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Protonic Ammonium Nitrate Ionic Liquids and Their Mixtures: Insights into Their Thermophysical Behavior José N. Canongia Lopes,†,‡ José M. S. S. Esperança,† André Maõ de Ferro,† Ana B. Pereiro,† Natalia V. Plechkova,§ Luis P. N. Rebelo,*,† Kenneth R. Seddon,*,†,§ and Isabel Vázquez-Fernández§ †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, 2780-157, Oeiras, Portugal Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal § The QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, U.K. ‡

S Supporting Information *

ABSTRACT: This study is centered on the thermophysical characterization of different families of alkylammonium nitrate ionic liquids and their binary mixtures, namely the determination at atmospheric pressure of densities, electric conductivities and viscosities in the 288.15 < T/K < 353.15 range. First, measurements focusing on ethylammonium, propylammonium and butylammonium nitrate systems, and their binary mixtures, were determined. These were followed by studies involving binary mixtures composed of ethylammonium nitrate (with three hydrogen bond donor groups) and different homologous ionic liquids with differing numbers of hydrogen bond donor groups: diethylammonium nitrate (two hydrogen bond donors), triethylammonium nitrate (one hydrogen bond donor) and tetraethylammonium nitrate (no hydrogen bond donors). Finally, the behavior of mixtures with different numbers of equivalent carbon atoms in the alkylammonium cations was analyzed. The results show a quasi-ideal behavior for all monoalkylammonium nitrate mixtures. In contrast, the other mixtures show deviations from ideality, namely when the difference in the number of carbon atoms present in the cations increases or the number of hydrogen bond donors present in the cation decreases. Overall, the results clearly show that, besides the length and distribution of alkyl chains present in a cation such as alkylammonium, there are other structural and interaction parameters that influence the thermophysical properties of both pure compounds and their mixtures.

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INTRODUCTION

Protonic ionic liquids are usually obtained by the transfer of a proton from a Brønsted acid, HA, to a Brønsted base, B, eq 1.

The first reported ionic liquids, ethanolammonium nitrate (1888)1 and ethylammonium nitrate (1914),2 were both protonic3 salts. Although these can be considered as the forerunners of modern ionic liquids, aprotonic ionic liquids have received much more attention.4 However, there has been a recent surge in interest in protonic ionic liquids due to their excellent conductivity, viscosity, and electrochemical properties.5 Indeed, the first publicised industrial ionic liquid application, the BASIL process, produces a protonic ionic liquid, 1-methylimidazolium chloride, [Hmim]Cl.6 An attractive feature of protonic ionic liquids is that they are very accessible, both synthetically and computationally, but, to date, studies have tended to focus on individual salts, where hydrogen bonding is well recognized.7 Binary mixtures of these materials8 are a more complex issue, and are little studied in the literature.9 Here, we report a detailed study of the physicochemical properties of a set of related simple protonic alkylammonium ionic liquids (siblings of the first room temperature ionic liquid, ethylammonium nitrate)10 and their binary mixtures. © XXXX American Chemical Society

HA + B ⇌ [BH]+ + A−

(1)

They have very different properties from their aprotonic analogues due to the presence of the exchangeable proton. As ionic liquids are entirely comprised from ions, in principle, the equilibrium in eq 1 must lie substantially toward the ionised forms in order for this salt to be considered a pure ionic liquid. It was discussed in the literature that the equilibrium should lie at least 99% toward the ionic species for the mixture to be considered a “‘pure’” ionic liquid.11 The presence of more than 1% of the neutral acid and base species would be considered a significant impurity. However, the degree to which the proton transfer occurs in eq 1 has not been quantified. Protonic ionic liquids are generally considered as a welldefined ionic liquid subclass due to the existence of one or more Received: December 5, 2015 Revised: January 15, 2016

A

DOI: 10.1021/acs.jpcb.5b11900 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Structure, name and acronym of (a) pure monoalkylammonium nitrate ionic liquids and their mixtures, and (b) pure di-, tri, and tetraethylammonium nitrate and their mixtures with ethylammonium nitrate.

Two main aspects of binary systems have been tackled here. First, the thermophysical properties of monoalkylammonium nitrate ionic liquids and their mixtures with three hydrogen bond donor groups in the cation, viz: ethylammonium nitrate ([N0 0 0 2][NO3]), propylammonium nitrate ([N0 0 0 3][NO3]), and butylammonium nitrate ([N0 0 0 4][NO3]), cf. Figure 1(a), have been determined. In order to study possible deviations from ideality, the thermophysical behaviors (namely, density, ionic conductivity and dynamic viscosity) of selected mixtures have been compared with systems with equivalent numbers of carbon atoms in the cation (henceforth simply referred as composite cations, and denoted by enclosure in curly brackets, {...}): {[N0 0 0 2]0.75[N0 0 0 4]0.25}[NO3] and {[N0 0 0 2]0.5[N0 0 0 3]0.5}[NO3] (2.5 equiv carbons); {[N0 0 0 2]0.25[N0 0 0 4]0.75}[NO3], {[N0 0 0 3]0.5[N0 0 0 4]0.5}[NO3] (3.5 equiv carbons). In addition, the mixture {[N0 0 0 2]0.5[N0 0 0 4]0.5}[NO3] (3 equiv carbons) was also studied and compared with the pure [N0 0 0 3][NO3]. Second, homologous binary mixtures based on [N0 0 0 2][NO3] have been studied, in which it is combined with similar cations with different numbers of hydrogen-bond donor groups, cf. Figure 1(b): diethylammonium nitrate, [N0 0 2 2][NO3] (two hydrogen-bond donors); triethylammonium nitrate, [N0 2 2 2][NO3] (one hydrogen-bond donor); and tetraethylammonium nitrate, [N2 2 2 2][NO3] (no hydrogen bond donors). The latter salt is an aprotonic ionic liquid, and was studied for comparison purposes. The thermophysical responses of these systems were measured over a relatively large temperature range. The results were correlated to the overall ionicity of the systems, using representations of the corresponding Walden plots.2,5a,17 Since the binary mixtures of ionic liquids studied here have three constituents (each ionic liquid has a specific alkylammonium cation, but all have the nitrate anion in common), the adopted nomenclature is based on the number of constituents: {[A]x[B](1‑x)}[Y].8 In these examples, x is the mole fraction of

