Density, Viscosity, and Refractive Index of Ionic Liquid Mixtures

Article pubs.acs.org/jced

Density, Viscosity, and Refractive Index of Ionic Liquid Mixtures Containing Cyano and Amino Acid-Based Anions Andreia S. L. Gouveia, Liliana C. Tomé, and Isabel M. Marrucho* Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, 2780-157 Oeiras, Portugal S Supporting Information *

ABSTRACT: In this work, mixtures of five ionic liquids (ILs) based on a common cation, 1-ethyl-3-methylimidazolium ([C2mim]+), and amino acid anions, namely glycinate ([Gly]−), L-alaninate ([L-Ala]−), taurinate ([Tau]−), L-serinate ([L-Ser]−), and L-prolinate ([L-Pro]−) with 1-ethyl-3-methylimidazolium tricyanomethane, [C2mim][C(CN)3], were prepared. The thermophysical properties, namely density, viscosity, and refractive index, of the neat ILs and their mixtures were measured in the temperature range of T = (293.15 up to 353.15) K. The thermal expansion coefficients were calculated for the neat ILs and were considered to be independent of temperature in the working temperature range. Overall, experimental density, viscosity, and refractive index data of the neat AAILs were in a good agreement with those reported in literature. A dramatic decrease in the viscosity was observed for the IL mixtures as the [C2mim][C(CN)3] content increased. The obtained results indicate that mixing [C2mim][C(CN)3] with amino acid-based ILs is a potential mean to further increase flexibility and the fine-tune capacity of the physical and chemical properties of amino acid-based ILs.

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prolinate, and amino acid ester salts. Moreover, Ohno et al.5,6 introduced several ILs having amino acid anions combined with different cations, such as imidazolium, phosphonium, tetraalkylammonium, pyrrolidinium, and pyridinium, in order to explore AAILs with lower viscosity and better thermal properties.5,6 In addition, the thermophysical properties of different AAILs based on [N4441]+ and [P4444]+ cations and different amino acid anions, such as, [Lys]−, [Tau]−, [Thr]−, [Ser]− and [Pro]− were also explored by Gardas et al.7 Owing to their growing interest AAILs have been tested for several applications, namely as extraction media,8,9 intermediates in the synthesis of chiral compounds10 and peptides,11 but especially as absorbents for CO2 capture.12−16 Solvent absorption with amines is certainly the most common and efficient technology but despite their advantages, such as high reactivity and good absorption capacity, the use of amines is of environmental and economic concern due to their corrosive nature, volatility, toxicity, and high energy demand for regeneration.17,18 To circumvent these shortcomings, AAILs have been explored as alternative solvents to amine-based aqueous solutions. Furthermore, and considering the engineering and economic advantages of membranes compared to absorption processes, the potential of AAILs have also been tested as liquid phases in supported ionic liquid membranes (SILMs) for CO2 separation.19−21 Because AAILs present an amino-functionalized group, they can react with CO2, form

INTRODUCTION Within the ionic liquids (ILs) synthesis context, amino acids have been shown to have a range of useful properties, such as their strong hydrogen bonding ability that is useful for dissolving biomaterials, namely cellulose,1 DNA, and other carbohydrates.2 Amino acids are very versatile compounds in IL’s synthesis because they present both carboxyl and amine functional groups in one molecule and thus they can be used as either anions or cations. However, the use of amino acids as anions has been more commonly explored, especially due to their easy synthesis. A number of amino acid-based ILs (AAILs) composed of a common imidazolium cation and different amino acid derived anions were first proposed by Fukumoto et al.3 as a new platform to prepare functional ILs for a variety of applications. These authors prepared AAILs using a simple two step procedure that consists first of the preparation of imidazolium hydroxide, followed by its neutralization with the desired amino acid. This synthetic route was proposed and efficiently used to prepare different AAILs because the conventional method of preparing ILs by the anion exchange of halide salts with metal salts seemed to be unsuitable to prepare neat AAILs due not only to the contamination of halide salts but also to the limited variety of commercially available metal salts with amino acids. Additionally, in the same work Fukumoto et al.3 studied the glass transition temperature, the ionic conductivity, and the miscibility of the amino acid-based ILs with organic solvents and explored the effects of anion structure on these properties. On the other hand, Tao et al.4 reported a new generation of AAILs in which the cations are derived from amino acids, such as glycinate, alaninate, and © XXXX American Chemical Society

Received: March 12, 2015 Accepted: November 4, 2015

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DOI: 10.1021/acs.jced.5b00242 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Chemical structures of the ions used and composition matrix of the prepared IL mixtures.

Table 1. Composition Descriptions, Thermophysical Properties, Viscosity (η), Density (ρ), and Calculated Molar Volume (Vm), at 298.15 K and p = 0.1 MPa, as well as Water Content of the Neat ILs and Their Mixturesa composition

wt % of water

(mole fraction) [C2mim][Gly] [C2mim][L-Ala] [C2mim][Tau] [C2mim][L-Ser] [C2mim][L-Pro] [C2mim][C(CN)3]0.25 [Gly]0.75 [C2mim][C(CN)3]0.25 [L-Ala]0.75 [C2mim][C(CN)3]0.25 [Tau]0.75 [C2mim][C(CN)3]0.25 [L-Ser]0.75 [C2mim][C(CN)3]0.25 [L-Pro]0.75 [C2mim][C(CN)3]0.5 [Gly]0.5 [C2mim][C(CN)3]0.5 [L-Ala]0.5 [C2mim][C(CN)3]0.5 [Tau]0.5 [C2mim][C(CN)3]0.5 [L-Ser]0.5 [C2mim][C(CN)3]0.5 [L-Pro]0.5 [C2mim][C(CN)3]0.75 [Gly]0.25 [C2mim][C(CN)3]0.75 [L-Ala]0.25 [C2mim][C(CN)3]0.75 [Tau]0.25 [C2mim][C(CN)3]0.75 [L-Ser]0.25 [C2mim][C(CN)3]0.75 [L-Pro]0.25 [C2mim][C(CN3)]

neat neat neat neat neat x [C2mim][C(CN)3] = 0.2498 + x [C2mim][Gly] = 0.7502 x [C2mim][C(CN)3] = 0.2506 + x [C2mim][L-Ala] = 0.7494 x [C2mim][C(CN)3] = 0.2503 + x [C2mim][Tau] = 0.7497 x [C2mim][C(CN)3] = 0.2502 + x [C2mim][L-Ser] = 0.7498 x [C2mim][C(CN)3] = 0.2503 + x [C2mim][L-Pro] = 0.7497 x [C2mim][C(CN)3] = 0.5001 + x [C2mim][Gly] = 0.4999 x [C2mim][C(CN)3] = 0.5000 + x [C2mim][L-Ala] = 0.5000 x [C2mim][C(CN)3] = 0.5001 + x [C2mim][Tau] = 0.4999 x [C2mim][C(CN)3] = 0.5005 + x [C2mim][L-Ser] = 0.4995 x [C2mim][C(CN)3] = 0.5005 + x [C2mim][L-Pro] = 0.4995 x [C2mim][C(CN)3] = 0.7501 + x [C2mim][Gly] = 0.2499 x [C2mim][C(CN)3] = 0.7499 + x [C2mim][L-Ala] = 0.2501 x [C2mim][C(CN)3] = 0.7496 + x [C2mim][Tau] = 0.2504 x [C2mim][C(CN)3] = 0.7497 + x [C2mim][L-Ser] = 0.2503 x [C2mim][C(CN)3] = 0.7498+ x [C2mim][L-Pro] = 0.2502 neat

