (3)

U_POT/kJ mol⁻¹ = 1981.2 (ρ/M)^1/3 + 103.8 (4)

where M is the molecular weight and N_A is Avogadro’s number.

Table 6 The calculated values of the volumetric properties of the ILs

ILs	ρ (kg m^−3)	Vm (nm^3)	S^0 (J K^−1 mol^−1)	UPOT (kJ mol^−1)
[MPy][HSO4]	1362.3	2.330 × 10^−1	3.200 × 10^2	3.916 × 10^3
[MPyr][HSO4]	1353.9	2.247 × 10^−1	3.096 × 10^2	3.963 × 10^3
[MIM][HSO4]	1455.5	2.055 × 10^−1	2.857 × 10^2	4.079 × 10^3
[BSMIM][HSO4]	1432.3	3.667 × 10^−1	4.866 × 10^2	3.381 × 10^3
[BSMIM][CF3SO3]	1441	4.244 × 10^−1	5.585 × 10^2	3.226 × 10^3
[BSPy][HSO4]	1470.4	3.538 × 10^−1	4.705 × 10^2	3.421 × 10^3
[BSPy][CF3SO3]	1474.4	4.114 × 10^−1	5.423 × 10^2	3.258 × 10^3

---

From Table 6 it can be seen that the molar volume of the [BSMIM][CF₃SO₃] and [BSPy][CF₃SO₃] ILs is larger than that of the rest of the ILs because of the larger volume and the higher molecular weight of the trifluoromethanesulfonate anion. The lattice energy values of [MPy][HSO₄], [MPyr][HSO₄], [MIM][HSO₄], [BSMIM][HSO₄], [BSMIM][CF₃SO₃], [BSPy][HSO₄], and [BSPy][CF₃SO₃] are 3.916 × 10³, 3.963 × 10³, 4.079 × 10³, 3.381 × 10³, 3.226 × 10³, 3.421 × 10³, and 3.258 × 10³ kJ mol⁻¹, respectively, and similar to previously reported ionic liquids.^37–39 The studied ionic liquids have significantly lower lattice energies than alkali halides.⁴⁰ The lower lattice energy values for ionic liquids render them as liquid at room temperature.^38,41

Surface tension

The ILs containing organic moieties are expected to exhibit surface activities. The measured surface tension data is tabulated in Table 7 and shown in Fig. 3. These selected ILs have surface tension values that are higher than most of the common organic solvents and lower than water. The surface tension values at 293.15 K for methanol, acetone and water are 22.6, 23.7 and 72.7 mN m⁻¹, respectively.^31,42 The reported ILs show higher surface tension values even at a higher temperature, while the lowest value is 19.62 for the [MPyr][HSO₄] IL which is very close to the values of common organic solvents. All these values are in an acceptable range which is very close to the reported values for ILs.⁴³ The lower surface tension values are attributed to a weakening of the Coulombic interactions. The experimentally obtained data shows that both the cation and anion have an effect on the surface tension. Surface tension is a measure of the surface cohesive energy and is related to the interaction strength of the cation and anion of the ILs. For these high values of surface tension the only explanation is the hydrogen bonding that exists between the cations, and the cations and anions, since the interaction increases between the ions leading to enhanced surface tension values.

Table 7 Experimental values of surface tension (γ) in the temperature range of 293.15–353 K at atmospheric pressure^a

T (K)	Surface tension-10^2γ (J m^−2)
	[MPy][HSO4]	[MPyr][HSO4]	[MIM][HSO4]	[BSMIM][HSO4]	[BSMIM][CF3SO3]	[BSPy][HSO4]	[BSPy][CF3SO3]
a Standard uncertainty μ is μ(T) = 0.04 K and the combined expanded uncertainty is μc(γ) = 1.2% mN m^−1, (level of confidence = 0.95).
293.15	5.111	3.260	5.394	4.465	4.529	4.362	4.421
303.15	4.973	3.042	5.320	4.318	4.424	4.242	4.298
313.15	4.840	2.81	5.227	4.172	4.324	4.124	4.181
323.15	4.658	2.586	5.127	4.029	4.227	3.997	4.062
333.15	4.480	2.352	5.032	3.889	4.117	3.869	3.957
343.15	4.329	2.180	4.903	3.749	4.020	3.738	3.823
353.15	4.210	1.962	4.790	3.618	3.912	3.612	3.702

---

Fig. 3 Surface tension (γ) as a function of temperature for the protic ionic liquids, ■ [MPy][HSO₄], ● [MPyr][HSO₄], ▲ [MIM][HSO₄], ▼ [BSMIM][HSO₄], ◄ [BSMIM][CF₃SO₃], ► [BSPy][HSO₄], ♦ [BSPy][CF₃SO₃].

