The Effect of the Methylation and N-H Acidic Group on the Physicochemical Properties of Imidazolium-Based Ionic Liquids

Abstract

This work presents and highlights the differentiation of the physicochemical properties of the [C₁Him][NTf₂], [C₂Him][NTf₂], [¹C₁²C₁Him][NTf₂], and [¹C₄²C₁³C₁im][NTf₂] that are related with the strong bulk interaction potential, which highlights the differentiation on the physicochemical arising from the presence of the acidic group (N-H) as well as the methylation in position 2 (C(2)) of the imidazolium ring. Densities, viscosities, refractive indices and surface tensions in a wide range of temperatures, as well as, isobaric heat capacities at 298.15 K, for this IL series are presented and discussed. It was found that the volumetric properties are barely affected by the geometric and structural isomerization, following a quite regular trend. A linear correlation between the glass transition temperature, T_g, and the alkyl chain size was found; however, ILs with the acidic N-H group present a significant higher T_g than the [¹C_N-1³C₁im][NTf₂] and [¹C_N³C_Nim][NTf₂] series. It was found that the most viscous ILs, ([¹C₁Him][NTf₂], [¹C₂Him][NTf₂] and [¹C₁²C₁Him][NTf₂]) have an acidic N-H group in the imidazolium ring in agreement with the observed increase of energy barrier of flow. The methylation in position 2, C(2), as well as, the N-H acidic group in the imidazolium ring, contribute to a significant variation in the cation-anion interactions and their dynamics, which is reflected in their charge distribution and polarizability leading to a significant differentiation of the refractive indices, surface tension and heat capacities. The observed differentiation of the physicochemical properties of the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₁²C₁Him][NTf₂], and [¹C₄²C₁³C₁im][NTf₂] are an indication of the stronger bulk interaction potential, which highlights the effect that arises from the presence of the acidic group (N-H) as well as the methylation in position 2 of the imidazolium ring.

1. Introduction

There is an increasing interest in Ionic Liquids (ILs) due to their unusual physical and transport properties which result from their peculiar type of cohesive interactions, charge distribution and nanostructuration. The molecular structure and supramolecular organization of an ionic liquid is complex, comprising polar and non-polar domains. This structural heterogeneity leads to nanostructuration in the bulk which was already observed both theoretically and experimentally.^1–9 Among their unique properties, the high thermal and chemical stabilities, negligible vapour pressure at room temperature, high ionic conductivity, and improved solvation ability makes them good candidates in a wide variety of applications in the chemical industry, as well as, models for the academic understanding of the structural features and interionic interactions in the bulk phase. Most of the studies reported in the literature were focused on the effect of the alkyl chain length and the chemical nature of the ion pairs on the thermophysical properties. There are some work in literature that focus on the effect of the structural isomerism of ionic liquids and how this can affect their thermodynamic properties,^10–15 however there is a lack of understanding of the effect of important features like the methylation and N-H acidic bond on physicochemical properties.

The variation in the substituent groups at the cation is found to have a drastic impact in the charge distribution and accessibility⁸. As a consequence, the anion-cation interaction potential can vary significantly. A case to point is the cation methylation in the position 2, C(2), of the imidazolium ring. This structural feature has been investigated by several groups, both experimentally and theoretically, in order to understand its effect on the thermophysical properties and transport properties.^16–21 This structural modification has been shown to induce a change in the melting temperature, thermal stability, viscosity, surface tension, and heat capacities. The experimental evidence published by Bonhôte et al.¹⁶ showed that the methylation in C(2) of the imidazolium ring of NTf₂-based ionic liquids increases the viscosity by a factor of 2.6, relative to the hydrogenated cation based IL. They also found that the methylation in the C(4) and C(5) positions of the imidazolium have a minor effect¹⁶. Later on, Hunt¹⁷ used quantum chemical calculations to investigate the effect of the C(2) methylation in the viscosities of imidazolium-based ionic liquids. They found that the increase in the viscosities and melting points is explained by the decrease in melting entropy of the methylated cations, due to a reduction of the number of stable conformers (cation-anion interactions) leading to a decrease of the absolute liquid entropy. Additionally, the free rotation on the butyl chain is restricted by the steric bulk methyl, which limits the position of the anion around the cation. Noack et al.¹⁸ observed the great influence of the C(2) position on the electron density distribution of the molecular structure and on the macroscopic behavior of imidazolium–based Ils, using vibrational and NMR spectroscopy. According to Fumino et al., ^19,20 the suppression of the hydrogen bonds formed in C(2) position upon methylation enhances the overall Coulomb interactions between anion and cation. They found that the hydrogen bond formed in C(2)-H disrupted the ionic network by altering the charge symmetry of the ions and thus fluidize imidazolium ILs. These results were supported experimentally by IR spectroscopy where a red-shift and an intensity change of the bands were observed. A theoretical study by Izgorodina et al.²¹ revealed that there is a restriction in the anion movement around the methylated C(2) cation relative to the hydrogenated one with a potential energy barrier exceeding 40 kJ∙mol^-1. Another structural feature that is scarcely addressed in the literature is the acidic hydrogen present in N-H of the imidazolium ring. The ILs comprising this acidic character are more likely to establish hydrogen bonds with the anions and with solvents, such as water.¹⁰

In this work, we explore the structural isomerization effect on the properties of the imidazolium NTf₂-based ILs. Heat capacity, density, viscosity, refractive index, surface tension and thermal behavior (glass transition, crystallization temperatures/profile, melting temperature, enthalpies and entropies of fusion) are here studied. The change of a group position (such as a methyl group), as well as, the presence of the acidic N-H group in the ionic liquid were used as an in situ probe to explore the molecular effects on the overall ionic liquid interaction potential, which will be reflected in their physicochemical properties. Table 1, presents the list and abbreviation of the studied ILs.