1.44 1.23 0.20 0.67 0.65 2.22 0.43 0.14 1.68 1.05 0.65 0.10 0.05 0.29 0.17 0.54 0.10 0.07 0.55 0.08 0.01

M

η

ρ

Vm

(g· mol−1)

(mPa·s)

(g· cm−3)

(cm3· mol−1)

185.22 199.25 235.30 215.25 225.29 189.22 199.75 226.78 211.75 219.28 193.23 200.24 218.27 208.24 213.26 197.23 200.74 205.49 204.74 207.25 201.23

171.97 263.98 514.67 2142.10 1194.63 70.85 107.20 176.64 384.17 474.82 43.40 60.26 64.77 120.68 131.11 23.44 25.68 28.77 33.37 38.60 14.19b

1.161 1.123 1.252 1.204 1.143 1.137 1.110 1.204 1.174 1.128 1.115 1.099 1.161 1.138 1.113 1.098 1.089 1.121 1.109 1.097 1.081b

159.51 177.43 188.01 178.82 197.14 166.43 179.89 188.29 180.42 194.33 173.30 182.22 187.93 182.92 191.60 179.66 184.27 187.17 184.56 188.88 186.04

a Relative standard uncertainties ur are ur(x) = 0.0003; ur(η) = 0.05; for the IL samples studied, the relative standard uncertainty was determined considering the purity of the IL samples: ur(ρ) = 0.004 for [C2mim][Gly], 0.005 for [C2mim][L-Ala], [C2mim][Tau], [C2mim][L-Ser], 0.003 for [C2mim][L-Pro], and 0.002 for [C2mim][C(CN)3], ur(Vm) = 0.05 bValues taken from Tomé et al.26

desired chemical characteristics for the active transport while the other maintains the low viscosity. Bearing in mind that the anions of ILs have a stronger influence on CO2 solubility than the cations,22 and considering that CO2 molecules have a larger affinity for anion versus cation associations,23,24 this work addresses IL mixtures of a common cation and different anions. In this context, five AAILs, namely 1-ethyl-3-methylimidazolium 2-aminoacetate ([C2mim][Gly]), 1-ethyl-3-methylimidazolium (S)-2-aminopropanoate ([C2mim][L-Ala]), 1-ethyl-3-methylimidazolium 2-aminoethanesulfonate ([C2mim][Tau]), 1-ethyl-3-methylimidazolium (S)-2-amino-3-hydroxypropanoate ([C2mim][L-Ser]), and 1ethyl-3-methylimidazolium (S)-pyrrolidine-2-carboxylate ([C2mim][L-Pro]) were mixed with 1-ethyl-3-methylimidazolium tricyanomethane ([C2mim][C(CN)3]), a very low viscous IL25 with high CO2 permeability and selectivity.26 The thermophysical properties, namely density, viscosity, and

CO2 complexes, and consequently act as CO2 carriers. For instance, Kasahara et al.20 investigated the CO2 permeability and CO2/N2 selectivity of AAILs, namely [P4444][Gly] and [C2mim][Gly], through the use of SILMs. On the contrary to what happened for the SILM containing bis(trifluoromethylsulfonyl)imide, the results showed that SILMs of the AAILs present facilitated CO2 transport because a significant increase of the CO2 permeability and CO2/N2 selectivity was observed, not only as the temperature increases, but also as the feed pressure decrease.20 Nevertheless, the major drawback in the use of AAILs for CO2 capture and separation is their high viscosity, leading to low sorption and desorption rates, even though the CO2 absorption in AAILs is substantially improved.13 These aspects motivated us to explore the tailoring of AAIL thermophysical properties, namely viscosity, through the use of IL mixtures, so that one IL component provides the B



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standard uncertainty for the temperature is 0.02 K. The repeatability of density and dynamic viscosity of this equipment is 0.0005 g·cm−3 and 0.35%, respectively. Further details on the equipment can be found elsewhere.27 Triplicates of each sample were performed to ensure accuracy and the reported results are the average values. Moreover, the relative standard uncertainty in density and viscosity was calculated by the ratio of the standard deviation and the average of the three replicates of each sample, where the highest relative standard uncertainty registered for the density and dynamic viscosity measurements was 4·10−4 and 0.05, respectively. However, considering the purity of the IL samples studied in this work as discussed by Chirico et al.,28 the relative standard uncertainty is ur(ρ) = 0.004 for the pure [C2mim][Gly], 0.005 for [C2mim][L-Ala], [C2mim][Tau], and [C2mim][L-Ser], 0.003 for [C2mim][LPro], and 0.002 for [C2mim][C(CN)3]. The measured viscosity, η, and density, ρ, and the calculated molar volume, Vm, of the neat ILs and their mixtures at 298.15 K are presented in Table 1. Refractive Index Measurements. The refractive indices were measured at atmospheric pressure in the temperature range between 293.15 and 353.15 K using an automated Anton Paar Refractometer Abbemat 500 with an absolute uncertainty of 0.00005. Triplicates of each sample were measured and the results presented are average values, where the highest relative standard uncertainty registered was 0.002. Thermogravimetric Analysis (TGA). The onset and decomposition temperatures of the neat ILs and their mixtures at 0.5 of mole fraction were measured using a thermal gravimetric analyzer (TA Instruments Model TGA Q50). The samples were placed inside aluminum pans and heated up to 773 K at a heating rate of 10 K·min−1 until complete thermal degradation was achieved. All samples were recorded under a nitrogen atmosphere. Universal Analysis, version 4.4A software, was used to determine the onset and the decomposition temperatures, as the temperatures at which the baseline slope changes 5% during the heating and at which the point of greatest rate of change on the weight loss curve (first derivative peak) is observed, respectively.

refractive index of the neat ILs and their mixtures were measured in the temperature range of T = (293.15 to 353.15) K. The thermal expansion coefficients were determined from the density values as a function of temperature and the energy barrier was calculated from the viscosity dependence with temperature. Moreover, the thermogravimetric analysis of the neat ILs and the IL mixtures at 0.5 of mole fraction was also performed in order to establish their degradation temperature and thus their upper working temperature limit.