The standard deviation (SD) and the calculated correlation coefficient values for surface tension using the equations in footnotes ‡ and || are shown in Table 8.

Table 8 The fitting parameter values with R² and the standard deviation (SD)‡ for the empirical correlation of the surface tension‖ of the measured ILs

ILs	SD	R^2	A4	A5
[MPy][HSO4]	1.719 × 10^−2	0.9969	−0.01554	9.6788
[MPyr][HSO4]	1.562 × 10^−2	0.9987	−0.0217	9.6112
[MIM][HSO4]	1.577 × 10^−2	0.994	−0.0101	8.3921
[BSMIM][HSO4]	4.905 × 10^−3	0.9997	−0.0142	8.6069
[BSMIM][CF3SO3]	3.147 × 10^−3	0.9998	−0.01025	7.5295
[BSPy][HSO4]	4.582 × 10^−3	0.9997	−0.0125	8.0464
[BSPy][CF3SO3]	5.456 × 10^−3	0.9995	−0.0119	7.9078

---

The surface tension and the temperature are correlated using the following equation:

σ/(mN m⁻¹) = A₄ − A₅T (5)

where σ represents the surface tension, A₄ and A₅ are the fitting parameters, and T is the absolute temperature. Table 8 shows the estimated values of the fitting coefficients along with the standard deviation (SD). The measured surface tension values were applied to calculate the entropy and enthalpy of surface formation. It was possible to determine the surface entropy from the slope, S_a, of eqn (6). The estimated entropies are listed in Table 9, and the results show lower values compared with the other ionic liquids^37,44 but close to the reported ionic liquids, for example [C₁₆-mim][PF₆] (109 J K⁻¹ mol⁻¹) and [(C₁₆)2-Bim][Cl] (255 J K⁻¹ mol⁻¹).⁴⁵ The lower value of entropy leads to greater ordering within the liquids.

Table 9 Surface thermodynamic functions of the pure ionic liquids at a temperature of 303.15 K: the surface entropy, S_a, and the surface enthalpy, E_s

ILs	10^3Sa (mJ K^−1 m^−2)	Es (mJ m^−2)
[MPy][HSO4]	155.4	54.44095
[MPyr][HSO4]	217	36.99836
[MIM][HSO4]	101	56.26182
[BSMIM][HSO4]	142	47.48473
[BSMIM][CF3SO3]	102.5	47.34729
[BSPy][HSO4]	125	45.52729
[BSPy][CF3SO3]	119	46.58749

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The surface enthalpy (E_s) was calculated from the surface tension values using eqn (7) at 303.15 K and the results are tabulated in Table 9.

NaNO₃ has a surface enthalpy (E_s) value of 146 mJ m⁻² which is noticeably higher than that for ionic liquids.⁴⁶ It is a sign of the lower degree of interaction among the ions in ionic liquids. The surface enthalpy values of the ionic liquids are close to those of common organic solvents such as octane (51.1 mJ m⁻²) and benzene (67 mJ m⁻²).

Critical temperature, enthalpy of vaporization and normal boiling point

Critical temperature (T_c), is an important parameter in correlating the equilibrium and transport properties of liquids. Because of the intrinsic nature of ionic liquids, it is difficult to get reliable data of their critical temperature.⁴⁷ To predict the critical temperature values for ILs, usually some estimated methods are used.⁴⁸ Hence in this work, Guggenheim⁴⁹ (eqn (8)) and Eötvos (eqn (9))⁵⁰ empirical equations were used to predict the critical temperature of the ILs and the results are shown in Table 10. The enthalpy of vaporization of ionic liquids was estimated using (eqn (10)). The enthalpies of vaporization of the [BSMIM][HSO₄], [BSMIM][CF₃SO₃], [BSPy][HSO₄], and [BSPy][CF₃SO₃] ILs are higher compared to the other ILs without a sultone group, implying that these ILs show lower volatility, especially the trifluoromethanesulfonate anion-containing ILs. A similar agreement has been reported in the literature.⁴⁸