Table 1.

Schematic structures and abbreviations of the ionic liquids under study.

Imidazolium-Based Cations			Abbreviations
R1: methyl	R2: hydrogen	R3: hydrogen	[1C1Him][NTf2]
R1: ethyl	R2: hydrogen	R3: hydrogen	[1C2Him][NTf2]
R1:methyl	R2:methyl	R3:hydrogen	[1C12C1Him][NTf2]
R1:methyl	R2:hydrogen	R3:methyl	[1C13C1im][NTf2]
R1:ethyl	R2:hydrogen	R3:methyl	[1C23C1im][NTf2]
R1:propyl	R2:hydrogen	R3:methyl	[1C33C1im][NTf2]
R1:ethyl	R2:hydrogen	R3:ethyl	[1C23C2im][NTf2]
R1:ethyl	R2:hydrogen	R3:propyl	[1C23C3im][NTf2]
R1:butyl	R2:hydrogen	R3:methyl	[1C43C1im][NTf2]
R1:pentyl	R2:hydrogen	R3:methyl	[1C53C1im][NTf2]
R1:propyl	R2:hydrogen	R3:propyl	[1C33C3im][NTf2]
R1:butyl	R2:methyl	R3:methyl	[1C42C13C1im][NTf2]

2. Experimental Section

2.1. Materials and purification

The twelve ILs samples were acquired from IOLITEC with the following state purity: [¹C₁Him][NTf₂] (1-methylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 353239-08-4), >98 %; [¹C₂Him][NTf₂] (1-ethylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 353239-10-8), >98 %; [¹C₁²C₁Him][NTf₂] (1,2-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 353239-12-0), 98%; [¹C₁³C₁im][NTf₂] (1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 174899-81-1), 99%; [¹C₂³C₁im][NTf₂] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 174899-82-2), >99; [¹C₃³C₁im][NTf₂] (1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 216299-72-8), 99%; [¹C₂³C₂im][NTf₂] (1,3-diethylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 174899-88-8), >99%; [¹C₂³C₃im][NTf₂] (1-ethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 347882-21-7, >99%; [¹C₄³C₁im][NTf₂] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 174899-83-3, >99%;[¹C₅³C₁im][NTf₂] (1-methyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: n.a., >99; [¹C₃³C₃im][NTf₂] (1,3-dipropylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: n.a., >99; [¹C₄²C₁³C₁im][NTf₂] (1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, CAS RN: 350493-08-2, 99%. The relative atomic masses used in this work were those recommended by the IUPAC Commission in 2007.²²

The commercial ILs samples were purified under vacuum (0.1 Pa) at moderate temperature (323 K) and constant stirring for 48 hours in order to remove traces of volatile impurities. The water mass fraction content was determined in a 151 Metrohm 831 Karl Fischer coulometer, using a Hydranal-152 Coulomat AG from Riedel-de Haën. The water content, in all ILs, was below 100 ppm. This process was performed systematically, before any experimental measurement.

2.2. Thermal behavior

Temperatures and the standard molar enthalpies of fusion for the ILs were measured in a power compensation differential scanning calorimeter, PERKIN ELMER model Pyris Diamond DSC, using hermetically sealed aluminum crucibles with a constant flow of nitrogen (50 mL∙min⁻¹). Samples of about 15 mg were used in each experiment. The temperature and heat flux scales of the power compensation DSC were calibrated by measuring the temperature and the enthalpy of fusion of reference materials,^23,24 namely benzoic acid, 4-metoxybenzoic acid, triphenylene, naphthalene, anthracene, 1,3,5-triphenylbenzene, diphenylacetic acid, perylene, o-terphenyl and 9,10-diphenylanthracene, at different scanning rates (2, 5 and 10 K∙min^-1). Each IL sample was previously heated above the melting temperature for 30 minutes, followed by a quenching step consisting of a fast cooling (~50 K∙min^-1) until 173 K. This procedure avoids the crystallization on cooling and promotes the glass formation. After that, the ILs samples were heated (5 K∙min^-1) to promote crystallization followed by cooling (~50 K∙min^-1) and heating cycles (5 K∙min^-1) in the crystallization region, exceeding the glass transition and below the temperature of melting to assure complete crystallization before the melting. A final scan at 5 K∙min^-1 rate was performed to determine the temperatures and enthalpies of the solid-solid and isotropization phase transitions.

2.3. Densities and Viscosities

The density, ρ, and viscosity,η, for the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs were measured using an automated SVM 3000 Anton Paar rotational Stabinger viscometer–densimeter. The measurements were carried out at 0.1 MPa in the temperature range from (278.15 to 363.15) K. Only the [¹C₁Him][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs, which are solids at room temperature, were measured in a narrower temperature range of (323.15 to 363.15) and (303.15 to 363.15) K, respectively. For each ionic liquid, at least two independent measurements were performed using the same experimental conditions and different samples. The apparatus was calibrated using the three standard calibration samples, APN7.5, APN26 and APN415 in the same experimental conditions of the ionic liquid measurements. The reproducibility of the viscosity and density measurements is, according to the manufacturer, 0.35 % and ± 0.5 kg∙m^-3, respectively from 288.15 to 378.15 K and the uncertainty of temperature is within ± 0.02 K. Further details regarding the equipment and method are available in the literature.^25,26

2.4. Heat Capacities

The heat capacities at T = 298.15 K of the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs were measured by a high-precision heat capacity drop calorimeter, described in the literature.^27–30 The calorimeter was calibrated with water and sapphire (α-Al₂O₃). ²³ The calibration constant was found to be ε = (6.6040 ± 0.0036) W·V^–1. The accuracy of the apparatus for measurements of heat capacities of liquids and solids was evaluated before, using benzoic acid, hexafluorobenzene, p-terphenyl and [¹C₆³C₁im][NTf₂].²⁹ The ampoules were weighted in a Mettler Toledo AG245 dual range analytical balance (sensitivity of 1×10⁻⁶ g and repeatability of 2×10⁻⁶ g) both empty and after filling with the ionic liquid. All the uncertainties are given as twice of the standard deviation of the mean, and include the calibration uncertainty. The buoyancy effect correction was considered both for the calibration and experiments of the ILs.