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EXPERIMENTAL SECTION Materials. Glycine (100 w ≥ 98.5), L-alanine (100 w ≥ 99.5), taurine (100 w ≥ 99), L-serine (100 w ≥ 99.5) and Lproline (100 w ≥ 99), acetonitrile (99.8%), and methanol (99.8%) were provided by Sigma-Aldrich. The 1-ethyl-3methylimidazolium tricyanomethane ([C2mim][C(CN)3]) (100 w > 98), and the 1-ethyl-3-methylimidazolium chloride [C2mim][Cl] (100 w > 98), were supplied by IoLiTec GmbH. Synthesis of AAILs. The ILs used in this work were synthesized via a two-step anion exchange reaction, following an established procedure developed by Ohno et al.3 Briefly, an aqueous solution of 1-ethyl-3-methylimidazolium hydroxide ([C2mim][OH]) was prepared by passing an aqueous solution of 1-ethyl-3-methylimidazolium chloride, [C 2 mim][Cl], through a column filled with anion exchange resin (SUPELCO AMBERLITE IRN-78) in the hydroxide form. Afterward, [C2mim][OH] was neutralized by the dropwise addition of a slight excess of the corresponding equimolar amino acid aqueous solution, using an ice bath for cooling. The obtained mixtures were stirred at ambient temperature and pressure for 12 h. Excess water was then removed by rotary evaporation under reduced pressure. A mixture of acetonitrile and methanol (9:1 v/v) was added in order to precipitate the unreacted amino acid. After filtration, the solvents were removed by rotary evaporation and the obtained crude products were dried under vacuum (1 Pa) and subjected to vigorous stirring at moderate temperature (≃ 318 K) for at least 4 days immediately prior to use. The chemical structures of the prepared ILs were confirmed by 1H and 13C NMR analysis (provided as Supporting Information) and their purities were determined: [C2mim][Gly] (96 mol %), [C2mim][L-Ala] (95 mol %), [C2mim][Tau] (95 mol %), [C2mim][L-Ser] (95 mol %), and [C2mim][L-Pro] (97 mol %). Preparation of the Ionic Liquid Mixtures. The IL mixtures (Figure 1) were prepared using an analytical highprecision balance with an uncertainty of ± 10−5 g by syringing known masses of the IL components into glass vials. Good mixing was assured by magnetic stirring for at least 30 min. Then, the prepared IL mixtures at 0.25, 0.5, and 0.75 mole fraction of [C2mim][AA] were dried under vacuum (1 Pa) at a moderate temperature (≈ 318 K) for at least 4 days. Because AAILs are very hydrophilic, all the samples were handled in a glovebox under nitrogen atmosphere. The samples were prepared immediately prior to the measurements to avoid variations in composition. The water contents of the neat ILs and their mixtures, measured by Karl Fischer titration, are presented in Table 1, as well as the composition descriptions of all the ionic liquid samples. Density and Viscosity Measurements. The measurements of density and viscosity of the neat ILs and their mixtures were performed in the temperature range between 293.15 and 353.15 K at atmospheric pressure using an SVM 3000 Anton Paar rotational Stabinger viscometer-densimeter, where the

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RESULTS AND DISCUSSION The results from the TGA experiments, specifically the onset, Tonset (5%), and decomposition, Tdeg 1 and Tdeg 2, temperatures of the neat ILs and their mixtures at 0.5 of mole fraction, are presented in Table 2. Two different decomposition temperTable 2. Onset (Tonset (5%)) and Decomposition (Tdeg 1 and Tdeg 2) Temperatures of the Neat ILs and Their Mixtures

IL sample [C2mim][C(CN)3] [C2mim][C(CN)3]0.5 [C2mim][C(CN)3]0.5 [C2mim][C(CN)3]0.5 [C2mim][C(CN)3]0.5 [C2mim][C(CN)3]0.5 [C2mim][Gly] [C2mim][L-Ala] [C2mim][Tau] [C2mim][L-Ser] [C2mim][L-Pro] C

[Gly]0.5 [L-Ala]0.5 [Tau]0.5 [L-Ser]0.5 [L-Pro]0.5

Tonset (5%)

Tdeg 1

Tdeg 2

(K)

(K)

(K)

606 484 484 530 488 476 430 455 500 476 416

634 518 512 540 524 517 500 509 574 514 536

624 628 597 631 551



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Table 3. Measured Density Values, ρ (g·cm−3), of the Neat ILs and the IL Mixtures at 0.5 of Mole Fraction (p = 0.1 MPa)a ρ (g·cm−3)

T

(K)

[C2mim] [Gly]

[C2mim] [L-Ala]

[C2mim] [Tau]

[C2mim] [L-Ser]

[C2mim] [L-Pro]

[C2mim] [C(CN)3]0.5Gly]0.5

[C2mim] [C(CN)3]0.5 [L-Ala]0.5

[C2mim] [C(CN)3]0.5 [Tau]0.5

[C2mim] [C(CN)3]0.5 [L-Ser]0.5

[C2mim] [C(CN)3]0.5 [L-Pro]0.5

[C2mim] [C(CN)3]b

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 ur(ρ)a

1.164 1.161 1.158 1.155 1.152 1.149 1.146 1.143 1.140 1.137 1.134 1.131 1.127 0.004

1.126 1.123 1.120 1.117 1.114 1.110 1.107 1.104 1.101 1.098 1.095 1.092 1.089 0.005

1.255 1.252 1.248 1.245 1.242 1.239 1.236 1.232 1.229 1.226 1.223 1.220 1.217 0.005

1.207 1.204 1.201 1.197 1.194 1.191 1.188 1.185 1.182 1.179 1.176 1.173 1.170 0.005

1.146 1.143 1.140 1.136 1.133 1.130 1.127 1.124 1.121 1.118 1.115 1.112 1.109 0.003

1.118 1.115 1.112 1.108 1.105 1.102 1.099 1.096 1.092 1.089 1.086 1.083 1.080 0.006

1.102 1.099 1.096 1.092 1.089 1.086 1.083 1.080 1.076 1.073 1.070 1.067 1.063 0.007

1.165 1.161 1.158 1.155 1.151 1.148 1.145 1.142 1.138 1.135 1.132 1.129 1.126 0.007

