Thermophysical properties of two ammonium-based protic ionic liquids

Abstract

Experimental data for density, viscosity, refractive index and surface tension are reported, for the first time, in the temperature range between 288.15 K and 353.15 K and at atmospheric pressure for two protic ionic liquids, namely 2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate, [N_11{2(N11)}H][CH₃CO₂], and N-ethyl-N,N-dimethylammonium phenylacetate, [N_112H][C₇H₇CO₂]. The effect of the anion aromaticity and the cation’s aliphatic tails on the studied properties is discussed. From the measured properties temperature dependency the derived properties, such as the isobaric thermal expansion coefficient, the surface entropy and enthalpy, and the critical temperature, were estimated.

1. Introduction

Ionic liquids (ILs) are an exciting class of solvents, receiving enormous attention in recent years as potential replacements for conventional organic solvents in a wide range of applications. They are defined as salts with a melting point lower than 100^°C [1] and have emerged as unique and versatile solvents and materials with unique properties. In contrast to conventional organic solvents, ILs usually have extremely low volatility, good thermal and chemical stabilities, a large liquidus temperature range and a very good solvation ability for a wide variety of compounds [1]. ILs have been seen as “designer solvents” or “task-specific fluids” due to the fact that different combinations of their ions and the introduction of specific and functionalized groups lead to significant changes in their thermophysical properties and phase behaviour, adapting them to use in specific applications. This feature created a tremendous potential of ILs as alternatives for conventional solvents in catalysis, [2, 3] electrochemistry, [4, 5] organic reactivity, [6] biocatalysis and enzymes, [7, 8] fuel cells [9], batteries, [10] sensors [11] and solar cells [12].

ILs can be divided into two main families, viz. aprotic ionic liquids (AILs) and protic ionic liquids (PILs). AILs can be synthesized by transferring any group other than a proton to a basic site on the basic parent molecule. PILs are complex liquids, formed by proton transfer from a Brønsted acid to a Brønsted base and their main difference, compared to AILs, is the presence of at least a proton which is/are able to endorse extensive hydrogen bonding [13]. Greaves et al. [14] discussed the known range of PILs, including their reported physicochemical properties and the applications where they have been used. To date, AILs have received more attention than PILs but, recently, there has been an increasing interest in PILs due to their highly mobile proton and its impact on the common tailored properties. The emphasis on PILs lies in their low cost, simplicity of synthesis and variety of applications of this new family of ILs [14–18]. Moreover, this kind of ionic liquids seem to yield low toxicities [19]. Their protic nature is determinant in a number of uses including biological applications [20], organic synthesis [21–23], chromatography [24], as electrolytes for polymer membrane fuel cells [25], as reactants in biodiesel production [26], and as propellant or explosives [27, 28]. Consequently, available physicochemical properties or thermodynamic databases of PILs can be of technological and/or theoretical interest. Experimental studies for thermophysical properties of ILs have been largely reported for imidazolium-, pyridinium-, piperidinium-, pyrrolidinium-, sulfonium- and for some ammonium-based ILs which are based in primary ammine cations of the form R₄N⁺. Recently, the general properties of ammonium-based PILs have been the subject of some structure-property relationship studies [14, 17, 29–41].

The objective of this work is to investigate and thus provide an insight on the cation and anion effects through the measured and derived thermophysical properties of PILs, and to study the relationship between the measured properties and their ionic structures aiming at establishing fundamental principles for the molecular design of ILs. In this work, density, viscosity, refractive index and surface tension data of two PILs, namely 2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate [N_11{2(N11)}H][CH₃CO₂] and N-ethyl-N,N-dimethylammonium phenylacetate [N_112H][C₇H₇CO₂] were measured as a function of temperature in the temperature range between 283.15 K and 353.15 K and at atmospheric pressure. Additional properties, such as the isobaric thermal expansion coefficient, the surface thermodynamics properties, and critical temperature of both PILs were also estimated.

2. Materials and Methods

2.1. Materials

Two PILs were studied in this work, namely 2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate, [N_11{2(N11)}H][CH₃CO₂] (CAS Registry No. CS-0463-HP-0100), and N-ethyl-N,N-dimethylammonium phenylacetate, [N_112H][C₇H₇CO₂] (CAS Registry No. CS-0466-HP-0100). These PILs were acquired from IoLiTec with mass fraction purities higher than 98%. The ionic structures and description of the studied PILs are presented in Table 1.

Table 1.