2.5. Refractive indices

The refractive indices for all the ILs presented in Table 1 were measured at the sodium D-line using a Bellingham model RFM340 refractometer (± 3×10^-5 stated precision), as a function of temperature. The refractometer features a presser with a seal ring made of fluoropolymer Kalrez® which is closed over the sample on the prism preventing/decreasing the IL sample contact with water and atmospheric gases in particular, CO₂. The presser incorporates a micro flow cell, which is used to introduce the sample into the refractometer, without opening it to the atmosphere in order to avoid moisture and other gases contamination of the ILs samples. For the studied samples no time drift was detected along the measurements. The presser and the internal prism water jacket assembly is temperature controlled by an external bath through the presser hinge and integral channels in the presser arm. The temperature in the refractometer cell is controlled using an external thermostatic bath within a temperature fluctuation of (± 5×10^-3) K, measured with a resolution better than 1×10^-3 K and an uncertainty within ± 0.02 K. The apparatus was calibrated with degassed water (Millipore quality) and toluene (Spectranal, 99.9 %). Samples were directly introduced into the flow cell (prism assembly) using a syringe; the flow cell was kept closed after sample injection. For each ionic liquid at least two independent experiments were performed and in each experiment at least three measurements were taken at each temperature. The refractive indices were measured with respect to air and no corrections were applied.

2.6. Surface Tension

The surface tension of each ionic liquid sample was determined by the analysis of the shape of a pendant drop and measured using a Dataphysics (model OCA-20) contact angle system. Drop volumes of (9 ± 0.5) µL were obtained using a Hamilton DS 500/GT syringe connected to a Teflon coated needle placed inside an aluminium air chamber able to maintain the temperature of interest within ± 0.1 K. The surface tension measurements were performed in the temperature range from (298.15 to 343.15) K, with the exception of [¹C₁Him][NTf₂] and [¹C₁²C₁Him][NTf₂], which was performed in the temperature range from (328.15 to 343.15) K and (303.15 to 343.15) K, respectively, due to their higher melting temperature. The temperature of the ionic liquid in each surface tension measurement was considered to be the same than as measured inside the aluminium chamber with a Pt100 within ± 0.1 K placed at a distance of approximately 20 mm from the liquid drop. After reaching a specific temperature inside the aluminium chamber, the measurements were carried out after 40 min to guarantee the thermal stabilization. Silica gel was kept inside the air chamber to maintain a dry environment.

For the surface tensions determination at each temperature, and for each ionic liquid, at least 5 drops were formed and analysed. For each drop, an average of 150 images was captured. The analysis of the drop shape was done with the software modules SCA 20 where the gravitational acceleration (g = 9.8018 m∙s²) and latitude (lat. = 40º) were used according to the location of the assay. The surface tensions were calculated using the measured density data. Further details on the equipment and its validity to measure surface tensions of ILs were previously addressed.^31–33

3. Results and Discussion

3.1. Thermal behavior

The experimental results of the onset temperatures of glass, cold crystallization and (solid-solid and fusion) transitions are presented in Table 2 along with along with some available literature data for comparison. The enthalpy of transitions were obtained by numerical integration of the peak thus obtained. Estimation errors of ± 0.5 K for the onset temperature and ± 0.7 kJ∙mol^-1 for enthalpies were assumed taking into account the combined uncertainty of the calibration and the ILs experiments. The values found in the literature concerning the phase behavior of the ionic liquids here studied are scarce and in poor agreement with each other.^6,34–36 This discrepancy in the phase transitions comes from the fact that the process by which liquids are cooled to form a supercooled liquid and glasses affects the crystallization kinetics, crystal perfection /relaxation level and, as a consequence, affects the experimental results.^37–39 The differences in the glass transition temperatures, T_g, determined in this work and those reported in literature do not exceed 5 K, which may result from the different cooling rates applied in the formation of the glass.³⁷ The structural symmetry of a supercooled liquid and the molecular weight affect the glass transition.

Table 2.

Experimental glass, T_g, cold crystallization, T_cc, solid-solid, T_ss, melting, T_m, temperatures, enthalpies, ΔH(T), and entropies of transitions, ΔS(T) for the studied ILs.