1.142 1.138 1.135 1.132 1.128 1.125 1.122 1.118 1.115 1.112 1.109 1.105 1.102 0.007

1.116 1.113 1.110 1.107 1.103 1.100 1.097 1.094 1.090 1.087 1.084 1.081 1.078 0.005

1.085 1.081 1.077 1.074 1.070 1.067 1.064 1.060 1.057 1.054 1.050 1.047 1.043 0.002

a ur(ρ) are the relative standard uncertainties ur, considering the purity of the IL samples and u(T) = 0.02 K is the standard uncertainty u. bValues taken from Tomé et al.26

atures were considered given that two different peaks were obtained in the first derivative weight loss curve for the five IL mixtures studied. Table 2 shows that the decomposition temperature (Tdeg 1) for all IL samples with the exception of the neat [C2mim][C(CN)3] is near to 500 K. The decomposition temperature (Tdeg 1) of the neat AAILs can be ordered as [C 2mim][Gly] < [C2mim][L-Ala] < [C2mim][L-Ser] < [C2mim][L-Pro] < [C2mim][Tau]. [C2mim][C(CN)3] is the IL with the highest decomposition temperature (Tdeg 1). The density, ρ (g·cm−3), viscosity, η (mPa·s), and refractive indices, nD, measured in this work for the neat ILs and the IL mixtures at 0.5 of mole fraction as a function of temperature are presented in Tables 3, 5, and 7, respectively. The density, viscosity, and refractive indices for the remaining IL mixtures are provided in Supporting Information (Tables S1 and S2; S11 and S12; and S16 and S17, respectively). In Figure 2a,b,c, the density, viscosity, and refractive index values as a function of temperature are compared to those available in literature for neat AAILs29 with the exception of [C2mim][Tau] because no data was found for this AAIL. Figure 2a shows that small but constant deviations between the density values measured in the present work and those reported by Muhammad et al.,29 displaying positive deviations of around 0.5% and 0.7% for [C2mim][Gly] and [C2mim][L-Ser], respectively, and negative deviations of around 1.2% for [C2mim][L-Ala] and [C2mim][LPro]. On the other hand, the experimental viscosity values measured (Figure 2b) display high positive deviations from literature data.29 For instance, the neat [C2mim][L-Ser] shows the highest viscosity deviations, while [C2mim][L-Ala] presents the lowest deviations. This difference may be explained by several factors such as the purity of the sample and most often by different water contents in the ILs, since this thermophysical property is significantly affected by the presence of water,30,31 particularly in this case, due to the hydrophilic nature of the amino acid anions. Although the water contents of the different neat AAILs reported by Muhammad et al.29 are lower (between 100 w = 0.023 and 100 w = 0.03) than those obtained in the present work (Table 1), higher viscosities were obtained here. These inexplicable differences highlight the importance of measuring the thermophysical properties of all the neat ILs so

Figure 2. Experimental (a) density, (b) viscosity, and (c) refractive index values measured in this work ( ■, [C2mim][Gly]; ▲, [C2mim][L-Ala]; ●, [C2mim][L-Ser]; ◆, [C2mim][L-Pro]) and those reported by Muhammad et al.29 ( □, [C2mim][Gly]; ▲, [C2mim][L-Ala]; ○, [C2mim][L-Ser]; ◊, [C2mim][L-Pro]) as a function of temperature (T).

that comparisons between the neat ILs and their mixtures can be confidently taken and trends clearly established. D



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Additionally, it can be seen from Figure 2c that the measured refractive index values of all the neat AAILs are in good agreement to those reported by Muhammad et al.,29 showing positive deviations between 0.5% and 1%. The experimental refractive index values of the neat [C2mim][C(CN)3] were also compared to the literature data.25,32,33 From Figure 3, it can be observed that the experimental refractive indices are in a good agreement to those reported in the literature,25,32,33 showing deviations lower than ±0.09%.

Figure 3. Experimental refractive index values of the neat [C2mim][C(CN)3] measured in this work (×) and those reported by Neves et al.25 (shaded square), Froba et al.32 (○). and Koller et al.33 (◆) as a function of temperature (T). Figure 4. (a) Densities (ρ) of the neat ILs as a function of temperature (T): ×, [C2mim][C(CN)3]; □, [C2mim][Gly]; ▲, [C2mim][L-Ala]; ○, [C2mim][Tau]; ●, [C2mim][L-Ser]; ■, [C2mim][L-Pro]. (b) Density values of the IL mixtures with different compositions at T = 298.15 K. *Density values of [C2mim][C(CN)3] were taken from Tomé et al.26

Density. The density values, ρ (g·cm−3), of the neat ILs and the IL mixtures at 0.5 of mole fraction as a function of temperature are presented in Table 3, while those obtained for the IL mixtures at 0.25 and 0.75 mole fraction of [C2mim][AA] are provided in Supporting Information (Tables S1 and S2). The density values of the neat ILs are illustrated in Figure 4a, where it can be observed that the density decreases linearly with temperature for all neat ILs in the whole temperature range studied. The [C2mim][C(CN)3] displays the lowest density, while for the neat AAILs the following order can be established: [C2mim][L-Ala] < [C2mim][L-Pro] < [C2mim][Gly] < [C2mim][L-Ser] < [C2mim][Tau]. An overall comparison of the densities for the different IL mixtures, as a function of composition, as well as for the neat fluids at a fixed temperature of 298 K is presented in Figure 4b. As expected, the density values of the IL mixtures are in between those of the neat ILs for the five IL mixtures studied. Additionally, within one mixture, the density decreases as the mole fraction of [C2mim][C(CN)3] increases in the IL mixture. This behavior is common to all five AAILs studied, leading to the conclusion that for a fixed composition, the AAILs density order is maintained within the mixtures. Another interesting conclusion is that if a specific density is needed, several mixtures of different AAILs with different compositions can be used. This fact illustrates the flexibility provided by the use of IL mixtures. The density values were fitted as a function of temperature, T (K), by the least-squares method using the linear expression given by eq 1 ρ = a + b(T )