labile hydrogen atoms in the cationic species. Thus, strong hydrogen bonding, together with Coulombic and van der Waals interactions, are regarded as key forces that control many of their physicochemical properties. Indeed, the hydrogen bonding rôle in protonic ionic liquids has been frequently addressed.12 In a study of protonic monoalkylammonium ionic liquids, Atkin and co-workers suggested a clear relationship between hydrogen bond strength and several physical properties. They claimed that when the hydrogen bond is enhanced, glass transitions, melting points, and viscosity increase while ionic conductivity decreases. This work also suggested that protonic ionic liquids containing strong linear hydrogen bonds have more “solid-like” behavior, whereas those containing weaker, bent hydrogen bonds present a more “liquid-like” character.12b Despite the fact that the behavior of individual protonic ionic liquids have been studied thoroughly,13 further research on this topic is still necessary. Oversimplifications and generalizations of the rôle of the hydrogen bond in ionic liquids should be avoided.14 Even though conventionally strong hydrogen bonding leads to strong lattice, it has been emphasized that, in specific cases, the elimination of a hydrogen bond leads to an increase in the viscosity and melting point temperatures of protonic ionic liquids instead of a decrease.15 These intriguing observations launched a vigorous debate, and motivated several authors to question the impact of hydrogen bonding in respect to other intermolecular interactions.15b,16 The fundamental motivation of this current work is to deepen our understanding of the behavior of alkylammonium nitrates and their mixtures, including some long-known protonic ionic liquids. Throughout this manuscript, [Nw x y z]+ represents a substituted ammonium cation, using a long-established convention for quaternary alkylammonium salts: w, x, y, and z represent the number of carbon atoms for each of the alkyl chains; a subscript 0 thus represents a proton. Hence, [N0 0 0 2]+ represents the ethylammonium cation, [NEtH3]+. B


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The Journal of Physical Chemistry B the salt [A][Y] in the mixture, while (1 − x) is the corresponding mole fraction of [B][Y]. It should be noted that the reason binary mixtures of alkylammonium nitrate ionic liquids were studied, as opposed to the conventional imidazolium ionic liquids, is that alkylammonium ionic liquids form a simpler computational model, with a smaller number of forces in the system, for future study.

