Measurements, Correlations, and Predictions of Thermodynamic

Article pubs.acs.org/jced

Measurements, Correlations, and Predictions of Thermodynamic Properties of N‑Octylisoquinolinium Thiocyanate Ionic Liquid and Its Aqueous Solutions Marta Królikowska,* Kamil Paduszyński, and Maciej Zawadzki Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland S Supporting Information *

ABSTRACT: N-Octylisoquinolinium thiocyanate, [C8iQuin][SCN], has been synthesized in our laboratory from [C8iQuin][Br] as a substrate. The compound was characterized using 1H-NMR, 13C-NMR, and elemental analysis. Karl Fisher titration was used to determine the water content. The density and viscosity of pure ionic liquid were determined over a wide temperature range from (288.15 to 348.15) K. The basic thermal properties of the pure IL have been measured using by means of a differential scanning microcalorimetry technique (DSC). Decomposition of the IL was detected by the simultaneous TG/DTA experiments. (Liquid + liquid) phase equilibria (LLE) have been measured for binary mixtures {[C8iQuin][SCN] (1) + water (2)} using a dynamic method at atmospheric pressure. The experiments have been done over a broad range of ionic liquid mole fraction, x1, and temperatures from 250 K to the boiling point of water. Experimental heat capacities and excess enthalpies were determined for {[C8iQuin][SCN] (1) + water (2)} binary mixtures for different IL mole fractions at a wide range of the temperature at ambient pressure. The influence of temperature and composition was assessed, and suitable equations were used to correlate the experimental data. The experimental results were compared with data for other ionic liquids available in the open literature.

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INTRODUCTION Ionic liquids (ILs) are defined as substances composed of an organic cation and inorganic or organic anion. Today an IL means an organic salt with a melting point below 100 °C. They generally feature good stability in air and water, wide liquid temperature range, high chemical and thermal stability, negligible vapor pressure, and relatively favorable viscosity and density characteristics. Due to the unique properties, ILs create many interesting possibilities as functional fluids and performance additives in various materials and technologies.1−5 From the viewpoint of green chemistry and chemical engineering, the possibility of using ILs in many technologies including the absorption refrigeration is essential. This is due to the fact that the absorption chillers are able to use the lowtemperature heat streams that are normally wasted and expelled into the atmosphere. The physicochemical and thermodynamic properties of the working pair fluid are one of the key properties for determining the performance of absorption refrigerators. Generally speaking, in absorption chillers the fluid with strong volatility is used as a refrigerant, while the second one with much lower volatility acts as an absorbent. Up to now, the most commonly used working fluids are {water + lithium bromide} or {ammonia + water}.6 However, these systems show a number of drawbacks, such as: corrosiveness, toxicity, high working pressure, the need © 2013 American Chemical Society

for {NH3 + H2O} rectification, or restricted range of the temperature due to the {H2O + LiBr} crystallization. Due to the unique properties it is possible to use the IL as a new absorbent of coolants for absorption refrigerators or absorption heat pumps. In fact, the knowledge of thermodynamic and physicochemical properties of pure IL and their aqueous solution is essential to determine the potential applications in this area. Several research groups have studied the phase equilibria, heat capacities, and excess enthalpies for the {IL + water} binary systems.7−26 This article is an addition to the database in this area and a continuation of our systematic investigation of thiocyanate-based ILs. We present the synthesis, analysis, and thermophysical and physicochemical properties of pure IL Noctylisoquinolinium thiocyanate, [C8iQuin][SCN], as well as the liquid−liquid phase equilibria, excess enthalpies, and isobaric heat capacities of {[C8iQuin][SCN] + water} binary mixtures over a wide temperature range. These data are very important and play a key role in evaluating IL and water binary systems for absorption refrigeration applications. Apart from Received: August 1, 2012 Accepted: December 18, 2012 Published: January 2, 2013 285

