Thermophysical Characterization of the Mixtures of the Ionic Liquid 1

Article pubs.acs.org/jced

Thermophysical Characterization of the Mixtures of the Ionic Liquid 1‑Ethyl-3-Methylimidazolium Acetate with 1‑Propanol or 2‑Propanol María C. Castro, Alberto Arce, Ana Soto, and Héctor Rodríguez* Departamento de Enxeñería Química, Universidade de Santiago de Compostela, E-15782, Santiago de Compostela, Spain S Supporting Information *

ABSTRACT: A thermal and physical characterization of the binary mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) with 1-propanol or with 2-propanol was carried out. Thermogravimetric analyses confirmed that the stability of the ionic liquid is not reduced by the presence of the propanols. The density, viscosity, refractive index, and surface tension of the two binary systems were experimentally determined over the entire composition range, at atmospheric pressure, and from 288.15 to 348.15 K (for the system [C2mim][OAc] + 1-propanol) or to 338.15 K (for the system [C2mim][OAc] + 2-propanol). All properties were found to decrease with an increase in temperature and in the concentration of alcohol in the system. Negative excess molar volume, positive logarithm viscosity change of mixing, and mostly negative surface tension change of mixing were obtained from the experimental data for the isotherms investigated. An analysis of the trends of these property changes of mixing in systems [C2mim][OAc] + alcohol was carried out. In assessing the trend for the surface tension change of mixing, experimental data for the surface tension of the system [C2mim][OAc] + methanol were also obtained.

1. INTRODUCTION Ionic liquids are salts with a low melting or glass transition temperature, namely, below the arbitrary mark of 100 °C according to the currently accepted definition.1 The properties of the compounds of this family of substances are widely diverse, as a result of the varied chemical structures of the ions that are typically comprised in the very large number of cation− anion combinations that give rise to ionic liquids. Thus, some ionic liquids are hydrophilic and some others are hydrophobic; some are solid at room temperature and some remain liquid down to several dozens of negative degrees centigrade; etc. Nevertheless, many ionic liquids present a set of properties that include the following: extremely low vapor pressure, good thermal and chemical stability, wide liquid range, and great ability to dissolve a broad variety of solutes. In addition, the properties of ionic liquids can be tuned to a considerable extent by judicious tailoring of the chemical structures of their constituting ions, enabling the possibility of creating customized ionic liquids to meet the requirements of a particular application.1,2 These characteristics led to the suggestion of ionic liquids as a safer and more sustainable alternative to volatile organic compounds as solvents in reactions and separation processes, being a major driver of research on ionic liquids in the past decade. Given the nonvolatile nature of ionic liquids, the recovery of volatile solutes from the ionic liquid solution can be easily done by distillation or even by simple evaporation. However, when dealing with nonvolatile solutes, alternative strategies are needed.3 A common approach is the use of an antisolvent, © XXXX American Chemical Society

that is, a substance that is miscible with the ionic liquid but does not act as a solvent of the dissolved solutes. Thus, upon addition of the antisolvent, the solute will be precipitated out of the solution and will be recoverable from the medium by filtration or other unit operation for solid−liquid separation. Paradigmatic examples of handling of nonvolatile solutes in ionic liquid solution are the dissolution of cellulose and the pretreatment of lignocellulosic biomass with ionic liquids.4,5 An archetypical ionic liquid for this type of processes is 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]). The most common choice of antisolvent for regeneration of the (ligno)cellulosic solutes is water, which is entirely miscible with [C2mim][OAc] in any proportion.6 However, the relatively high boiling temperature and specific heat of water pose an important energy penalty at the stage of recovering the ionic liquid from its mixture with the antisolvent by vaporization of the latter. The use of light alcohols as antisolvents has also been reported,7−9 and may constitute a more viable alternative, while keeping the green credentials of the overall process at an acceptable level. Alternatively, alcohols mixed with the ionic liquid can act as cosolvents in more general applications. The use of molecular cosolvents is a possible approach to partially palliate the higher cost and higher viscosity of ionic liquids as compared to conventional solvents,2 provided that the introduction of a volatile component in the Received: December 1, 2015 Accepted: June 3, 2016

A

DOI: 10.1021/acs.jced.5b01023 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Source, Initial Mass Fraction Purity (wi), Method of Purification, Final Mass Fraction Purity (wf), and Method of Analysis for the Chemical Samples Used in This Work

a

chemical name

source

wi

purification method

wf

analysis method

1-propanol 2-propanol methanol [C2mim][OAc]b

Riedel-de Haën Sigma-Aldrich Aldrich IoLiTec

0.999 0.995 0.999 >0.95

none none none high vacuum, heating

0.999 0.995 0.999 >0.997

GCa GCa GCa 1 H NMR spectroscopy

Gas chromatography. b1-Ethyl-3-methylimidazolium acetate.