viscosity effect using the internal calibration of the densimeter. The repeatability of this densimeter is known to be very good (±1 μg cm−3). However, taking into account sample handling, the repeatability here is nearly 500 μg cm−3, mainly due to their high hygroscopicity. Measurements of dynamic viscosity were performed at atmospheric pressure using an automated SVM 3000 Anton Paar rotational Stabinger viscometer-densimeter. In this equipment, the precision of the dynamic viscosity measurements is ±0.5%. However, it is known that small amounts of water result in a significant decrease in the viscosity of ionic liquids.18 Using different batches, deviations smaller than 2% were obtained for the large majority of samples, but some exceptional cases suggest a higher lack of precision (±6%). It is important to stress that, for each sample, the viscosity measurements were repeated three times, and the data here always agreed within a 1% deviation. Ionic conductivity measurements of the ionic liquid systems were performed at atmospheric pressure using a CDM210 Radiometer Analytical conductivity meter. The measurements were carried out using a jacketed glass cell with a volume of 1.5 cm3 containing a magnetic stirrer. A water bath, controlled to ±0.01 °C, was used to thermostat the cell, whose temperature was measured by means of a platinum resistance thermometer coupled to a previously calibrated Keithley 199 System DMM/Scanner. This conductivity meter uses an alternating current of 12 V and a frequency of 2.93 kHz for the range of conductivities measured. The use of highfrequency (above 600 Hz) alternating currents, and the fact that the electrodes are platinum coated, avoids polarization phenomena at the surface of the cell electrodes. The conductivity meter was previously calibrated at each temperature with certified 0.01, 0.1, and 1 D aqueous KCl standard solutions supplied by Radiometer Analytical.19 This calibration was also checked using different aprotonic ionic liquids.20 The repeatability of measurements is estimated to be less than 1%. Using different batches, the uncertainty of the ionic conductivity measurements ranged within this value for the vast majority of samples, however we obtained a deviation slightly higher for some measurements (±6%). In addition, NMR spectra were recorded at ambient temperature on Bruker Avance spectrometer (300 MHz) Bruker-Spectrospin 300. 1H and 13C NMR spectra were referenced with respect to tetramethylsilane. The melting points and phase transitions of the salts were measured by DSC, using a TA Instruments Modulated DSC Q 2000 V24.4 Build 116 with refrigerated cooling system RCS 90 capable of controlling the temperature down to 220 K. Dry dinitrogen gas was purged through the DSC cell with a flow rate of ca. 20 cm3 min−1. All samples, due to their hygroscopic character, were prepared in a glovebox, using Tzero hermetic platinum pans to avoid any contact with air and measured under the same conditions (standard heating and cooling ramp of 5 K min-1 over five cycles). Water content was determined using a Cou-Lo Compact Karl Fischer titrator, and titration solutions from Riedel-de-Haën. Ionic liquids were tested immediately after the freeze-drying cycle. The method, first proposed by Karl Fischer in 1935 based on water titration by coulometry. The water reacts stoicheiometrically with iodine generated in situ by oxidation of iodide. As soon as no water is reacting, the excess of iodine is detected by the electrode and the reaction is finished. The measured amount of water is calculated from the amount of electrolysis current produced to in order to generate the required amount of iodine.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Pure Compounds. Amines were sourced from Alfa Aesar, Aldrich, or Fluka. Ethylammonium nitrate ([N0 0 0 2][NO3]; colorless liquid; mp(DSC) 283 K; H2O < 1200 ppm), propylammonium nitrate ([N0 0 0 3][NO3]; pale yellow liquid; mp(DSC) 278 K; H2O < 600 ppm), butylammonium nitrate ([N0 0 0 4][NO3]; white crystals; mp(DSC) 456 K; H2O < 200 ppm), diethylammonium nitrate ([N0 0 2 2][NO3]; pale yellow powder; mp(DSC) 369.15 K; H2O < 300 ppm), and triethylammonium nitrate ([N0 0 2 2][NO3]; white crystals; mp(DSC) 393 K; H2O < 600 ppm) were prepared by treating the frozen base (−196 °C) with a stoicheiometric amount of concentrated nitric acid (65%), allowing the mixture to warm to room temperature, with stirring, for 2 h. This procedure was necessary because the reaction is highly exothermic, see eq 2. Then, the mixture was refrozen to −196 °C and attached to a freeze drier (Alpha 1-2 LD plus), evacuated to 0.021 mbar, and continuously evacuated for between two and five days. The sample purity was checked by 1H and 13C NMR spectroscopies, and Karl Fischer titration. All the samples were free of halide. Despite the fact that these systems are regarded as highly hygroscopic, coulometric Karl Fischer titrations revealed water content between 200 and 1200 ppm. Typical yields were 92− 98%. Nx y z + HNO3 → [N0 x y z][NO3]

(2)

Tetraethylammonium nitrate ([N2 2 2 2][NO3]; white crystals; m.pt.(DSC) 513 K; H2O < 800 ppm) was prepared by an analogous procedure, in which aqueous tetraethylammonium hydroxide (35%) was treated with concentrated nitric acid, eq 3, requiring 4 days of freeze-drying. [N2222][OH] + HNO3 → [N2222][NO3] + H 2O

(3)

All the samples were stored in a glovebox. 2.2. Preparation of Binary Mixtures. Binary mixtures were prepared in a dinitrogen-filled glovebox to prevent water vapor uptake, using an analytical high-precision (±10 μg) balance, by adding known masses of the each component into stoppered bottles. The binary systems, composed of di-, tri- or tetraethylammonium nitrate mixed with ethylammonium nitrate, were prepared in two different series, where the molar concentration (x) ranged either from x = 0.05 until solidification, or from x = 0.1 until solidification, both having molar concentration increments in steps of x = 0.1. This procedure allowed us to track possible systematic errors. 2.3. Instrumental Measurements. Density was measured at atmospheric pressure with an Anton Paar DMA 5000 digital vibrating tube densimeter from 288.15 up to 353.15 K. The calibration of the densimeter was periodically checked by measuring the densities of atmospheric air and Millipore-water, according to the manufacturer recommendations. Temperature stability during measurements was always better than ±2 mK, and the reported density data undertook a correction for the C


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3. RESULTS AND DISCUSSION 3.1. Monoalkylammonium Nitrates. The density, dynamic viscosity and ionic conductivity data of pure protonic ionic liquids: [N0 0 0 2][NO3], [N0 0 0 3][NO3] and [N0 0 0 4][NO3] are depicted in Figure 2. These are similar to values previously

ln(η /mPa s) = ln(A η) +

Bη T − T0η

(5)