Table 4. Fitted Parameters of the Linear Expression Given by Equation 1 and Respective Correlation Coefficient, R2a IL Sample

b·104

a −3

[C2mim][Gly] [C2mim][L-Ala] [C2mim][Tau] [C2mim][L-Ser] [C2mim][L-Pro] [C2mim][C(CN)3]0.25 [Gly]0.75 [C2mim][C(CN)3]0.25 [L-Ala]0.75 [C2mim][C(CN)3]0.25 [Tau]0.75 [C2mim][C(CN)3]0.25 [L-Ser]0.75 [C2mim][C(CN)3]0.25 [L-Pro]0.75 [C2mim][C(CN)3]0.5 [Gly]0.5 [C2mim][C(CN)3]0.5 [L-Ala]0.5 [C2mim][C(CN)3]0.5 [Tau]0.5 [C2mim][C(CN)3]0.5 [L-Ser]0.5 [C2mim][C(CN)3]0.5 [L-Pro]0.5 [C2mim][C(CN)3]0.75 [Gly]0.25 [C2mim][C(CN)3]0.75 [L-Ala]0.25 [C2mim][C(CN)3]0.75 [Tau]0.25 [C2mim][C(CN)3]0.75 [L-Ser]0.25 [C2mim][C(CN)3]0.75 [L-Pro]0.25 [C2mim][C(CN)3]

(1)

where a and b are adjustable parameters that are listed in Table 4. Despite the controversy surrounding the application of a linear correlation or a second order polynomial equation to describe the experimental density data,25,34 we found that the use of a linear function adequately describes the measured

a

R2

−3 −1

(g·cm )

(g·cm K )

1.3431 1.3072 1.4398 1.3880 1.3254 1.3241 1.2972 1.3937 1.3668 1.3160 1.3065 1.2916 1.3557 1.3363 1.3055 1.2949 1.2866 1.3194 1.3101 1.2965 1.2846

−6.10 −6.18 −6.32 −6.19 −6.13 −6.28 −6.27 −6.35 −6.48 −6.30 −6.42 −6.46 −6.52 −6.64 −6.46 −6.61 −6.62 −6.67 −6.74 −6.69 −6.83

0.9999 0.9999 0.9997 0.9997 0.9998 0.9999 0.9999 1.0000 0.9998 0.9999 0.9999 0.9999 0.9999 1.0000 1.0000 0.9999 0.9999 1.0000 0.9999 0.9999 0.9999

Standard uncertainties u are u(a) = 0.001, u(b) = 3·10−6.

density data, evaluated by the correlation coefficient, R2, listed in Table 4, for the temperature range studied in this work. E



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where ρ corresponds to the density (g·cm−3), x is the molar fraction, and the M corresponds to the molar mass (g·mol−1). The subscripts 1 and 2 refer to the neat ILs and the subscript M denotes the IL mixtures. The calculated excess molar volume values are provided in Supporting Information (Tables S8, S9, and S10) and are depicted in Figure 6 at T = 298.15 K. Note that the excess

Furthermore, the use of a linear regression implies that the thermal expansion coefficient values are constant at a given pressure. Thus, thermal expansion coefficients (αP), were calculated from the fitting of the experimental data using the following eq 229,35 αP = −

⎛ ∂ ln ρ ⎞ 1 ⎛ ∂ ln ρ ⎞ ⎜ ⎟ = −⎜ ⎟ ⎝ ∂T ⎠P ρ ⎝ ∂T ⎠P

(2)

where ρ is the density in g·cm−3, T is the temperature in K, and P is the pressure in kPa. The thermal expansion coefficient values of all the studied neat ILs are provided in Supporting Information (Table S3). The thermal expansion coefficients of the neat ILs do not change considerably with temperature, which is in accordance to what has been observed for other ILs.36,37 Additionally, the thermal expansion coefficients for the neat AAILs can be ordered as [C2mim][Tau] < [C2mim][L-Ser] < [C2mim][Gly] < [C2mim][L-Pro] < [C2mim][L-Ala]. The molar volumes (Vm) of the neat ILs and their mixtures are provided in Supporting Information (Tables S4−S7) and were calculated through the density values, by eq 3: Vm =

x1M1 + x 2M 2 ρ

Figure 6. Excess molar volumes of the IL mixtures at 298.15 K: □, [C 2 mim][C(CN) 3 ][Gly]; ▲ , [C 2 mim][C(CN) 3 ][ L -Ala]; ○ , [C 2 mim][C(CN) 3 ][Tau]; ● , [C 2 mim][C(CN) 3 ][ L -Ser]; ■ , [C2mim][C(CN)3]L-Pro].

(3)

where ρ corresponds to the density (g·cm−3), x is the molar fraction, and M the molar mass (g·mol−1). The molar volumes (Vm) of the neat ILs are illustrated in Figure 5. The subscripts 1

molar volumes are very small (tens of the unit) in comparison to the molar volumes (in the order of hundreds of the unit) used in their calculations. Thus, the accuracy of the density measurements is very important in the discussion of the excess molar volumes, which are the result of contributions from several effects, namely, chemical, physical, and structural modifications.39 Physical contributions, that are nonspecific interactions between the species present in the mixture, originate positive VE values,40 while negative VE values are a result of chemical contributions (charge-transfer type forces, changes in hydrogen bonding equilibrium, or electrostatic interactions) or structural contributions (geometrical fitting or changes of free volume).41 All the studied IL mixtures show positive VE, except the [C2mim][C(CN)3][L-Pro] mixture that presents negative VE values in the whole composition range (Figure 6), which can probably be attributed to a packing effect due to the presence of the cyclic amine group. The [C2mim][C(CN)3][L-Ser] mixture shows a different behavior compared to the other IL mixtures because it exhibits positive VE values at low [C2mim][AA] mole fractions and negative VE values at high [C2mim][AA] mole fractions. There are two mixtures with very similar excess volumes in the whole composition range, [C2mim][C(CN)3][Gly] and [C2mim][C(CN)3][L-Ala]. In fact, [C2mim][C(CN)3][L-Ser] also presents very similar VE values to these two mixtures at the 0.25 and 0.5 mole fractions of [C2mim][AA], but at 0.75 mole fraction of [C2mim][AA] a substantially different behavior was obtained. The negative VE value at high [C2mim][L-Ser] mole fraction indicates that a more efficient packaging and/or attractive interaction occurred when 0.75 [C2mim][L-Ser] mole fraction is mixed with [C2mim][C(CN)3], possibly an effect of the hydroxyl group present in its structure. Viscosity. The viscosity values, η (mPa·s), of the neat ILs and the IL mixtures were measured in the temperature range from 293.15 to 353.15 K. The viscosity values of the neat ILs and the IL mixtures at 0.5 of mole fraction as a function of