dx.doi.org/10.1021/je300853z | J. Chem. Eng. Data 2013, 58, 285−293

Journal of Chemical & Engineering Data

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(298.15 to 348.15) K. The nominal uncertainty of the measured was estimated to be better than ± 5.0 % and reproducibility < 0.05 % for viscosities in the wide range of (0.3 to 2500) mPa·s. The temperature was controlled internally with a precision of ± 0.01 K. In this range of viscosity values the diameter of the capillary was 3.0 mm, and the error for this capillary was < 0.01 %. Differential Scanning Calorimetry (DSC). Basic thermal characteristics of the IL, that is, the glass transition temperature (Tg) and the change of heat capacity at the glass-transition temperature (ΔCp(g)), have been measured using a differential scanning calorimetry (DSC) technique at the 5 K·min−1 scan rate. The instrument (Perkin-Elmer Pyris 1) was calibrated with the 99.9999 mol % purity indium sample. The calorimetric accuracy was ± 3 %, and the calorimetric precision was ± 0.5 %. Decomposition of Compounds (TG/DTA). The thermal decomposition temperature for [C8iQuin][SCN] (Td) was determined by the SDT Q600 (TA Instruments) thermogravimetric analyzer. A sample of 7.51 mg was analyzed in a ceramic pan using argon as the purge gas with a flow rate of 100 mL·min−1. The temperature was linearly increased at 5 K·min−1 from (273.15 to 623.15) K. The uncertainty of decomposition temperature measurement was ± 1 K. Liquid−Liquid Equilibrium Measurements. The liquid− liquid phase equilibria (LLE) in binary systems {[C8iQuin][SCN] + water} have been determined using the dynamic method. The binary mixtures have been prepared by weighing into a Pyrex glass cell using Mettler Toledo AB204-S analytical balance with an accuracy of ± 0.0001 g. Then, the sample was placed into the thermostatic bath and heated very slowly (less than 2 K·h−1 close to the equilibrium temperature) with continuous stirring inside the cell. The temperature of the twophase disappearance was detected visually during an increasing temperature period. During the experiment, the temperature was controlled by calibrated electronic thermometer P550 (DOSTMAN electronic) with an uncertainty ± 0.5 K. The experiments have been done over a wide range of the IL’s mole fraction and temperature. Heat Capacity Measurements. Isobaric heat capacity (Cp) measurements were carried out applying modulated differential scanning calorimetry (MDSC) technique.28 The MDSC experiments were performed with DSC Q2000 (TA Instruments) calorimeter equipped with liquid nitrogen cooling system and operating in a heat-flux mode. The sample cell was constantly fluxed with high purity helium gas at constant flow rate of 25 mL·min−1. A sample size of about 20 mg was used throughout this study, and the heat flow was normalized by the actual weight of each sample. The experiments were carried out using 2 K·min−1 heating rate, 1 K sinusoidal temperature modulation amplitude, and 120 s modulation period. The MDSC data were analyzed using TA Universal Analysis software. A detailed description of the mentioned analysis have been presented by us earlier.52 Excess Enthalpy Measurements. Excess molar enthalpies of mixing (HE) for binary mixture {[C8iQuin][SCN] + water} were obtained by calorimetry, using an isothermal titration calorimeter (ITC, model TAM III, TA Instruments, USA). The titration and reference cells were placed in the test wells of the highly stable thermostatic oil bath. During the experiment, the temperature of the oil bath was maintained at T = (298.15, 303.15, 308.15) K respectively for 24 h with a stability of ± 100 μK. The whole apparatus was housed in a constant temperature booth, also maintained at very close to each temperature.

that, these data can give more detailed physical insight into the mutual affinity between IL and water.