Ionic Liquid	T / K	ΔH(T) / kJ∙mol-1	ΔS(T) / J∙K-1∙mol-1
[1C1Him][NTf2]	321.5 ± 0.5 (Tm)	24.1 ± 0.7	75.0 ± 2.2
[1C2Him][NTf2]	184.2 ± 0.5 (Tg)
	206.6 ± 0.5 (Tcc)
	263.5 ± 0.5 (Tss)	1.8 ± 0.7	6.7 ± 2.6
	275.5 ± 0.5 (Tm)	14.2 ± 0.7	51.5 ± 2.5
[1C12C1Him][NTf2]	311.0 ± 0.5 (Tm)	19.2 ± 0.7	61.7 ± 2.2
[1C13C1im][NTf2]	176.4 ± 0.5 (Tg)
	293.2 ± 0.5 (Tm)	22.0 ± 0.7	75.0 ± 2.4
	[299 (Tm)]34	[24.5]34	[81.7]34
[1C23C1im][NTf2]	178.3 ± 0.5 (Tg)
	[186 (Tg)]34
	206.8 ± 0.5 (Tcc)
	260.0 ± 0.5 (Tm)	21.3 ± 0.7	81.9 ± 2.7
	[291(Tm)]34	[24.8 ΔH(Tm)]34	[97.1 ΔS(Tm)]34
[1C33C1im][NTf2]	179.6 ± 0.5 (Tg)	-	-
	[184.0(Tg)]6
[1C43C1im][NTf2]	182.1 ± 0.5(Tg)	-	-
	[186 (Tg)]34
	[181.5 ± 0.1(Tg)]36
[1C23C2im][NTf2]	286.3 ± 0.5 (Tm)	29.1 ± 0.7	101.6 ± 2.4
	[262.6 ± 0.1(Tm)]35	[20.4 ± 0.3]35
	[285.5 (Tm)]6
[1C23C3im][NTf2]	179.5 ± 0.5(Tg)	-	-
[1C53C1im][NTf2]	183.8 ± 0.5(Tg)	-	-
	[186.5(Tg)]6
[1C33C3im][NTf2]	180.9 ± 0.5(Tg)	-	-
[1C42C13C1im][NTf2]	191.5 ± 0.5(Tg)	-	-

Figure 1, depicts the glass transitions and fusion temperatures of the studied ILs. As it was expected T_g increases with the molecular weight. The glass transition temperature of the [¹C_N-1³C₁im][NTf₂] series follows the order: [¹C₁³C₁im][NTf₂] < [¹C₂³C₁im][NTf₂] < [¹C₃³C₁im][NTf₂] < [¹C₄³C₁im][NTf₂] < [¹C₅³C₁im][NTf₂]. The [¹C₂³C₃im][NTf₂] and [¹C₃³C₃im][NTf₂] have slightly lower T_g than their respective isomers. These results are consistent with the symmetric features. Liquids composed of asymmetric molecules form glasses much more readily than those consisting of symmetric ones.³⁸ Additionally, there is a correlation between the T_g and the viscosities measured in this work. The most viscous ILs are those with the highest T_g, which could be related with glass cohesive energy and their relation with their higher energy barrier to flow.

Figure 1.

Graphic representation of the glass transitions temperatures, T_g /K, (I), and melting temperatures, T_m /K, (II) as a function of the total number of carbon atoms in the alkyl side chains of the ILs: - [¹C₂Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₃³C₃im][NTf₂]; - [¹C₄²C₁³C₁im][NTf₂]; - [¹C₁Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₂im][NTf₂]. The dash and dots view-lines highlight the [¹C_NHim][NTf₂] and [¹C_N-1³C₁im][NTf₂] series which have no physical meaning.

Among the short alkyl chain ILs with the acidic N-H group, only for [¹C₁Him][NTf₂] it was not possible to obtain a glass state temperature due to the fast and easy crystallization mechanism even at fast cooling rate before the glass formation. The fast and easy crystallization mechanism could be related with small hindrance effect and high cohesive energy due to the acidic N-H group on the crystallization process. The [¹C₄²C₁³C₁im][NTf₂] isomer with a methylation in the position 2, C(2), presents a higher T_g than the [¹C_N-1³C₁im][NTf₂] series due to the hindrance effect of the –CH₃ that decreases the cation-anion dynamics as will be discussed below. By other hand, the significant increase in the T_g in the [¹C₂Him][NTf₂] should be related to the increase of the cohesive energy due to the presence of the acidic N-H group. The empirical correlation between the T_g and T_m, [T_g/T_m = 2/3], observed for polymers,⁴⁰ was also found here for the [¹C₂Him][NTf₂] and [¹C₂³C₁im][NTf₂].

For the enthalpies and entropies of transitions, the results presented in this work show some deviations from the literature values.^34,35 The entropy of transition was determined by the relation: ΔS_trans. = ΔH_trans. / T_trans..

The discussion, comparison, and interpretation of the results with the literature data should be carried out with caution since the obtained of temperatures and enthalpies of transitions are affected by the thermal history and the methodology applied to the phase behavior studies (e.g., temperature scan rate, quenching procedure, time of stabilization, impurities, crucible material, and sample size).

Figure 2, depicts the enthalpies and entropies of melting as a function of the total number of carbon atoms, N, in the alkyl side chains of the ILs. The initial decrease in the enthalpy of melting is reflected in the initial decrease of the melting temperatures along the total number of carbons of the cation. The enthalpic differentiation between the series [¹C_N-1³C₁im][NTf₂] and [¹C_NHim][NTf₂] is, however, partially cancelled in the melting temperatures due to the entropic compensation. The higher temperature of melting of the [¹C₁²C₁Him][NTf₂] relative to the [¹C₂Him][NTf₂] and [¹C₁³C₁im][NTf₂] isomers is ruled by the lower entropy of melting in agreement with the decrease of the anion-cation dynamics due to the methylation in the position 2, C(2), of the imidazolium ring. The melting temperature of the [¹C₂³C₂im][NTf₂] is similar to the [¹C₁³C₁im][NTf₂], besides their significant higher enthalpy of melting, reflecting the strong entropic compensation effect.

Figure 2.

Graphic representation of the enthalpies (I) and entropies (II), as a function of the total number of carbon atoms in the alkyl side chains of the ILs, N.: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]. The dash and dots view-lines highlight the [¹C_NHim][NTf₂] and [¹C_N-1³C₁im][NTf₂] series which have no physical meaning.