Figure 5. Molar Volumes (Vm) of the neat ILs as a function of temperature (T): ×, [C2mim][C(CN)3]; □, [C2mim][Gly]; ▲, [C 2 mim][ L -Ala]; ○ , [C 2 mim][Tau]; ● , [C 2 mim][ L -Ser]; ■ , [C2mim][L-Pro]. Molar volumes of [C2mim][C(CN)3] were taken from Tomé et al.26

and 2 refer to the neat ILs. The molar volumes of all samples increase with increasing temperature. The neat [C2mim][LPro] presents the highest molar volume due to the presence of the nitrogen containing ring, while the neat [C2mim][Gly] present the lowest, which is in accordance to its small chemical structure in the whole range of temperatures studied. It is important to be mentioned that [C2mim][C(CN)3] and [C2mim][Tau] present very similar molar volumes. The same behavior was also found for [C2mim][L-Ser] and [C2mim][LAla]. The excess molar volume (VE) of the IL mixtures was calculated by eq 438 VE =

x1M1 + x 2M 2 xM xM − 1 1 − 2 2 ρM ρ1 ρ2

(4) F



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Table 5. Measured Viscosity Values, η (mPa·s), of the Neat ILs and the IL Mixtures at 0.5 of Mole Fraction (p = 0.1 MPa)a η (mPa·s)

T

a

(K)

[C2mim] [Gly]

[C2mim] [L-Ala]

[C2mim] [Tau]

[C2mim] [L-Ser]

[C2mim] [L-Pro]

[C2mim] [C(CN)3]0.5 [Gly]0.5

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 ur(η)a

240.18 171.97 126.19 95.04 73.50 57.66 46.21 37.66 31.24 26.08 22.11 18.95 16.45 0.01

382.06 263.98 187.79 137.47 103.54 79.34 62.21 49.69 40.50 33.22 27.72 23.38 20.03 0.01

760.89 514.67 359.36 258.04 190.44 143.31 110.35 86.59 69.36 56.05 46.10 38.40 32.53 0.01

3796.80 2235.55 1376.25 882.82 588.27 405.12 287.60 209.91 157.29 119.86 93.36 74.03 59.91 0.01

1928.43 1194.63 771.51 517.61 359.21 256.92 188.66 141.76 108.85 85.12 67.69 54.73 44.89 0.01

53.975 42.139 33.550 27.193 22.254 18.687 15.895 13.839 11.884 10.468 9.233 8.191 7.189 0.05

[C2mim] [C(CN)3]0.5 [L-Ala]0.5

[C2mim] [C(CN)3]0.5 [Tau]0.5

[C2mim] [C(CN)3]0.5 [L-Ser]0.5

[C2mim] [C(CN)3]0.5 [L-Pro]0.5

[C2mim] [C(CN)3]b

78.60 60.26 46.89 37.29 30.22 24.90 20.83 17.64 15.08 13.09 11.45 10.10 8.93 0.01

84.50 64.77 50.77 40.59 33.04 27.27 22.83 19.37 16.60 14.40 12.59 11.10 9.83 0.03

168.52 120.68 88.95 67.28 52.28 41.25 33.28 27.29 22.78 19.18 16.37 14.12 12.31 0.02

183.74 131.11 96.60 73.15 56.72 44.93 36.25 29.73 24.75 20.87 17.80 15.35 13.37 0.02

16.62 14.19 12.18 10.58 9.02 8.20 7.31 6.56 5.72 5.38 4.91 4.50 3.98 0.02

ur(η) are the relative standard uncertainties ur and u(T) = 0.02 K is the standard uncertainty u. bValues taken from Tomé et al.26

to the presence of the hydroxyl group, increasing its hydrogen bond capacity. In fact, the viscosity values for the five AAILs can be ordered as [C2mim][Gly] < [C2mim][L-Ala] < [C2mim][Tau] < [C2mim][L-Pro] < [C2mim][L-Ser]. A similar trend was reported by Ghanem et al.42 for AAILs with a different imidazolium cation ([C8mim]+) and the same AA anions, with the exception of [Tau]− anion. The viscosity values for all the IL series at T = 298.15 K are presented in Figure 7b. The viscosity values of the prepared IL mixtures are in between those of the neat ILs for the five AAILs. Indeed, this behavior can be observed in the whole temperature range studied. A very marked decrease in viscosity with increasing mole fraction of [C2mim][C(CN)3] in the IL mixture can be observed especially for the two most viscous AAILs, [C2mim][L-Ser] and [C2mim][L-Pro]. The experimental viscosity values were fitted as a function of temperature, using the Vogel−Fulcher−Tammann (VFT) model described in eq 5

temperature are presented in Table 5, while those obtained for the IL mixtures at 0.25 and 0.75 mole fraction of [C2mim][AA] are provided in Supporting Information (Tables S11 and S12). Figure 7a illustrates the viscosity values of the neat ILs and, as expected, they decrease linearly with temperature for all neat ILs, in the whole temperature range studied. As it can be observed from Figure 7a, the neat [C2mim][L-Ser] exhibits higher viscosity compared to the other neat ILs, probably due

ln η = A η +

Bη (T − Cη)

(5)

where η is the viscosity in mPa·s, T is the temperature in K, and Aη, Bη, and Cη are adjustable parameters. The adjustable parameters, which were determined from the fitting of the experimental data, are listed along with the absolute average relative deviation (ARD) and the energy barrier of a fluid to shear stress, E, (kJ·mol−1) at T = 298.15 K, in Table 6. The average relative deviation was calculated by eq 6 ARD (%) =

1 N

N

∑ i=1

ηcalc, i − ηexp, i ηexp, i

× 100 (6)

where N is the total number of experimental points, ηexp is the experimental data and ηcalc is calculated from the correlation. The energy barrier was determined based on the viscosity dependence with temperature using eq 7, as follows25 Figure 7. (a) Measured viscosity (η) values of the neat ILs as a function of temperature (T): ×, [C2mim][C(CN)3]; □, [C2mim][Gly]; ▲, [C2mim][L-Ala]; ○, [C2mim][Tau]; ●, [C2mim][L-Ser]; ■, [C2mim][L-Pro]. (b) Comparison of the viscosity values of the IL mixtures containing different compositions at T = 298.15 K. *Viscosity values of [C2mim][C(CN)3] were taken from Tomé et al.26

⎛ ⎞ ⎜ ⎟ Bη ∂(ln η) ⎜ ⎟ E = R· R = · 1 ⎞⎟ 2C η ⎜ ⎛⎜ Cη2 ∂ T ⎜ 2 − T + 1⎟ ⎟ ⎠⎠ ⎝ ⎝T

()

G

(7)