Ionic structure, compound description, molecular weight, and water content of the studied ILs.

IL	Ionic structure
2-(dimethylamino)-N,N-dimethylethan-1-ammonium acetate [N11{2(N11)}H][CH3CO2] (176.3 g·mol−1; water wt % = 0.0678%)
N-ethyl-N,N-dimethylammonium phenylacetate [N112H][C7H7CO2] (207.3 g·mol−1; water wt % = 0.0278%)

The [N_11{2(N11)}H][CH₃CO₂] was distilled at room temperature, under constant stirring and high vacuum (≈ 10⁻¹ Pa). The initial fraction, circa to 5 mL, was discarded in order to remove water and solvents derived from the synthesis of the IL. The remaining IL was further distilled and the distillate used. Before each measurements the IL was further distilled (discarding small initial fractions) to remove traces of water adsorbed during the IL manipulation. The [N_112H][C₇H₇CO₂] was dried at moderate temperature (≈ 313 K), vacuum (≈ 10⁻¹ Pa) and under continuous stirring, for a minimum period of 48 h in order to remove traces of water and volatile compounds.

The purity of each IL was checked by ¹H and ¹³C NMR, both before and after the measurements, to assure that no degradation occurred. The final IL water content, after the drying step and immediately before the measurements, was determined with a Metrohm 831 Karl Fischer coulometer (using the Hydranal - Coulomat AG from Riedel-de Haën as analyte). The average water content, the molecular weight and the mass purity of each PIL are presented in Table 1.

2.2. Experimental Section

2.2.1. Density and Viscosity

Density (ρ) and dynamic viscosity (η) measurements were carried out using an automated SVM3000 Anton Paar rotational Stabinger viscometer-densimeter in the temperature range from 283.15 K to 353.15 K and at atmospheric pressure (≈ 0.1 MPa). The absolute uncertainty in density is ±5 × 10⁻⁴ g·cm⁻³, and the relative uncertainty in dynamic viscosity is ±1%. The relative uncertainty in temperature is within ±0.02 K. Further details regarding the use of the equipment and methodologies for the determination of densities and viscosities can be found elsewhere [42, 43].

2.2.2. Refractive Index

Measurements of refractive index (n_D) were performed at 589.3 nm using an automated Abbemat 500 Anton Paar refractometer. Refractive index measurements were carried out in the temperature range from 283.15 K to 353.15 K and at atmospheric pressure. The Abbemat 500 Anton Paar refractometer uses reflected light to measure the refractive index, where the sample on the top of the measuring prism is irradiated from different angles by a light-emitting diode (LED). The maximum deviation in temperature is ±0.05 K, and the maximum uncertainty in the refractive index measurements is ±2·10⁻⁴ n_D. The capability of the equipment to determine accurate refractive indices of ILs has been previously verified [36, 44–46].

2.2.3. Surface Tension

The surface tensions of each IL were determined through the analysis of the shape of a pendant drop using a Data Physics OCA-20 (Data Physics Instruments GmbH, Germany). Pendant drops were created using a Hamilton DS 500/GT syringe connected to a Teflon coated needle placed inside an aluminium air chamber capable of maintaining the temperature within ±0.1 K. The temperature was attained by circulating water in the double jacketed aluminium cell by means of a Julabo F-25 water bath. The temperature inside the aluminium chamber was measured with a Pt100 which is at a distance of approximately 2 cm of the drop. The surface tension measurements were performed in the temperature range from 293 K to 344 K and at atmospheric pressure. After reaching a specific temperature, the drop was formed and the measurements were carried out after 30 min to guarantee the thermal equilibrium. Silica gel was kept inside the air chamber to maintain a dry atmosphere. For the surface tension determination, at each temperature and for each IL, at least three drops were formed and analyzed. For each drop an average of 100 images were additionally captured. The analysis of the drop shape was achieved with the software module SCA 20. The IL density data required to the surface tension determination were those determined in this work. The equipment was previously validated through the measurement of the surface tension of ultra-pure water, n-decane and n-dodecane, as well as for a large number of ILs and ILs families [45–47].

3. Results and discussion

3.1. Density

The experimental density values of the two studied PILs are reported in Table 2 and depicted in Fig. 1. To the best of our knowledge no literature data are available. As commonly observed, the alkyl chains size and an increase on the IL asymmetry lead to weaker intermolecular interactions and a more bulk entanglement and thus to lower densities displayed by [N_11{2(N11)}H][CH₃CO₂] [36, 46, 48–51]. Furthermore, the presence of an aromatic ring is known to lead to higher intermolecular interactions, and thus to the higher densities of [N_112H][C₇H₇CO₂] compared to the [N_112H][CH₃CO₂], previously measured [36], or the [N_11{2(N11)}H][CH₃CO₂]. Nevertheless, it should be remarked that the lack of data for other anions precludes more conclusive analysis.