3.2. Densities

The experimental raw data for density for the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs are presented in the Supporting Information, Table SI.2. The density data (ρ), in the studied temperature (T) range, was further correlated using a second order polynomial equation:

$$\text{ln}\left(\rho /\text{kg}\cdot {\text{m}}^{-3}\right)=a+b\cdot T+c\cdot T^2$$ (1)

where a, b, and c are the coefficients obtained from the least square fitting of equation (1), T is the temperature in K. The graphic representation of the logarithm of density as function the temperature is presented in Figure 3, together with literature data¹⁵ for [¹C_N-1³C₁im][NTf₂] (where N = 3 – 6) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6).

Figure 3.

Logarithm of density as a function of temperature. This work: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; Literature¹⁵: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂].

The isobaric thermal expansion coefficient, α_p, which considers the volumetric changes with temperature, was derived using the equation (2):

$${\alpha }_p=-\frac{1}{\rho }{\left(\frac{\partial \rho }{\partial T}\right)}_p=-{\left(\frac{\partial \text{ln}\rho }{\partial T}\right)}_p=-[b+2c\cdot (T/\text{K})]$$ (2)

where, ρ is the density in kg·m^-3, T is the temperature in K, p is the standard pressure (0.1 MPa), and b and c are the fitting coefficients of equation (1). The fitting parameters such as the molar volume and the thermal expansion coefficients, at T = 323.15 K and 0.1 MPa, for all the studied ILs are listed in Table 3. Due to the fact that some of the ILs are solid at room temperature, the comparison of the data was done at T = 323.15 K.

Table 3.

List of the fitted parameters (equation (1)), density, molar volume, and the thermal expansion coefficients, α_p, at 323.15 K and 0.1 MPa for the studied ILs.

Ionic Liquid	a	104 × b / K-1	107 × c / K-2	ρ / (kg·(m-3)	Vm / (cm3·mol-1)	103× αp / K–1
				T=323.15 K
[1C1Him][NTf2]	7.6198 ± 0.0036	-8.42 ± 0.21	2.90 ± 0.30	1600.6	227.0	0.654 ± 0.029
[1C2Him][NTf2]	7.5678 ± 0.0010	-7.59 ± 0.07	1.43 ± 0.10	1537.0	245.5	0.666 ± 0.009
[1C23C3im][NTf2]	7.4844 ± 0.0014	-7.62 ± 0.09	1.35 ± 0.14	1411.4	297.1	0.675 ± 0.013
[1C12C1Him][NTf2]	7.5632 ± 0.0032	-7.07 ± 0.19	0.79 ± 0.29	1545.2	244.2	0.656 ± 0.027

The graphic representations of the density and molar volume and thermal expansion coefficients at 323.15 K and 0.1 MPa, against the total number of carbon atoms in the alkyl side chains of the cation, N, are presented in Figures 4 and 5, respectively. No significant differentiation in the isomers could be detected in the density and thermal expansion coefficients. Thus, it can be concluded that the volumetric properties are barely affected by the geometric and structural isomerization for these ILs, following a quite regular trend.

Figure 4.

Density(I) and molar volume(II), at 323.15 K and 0.1MPa, as a function of the total number of carbon atoms in the alkyl side chains of the cation, N. This work: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂].¹⁵

Figure 5.

Thermal expansion coefficient, α_p, at 323.15 K and 0.1 MPa, as a function of the total number of carbon atoms in the alkyl side chains of the cation, N. This work: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]. ¹⁵

3.3. Viscosities

The experimental viscosity data for the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂], and [¹C₁²C₁Him][NTf₂] ILs are presented in the Supporting Information, Table SI.3. Figure 6 depicts the graphic representation of the ln(η/mPa·s) against the temperature, together with literature data for [¹C_N-1³C₁im][NTf₂] (where N = 3 – 6) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6) series.^15,41

Figure 6.

Logarithm of viscosity as a function of temperature for studied ILs. The solid lines represents the Vogel-Tammann-Fulcher fitting from equation (3) This work: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂].¹⁵,⁴¹

The experimental viscosity data was correlated using the Vogel-Tammann-Fulcher (VTF) model described in equation (3):

$$\text{ln}(\eta /\text{mPa}\cdot \text{s})=\text{ln}(A_{\eta }/\text{mPa}\cdot \text{s})+\frac{B_{\eta }}{(T-C_{\eta })}$$ (3)

Where, η is the viscosity in mPa·s, T is the temperature in K, and A_η, B_η and C_η are the fitting parameters from the fitting of the experimental data. The energy barrier of the fluid to a shear stress was evaluated based on the viscosity dependence with the temperature using the following equation (4)

$$E=R\cdot \frac{\partial (\text{ln}\eta )}{\partial (1/T)}=R\cdot \left(\frac{B_{\eta }}{\left(\frac{C_{\eta }^2}{T^2}-\frac{2\cdot C_{\eta }}{T}+1\right)}\right)$$ (4)

The derived coefficients of the VTF equation (3), the viscosity and the derived energy barrier, E, at T = 323.15 K, for the studied ILs, are presented in Table 4.

Table 4.

Fitting parameters of VTF equation for the viscosity data of the studied ILs, viscosity and the derived energy barrier at T = 323.15 K.