Table 2. CAS Number, Water Content (In Mass Fraction, wH2O), and Selected Physical Properties (Density (ρ), Viscosity (η), Refractive Index (nD), and Surface Tension (σ)) at 298.15 K and 101 kPa (Atmospheric Pressure) For 1-Propanol, 2-Propanol, Methanol, and [C2mim][OAc]. Literature Values (lit.) Were Selected and Are Displayed for Comparison with the Experimental Values (exp) Obtained in This Worka ρ/g·cm−3

η/mPa·s

σ/mN·m−1

nD

cmpd

CAS no.

w H2 O

exp

lit.

exp

lit.

exp

lit.

exp

lit.

1-propanol

71-23-8

0.0005

0.79956

1.952

0.0004

0.78084

1.38370b 1.3850c 1.3752b 1.3776c

23.4

67-63-0

methanol

64-17-5

n.a.i

0.78674

[C2mim][OAc]

143314-17-4

0.0012

1.09903

1.0993d 1.09778e

139.0

1.9430b 1.945c 2.0436b 2.038c 0.5513b 0.544c 143.61d 132.91e

1.38303

2-propanol

0.79960b 0.7997c 0.78126b 0.7809c 0.78637b

23.10b,h 23.32c 21.24b,h 20.93c 22.30b 22.07c 42.9e 47.1g

2.018 0.533

1.37517 n.a.i 1.50069

21.2 22.7

1.50091d 1.49992f

47.1

Standard uncertainties: u(ρ) = 1 × 10−4 g·cm−3, u(η) = 0.5%, u(nD) = 1 × 10−4, u(σ) = 0.3 mN·m−1, u(T)ρ = 0.002 K, u(T)η = 0.05 K, u(T)nD = 0.02 K, u(T)σ = 0.1 K, u(p) = 5 kPa. bReference 10. cReference 11. dReference 12. eReference 6. fReference 13. gReference 14. hValue interpolated linearly for 298.15 K from data at other temperatures. iNot available. a

The ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), with a nominal purity higher than 95%, was purchased from IoLiTec. It was subjected to reduced pressure ( ethanol ≈ 2-propanol > 1-propanol. This trend is quite similar to that discussed above for VE, basically with just the difference that Δ ln(η/η0) for the systems with ethanol and with 2-propanol is essentially coincident. Regarding ΔRM, its relatively small values prevented us from getting sufficiently smooth trends as to solidly discuss any composition or temperature effect in this magnitude for the studied systems. Figure 8 shows the surface tension change of mixing. In the case of [C2mim][OAc] + 1-propanol, the uncertainty of the calculated values of Δσ is proportionally high in comparison to their magnitude. For this reason, considerably scattered plots are obtained in Figure 8a, with no particularly smooth trend with composition, nor evident evolution with temperature for a given composition. Despite this fact, general trends can still be intuited, with Δσ being typically negative and reaching a minimum (i.e., a maximum in absolute value) in the band of J



Article

where Q is the excess property or property change of mixing, x1 and x2 are the mole fractions of the components of the mixture, and Ak represents the m + 1 polynomial coefficients. Root mean square deviations (rmsd) were evaluated for correlation of the properties with the Redlich−Kister polynomials, and a value of m = 3 was finally selected for VE and Δln(η/η0), and m = 2 for Δσ. The correlations thus obtained are depicted in Figures 6 to 9 as solid lines for each of the isotherms experimentally investigated. The correlation coefficients Ak, together with the corresponding rmsd values, are reported in Tables S7 to S9 in the Supporting Information.

Table 6. Experimental Surface Tension (σ) for the Binary System [C2mim][OAc] + Methanol, In the Temperature Range (278.2 to 318.2) K and at 101 kPa (Atmospheric Pressure), as a Function of the Mole Fraction of [C2mim][OAc] (xIL)a T/K xIL