Table 2.

Densities (ρ), molar volumes (V_m), viscosities (η) and refractive index values (n_D) of the studied PILs as a function of temperature and at atmospheric pressure.

[N11{2(N11)}H][CH3CO2]					[N112H][C7H7CO2]
T /K	ρ /kg·m−3	106Vm /m3·mol−1	η /mPa·s	n D	ρ /kg·m−3	106Vm /m3·mol−1	η /mPa·s	n D
283.15	1017.9	173.2	64.8	1.44000	1109.9	188.6	413.1	1.53585
288.15	1013.5	173.9	46.3	1.43807	1106.1	189.2	269.6	1.53415
293.15	1009.1	174.7	34.0	1.43612	1102.5	189.8	185.3	1.53239
298.15	1004.7	175.4	25.9	1.43401	1098.9	190.4	130.8	1.53064
303.15	1000.4	176.2	20.2	1.43222	1095.4	191.1	96.00	1.52889
308.15	996.3	176.9	16.0	1.43024	1091.8	191.7	72.45	1.52707
313.15	991.9	177.7	12.9	1.42834	1088.3	192.3	56.16	1.52530
318.15	987.6	178.5	10.7	1.42642	1084.8	192.9	44.27	1.52340
323.15	983.3	179.3	8.92	1.42443	1081.3	193.5	35.62	1.52153
328.15	978.9	180.1	7.54	1.42250	1077.8	194.2	29.13	1.51965
333.15	974.6	180.9	6.44	1.42055	1074.3	194.8	24.22	1.51774
338.15	970.2	181.7	5.58	1.41861	1070.8	195.4	20.30	1.51590
343.15	965.9	182.5	4.86	1.39616	1067.3	196.1	17.25	1.51410
348.15	961.6	183.3	4.28	1.39486	1063.8	196.7	14.80	1.51216
353.15	957.2	184.1	3.78	1.39358	1060.4	195.5	12.85	1.51032

^aStandard temperature uncertainty is u (T) = ±0.02 K and the combined expanded uncertainties, U are U (ρ) = ±0.5 kg·m⁻³; U (η) = 0.1%; U (n_D) = 2×10⁻⁴ with 95% confidence level.

Fig. 1.

Density as function of temperature for the studied PILs. The dotted lines represent the linear fit to the experimental data.

Molar volumes (V_m) were calculated as function of temperature and up to 353.15 K and are reported in Table 2. As commonly observed, the molar volumes increase with the temperature increase and follow the opposite trend of the densities, with the [N_11{2(N11)}H][CH₃CO₂] presenting lower molar volumes than [N_112H][C₇H₇CO₂] due to the larger volume of phenylacetate (1133.79 nm³) when compared with acetate (488.49 nm³), as obtained from COSMO-RS [52, 53].

The isobaric thermal expansion coefficients (α_p), which consider the volumetric changes with temperature, were estimated from the density linear dependency with temperature using the following equation:

$${\alpha }_p=-\frac{1}{\rho }{\left(\frac{\partial \rho }{\partial T}\right)}_p=-{\left(\frac{\partial \ln \rho }{\partial T}\right)}_p$$ (1)

where ρ is the density in kg·m⁻³, T the temperature in K, and p the pressure in MPa.

In Table 3, α_p is presented for the studied PILs. The value of α_p for [N_112H][C₇H₇CO₂] (6.499 ± 0.001) × 10⁻⁴ K⁻¹ and [N_112H][CH₃CO₂] (7.7 × 10⁻⁴ K⁻¹) [36] are lower than that of [N_11{2(N11)}H][CH₃CO₂] (8.762 ± 0.001) × 10⁻⁴ K⁻¹, denoting the lower thermal expansion of the smaller alkyl chains ILs, compared to those with longer chains, as commonly observed. Furthermore, if one compares the thermal expansion coefficient of [N_112H][C₇H₇CO₂] and [N_112H][CH₃CO₂] [36], it is shown that the phenylacetate IL presents lower thermal expansion.

Table 3.