Ionic Liquid	Aη / (mPa.s)	Bη / K	Cη / K	η / (mPa·s)	E/ (kJ·mol–1)
				(T=323.15 K)
[1C1Him][NTf2]	0.177 ± 0.005	833 ± 9	163.0 ± 1.0	32.31	28.22 ± 0.76
[1C2Him][NTf2]	0.214 ± 0.004	724 ± 5	168.5 ± 0.5	23.15	26.30 ± 0.36
[1C23C3im][NTf2]	0.150 ± 0.003	789 ± 7	157.1 ± 0.6	17.43	24.85 ± 0.42
[1C12C1Him][NTf2]	0.234 ± 0.005	678 ± 6	188.2 ± 0.6	35.66	32.34 ± 0.78

The graphic representation of viscosity, η, at T = 323.15 K, as a function of the total number of carbon atoms in the alkyl chains in the cation, for the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs and their comparison with the literature data^15,41 for the [¹C_N-1³C₁im][NTf₂] (where N = 3 – 6) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6) series, are depicted in Figure 7(I). The energy barrier at T = 323.15 K, E (T = 323.15 K), as a function of the total number of carbon atoms in the alkyl side chains of the cation, N, are presented in Figure 7 (II).

Figure 7.

Viscosity (η /mPa.s) at T = 323.15K and 0.1 MPa, (I) and energy barrier (E / kJ∙mol^-1) at 323.15 K, (II) for ILs under study as function of the total number of carbons, N. This work: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]; ¹⁵,⁴¹ - [¹C₄²C₁³C₁im][NTf₂]⁴². The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

The [¹C_N-1³C₁im][NTf₂] series is systematically more viscous than the [¹C_N/2³C_N/2im][NTf₂] series, as described in a previous work.¹⁵ A strong differentiation in the viscosities of the isomers with short alkyl chains, such as [¹C₂Him][NTf₂], [¹C₁²C₁Him][NTf₂] and [¹C₁³C₁im][NTf₂] was observed. The shorter alkyl substituted ILs present higher viscosities when compared with the long alkyl chain series. This trend of the viscosity along the total number of carbons of the cation is very similar to that observed in the energy barrier profile, as depicted in Figure 7, which is an indication that the viscosity trend is ruled by the differentiation in the cohesive energy. The most viscous ILs, [¹C₁Him][NTf₂], [¹C₂Him][NTf₂] and [¹C₁²C₁Him][NTf₂], have an acidic N-H group in the imidazolium ring. The acidic hydrogen atom is able to establish hydrogen bonds with the oxygen atoms of the [NTf₂]^- anion, leading to an increase of energy barrier of shear stress. The analysis of the IR spectra of the [¹C₁³C₁im][NTf₂], [¹C₁²C₁Him][NTf₂] and [¹C₁Him][NTf₂] (data is presented as support information) in the range of 400- 4000 cm^-1 do not show any significant differentiation that could be associated to a change in the sulfonyl vibrations, however the enhancement of the anion-cation interaction by increasing the number and the strength of H-bond abilities was previously proposed by Ludwig et al. ⁴³ based on FIR (far IR) spectroscopy in agreement with our conclusion and remarks concerning the effect of the acid N-H group in the thermophysical properties.

The effect of methylation of the C(2) position in the viscosity was previously discussed by Hunt¹⁷, based on computational studies, considering the reduction of ion-pair configurational variation, which leads to an additional increase of the energy barrier in relation to other isomers, as observed in this work. Our results also indicate the differentiation (increase) in the viscosity by the methylation in C(2) in the [¹C₄²C₁³C₁im][NTf₂] with the same magnitude that was observed in [¹C₁²C₁Him][NTf₂], giving thus an additional support for the previous rationalization concerning the effect of the substitution in the position 2 of the imidazolium.

3.4. Heat Capacities

The molar, $C_{p,\mathrm{m}}^{\mathrm{o}}(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{mol}}^{-1}),$ specific, $c_p^0(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{g}}^{-1}),$ and volumetric, $C_p^{\mathrm{o}}/V(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{cm}}^{-3})$ heat capacities at T = 298.15 K and 0.1 MPa, of the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs are presented in Table 5 together with the total number of drop experiments, N_drop, for each ionic liquid. Due to the high metastability of the supercooled liquid [¹C₁²C₁Him][NTf₂], it was possible to measure the heat capacity in the liquid phase, which is also presented in Table 5.

Table 5.

Number of drop experiments, N_drop, the molar heat capacity, $C_{p,\mathrm{m}}^{\mathrm{o}}(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{mol}}^{-1}),$ specific heat capacities, $c_p^{\mathrm{o}}(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{g}}^{-1}),$ and volumic heat capacities, $C_p^0/V(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{cm}}^{-3})$ at 298.15 K.

Ionic Liquid	Ndrop	$C_{p,\mathrm{m}}^{\mathrm{o}}(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{mol}}^{-1}),$	$C_p^0(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{g}}^{-1}),$	$C_p^{\mathrm{o}}/V(\text{J}\cdot {\text{K}}^{-1}\cdot {\text{cm}}^{-3})$
[1C1Him][NTf2] (cr)	12	441.64 ± 0.33	1.2158 ± 0.0009	1.9785 ± 0.0015
[1C2Him][NTf2] (l)	33	484.08 ± 0.44	1.2831 ± 0.0012	2.0055 ± 0.0019
[1C23C3im][NTf2] (l)	19	565.08 ± 0.57	1.3475 ± 0.0014	1.9342 ± 0.0020
[1C12C1Him][NTf2] (l)	23	483.08 ± 0.38	1.2804 ± 0.0010	2.0113 ± 0.0016

The heat capacity data obtained for the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂], was compared with the data for the [¹C_N-1³C₁im][NTf₂] (where N = 3 – 5) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6) ILs available in the literature.^44,45 Figure 8 (I) shows the representation of the molar heat capacity data $\left(C_{p,\mathrm{m}}^{\mathrm{o}}\right)$ against the total number of carbon atoms in the alkyl side chains of the cation, N, of the [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂] and [¹C₁²C₁Him][NTf₂] ILs, together with the data for the [¹C_N-1³C₁im][NTf₂] (where N = 3 – 5)⁴⁴ and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6)⁴⁵ and (II) the deviation from the linear fitting of the molar heat capacities. Figure 9 presents the plots of the specific heat capacity (I) and the volumetric heat capacities (II) against the total number of carbon atoms in the alkyl side chains of the cation, N, of the considered ILs.