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Table 6. Fitted Parameters of VFT Expression Given by Equation 5, Respective ARD, Correlation Coefficient, R2, and Energy Barrier Values at T = 298.15 K parameters IL sample [C2mim][Gly] [C2mim][L-Ala] [C2mim][Tau] [C2mim][L-Ser] [C2mim][L-Pro] [C2mim][C(CN)3]0.25 [Gly]0.75 [C2mim][C(CN)3]0.25 [L-Ala]0.75 [C2mim][C(CN)3]0.25 [Tau]0.75 [C2mim][C(CN)3]0.25 [L-Ser]0.75 [C2mim][C(CN)3]0.25 [L-Pro]0.75 [C2mim][C(CN)3]0.5 [Gly]0.5 [C2mim][C(CN)3]0.5 [L-Ala]0.5 [C2mim][C(CN)3]0.5 [Tau]0.5 [C2mim][C(CN)3]0.5 [L-Ser]0.5 [C2mim][C(CN)3]0.5 [L-Pro]0.5 [C2mim][C(CN)3]0.75 [Gly]0.25 [C2mim][C(CN)3]0.75 [L-Ala]0.25 [C2mim][C(CN)3]0.75 [Tau]0.25 [C2mim][C(CN)3]0.75 [L-Ser]0.25 [C2mim][C(CN)3]0.75 [L-Pro]0.25 [C2mim][C(CN)3]

Aη

Bη

Cη

ARD

(mPa.s)

(K)

(K)

(%)

−1.766 −1.971 −2.049 −2.457 −2.300 −1.681 −1.702 −1.965 −2.261 −1.852 −1.598 −1.606 −1.596 −1.681 −1.590 −1.478 −1.840 −1.535 −1.570 −1.497 −1.898

738.601 798.653 910.854 1009.588 959.539 654.712 678.486 825.404 833.326 778.949 601.348 625.181 653.691 653.298 651.444 546.202 652.869 595.406 582.662 576.639 663.573

191.275 192.303 188.296 198.846 195.905 187.962 191.746 182.502 196.652 200.950 185.477 188.483 184.787 197.224 197.405 180.340 169.726 176.467 183.379 186.178 152.177

0.05 0.05 0.05 0.06 0.15 0.03 0.12 0.07 0.11 0.02 0.23 0.05 0.02 0.03 0.01 0.13 0.39 0.23 0.21 0.01 0.64

R2

E298.15K (kJ·mol‑1)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9999 1.0 1.0 1.0 1.0 1.0 0.9996 0.9999 0.9999 0.9999 1.0

47.79 52.69 55.79 75.67 67.84 39.86 44.29 45.61 59.79 60.94 35.01 38.42 37.60 47.40 47.44 29.09 29.26 29.72 32.69 33.99 23.11

terminal −OH group in [C2mim][L-Ser], providing an extra hydrogen bonding point, might possibly account for this behavior. Also, the high energy barrier calculated for [C2mim][L-Pro] indicates that it requires more energy for the ions to move in the bulk, probably due to its large molar volume (Figure 5), arising from cyclic amine present in its structure. It might be a possible explanation for its high viscosity, when compared to the other AAILs, such as [C2mim][Gly], [C2mim][L-Ala] and [C2mim][Tau]. Note that, very similar E values were obtained for all the [C2mim][C(CN)3]0.75 [AA]0.25, indicating the disruption of most the AAIL network. Conversely, at [C2mim][C(CN)3]0.25 [AA]0.75, large E values can be observed for both [C2mim][L-Ser] and [C2mim][L-Pro], showing the strong structural bulk effects present in these mixtures. The viscosity deviations, Δη (mPa·s), for the IL mixtures were calculated using eq 8

where η is the viscosity, T is the temperature, Bη and Cη are the adjustable parameters obtained from eq 5, and R is the universal gas constant. The higher the energy barrier value, the more difficult it is for the ions to move past each other. This fact can be a direct consequence of the size or entanglement of the ions and/or of the presence of stronger interactions, such as H-bonding and Coulombic interactions, in the fluid. Looking at Figure 8, where

Δη = ηM − [x1η1 + (1 − x1)η2]

(8)

where η corresponds to viscosity (mPa·s) and x is the mole fraction. The subscripts 1 and 2 correspond to the two neat ILs and the subscript M denotes the IL mixture. Although all the calculated viscosity deviations values for the studied IL mixtures are provided in Supporting Information (Tables S13−S15), the values at T = 298.15 K are represented in Figure 9. Regarding the viscosity deviations, positive Δη values are related to the charge transfer and hydrogen bonding interactions, while negative Δη values are usually obtained for systems where molecular size and shapes of the components, dispersion and dipolar interactions are considered. As it can be observed from Figure 9, all the IL mixtures studied show negative viscosity deviations, within the entire range of temperatures. Refractive Index. The refractive indices (nD) of the neat ILs and the IL mixtures were measured in the temperature

Figure 8. Calculated energy barrier (E) of IL series at 298.15 K: □, [C 2 mim][C(CN) 3 ][Gly]; ▲ , [C 2 mim][C(CN) 3 ][ L -Ala]; ○ , [C 2 mim][C(CN) 3 ][Tau]; ● , [C 2 mim][C(CN) 3 ][ L -Ser]; ■ , [C2mim][C(CN)3][L-Pro]; ×, [C2mim][C(CN)3].

the energy barrier (E) values for the five IL series at T = 298.15 K are represented, it can be observed that, for the five AAILs, the E decreases as the mole fraction of [C2mim][C(CN)3] increases in the IL mixture. The [C2mim][L-Ser] shows the highest E value, while [C2mim][Gly] displays the lowest, indicating the establishment of stronger interactions in the mixture containing [C2mim][L-Ser] than in that having [C2mim][Gly]. As mentioned before, the presence of the H



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Figure 9. Viscosity deviations (Δη) of the IL mixtures at 298.15 K: □, [C 2 mim][C(CN) 3 ][Gly]; ▲ , [C 2 mim][C(CN) 3 ][ L -Ala]; ○ , [C 2 mim][C(CN) 3 ][Tau]; ● , [C 2 mim][C(CN) 3 ][ L -Ser]; ■ , [C2mim][C(CN)3][L-Pro].

range from 293.15 to 353.15 K. The refractive indices of the neat ILs and the IL mixtures at 0.5 of mole fraction as a function of temperature are presented in Table 7, while those obtained for the IL mixtures at 0.25 and 0.75 mole fraction of [C2mim][AA] are provided in Supporting Information (Tables S16 and S17). The refractive index values of the IL mixtures are in between those of the neat ILs, for the five AAILs studied. From Figure 10a), it can be observed that the refractive indices (nD) of the neat AAILs can be ordered as [C2mim][Tau] ∼ [C2mim][L-Ala] < [C2mim][Gly] < [C2mim][L-Pro] < [C2mim][L-Ser]. The molar refraction or molar polarizability (Rm) was calculated from experimental data of both molar volume (Vm) and refractive index (nD) at the studied range of temperatures using the Lorentz−Lorenz relation (eq 9): ⎛ n2 − R m = ⎜ D2 ⎝ nD +

Figure 10. Measured (a) refractive indices (nD) and (b) free molar volumes ( f m) of the neat ILs as a function of temperature (T): ×, [C2mim][C(CN)3]; □, [C2mim][Gly]; ▲, [C2mim][L-Ala]; ○, [C2mim][Tau]; ●, [C2mim][L-Ser]; ■, [C2mim][L-Pro].