⎛ −Bσ ⎞ σ /mS cm−1 = Aσ exp⎜ ⎟ ⎝ T − T0σ ⎠

(6)

where A2, A1, A0, Aη, Bη, T0η, Aσ, Bσ, and T0σ are fitting parameters and T is the absolute temperature. All fitting parameters and corresponding standard deviations of each fitting are presented in Table S4 of Supporting Information. In this work, we have chosen to use a second order polynomial equation since the standard deviation of the linear fitting is greater than the repeatability of the density measurements. As expected, the lengthening of the alkyl chain of the pure compounds induces a decrease either in density or ionic conductivity, and an increase in dynamic viscosity, as can be seen in Figure 2. If one considers the number of equivalent carbons in the composite cations of the monoalkylammonium nitrate mixtures, these trends are maintained. The density values of the binary mixture [N0 0 0 2]0.5[N0 0 04]0.5[NO3] (three carbons in the composite cation) perfectly match with the ones for pure [N0 0 0 3][NO3], once the deviation between these series is below the uncertainty in the density measurements. The viscosity and conductivity values differ by 12 and 19%, respectively. This result seems to indicate a nonideal behavior of both viscosity and conductivity. It is also important to highlight that the latter result is the only one based on the direct comparison between a mixture and a pure compound. Comparing the mixtures {[N0 0 0 2]0.25[N0 0 0 4]0.75}[NO3] with {[N0 0 0 3]0.5[N0 0 0 4]0.5}[NO3] (3.5 carbons in the composite cation) and {[N0 0 0 2]0.75[N0 0 0 4]0.25}[NO3] with [N0 0 0 2]0.5[N0 0 0 3]0.5[NO3] (2.5 carbons in the composite cation) we observed a maximum deviation percentage of the experimental data of 0.1, 4.6, and 2.3% for density, viscosity and conductivity, respectively. The deviation values obtained are within the uncertainty on the properties measurements which indicate a quasi-ideal behavior of these mixtures. The isobaric thermal expansion coefficient, αp, was calculated using eq 7, which is obtained from the first derivative of eq 4: ⎛ ∂[ln ρ] ⎞ αp = − ⎜ ⎟ = −(2A 2 T + A1) ⎝ ∂T ⎠ p

Figure 2. Experimental (a1) density, ρ, (b1) dynamic viscosity, η, and (c1) ionic conductivity, σ, of pure [N0 0 0 2][NO3], [N0 0 0 3][NO3], and [N0 0 0 4][NO3] as well as their selected binary mixtures versus temperature, and (a2, b2, and c2) versus the number of carbons in the cation/composite cation at constant temperature.

The values of αp (cf. Figure 3) are within 4.8- 5.6 × 104 K−1 usual range for ionic liquids found in literature,22b and increase with the alkyl chain length of the cation or composite cation. The molar volumes (Vm) at 298.15 K of [N0 0 0 2][NO3], [N0 0 0 3][NO3], and [N0 0 0 4][NO3] and their binary mixtures are depicted in Table 1. The average increase per CH2 group in this family of ionic liquids and mixtures is 16.97 cm3 mol−1. These values are perfectly consistent with the recognized increase of 17.2 ± 0.3 cm3 mol−1 per each CH2 increment in the cation (or anion) at 298.15 K.22b The excess molar volume (VE) of the binary monoalkylammonium nitrate mixtures was calculated, eq 8, as the difference between the molar volume of the mixture and molar volume of the hypothetical ideal mixture:

reported in the literature21 (cf. Tables S1 and S3 of the Supporting Information), the absolute differences indicating a higher purity for the samples prepared for this work (as the presence of water as an impurity lowers both the density and the viscosity of an ionic liquid, the higher values represent the purest materials). Figure 2 also shows a comparison of these pure protonic ionic liquids with mixtures containing equivalent number of carbons in the cation: {[N0 0 0 2]0.75[N0 0 0 4]0.25}[NO3] and {[N0 0 0 2]0.5[N0 0 0 3]0.5}[NO3]; {[N0 0 0 2]0.25[N0 0 0 4]0.75}[NO 3 ], {[N 0 0 0 3 ] 0.5 [N 0 0 0 4 ] 0.5 }[NO 3 ] and {[N 0 0 0 2 ] 0.5 [N0 0 0 4]0.5}[NO3]. Numerical experimental data for the binary mixtures are given in Table S3 of Supporting Information. Density, ionic conductivity and dynamic viscosity data were fitted with commonly used eqs 4-6:22 ln(ρ /g cm−3) = A 2 T 2 + A1T + A 0

(7)

V E = Vmix − (V1x1 + V2x 2)

(8)

where Vmix, V1, and V2 are the molar volumes of the mixture and of the two pure components, respectively, while x1 and x2 are the corresponding molar fractions. For solids, hypothetical liquid

(4) D


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temperature: density and viscosity values decrease with temperature, while conductivity values increase. Of more interest, for a given temperature, the values of density or ionic conductivity decline when the mole ratio of [N0 0 2 2][NO3] increases in the binary mixture. In surprising contrast, the viscosity remains effectively constant when the mole fraction of [N0 0 2 2][NO3] increases, Figure 4b1. It is worth emphasizing that [N0 0 2 2][NO3] is solid at room temperature, with a melting point at 369.15 K. Hence, it was not possible to measure its density, and the direct calculation of the VE for the mixtures of {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3] is simply not feasible. To overcome this, a calculation method based on the relation of known variablesVm of [N0 0 0 2][NO3], Vm of the binary mixture (Vmix), and the corresponding molar concentration of each componentwas used. It was assumed that the VE for a given mixture can be described by a parabolic function with a downward concavity:

Figure 3. Isobaric thermal expansion coefficients (a), αP, of pure [N0 0 0 2][NO3], [N0 0 0 3][NO3] and [N0 0 0 4][NO3] as well as their selected binary mixtures versus temperature and (b) versus the number of carbons in the cation/composite cation at constant temperature.