Figure 8.

Molar heat capacities, at 298.15 K, as a function of the total number of carbon atoms in the alkyl side chains of the cation, N (I): - [¹C₁Him][NTf₂] (cr); - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂], - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]; ⁴⁴ - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]. ⁴⁵ Deviation from the $C_{p,\mathrm{m}}^{\mathrm{o}}$ (linear fitting) of the ILs presented in (I) as a function of total number of carbons in the alkyl side chains of the cation (II). The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

Figure 9.

Specific heat capacities (I) and the volumic heat capacities (II), at 298.15 K, as a function of the total number of carbon atoms in the alkyl side chains of the cation, N. - [¹C₁Him][NTf₂] (cr); - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂], - [¹C₂³C₃im][NTf₂]; Literature: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂];⁴⁴ - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂].⁴⁵ The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

From Figure 8 (II) it can be seen that the N = 2 isomers, namely [¹C₂Him][NTf₂] [¹C₁²C₁Him][NTf₂], are outliers, with a significant positive deviation from the linear trend of the ILs series presented in this work. From the analysis of the volumetric heat capacities depicted in Figure 9 (II), the differentiation is more evident. The heat capacity per volume units is significantly higher for the [¹C₂Him][NTf₂] and [¹C₁²C₁Him][NTf₂]. The presence of the acidic N-H contributes to a significant interaction potential profile which leads to a heat capacity increment of ~12 J∙K^-1∙mol^-1. In agreement with the previous findings, the [¹C₂³C₂im][NTf₂] (symmetrical series) presents a slightly lower heat capacity than the asymmetric series.⁴⁵ However, the [¹C₂³C₃im][NTf₂] nicely fits the trend of the asymmetric series, as expected.

3.5. Refractive indices

The refractive indices fitting data for [¹C₁Him][NTf₂], [¹C₂Him][NTf₂], [¹C₂³C₃im][NTf₂], [¹C₁²C₁Him][NTf₂], [¹C_N-1³C₁im][NTf₂] (where N = 3 – 6) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6), in the temperature range from 289 K to 342 K, are presented in supporting information, Table SI.4. The graphic representation of the refractive indices, as function of the temperature for the studied ILs, is depicted in Figure 10 (I). Table 6, lists the refractive indices of all studied ILs, at T = 298.15 K, together with available literature values, and the temperature derivative of the temperature dependence of the refractive index, dn_D/dT. The plots of the refractive indices, at T = 298.15K, as a function of the total number of carbon atoms in the alkyl chains in the imidazolium cations, for the measured ILs, are shown in Figure 10 (II). The refractive indices obtained in this work are in good agreement with the available literature values, with relative deviations under 2%. ^46–57

Figure 10.

Refractive indices as a function of temperature for studied ILs (I): Refractive indices, n_D (T = 298.15 K) as function of the total number of carbon atoms in the alkyl side chains of the cation, N (II). - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂].The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

Table 6.

Experimental refractive indices at the sodium D-line, n_D ^a, for the studied ILs as a function of temperature T at 0.1 MPa. Available literature data at T = 298.15 K.

Ionic liquid	nD (298.15 K)	104·(dnD/dT) / K-1	nD (298.15 K) Literature
[1C1Him][NTf2]	1.42203	-2.93 ± 0.01	n.a.
[1C2Him][NTf2]	1.42317	-3.012 ± 0.002	n.a.
[1C12C1Him][NTf2]	1.42490	-2.959± 0.003	n.a.
[1C13C1im][NTf2]	1.42101	-2.97 ± 0.02	n.a.
[1C23C1im][NTf2]	1.42319	-2.99 ± 0.01	1.4230(9)58 1.422047 1.4225148 1.4230749–51 1.4229852
[1C33C1im][NTf2]	1.42534	-3.02 ± 0.01	1.4252553
[1C23C2im][NTf2]	1.42512	-3.04 ±0.02	n.a.
[1C23C3im][NTf2]	1.42668	-3.08 ± 0.01	n.a.
[1C43C1im][NTf2]	1.42705	-3.10 ± 0.01	1.4265348 1.4269254,55 1.42756 1.451 57 1.4267250,51
[1C33C3im][NTf2]	1.42805	-3.10 ± 0.01	n.a.

^ain the temperature interval, n_D, at a specific temperature, T, can be estimated using the following equation: n_D(T / K) = n_D (298.15 K) + dn_D/dT·(T / K - 298.15 K).

As shown in Figure 10 (II), the ILs with shorter alkyl chain length are clearly differentiated with respect to long-chain isomers. A regular increase trend in the refractive indices from [¹C_N-1³C₁im][NTf₂] (where N = 3 – 6) and [¹C_N/2³C_N/2im][NTf₂] (where N = 2, 4, 6) was observed. Concerning the short alkyl chain ILs, the following order for the refractive indices was found: [¹C₁²C₁Him][NTf₂] > [¹C₂Him][NTf₂] > [¹C₁³C₁im][NTf₂] . The methylation in C(2) position, as well as the N-H acidic group in the imidazolium ring contributes to a significant differentiation in the cation-anion interactions which is reflected in their charge distribution and polarizability compared with their respective isomers.