The unoccupied fraction of the molar volume of an IL is defined as molar free volume ( f m), which can be estimated by eq 10 fm = (Vm − R m)

1⎞ ⎟Vm 2⎠

(10)

where Vm and Rm are the molar volume and the molar refraction of the IL, respectively. The calculated molar refraction (Rm) values as well as the molar free volumes ( f m) of all the studied IL samples are listed

(9)

where Vm is the molar volume (cm3·mol−1) and nD is the refractive index.

Table 7. Measured Refractive Indices, nD, of the Neat ILs and the IL Mixtures at 0.5 of Mole Fraction (p = 0.1 MPa)a T

a

nD

(K)

[C2mim] [Gly]

[C2mim] [L-Ala]

[C2mim] [Tau]

[C2mim] [L-Ser]

[C2mim] [L-Pro]

[C2mim] [C(CN)3]0.5 [Gly]0.5

[C2mim] [C(CN)3]0.5 [L-Ala]0.5

[C2mim] [C(CN)3]0.5 [Tau]0.5

[C2mim] [C(CN)3]0.5 [L-Ser]0.5

[C2mim] [C(CN)3]0.5 [L-Pro]0.5

[C2mim] [C(CN)3]

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 ur(nD)a

1.52134 1.51997 1.51860 1.51724 1.51585 1.51450 1.51313 1.51179 1.51041 1.50904 1.50769 1.50637 1.50495 0.0001

1.51366 1.51223 1.51081 1.50937 1.50793 1.50651 1.50505 1.50358 1.50217 1.50074 1.49932 1.49795 1.49656 0.0001

1.51350 1.51217 1.51084 1.50950 1.50816 1.50680 1.50543 1.50403 1.50270 1.50136 1.49997 1.49860 1.49726 0.0008

1.52699 1.52565 1.52429 1.52294 1.52159 1.52022 1.51887 1.51755 1.51620 1.51486 1.51353 1.51220 1.51084 0.0020

1.52291 1.52151 1.52009 1.51870 1.51731 1.51591 1.51454 1.51315 1.51177 1.51035 1.50897 1.50757 1.50620 0.0001

1.51815 1.51656 1.51505 1.51355 1.51205 1.51057 1.50908 1.50760 1.50613 1.50462 1.50315 1.50170 1.50021 0.0003

1.51409 1.51258 1.51107 1.50950 1.50796 1.50643 1.50488 1.50338 1.50191 1.50039 1.49888 1.49739 1.49589 0.0001

1.51322 1.51166 1.51020 1.50871 1.50723 1.50574 1.50428 1.50286 1.50142 1.49996 1.49854 1.49709 1.49563 0.0007

1.51865 1.51718 1.51563 1.51409 1.51254 1.51099 1.50943 1.50784 1.50632 1.50475 1.50324 1.50173 1.50018 0.0001

1.51910 1.51760 1.51611 1.51460 1.51306 1.51153 1.51006 1.50858 1.50710 1.50564 1.50412 1.50269 1.50126 0.0003

1.51454 1.51286 1.51114 1.50938 1.50765 1.50595 1.50419 1.50255 1.50087 1.49921 1.49757 1.49593 1.49429 0.0002

ur(nD) are the relative standard uncertainties ur and u(T) = 0.02 K is the standard uncertainty u. I



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spectrometers are part of the National NMR Facility supported by Fundaçaõ para a Ciência e a Tecnologia (RECI/BBB-BQB/ 0230/2012).

in Supporting Information (Tables S18, S19, S20, and S21). The f m values as a function of T for all the neat ILs are shown in Figure 10b. The neat [C2mim][L-Pro] presents the highest free molar volume, while the neat [C2mim][Gly] has the lowest free molar volume. In fact, this behavior was also observed for all mixtures containing different mole fractions of AAIL. Moreover, the neat [C2mim][Gly], [C2mim][L-Ala], and [C2mim][L-Ser] display lower free molar volumes compared to those of the neat [C2mim][C(CN)3].

Notes

The authors declare no competing financial interest.

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CONCLUSION In the present work, five AAILs, namely [C2mim][Gly], [C 2 mim][ L -Ala], [C 2 mim][Tau], [C 2 mim][ L -Ser], and [C2mim][L-Pro], were synthesized. Mixtures of varying concentrations of these AAILs with [C2mim][C(CN)3] were prepared and their thermophysical properties (density, viscosity, and refractive index) were evaluated. Both the chemical structure of the amino acids and the presence of [C2mim][C(CN)3] can dramatically influence the thermophysical properties of the studied [C2mim][C(CN)3]x[AA](1−x) mixtures. In terms of the amino acid chemical structure, it was observed that the presence of the sulfonic acid group in the AAIL increased its density, as well as the density of its mixtures, but did not show significant effect on viscosity. On the other hand, the hydroxyl group in [C2mim][L-Ser] caused an increase in both the density and the viscosity, probably due to the extra hydrogen bond capacity. The bulky cyclic secondary amine in [C2mim][L-Pro] showed increased viscosity, while density remains similar to that of [C2mim][Gly] and [C2mim][L-Ala]. Overall, the results obtained in this work lead to the conclusion that mixing ionic liquids is a promising strategy to tune the AAIL’s thermophysical properties, which can thus be advantageously explored to further improve the performance of these ILs for specific applications, such as, for example, CO2 capture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00242. 1 H and 13C NMR of the synthesized amino acid-based ionic liquids. Tables with all the experimental data for the density, viscosity, and refractive index for the IL mixtures at 0.25 and 0.75 mole fraction of [C2mim][AA], as well as the thermal expansion coefficients, molar volume, excess molar volume, viscosity deviation, molar refraction, and free molar volume for all the neat ILs and their mixtures. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +351 21 4469724. Fax: +351 21 4411277. Funding

L.C.T. is grateful to FCT (Fundaçaõ para a Ciência e a Tecnologia) for her postdoctoral research grant (SFRH/BPD/ 101793/2014). I.M.M. acknowledges FCT/MCTES (Portugal) for a contract under Investigador FCT 2012. This work was partially supported by FCT through the project PTDC/QEQFTT/1686/2012 and Research Unit GREEN-it “Bioresources for Sustainability” (UID/Multi/04551/2013). The NMR J



Article

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