V E ≡ Ax(1 − x)

(9)

where A is a fitting parameter. Taking into account eq 9, it is possible to rearrange eq 8, to give:

molar volumes have been used, which can be interpreted as the values of supercooled [N0 0 2 2]+, [N0 2 2 2]+, and [N2 2 2 2]+ salts. Discussions over ideal solutions in mixtures of ionic liquids and their excess volume of mixing have been presented elsewhere,9a and more recently discussed in a critical review by Welton and co-workers.8 However, to the best of our knowledge, this is the first work reporting VE values of protonic ionic liquids mixtures solely composed of ammonium cations and nitrate anions. The obtained VE values range between −0.1 < VE/cm3 mol−1 < 0.1. The almost zero VE values presented here clearly suggest a quasiideal behavior for all binary mixtures in accordance with the data for binary mixtures containing the imidazolium cation combined with different anions.9a 3.2. Binary Mixtures of [N0 0 0 2][NO3] with Di-, Tri- or Tetraethylammonium Nitrates. The experimental data for binary mixtures of ethylammonium nitrate, [N0 0 0 2][NO3], with nitrates of the analogous diethyl-, triethyl- and tetraethylammonium cations: [N0 0 2 2][NO3], [N0 2 2 2][NO3], and [N2 2 2 2][NO3], are presented in Figure 4. For greater insight, instead of plotting density, viscosity or conductivity versus temperature, they are now plotted versus the mole fraction (x) of the di-, tri- or tetraethylammonium ionic liquid, in steps of approximately 0.05 mol fraction (the upper limit being determined by solidification). The density and dynamic viscosity were measured from 288.15 to 353.15 K, while ionic conductivity was measured from 288.15 to 323.15 K. The full data sets are presented in Table S5 of Supporting Information. As seen in parts a1, b1 and c1 of Figure 4, the experimental data for {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3] follow the expected trend with

Vmix = V1x + V2(1 − x) + Ax(1 − x)

(10)

where V1 and V2 are the molar volumes of the component added to [N0 0 0 2][NO3], and the molar volume of [N0 0 0 2][NO3], respectively. By rearranging eq 10, the following expression is obtained: Vmix − V2 = (V1 − V2) + A(1 − x) x

(11)

Thus, eq 11 describes a linear function where (V1 − V2) and A are the intercept and slope, respectively, while (1 − x) is the dependent variable. The Vm values for [N0 0 2 2][NO3] can be found in Table S6 of the Supporting Information. Interestingly, the calculated values of Vm for [N0 0 2 2][NO3] are very similar to those for [N0 0 0 4][NO3] (which has the same molecular weight), having a deviation less than 0.58% over the entire temperature range. At 298.15 K, this indicates an increase of 17.02 cm3 mol−1 per each CH2 in the cation, which is in agreement with the literature value of 17.2 ± 0.3 cm3 mol−1.22b The corresponding VE values of the binary mixture {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3], also presented in Table S6 of Supporting Information, deviate from 0.12 to 0.75 cm3 mol−1 from the ideal molar volume. These values are slightly higher values than those observed for the monoalkylammonium mixtures but, still, suggest a quasi-ideal behavior. One particular aspect of the system {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3] is the formation of bubbles at high temperatures and higher values of x, which precludes the correct measurement of the physical properties under those conditions.

Table 1. Molar Volume (Vm) of Protonic Monoalkylammonium Nitrates and Their Selected Binary Mixtures at 298.15 Kb pure compounds

Vm/cm3·mol−1 number of carbons (n) ΔV m /cm3·mol−1 n−2

a

mixtures

[N0 0 0 2]

[N0 0 0 3]

[N0 0 0 4]

[N0 0 0 2]0.75[N0 0 0 4]0.25

[N0 0 0 2]0.5[N0 0 0 3]0.5

[N0 0 0 2]0.5[N0 0 0 4]0.5

[N0 0 0 2]0.25[N0 0 0 4]0.75

[N0 0 0 3]0.5[N0 0 0 4]0.5

89.16 2 −

106.04 3 16.89

123.07a 4 16.96

97.74 2.5 16.94

97.64 2.5 16.90

106.05 3 17.17

114.60 3.5 16.98

114.54 3.5 16.92

Assuming a linear extrapolated density value at 298.15 K; b

ΔV m /cm3·mol−1 n−2

gives the increase in Vm per methylene group added to the alkyl side

chain of the cation. The [NO3] anions omitted from the table heading. E


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Figure 4. Experimental density, ρ, dynamic viscosity, η, and ionic conductivity, σ, of (1) {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3], (2) {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3], and (3) {[N2 2 2 2]x[N0 0 0 2](1‑x)}[NO3]. The line of each plot is only a guide to the eye.