3.6. Surface Tensions

The surface thermodynamic properties, namely surface entropy and surface enthalpy, were estimated using the quasi-linear dependence of the surface tension with temperature.⁵⁹

The surface entropy, S^γ(T), was evaluated according to equation (5):

$$S^{\gamma }(T)=-{\left(\frac{d\gamma }{dT}\right)}_T$$ (5)

Whereas the surface enthalpy, H^γ(T), was determined according to equation (6):

$$H^{\gamma }(T)=\gamma -T{\left(\frac{d\gamma }{dT}\right)}_T$$ (6)

Where γ stands for the surface tension and T for the temperature.

The values of the surface tensions and the thermodynamic functions, at T = 330K, of all the bis[(trifluoromethyl)sulfonyl]imide based ILs derived from the temperature dependence of the surface tension, γ = f(T), in combination with the associated deviation⁶⁰ are presented in Table 7. Figure 11, depicts the dependence of the surface tension with temperature among the ILs. The experimental raw data for the ILs studied is presented in the Supporting Information, Table SI.5. The surface tension, at T = 330 K, as a function of the total number of carbons is depicted in Figure 12. A strong differentiation in the surface tension between the N = 2 isomers was found to follow the trend: [¹C₁³C₁im][NTf₂] > [¹C₁²C₁Him][NTf₂] > [¹C₂Him][NTf₂].

Table 7.

Values of the surface tension γ (mN·m^-1) at 330.0 K, and surface thermodynamic functions S^γ (J·K^-1·m^-2) and H^γ (J·m^-2).

Ionic liquid	γ (330 K)/ mN·m−1	(Sγ ± σa) ×105 / (J·K−1·m−2)	(Hγ ± σa) ×102 / (J·m−2)
[1C1Him][NTf2]	36.2	5.1 ± 0.5	5.3 ± 0.2
[1C2Him][NTf2]	33.1	5.5 ± 0.2	5.13 ± 0.08
[1C23C3im][NTf2]	32.0	4.6 ± 0.1	4.71 ± 0.02
[1C12C1Him][NTf2]	36.7	6.2 ± 0.1	5.73 ± 0.04
[1C42C13C1im][NTf2]	32.7	5.0 ± 0.1	4.91 ± 0.03

^aStandard deviation.

Figure 11.

Surface tension values for the ILs as function of temperature: - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; [¹C₄²C₁³C₁im][NTf₂]; Literature data: - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]⁶¹; - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]¹³.

Figure 12.

Surface tension dependence, at 330 K, as a function of the total number of carbons in the aliphatic chains, N, - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; [¹C₄²C₁³C₁im][NTf₂]. Literature data: - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]¹³ ; - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]⁶¹; The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

The derived surface enthalpies and entropies of the ILs series are depicted in Figure 13. The higher values of surface tension of [¹C₁²C₁Him][NTf₂] and [¹C₁³C₁im][NTf₂] are in agreement with their higher interaction potential in the bulk. It is interesting to notice that the present behavior reflects the low basicity of the bistriflamide in where the full aprotic character of this ionic liquid is preserved even with the acidic N-H group when compared with the recent results concerning the acetate derivatives.³² the remaining isomers follow a regular trend as observed previously,¹³ with an initial decrease of the surface tension until N = 6 , critical alkyl chain length (CAL), reflecting the decrease of the polar interaction.

Figure 13.

Surface enthalpies (I) and entropies (II) as a function of the total number of carbon atoms N, - [¹C₁Him][NTf₂]; - [¹C₂Him][NTf₂]; - [¹C₁²C₁Him][NTf₂]; - [¹C₂³C₃im][NTf₂]; [¹C₄²C₁³C₁im][NTf₂]. Literature data: - [¹C₁³C₁im][NTf₂]; - [¹C₂³C₂im][NTf₂]; - [¹C₃³C₃im][NTf₂]¹³; - [¹C₂³C₁im][NTf₂]; - [¹C₃³C₁im][NTf₂]; - [¹C₄³C₁im][NTf₂]; - [¹C₅³C₁im][NTf₂]⁶¹;. The dash view-line highlight the [¹C_N-1³C₁im][NTf₂] series which has no physical meaning.

3.7. Final remarks

This work presents an extended study of the isomerization effect in the physicochemical properties of short chain length imidazolium [NTf₂]^- ionic liquid series. A strong differentiation in the physicochemical properties arising from the presence of the acidic group, N-H, as well the methylation of the position 2, C(2), in the imidazolium ring was found and interpreted. The observed differentiation of [¹C₁Him][NTf₂], the isomers [¹C₁²C₁Him][NTf₂], [¹C₂Him][NTf₂] and [¹C₄²C₁³C₁im][NTf₂], when compared with the regular trend of the remaining IL member series, is in agreement with their higher interaction potential in the bulk. This increase in the interaction potential arises from two main effects, which contribute in the same direction to the differentiation: the acidic hydrogen N-H, that is able to form hydrogen bonds with the oxygen atoms of the [NTf₂]^- anion; and the methylation in the C(2), which reduces the ion-pair configurational variation, leading to a more localized charge distribution and a significant decrease of the entropy of the liquid phase in agreement with the observed lower enthalpy of melting. The obtained results, and especially their comparative analysis, are in full agreement with the rationalization based in the higher interaction potential: higher heat capacities, higher viscosities, higher refractive indices, thermal behavior and the bulk structuration derived from the surface tension results.

Supplementary Material

Supporting Information Available

This information is available free of charge via the Internet at http://pubs.acs.org.