Coefficients of thermal expansion (α_p), refractive indices (n_D), isotropic polarizabilities, derived molar refractions (R_m) and free volumes (f_m), at 298.15 K, surface entropy (S^γ), surface enthalpy (H^γ) and estimated critical temperatures using both Eötvös (T_c)_Eot [68] and Guggenheim (T_c)_Gug [69] empirical equations for the studied ILs.

	[N11{2(N11)}H][CH3CO2]	[N112H][C7H7CO2]
104·(αp ± σ)a /K−1	8.762 ± 0.001	6.499 ± 0.001
n D	1.43401	1.53064
Polarizability /bohr3	122.22	157.54
Rm /cm3·mol−1	45.69	58.89
fm /cm3·mol−1	129.75	131.56
105· (Sγ ± σ)a /J·m−2·K−1	10.7 ± 0.33	11.0 ± 0.24
102· (Hγ ± σ)a /J·m−2	6.52 ± 0.10	7.61 ± 0.08
(Tc)Eot /K	673 ± 12	773 ± 11
(Tc)Gug /K	669 ± 14	762 ± 13

^aExpanded uncertainty with an approximately 95% level of confidence.

3.2. Viscosity

The viscosity is an important macroscopic property of ILs, which is the consequence of several microscopic interactions such as columbic, van der Waals and hydrogen bonding, and is quite dependent upon the shape and size of the constituent ions. It affects ionic conductivity and mass transport phenomena, thereby restricting their suitability for particular applications. The experimental viscosities of the PILs from 283.15 K to 353.15 K are shown in Fig. 2 and summarized in Table 2. As commonly observed in the literature, and as discussed in the density section, the size of the alkyl chains and the IL asymmetry leads to more entanglement [36, 45, 46, 48–51], that by its turn, leads to a higher resistance to shear stress and therefore to higher viscosities, as displayed by [N_11{2(N11)}H][CH₃CO₂] and [N_112H][CH₃CO₂] [36]. Furthermore, the presence of an aromatic ring, known to lead to higher intermolecular interactions, also contributes to the higher viscosities displayed by the [N_112H][C₇H₇CO₂] when compared to [N_11{2(N11)}H][CH₃CO₂] and [N_112H][CH₃CO₂] [36]. The [N_112H][C₇H₇CO₂]viscosity is one order of magnitude higher than that obtained for the [N_112H][CH₃CO₂] previously measured by us [36].

Fig. 2.

Viscosity data for the studied ILs. The solid lines represent the Vogel-Fulcher-Tammann (VFT) group contribution correlation [54].

The description of viscosities for the pure PILs was fitted using the group contribution method, based on the Vogel-Fulcher-Tammann (VFT) correlation [54], represented by the following equation:

$$\ln \eta =A_{\eta }+\frac{B_{\eta }}{(T-T_{0\eta })}$$ (2)

where η is the dynamic viscosity in mPa·s, T the temperature in K, and A_η , B_η and T_0η are adjustable parameters.

The parameters A_η , B_η and T_0η were determined from the fitting of the experimental data and are presented in Table 4. The fitting of the data against the experimental results is depicted in Fig. 2 and presents and average absolute relative deviations, between the experimental and the fitting data, of 0.15% for [N_11{2(N11)}H][CH₃CO₂] and 0.18% for [N_112H][C₇H₇CO₂].

Table 4.

Vogel-Fulcher-Tammann correlation parameters, A_η, B_η and T_0η, for the viscosity of the studied ILs.

IL	Aη	Bη /K	T0η /K
[N11{2(N11)}H][CH3CO2]	−2.4822	626.40	188.99
[N112H][C7H7CO2]	−1.7662	679.41	195.88

3.3. Refractive Index

Experimental refractive indexes for the studied compounds are presented in Table 2 and depicted in Fig. 3. The temperature interval was scanned upward and downward and no temperature hysteresis effects were observed. For the two PILs under study, the refractive index decreases with an increase in temperature. Furthermore, the refractive index values for the studied PILs increase in the following sequence: [N_112H][CH₃CO₂] [36] < [N_11{2(N11)}H][CH₃CO₂] < [N_112H][C₇H₇CO₂].

Fig. 3.

Refractive index data for the studied PILs. The solid lines represent the Gardas and Coutinho group contribution method [54].