3.3. An Overall Picture. The experimental data for the mixtures depicted in Figure 4, clearly show that, for a given mole fraction, density, dynamic viscosity, and ionic conductivity values change more dramatically as the number of ethyl chains in the cation increases. Figure 5 illustrates the relationship between molar volume, dynamic viscosity and ionic conductivity values and the number of carbon atoms in the cation or composite cation. The molar volume behavior shows a remarkable insensitivity to the chemical composition of the cations (less than 2.5% difference for the various systems), and depends only on the total number of carbon atoms in the cation or composite cation. However, the analysis of the viscosity and ionic conductivity data is not straightforward, and the results indicate a different behavior for each system studied. The increase in viscosity as the number of carbon atoms in the cation increases is more pronounced for the monoalkylammonium nitrate (pure and binary mixtures) and for {[N2 2 2 2]x[N0 0 0 2](1‑x)}[NO3]. This result contrasts with a smaller increase for the system {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3] atoms in the cation of the ionic liquid, these results prove that there are some structural features that are playing an important rôle in the distinct behaviors of the studied systems. It has been demonstrated that excess properties are prone to increase with the difference of the aliphatic chain length between two components.23 More recently, such a trend

Like the results for {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3], those for {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3] follow the expected universal trend with temperature. In contrast, {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3], Figure 4b2, shows that the viscosity behavior of these mixtures presents a clearer dependency on x. The effect on conductivity, Figure 4b3, is also more pronounced. Using eqs 9−11, the resulting Vm values of {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3], reported in Table S6 of the Supporting Information, are lower than expected. Compared with {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3], the increase in Vm per CH2 group is only 13.4 cm3 mol−1 at 298.15 K, instead of 17.2 cm3 mol−1. Moreover, the VE presents a maximum of 2.18 cm3 mol−1, which suggests a deviation from ideality. Parts a3, b3, and c3 of Figure 4 show the experimental data for {[N2 2 2 2]x[N0 0 0 2](1‑x)}[NO3] mixtures. The effect of increasing the mole ratio of [N2 2 2 2][NO3] on the physical properties of this mixture is more pronounced than for the other two systems. Using eqs 9 to 11, the calculated value of Vm at 298.15 K is 172.7 cm3 mol−1. This value for [N2 2 2 2][NO3] represents an increase of 11.6 cm3 mol−1 per CH2 group relative to [N0 2 2 2][NO3]. The exchange of the fourth ethyl group for a proton in the triethylammonium cation produces an even more pronounced deviation from ideality, with VE values that approach 2.8 cm3 mol−1. F


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Figure 6. (a) Molar volume, Vm, of the systems composed of ethylammonium nitrate with di-, tri- and tetraethylammonium nitratethe dashed line describes the ideal molar volume taking into account the calculated molar volume of di-, tri- and tetraethylammonium nitrate; (b) excess molar volume, VE of the systems mentioned above.

volume increment from [N0 0 0 2][NO3] to [N0 0 0 3][NO3] and [N0 0 0 4][NO3] follows the usual trend for ionic liquids. This also happens when changing from [N0 0 0 2][NO3] to [N0 0 2 2][NO3]. However, more intriguingly, this increment is much smaller when increasing the number of ethyl groups within the cation for more highly substituted systems. Thus, for [N0 2 2 2][NO3] and [N2 2 2 2][NO3], the steric hindrance at the central nitrogen atom significantly increases with the addition of the fourth ethyl group. Furthermore, the pronounced increase of the equimolar excess molar volume, VE1/2, is linearly related to the difference in the number of alkyl substituent groups attached to the nitrogen atom, ΔL, of mixtures composed of ethylammonium nitrate with propyl- or butylammonium nitrate (ΔL = 0), and with di-, triand tetraethylammonium nitrate (ΔL = 1, 2 and 3) as shown in Figure 7. One way to study the ionicity of ionic liquids is by comparison with an ideal electrolyte (fully dissociated potassium chloride ions in a dilute aqueous solution) in the well-known Walden plot.2,17 By relating the molar conductivity with the fluidity of the ionic liquid, it is possible to measure the ionicity, giving the deviation from the “ideal line”. According to the classification

Figure 5. Experimental (a) molar volume, Vm, (b) dynamic viscosity, η, and (c) ionic conductivity, σ, of pure and binary mixtures ionic liquids at 313.15 K versus the number of carbons in the cation/composite cation.

was also confirmed for the excess molar volume in binary mixtures of ionic liquids.9a,24 Moreover, the excess volumes in binary mixtures of ionic liquids are usually below 1 cm3 mol−1. Under the assumption that the VE is described by a parabolic function centered at a mole fraction of 0.5, the pseudo-liquid molar volume of the pure compounds [N0 0 2 2][NO3], [N0 2 2 2][NO3], [N2 2 2 2][NO3] (impossible to measure due to their high melting point) have been calculated, Figure 6a, along with the corresponding VE values of the binary mixtures composed of the above-mentioned ionic liquids with [N0 0 0 2][NO3], Figure 6b. Our results indicate a trend with the maximum VE changing from 0.75 cm3 mol−1 in {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3] to 2.8 cm3 mol−1 in {[N2 2 2 2]x[N0 0 0 2](1‑x)}[NO3], Figure 6b. Another result that is worth comparing is the increase in the molar volume of the pure compounds by both lengthening the alkyl chain of the monoalkylammonium nitrate, and by successively replacing the proton by ethyl groups in the ammonium cation. The molar G


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Figure 7. Equimolar excess molar volume, VE1/2, versus the difference in the number of alkyl substituent groups attached to the nitrogen atom, ΔL, of mixtures composed of ethylammonium nitrate with propyl- or butylammonium nitrate (ΔL = 0), and with di-, tri- and tetraethylammonium nitrate (ΔL = 1, 2 and 3, respectively).