Δ^g₁H^o_m = 0.01121(σV^2/3N^1/3_A) + 2.4 (10)

where E^σ is the total surface energy of ILs, which equals the surface enthalpy because of the tiny volume difference due to thermal expansion at temperatures that are not similar to the critical temperature T^G_c, K is a constant, σ is the surface tension, M is the molecular weight, ρ is the density, T is the measured surface tension temperature, and N_A is the Avogadro number.

It is also possible to calculate the boiling temperature, T_b, of ILs from the critical temperature, T_c, which has been proposed by Rebelo et al.⁵¹ According to them, the relation between T_c and T_b is that T_b ≈ 0.6T_c for an IL. The calculated T_b values of the ILs are given in Table 10.

Table 10 The critical temperature, T_c, the normal boiling temperature, T_b, and the enthalpy of vaporization of the ionic liquids at 303.15 K

ILs	Guggenheim		Eötvos		Δl^gHm^o kJ mol^−1
	Tc/K	Tb/K	Tc/K	Tb/K
[MPy][HSO4]	4247.178	2548.307	942.7021	565.6213	129.5467
[MPyr][HSO4]	2048.201	1228.921	684.9887	410.9932	78.31177
[MIM][HSO4]	6774.106	4064.464	932.3981	559.4389	127.4982
[BSMIM][HSO4]	4052.452	2431.471	1054.46	632.6761	151.7649
[BSMIM][CF3SO3]	5611.371	3366.823	1151.666	690.9999	171.0901
[BSPy][HSO4]	5394.323	3236.594	1023.838	614.303	145.6771
[BSPy][CF3SO3]	4750.39	2850.234	1110.589	666.3533	162.9236

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Interstice model for ionic liquids

A new theoretical model called the interstice model^52,53 was developed for ILs by abstracting the essence of the hole model for molten salts⁵⁴ The model is based on four assumptions which can be seen elsewhere.^48,55,56 In this model the interstice volume, v, for the ionic liquids was calculated using an equation from classical statistical mechanics.⁴⁸

The values of the average volumes of the interstices of the synthesized ILs are given in Table 11. The volume fractions of the interstice, ∑v/V, for all the measured ionic liquids are also given in Table 11. The values are between 10.76 and 12.72% and are in agreement with the substances which show a volume expansion of approximately less than 15% during the transformation from solids to liquids. The molar volume, V, is the summation of the inherent volume, V_i, and the sum of the volumes of all the interstices, ∑v = 2N_Av, i.e.⁴⁸

V = V_i + 2N_Av (12)

Table 11 Parameters of the interstice model for the protic ILs at 303.15 K

ILs	10^−24 v/cm^3	∑v/cm^3	∑v/v	10^4α(calc)/K^−1	10^4α(exp)/K^−1
[MPy][HSO4]	16.58758	19.98139	14.23674	7.04	5
[MPyr][HSO4]	34.67143	41.76521	30.86062	15.26	7
[MIM][HSO4]	14.99143	18.05868	14.58786	7.22	4
[BSMIM][HSO4]	20.50155	24.69616	11.18139	5.53	4
[BSMIM][CF3SO3]	19.76915	23.81391	9.316099	4.61	6
[BSPy][HSO4]	21.05497	25.36282	11.90154	5.89	5
[BSPy][CF3SO3]	20.64481	24.86874	10.03599	4.96	6

---

The thermal expansion coefficient (α) was calculated by assuming that the expansion of the ILs results only from the expansion of the interstice during the temperature change. Hence, the equation of α derived from the interstice model is given below:⁴⁸

The calculated and the experimental values of α for all the ionic liquids studied are shown in Table 11.