The refractive index for the PILs studied were also fitted with the group contribution method proposed by Gardas and Coutinho [54], as depicted in Fig. 3, which follows a linear temperature dependency of the form,

$$n_{\mathrm{D}}=A_{n_{\mathrm{D}}}-B_{n_{\mathrm{D}}}T$$ (3)

$$A_{n_{\mathrm{D}}}=\sum _{i=1}^k{n}_i{a}_{i,n_{\mathrm{D}}}$$ (4)

$$B_{n_{\mathrm{D}}}=\sum _{i=1}^k{n}_i{b}_{i,n_{\mathrm{D}}}$$ (5)

where n_i is the number of groups of type i and k is the total number of different groups in the molecule. The estimated parameters a_i,nD and b_i,nD for the studied ILs are given in Table 5. The average absolute relative deviation between the experimental and the fitting are 0.0043% for [N_11{2(N11)}H][CH₃CO₂] and 0.0062% for [N_112H][C₇H₇CO₂].

Table 5.

Group contribution parameters, a_i,nD and b_i,nD, determined using the Gardas and Coutinho group contribution method for the refractive index [54]

Ionic Species	a i,nD	104·bi,nD /K−1
	Cation
[N11{2(N11)}H]+	1.4101	3.28
[N112H]+ [36]	1.3792	2.78
	Anion
[CH3CO2]− [36]	0.1387	0.566
[C7H7CO2]−	0.2611	0.902

The derived molar refractions (R_m), free volumes (f_m), and polarizabilities were additionally determined as follows [55–57]:

$$\frac{{\alpha }_0}{4\pi {\epsilon }_0}=\left(\frac{{n_{\mathrm{D}}}^2-1}{{n_{\mathrm{D}}}^2+2}\right)\frac{3M}{4\pi \rho N_{\mathrm{A}}}$$ (6)

$$R_{\mathrm{m}}=\frac{N_{\mathrm{A}}{\alpha }_0}{3{\epsilon }_0}=\frac{{n_{\mathrm{D}}}^2-1}{{n_{\mathrm{D}}}^2+2}\times V_{\mathrm{m}}$$ (7)

$$f_{\mathrm{m}}=V_{\mathrm{m}}-R_{\mathrm{m}}$$ (8)

where α₀ the electronic polarizability, ε₀ the vacuum permittivity, ρ the compound density, M the molecular weight and N_A the Avogadro number. The obtained values for these properties are reported in Table 3. As for the molar volume, density and refractive index, the [N_112H][C₇H₇CO₂] also presents higher free volumes, molar refractions and polarizabilities than the [N_11{2(N11)}H][CH₃CO₂].

Recently, based on the premise that refractive indices are an indication of the dielectric response to an electrical field, induced by electromagnetic waves, and that refractive indices can be considered as the first order approximation response to electronic polarization within an instantaneous time scale, Seki et al. [58] evaluated the refractive indices of 17 ILs, as a function of temperature, against theoretical polarizabilities obtained through ab initio calculations and proposed a correlation between the refractive index and the polarizability normalized in terms of the molecular volume. Following the work of Seki et al. [58] the correlation was previously extended [45, 46, 59] for ammonium-, sulfonium-, phosphonium-, piperidinium-, and pyridinium-based ILs and now to the ILs studied here, showing that this type of correlation stands as a simpler approach to determine the refractive index from polarizability and vice-versa, within the uncertainty of the correlation, as depicted in Fig. 4.

Fig. 4.

Relationship between the refractive index and polarizability/molar volume for the two PILs at 303.15 K. The unfilled triangles and the solid line represent the experimental data and the correlation of Seki et al. [58]. The unfilled squares, diamonds and circles represent the experimental data reported in previous works [45, 46, 59].

3.4. Surface Tension

The experimental surface tension data was determined at atmospheric pressure in the temperature range of 293 K to 343 K for the [N_112H][C₇H₇CO₂] and 293 K to 323 K for the [N_11{2(N11)}H][CH₃CO₂], as presented in Table 6 and depicted in Fig. 5. As discussed above, the volatility of [N_11{2(N11)}H][CH₃CO₂] makes the surface tension determination, above 323 K, highly unstable, making the methodology and equipment used not adequate to accurately determine the compound’s surface tension. The [N_112H][C₇H₇CO₂] presents higher surface tensions than the [N_11{2(N11)}H][CH₃CO₂], indicating that despite the important role displayed by the cation through the air–liquid interface structural organization [60, 61], the anion nature and their impact on the IL structure and surface organization also presents a relevant role. The higher intermolecular interactions of the phenylacetate anion together with the smaller [N_112H]⁺ cation leads to a more organized arrangement and higher surface energy when compared with the [N_11{2(N11)}H][CH₃CO₂].