proposed by Xu and Angell,5a [N0 0 0 2][NO3], [N0 0 0 3][NO3], and [N0 0 0 4][NO3], including all their binary mixtures, can be classified as “good” ionic liquids based on their position on the Walden plot, see Figure 8. Similar behavior has been observed for several alkylammonium bis(triflamide) salts,25 and for ionic liquids based on the imidazolium cation combined with a range of anions.4b Excluding the two mixtures with the highest concentrations of [N2 2 2 2][NO3], the ionicity appears to decrease when temperature increases. A similar trend has been reported for other ionic liquids, such as ionic liquids based on pyrrolidinium,26 or even group 1 cations combined with oligoether carboxylate anions.27 Nevertheless a linear relationship should not be taken for granted, given that other ammonium bistriflamide data28 suggest an ionicity minimum for a given temperature. The Walden plot for pure monoalkylammonium nitrate ionic liquids and their mixtures, Figure 8a, also shows a direct relationship between ionicity and the alkyl chain length of the cation or composite cation. On the other hand, and somewhat surprisingly, the ionicity of the homologous mixtures di-, tri- and tetraethylammonium nitrate with [N0 0 0 2][NO3], cf. Figure 8b, remains essentially independent of the mole fraction or degree of alkylation of the first component. This behavior might be directly related to a change in the initial network of the pure ethylammonium nitrates, due to specific interactions between the two components present in the mixture, as had already been demonstrated for mixtures of ionic liquids with inorganic salts.20 Furthermore, a constancy of ionicity was observed for mixtures of 1-alkyl-3-methylimidazolium ethanoate with nucleobases, where the invariable ionicity was related to the hydrogen bonding network formed in these mixtures.29

Figure 8. Walden plots for (a) pure [N0 0 0 2][NO3], [N0 0 0 3][NO3] and [N0 0 0 4][NO3] as well as their selected binary mixtures and (b) for the mixtures {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3], {[N0 2 2 2]x[N0 0 0 2](1‑x)}[NO3] and {[N0 0 2 2]x[N0 0 0 2](1‑x)}[NO3].

conductivity followed the opposite trend. Moreover, small excess properties for all binary mixtures were obtained. These results confirm that ideal behavior is a good model for an approximate description of these binary ionic liquid mixtures. In addition, binary mixtures, composed of [N0 0 0 2][NO3] and an increasing mole fraction of homologous ionic liquids with decreasing numbers of hydrogen bond donors in the cation[N0 0 2 2][NO3], [N0 2 2 2][NO3], and [N2 2 2 2][NO3] were also investigated. The results show that, besides the expected influence of increasing the number of carbon atoms in an ionic liquid cation, there are other interactions affecting the thermophysical properties of these ionic liquid mixtures. This can be seen by direct comparison of the binary mixtures, where a divergent trend in both dynamic viscosity and ionic conductivity is revealed as the number of carbon atoms in the composite cation increases. In contrast, with the results of monoalkylammonium nitrate mixtures, the VE data also reveal a clear deviation from ideal behavior as the number of ethyl chains is increased on the ethylammonium cation. Furthermore, there is

4. CONCLUSIONS The thermophysical properties of three monoalkylammonium nitrate ionic liquids [N 0 0 0 2][NO3], [N0 0 0 3][NO3], and [N0 0 0 4][NO3], and their binary mixtures in selected concentrations (all having three bond donors in the cation) were studied. The measured thermophysical data follow the expected trends. The dynamic viscosity values increased with the length of the alkyl chain, and with the number of carbons present in the cation or composite cation. On the other hand, density and ionic H