Refractive index

Table 12 presents the refractive index values for the investigated protic ILs and the temperature effect on the refractive indices is shown in Fig. 4. The effects of the temperature on the refractive indices of the present ionic liquids were measured in the temperature range of 293.15–323.15 K. It is indicated that the refractive indices of these protic ILs makes an obvious reduction with an increase in temperature. As presented in Table 12, among these studied PILs, [MPy][HSO₄] has the highest refractive index value while [BSPy][CF₃SO₃] has the lowest refractive index value. The increasing order of the refractive indices of the investigated ILs is [MPy][HSO₄] > [BSMIM][HSO₄] > [BSPy][HSO₄] > [MIM][HSO₄] > [MPyr][HSO₄] > [BSMIM][CF₃SO₃] > [BSPy][CF₃SO₃]. The present refractive index values are very close to those for other reported ILs available in the literature.^57–61 Due to the limited literature on protic ionic liquids, the comparisons are made with aprotic ionic liquids. The standard deviations (SDs) and fitting parameters of the refractive indices for the present ionic liquids are given in Table 13.

Table 12 Experimental refractive indices, (n_D), for the present ionic liquids in the temperature range of 293.15–323 K at atmospheric pressure^a

T/K	nD
	[MPy][HSO4]	[MPyr][HSO4]	[MIM][HSO4]	[BSMIM][HSO4]	[BSMIM][CF3SO3]	[BSPy][HSO4]	[BSPy][CF3SO3]
a Standard uncertainty μ is μ(T) = ±0.05 K and the combined expanded uncertainty is μc(nD) = 3.5 × 10^−5, (level of confidence = 0.95).
293.15	1.5335	1.46458	1.48955	1.5038	1.46042	1.49878	1.45608
298.15	1.53283	1.46359	1.48865	1.50287	1.45947	1.49827	1.45525
303.15	1.53168	1.46254	1.48761	1.502	1.45835	1.49739	1.45399
308.15	1.53051	1.46149	1.48655	1.50118	1.45718	1.49638	1.45254
313.15	1.52931	1.4604	1.48552	1.50033	1.45595	1.4953	1.45093
318.15	1.52812	1.45934	1.48432	1.49949	1.45473	1.49422	1.4493
323.15	1.52692	1.45827	1.48311	1.49864	1.45355	1.4931	1.44748

---

Fig. 4 Refractive index (n_D) as a function of temperature for the protic ionic liquids, ■ [MPy][HSO₄], ● [MPyr][HSO₄], ▲ [MIM][HSO₄], ▼ [BSMIM][HSO₄], ◄ [BSMIM][CF₃SO₃], ► [BSPy][HSO₄], ♦ [BSPy][CF₃SO₃].

Table 13 Standard deviations (SDs) and the fitting parameters of the refractive index^a

ILs	SD (× 10^−3)	R^2	A6	A7
a Equation for fitting parameters: nD = A6 + A7.
[MPy][HSO4]	6.849	0.9956	1.5998	−0.0002
[MPyr][HSO4]	3.010	0.9999	1.5265	−0.0002
[MIM][HSO4]	4.057	0.998	1.5527	−0.0002
[BSMIM][HSO4]	7.957	0.9997	1.5538	−0.0002
[BSMIM][CF3SO3]	8.716	0.9987	1.5286	−0.0002
[BSPy][HSO4]	1.541	0.9907	1.5561	−0.0002
[BSPy][CF3SO3]	2.458	0.9893	1.5419	−0.0003