278.2

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.6999 0.8003 0.9003 1.0000

23.8 28.5 32.7 35.9 38.4 40.3 42.1 44.3 45.9 47.6 48.1

288.2 σ/mN·m 23.6 28.1 32.1 35.3 38.0 40.0 41.6 43.8 45.1 46.9 47.7

298.2

308.2

318.2

21.9 26.8 31.1 34.5 37.1 39.0 40.8 42.8 44.2 45.9 46.6

21.0 26.3 30.6 33.9 36.5 38.7 40.4 42.4 43.9 45.3 46.1

−1

22.7 27.5 31.6 35.0 37.5 39.6 41.1 43.3 44.7 46.4 47.1

4. CONCLUSIONS The binary systems [C2mim][OAc] + 1-propanol and [C2mim][OAc] + 2-propanol were investigated from a thermal and physical point of view, and the results compared with those previously obtained for analogous binary systems of the same ionic liquid plus lighter alcohols.8,9 By thermogravimetric analysis, it was found that the presence of the propanols in the liquid mixture does not have a negative effect on the thermal stability of the ionic liquid. Density, viscosity, refractive index, and surface tension of the mixtures of [C2mim][OAc] with each of the propanols were determined over the entire composition range at atmospheric pressure and in the temperature ranges (288.15 to 338.15) K, in the case of the system with 2-propanol, or (288.15 to 338.15) K, in the case of the system with 1-propanol. All these properties decrease with an increase in temperature and with an increase in the concentration of the alcohol. For a given composition, the variation of refractive index and surface tension varied linearly, whereas for the description of density with temperature a second-order polynomial fit was preferred (the quadratic term was found to be statistically significant). The evolution of viscosity with temperature obeyed the exponential decay of the VFT equation. Negative values of the excess molar volume are indicative of the predominance of attractive forces in the liquid mixtures of ionic liquid and alcohol. Nevertheless, the evolution of VE with temperature suggests the relevance of spatial packing effects in the volumetric behavior of the studied systems. The viscosity logarithm change of mixing in the studied systems was positive over the entire composition range. The surface tension change of mixing was mostly negative in the studied systems, with a greater magnitude in the case of [C2mim][OAc] + 2-propanol. For the latter, a trend with the polarity of the alcohols was identified: the less polar the alcohol, the lower (less positive or more negative) Δσ. From this fact, it can be proposed that with a reduction of the polarity of the alcohol, its tendency to exhibit a higher concentration at the surface as compared to the bulk increases. In helping assess this trend, new surface tension measurements for the system [C2mim][OAc] + methanol were determined.

Standard uncertainties: u(xIL) = 0.0001, u(σ) = 0.3 mN·m−1, u(T)σ = 0.1 K, u(p) = 3 kPa.

a

Figure 9. Surface tension change of mixing (Δσ) for the binary system [C2mim][OAc] + methanol, as a function of the mole fraction of [C2mim][OAc] (xIL), at different temperatures: ○, 288.2 K; ▼, 298.2 K; ∇, 308.2 K; ■, 318.2 K. Solid lines represent the corresponding correlations by Redlich−Kister polynomials.

in the Supporting Information.) As observed, Δσ is clearly positive throughout the entire composition range, with maxima in the band of composition xIL = 0.3−0.5 for all the isotherms. Therefore, by simultaneous consideration of Figures 8 and 9 as well as the equivalent figure reported in our previous work for the system with ethanol,8 a gradual evolution of Δσ with the varying polarity of the alcohol in the systems [C2mim][OAc] + alcohol can be inferred: from totally positive for methanol, to positive−negative for ethanol, a transition to clearly negative for 1-propanol, and totally negative for 2-propanol; with the maxima in the ethanol-rich region and the minima in the ionic liquid-rich region. The excess properties and property changes of mixing could be suitably correlated at each temperature by means of empirical Redlich−Kister polynomials.25 For binary mixtures, these polynomial expressions are of the form:

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b01023. 1 H and 13C NMR spectra of the purified ionic liquid; details on the individual viscosity measurements; determination of Td,5%onset ′ in the TGA thermograms; comparison of TGA curves for binary systems [C2mim][OAc] + alcohol; DSC thermograms; fit parameters of

m

Q = x1x 2 ∑ Ak (x1 − x 2)k k=0

ASSOCIATED CONTENT

(13) K



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Article

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correlations; numerical values of excess molar volume and property changes of mixing; comparison of excess and change of mixing properties in binary systems [C2mim][OAc] + alcohol; and additional information on the surface tension of the binary system [C2mim][OAc] + methanol (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 881816804. E-mail: [email protected]. Funding

The authors acknowledge Xunta de Galicia (Spain) for support through projects EM 2012/042 and GPC 2014/026, and the Galician Network on Ionic Liquids (REGALIS). Notes

The authors declare no competing financial interest.

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REFERENCES

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Thermophysical Characterization of the Mixtures of the Ionic Liquid 1

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