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S.; Kuhnel, R. S.; Balducci, A. The beneficial effect of protic ionic liquids on the lithium environment in electrolytes for battery applications. J. Mater. Chem. A 2014, 2, 8258−8265. (e) Timperman, L.; Vigeant, A.; Anouti, M. Eutectic mixture of Protic Ionic Liquids as an Electrolyte for Activated Carbon-Based Supercapacitors. Electrochim. Acta 2015, 155, 164−173. (f) Lopes, J. N. C.; Rebelo, L. P. N. Ionic liquids and reactive azeotropes: the continuity of the aprotic and protic classes. Phys. Chem. Chem. Phys. 2010, 12, 1948−1952. (6) (a) Maase, M.; Massonne, K. Biphasic Acid Scavenging Utilizing Ionic Liquids: The First Commercial Process with Ionic Liquids. In Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities Transformations and Processes; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 902; American Chemical Society: Washington D.C., 2005; pp 126−132. (b) Maase, M. BASIL process. In Multiphase Homogeneous Catalysis; Cornils, B., Herrmann, W. A., Horvath, I. T., Leitner, W., Mecking, S., Olivier-Bourbigou, H., Vogt, D., Eds.; WileyVCH: Weinheim, Germany, 2005; Vol. 2, 560−566. (c) Seddon, K. R. Ionic liquids: A taste of the future. Nat. Mater. 2003, 2, 363−364. (7) Hunt, P. A.; Ashworth, C. R.; Matthews, R. P. Hydrogen bonding in ionic liquids. Chem. Soc. Rev. 2015, 44, 1257−1288. (8) Niedermeyer, H.; Hallett, J. P.; Villar-Garcia, I. J.; Hunt, P. A.; Welton, T. Mixtures of ionic liquids. Chem. Soc. Rev. 2012, 41, 7780− 7802. (9) (a) Canongia Lopes, J. N.; Cordeiro, T. C.; Esperança, J. M. S. S.; Guedes, H. J. R.; Huq, S.; Rebelo, L. P. N.; Seddon, K. R. Deviations from ideality in mixtures of two ionic liquids containing a common ion. J. Phys. Chem. B 2005, 109, 3519−3525. (b) Łachwa, J.; Szydlowski, J.; Najdanovic-Visak, V.; Rebelo, L. P. N.; Seddon, K. R.; Nunes da Ponte, M.; Esperança, J. M. S. S.; Guedes, H. J. R. Evidence for lower critical solution behavior in ionic liquid solutions. J. Am. Chem. Soc. 2005, 127, 6542−6543. (10) Walden, P. Ueber die Molekulargrosse und elektrisehe Leitfahigkeit einiger gesehmolzenen Salze. Bull. Acad. Imp. Sci. SaintPétersbourg 1914, 8, 405−422. (11) MacFarlane, D. R.; Seddon, K. R. Ionic liquids - Progress on the fundamental issues. Aust. J. Chem. 2007, 60, 3−5. (12) (a) Abdul-Sada, A. K.; Al-Juaid, S.; Greenway, A. M.; Hitchcock, P. B.; Howells, M. J.; Seddon, K. R.; Welton, T. Upon the hydrogenbonding ability of the H(4) and H(5) protons of the imidazolium cation. Struct. Chem. 1990, 1, 391−394. (b) Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R. The Nature of Hydrogen Bonding in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2013, 52, 4623−4627. (13) (a) Belieres, J. P.; Angell, C. A. Protic ionic liquids: Preparation, characterization, and proton free energy level representation. J. Phys. Chem. B 2007, 111, 4926−4937. (b) Anouti, M.; Caillon-Caravanier, M.; Le Floch, C.; Lemordant, D. Alkylammonium-based protic ionic liquids. II. Ionic transport and heat-transfer properties: Fragility and ionicity rule. J. Phys. Chem. B 2008, 112, 9412−9416. (c) Angell, C. A.; Byrne, N.; Belieres, J.-P. Parallel Developments in Aprotic and Protic Ionic Liquids: Physical Chemistry and Applications. Acc. Chem. Res. 2007, 40, 1228−1236. (14) Kempter, V.; Kirchner, B. The role of hydrogen atoms in interactions involving imidazolium-based ionic liquids. J. Mol. Struct. 2010, 972, 22−34. (15) (a) Zahn, S.; Bruns, G.; Thar, J.; Kirchner, B. What keeps ionic liquids in flow? Phys. Chem. Chem. Phys. 2008, 10, 6921−6924. (b) Hunt, P. A. Why does a reduction in hydrogen bonding lead to an increase in viscosity for the 1-butyl-2,3-dimethyl-imidazolium-based ionic liquids? J. Phys. Chem. B 2007, 111, 4844−4853. (16) (a) Fumino, K.; Wulf, A.; Ludwig, R. Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water. Angew. Chem., Int. Ed. 2009, 48, 3184−3186. (b) Fumino, K.; Reichert, E.; Wittler, K.; Hempelmann, R.; Ludwig, R. Low-Frequency Vibrational Modes of Protic Molten Salts and Ionic Liquids: Detecting and Quantifying Hydrogen Bonds. Angew. Chem., Int. Ed. 2012, 51, 6236−6240. (c) Cremer, T.; Kolbeck, C.; Lovelock, K. R. J.; Paape, N.; Wolfel, R.; Schulz, P. S.; Wasserscheid, P.; Weber, H.; Thar, J.; Kirchner, B.; Maier, F.; Steinruck, H. P. Towards a Molecular Understanding of Cation-Anion Interactions-Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectros-

also an unexpected change in the (pseudo-liquid) molar volume increase of [N0 0 2 2][NO3], [N0 2 2 2][NO3], and [N2 2 2 2][NO3] when removing hydrogen-bond donors from the cation. This work reinforces the complicated behavior of simple and binary protonic ionic liquids and the need for further studies into the hydrogen bond networks in these, and related, systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b11900. Experimental density, ρ, dynamic viscosity, η, and ionic conductivity, σ, of pure and binary systems; comparison of these data with literature data; fitting parameters for experimental density, dynamic viscosity, and ionic conductivity; calculated excess molar volumes, VE, of the binary mixtures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(L.P.N.R.) E-mail: [email protected]. *(K.R.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Fundaçaõ para a Ciência e Tecnologia, FCT/ MEC (Portugal), for financial support through FCT Investigator Contracts (A.B.P. and J.M.S.S.E), Projects PTDC/CTM-NAN/ 121274/2010, PTDC/QUI-QUI/117340/2010, PTDC/EQUFTT/118800/2010, and UID/Multi/04551/2013, and COSTSTSM-CM1206-15844 for supporting a research visit (I.V.-F.). The NMR spectrometers are part of the Portuguese NMR Facility supported by FCT/MEC (RECI/BBB-BQB/0230/ 2012). K.R.S., I.V.-F. and N.V.P. thank the industrial advisory board of QUILL for their support and Darius Yeadon (QUILL) for his assistance with graphics.

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REFERENCES

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