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Thermal stability

Thermogravimetric analysis (TGA) was used to evaluate the thermal degradation temperature of the protic ionic liquids. It has been observed that ionic liquids based on an imidazole cation have higher stability compared to other cations both with a functionalized SO₃H group and without the functional group. In the functionalized four ILs, the CF₃SO₃⁻ anion-containing ILs show higher stability than those containing the HSO₄⁻ anion. The onset decomposition temperatures of the present protic ILs are 252.36, 299.07, 263.97, 246.51, 253.98, 323.56, 333.92, 288.74, 243.22, 338.60, and 282.46 °C for [MPip][HSO₄], [MPy][HSO₄], [Pi][HSO₄], [MPyr][HSO₄], [EAm][HSO₄], [P][HSO₄], [MIM][HSO₄], [BSMIM][HSO₄], [BSPy][HSO₄], [BSMIM][CF₃SO₃], and [BSPy][CF₃SO₃], respectively. These data are in good agreement with the literature.^62–64 Miran et al.⁶⁵ reported that the DBU-based protic ILs with different anions such as acetate, trifluoroacetate and methanesulfonate, etc., have onset decomposition temperatures in the temperature range from 171–451 °C. It is generally believed that protic ionic liquids (PILs) are thermally unstable relative to aprotic ionic liquids (AILs). The protic ionic liquids (PILs) are thermally unstable due to N–H bonding in the structure of their cations, compared to aprotic ionic liquids (AILs) such as 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C₂mim][NTf₂]).²³ Moreover, the acetate-based protic liquids were observed to be more unstable compared to other anion-based protic ionic liquids, which might be due to the degradation of the acetate anion also. The thermal stability measured for these PILs is in a reasonable range to be used in various applications. It is assumed that the decomposition temperature of the ionic liquids depends upon the structure of the cations and anions. The synthesized ILs decomposition temperature trend are shown in Fig. 5.

Fig. 5 Thermogravimetric analysis (TGA) of the protic ionic liquids.

The melting point and the glass transition temperature were determined for the prepared ionic liquids, in which the exothermic peak onset temperature was assigned to the glass transition and the endothermic peak onset temperature was assigned to the melting point. The obtained values are listed in Table 14.

Table 14 The glass transition temperature and the melting point of the synthesized ionic liquids

ILs	Tg	Tm
[MPip][HSO4]	−62.79	—
[MPy][HSO4]	−69.67	−18.95
[Pi][HSO4]	—	60.33
[MPyr][HSO4]	−96.24	—
[EAm][HSO4]	−74.63	51.47
[P][HSO4]	−73.73	45.20
[MIM][HSO4]	−73.49	20.88
[BSMIM][HSO4]	−34.72	—
[BSMIM][CF3SO3]	−54.75	—
[BSPy][HSO4]	−40.90	—
[BSPy][CF3SO3]	−55.87	—

---

Conclusions

In the present work, a series of protic ILs were prepared and their various physicochemical properties were measured to investigate the effect of temperature. The viscosity and density values decrease with an increase in temperature. In comparison, the SO₃H-functionalized ILs ([BSMIM][HSO₄], [BSMIM][CF₃SO₃], and [BSPy][HSO₄]) show higher viscosities. The pyridinium cation ([MPy][HSO₄]) protic IL with a HSO₄⁻ anion without any functional groups shows higher viscosity values compared to the imidazolium cation. In functionalised ILs, the CF₃SO₃⁻ anion-containing ILs show higher density values compared to those containing the HSO₄⁻ anion because the CF₃SO₃⁻ anion has a higher molecular weight. From the experimental density values some thermodynamic properties like the molecular volume, standard entropy, lattice energy and thermal expansion coefficient were calculated. The calculated values are in the range as reported for other ILs. The surface tension data was further used to calculate the surface entropy and surface enthalpy. With the help of the Guggenheim and Eötvos equations, the critical temperature and the normal boiling temperature were calculated for the prepared PILs. The volume fractions of the interstice, ∑v/V, are between 10.76 and 12.72% and are in good agreement with the substances which show a volume expansion of approximately less than 15% during the transformation from solid to liquid. The molar enthalpy of vaporization was also determined. It also suggested that the interstice model can be applied for an IL system and the calculated values are in good agreement with the experimental ones. The present ionic liquids have lower lattice energies and higher standard entropies, while the thermal stabilities were measured as higher compared to most of the reported PILs.

Acknowledgements

We especially acknowledge the Centre of Research in Ionic Liquid and all the research officers and postgraduate students for helping in all aspects.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07656k

‡ For each property, the standard deviation (SD) values were calculated using the equation where Z_exp and Z_cal are the experimental and calculated data values, respectively, and n_DAT is the number of experimental points.

§ Equation for the temperature dependence of viscosity: log η/(mPa s) = A₀ + (A₁/T).

¶ Equation for the temperature dependence of density: ln ρ/(kg m⁻³) = A₂ + A₃ (T-298.15).

|| Equation for the surface tension temperature dependence: σ/(mN m⁻¹) = A₄ + A₅T.

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