Table 6.

Surface tension (γ), for the studied ILs as function of temperature and at atmospheric pressure.

[N11{2(N11)}H][CH3CO2]		[N112H][C7H7CO2]
T /K	γ /mN·m−1	T /K	γ /mN·m−1
293.3	33.6	293.2	43.6
298.3	33.3	303.2	42.7
303.2	32.6	313.1	41.6
313.2	31.6	323.1	40.5
323.3	30.4	332.9	39.4
		343.0	38.2

^aStandard uncertainties u are u (T) = ±0.1 K and the combined expanded uncertainty U_c is U_c (γ) = 0.1 mN·m⁻¹, with an expanded uncertainty at 95% confidence level.

Fig. 5.

Surface tension data for the studied PILs as a function of temperature. The dotted lines represent the linear fit to the experimental data.

The surface thermodynamic properties, namely, the surface entropy and the surface enthalpy, were derived using the quasi linear dependence (with an uncertainty of ±0.1 mN·m⁻¹) of the surface tension with temperature.

The surface entropy (S^γ), can be calculated according to the following equation [62, 63],

$$S^{\gamma }=-\left(\frac{\mathrm{d}\gamma }{\mathrm{d}T}\right)$$ (9)

while the surface enthalpy, H^γ , according to [62, 63],

$$H^{\gamma }=\gamma -T\left(\frac{\mathrm{d}\gamma }{\mathrm{d}T}\right)$$ (10)

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

The values of the thermodynamic functions for all of the ILs studied and the respective expanded uncertainties, derived from the slope of the curve γ = f(T) in combination with the law of propagation of uncertainties, are presented in Table 3 [64].

The PILs display higher surface entropies than the aprotic ILs, suggesting a less structured liquid phase, in agreement with their less prevalent hydrogen bonding due to their iconicity and proton transfer ability. [36, 65]

The critical temperature of fluids is a significant thermophysical value regularly used in corresponding state correlations involving equilibrium and transport properties [66]. However, the direct determination of the critical temperatures of ILs is not feasible owing to the intrinsic nature of PILs together with negligible vapor pressures and relatively low decomposition temperatures. Rebelo et al. [67] proposed the use of the Eötvös [68] and Guggenheim [69] equations to estimate the hypothetical critical temperature of ILs, as described below,

$$\gamma {\left(\frac{M}{\rho }\right)}^{2∕3}=K_{\mathrm{Eot}}(T_c-T)$$ (11)

$$\gamma =K_{\mathrm{Gug}}{\left(1-\frac{T}{T_c}\right)}^{11∕9}$$ (12)

where T_c is the critical temperature, M the molecular weight, ρ the density, K_Eot and K_Gug are fitted parameters. T_c, K_Eot and K_Gug were determined by fitting Equations 11 and 12 to the experimental data. Both equations are based on the fact that the surface tension becomes null at the critical point and although an overestimation of the critical temperature is expected, since at the critical point the pressure becomes the critical pressure, these equations provide reasonable estimations [66, 70]. The critical temperature values estimated from the surface tension data are summarized in Table 3. In agreement with the results obtained for other PILs studied in our previous work [36] the compounds here reported also present significantly lower critical temperatures than the aprotic ionic liquids [45, 46, 60, 61].

4. Conclusions

Data for the density, viscosity, refractive index and surface tension for two ammonium-based PILs, [N_11{2(N11)}H][CH₃CO₂] and [N_112H][C₇H₇CO₂], were measured in the temperature range between 283.15 K and 353.15 K and at atmospheric pressure and are here reported for the first time. Group contribution methods proposed by Gardas and Coutinho and the VFT were used to fit the refractive index and viscosity of the investigated ILs. The effect of the anion aromaticity and of the cation’s aliphatic tails size on the studied properties was discussed. The presence of an aromatic ring contributes to a more organized and compact bulk distribution, resulting from higher intermolecular interactions, and thus exhibiting high density for [N_112H][C₇H₇CO₂]. This factor also leads to higher resistance to shear stress and therefore, higher viscosities of [N_112H][C₇H₇CO₂], compared to the [N_11{2(N11)}H][CH₃CO₂]. The higher intermolecular interactions of the phenylacetate anion together with the smaller [N_112H]⁺ cation leads to a more organized arrangement and higher surface energy, compared with the [N_11{2(N11)}H][CH₃CO₂] thus, presenting higher surface tensions for the former.