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EXPERIMENTAL SECTION Materials. The IL named: N-octylisoquinolinium thiocyanate, [C8iQuin][SCN] (> 97 %), was synthesized in our laboratory from N-octylisoquinolinium bromide, [C8iQuin][Br] (> 95 % from NMR analysis), as a substrate. For the synthesis of [C8iQuin][Br] a procedure described earlier was used.27 A portion of 40.20 g of N-octylisoquinolinium bromide (0.1247 mol) was exchanged on anion exchange resin charged with hydrocarbonate. Afterward 11.62 g of sodium thiocyanate (as received, Aldrich 98 %; 0.1433 mol, 1.15 equiv) was added. The mixture was extracted 5 times with 25 mL of dichloromethane. Solvents were removed in vacuum, and the product was dried under vacuum at 333.15 K for 72 h. The yield was 34.33 g (91.74 % of theoretical value). The structure of the [C8iQuin][SCN] is given below:

The IL was dried in vacuum at 300 K for approximately 24 h to remove volatile compounds and water. The final product was characterized with 1H NMR, 13C NMR, and elementary microanalysis. In respect to the product, the following information was determined: 1 H NMR (400 MHz, CDCl3): δ = 0.237 (t, 3H, J = 6.8); 0.637 (m, 6H); 0.791 (quint, 2H); 0.893 (quint, 2H, J = 7.6); 1.635 (quint, 2H, J = 7.6); 4.457 (t, 2H, J = 7.6); 7.417 (m, 1H); 7.615 (m, 1H); 7.676 (m, 1H); 7.970 (d, 1H, J = 7.2); 8.094 (d, 1H); 8.273 (m, 1H); 9.763 (s, 1H). 13 C NMR (400 MHz, CDCl3): δ = 13.316, 21.743, 23.437, 28.186, 28.213, 30.834, 31.052, 61.329, 125.928, 126.524, 126.961, 130.064, 130.703, 130.885, 133.723, 136.453, 148.720. The NMR spectra for [C8iQuin][SCN] are shown as Figures S1 to S5 in the Supporting Information. The elementary microanalysis. Found: C 71.20 %, H 7.95 %, N 9.35 %, S 10.74 %, Theory: C 71.95 %, H 8.05 %, N 9.32 %, S 10.67 %. Water Content. The water content of IL was analyzed by Karl Fischer titration technique (model SCHOTT Instruments TitroLine KF). A sample of [C8iQuin][SCN] was dissolved in dry methanol and titrated with steps of 2.5 μL. The analysis have shown that the water content was less than 230 ppm. Density Measurements. The density (ρ) of the [C8iQuin][SCN] was measured at ambient pressure and at temperature T = (288.15 to 348.15) K using an Anton Paar GmbH 4500 vibrating-tube densimeter (Graz, Austria). Two integrated Pt 100 platinum thermometers provided good precision of the internal control of temperature ± 0.01 K. The densimeter includes an automatic correction for the viscosity of the sample. The calibration for temperature and pressure was made by the producer. The apparatus is precise to within ± 10−5 g·cm−3, and the uncertainty of the measurements was estimated to be better than ± 10−4 g·cm−3. Viscosity Measurements. The Anton Paar BmbH AMVn (Graz, Austria) programmable rheometer was used to measured dynamic viscosity (η) of pure IL over temperature range from 286



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cationic and anionic structures, including isoquinolinium thiocyanates. Additionally, predictions from Paduszyński− Domańska model were compared those from with GC version of Patel−Teja equation of state developed by recently by Shen et al.31 It should be noted that the most of remaining models exhibit good accuracy of predictions as well. However, they were developed based on ionic, rather than functional group, contributions, and hence their range of applications is significantly limited. Heat Capacity Prediction. The models allowing predicting Cp of ILs comprises mainly some more or less elaborated correlations based on different ideas. Thus far, the most popular models are those involving other physical properties like molar volume V,32 or topological indices, for example, mass connectivity index λ.33,34 Group contributions (GC) methods35−38 and approaches employing artificial neural networks (ANN)39,40 seem to be attractive as well. In this work only selected models to predict Cp for pure IL investigated at different temperatures and Cp of the binary mixtures with water have been used. The models investigated were: Cp−V correlation of Paulechka et al.32 and Gardas et al.,35 Cp-λ correlations of Valderrama et al.33,34 and ANN-based method of Lashkarbolooki et al.39 Unfortunately, the other models (considering their current development status) are not capable of predicting Cp for systems involving ILs based on isoquinolinium and/or thiocyanate ions due to the lack of appropriate model parameters. HE Correlation with UNIQUAC Model. The universal quasichemical (UNIQUAC) activity coefficient model proposed and developed by Abrams and Prausnitz41 was adopted to describe experimental excess enthalpies for {[C8iQuin][SCN] (1) + water (2)}. The model provides the expression for the molar excess Gibbs free energy of the mixture, GE, as a function of mixture’s composition and temperature. The combinatorial (entropic) and residual (enthalpic) contributions are taken into account explicitly. The excess enthalpy is readily obtained by differentiation of GE with respect to temperature, using the well-known thermodynamic relation:

Measurements were started from placing about 0.5 mL of pure IL in the stainless steel ampule (titration cell), which was placed into the thermostatic oil bath and equilibrated a few hours. Depending on the change in mole fraction of IL, (2 to 15 ± 0.001) μL of water was injected into the titration cell using the precise syringe pump. The mixture was rigorously stirred during the titration. The stirring speed was set to 100 rpm. The number of moles of the injected fluid was calculated from the volume with the known density and molecular weight. The measured property is the difference in heat flow between sample and reference cells. The uncertainty of this measurement is about ± 0.2 %. Integration of the heat flow peaks results in the total amount of heat effect during the j-th injection (δqj). This quantity is readily transformed into total molar excess enthalpy of mixing corresponding to i injections (HiE): i

HiE

=

∑ j = 1 δqj i

n1 + ∑ j = 1 Δn2, j

(1)

where n1 is the number of moles of IL and Δn2,j is the number of moles of water injected during j-th titration (please note that in further text we will denote the IL and water as 1 and 2, respectively). The uncertainty of the HE data determined in the present study is estimated to be less than 0.5 %.

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THEORETICAL SECTION In this section a summary of equations used to regress and/or predict the obtained experimental data is presented. In particular, it is very important from scientific and practical point of view to test the available thermodynamic tools (models/correlations) to assess their predictive capacity and reliability. Therefore, we also provide a evaluation of performance of different, mostly empirical, approaches allowing predicting density and heat capacity of ILs. The results of calculations for [C8iQuin][SCN] are compared with the experimental data reported in this work. As a measure of quality of correlation/prediction of a given property X (where X = ρ, η, Cp, x1, HE) the root-mean-square deviation (RMSD) and average absolute relative deviation (AARD) were used: ⎡ 1 RMSD(X ) =⎢ ⎢⎣ N − k AARD(X )

=

1 N

N

∑ i

⎤1/2

N

∑

(Xicalcd

−

Xiexptl)2 ⎥

i

Xicalcd − Xiexptl Xiexptl

⎡ ∂(GE /RT ) ⎤ HE = −RT 2⎢ ⎥ ∂T ⎣ ⎦ p ,x

(3)

A comprehensive description and derivation of the UNIQUAC equations can be found elsewhere.41 In this paper only a brief summary of the model parameters and some methods for their determination is presented. The combinatorial term accounts for the effects of differences in size and shape of molecules forming mixture. Those two features are represented in UNIQUAC as geometric molecular parameters r (molecular size) and q (molecular surface area). Although this term does not contributes to HE, the parameter q remains relevant. The parameters for water were taken from literature:42 r2 = 0.920 and q2 = 1.400. For [C8iQuin][SCN], they were calculated from the experimental molar volume at T = 298.15 K, by using the following correlation:43

⎥⎦

·100 % (2)

where and N and k stand for number of data points and number of adjustable parameters. Density Prediction. The density of pure [C8iQuin][SCN] at ambient pressure can accurately described by assuming that thermal expansion coefficient is constant over the range of temperature covered by the experimental data. Then, the linear dependence of logarithm of density versus temperature is obtained. The density of ILs can be also predicted by using numerous correlations reported in literature. According to the recent review of Coutinho et al.,29 the best predictive model was group contribution (GC) method proposed by Paduszyński and Domańska,30 and hence, this model was used in this work. The advantage of this model over the others is that it is capable of accurately describe a huge number of simple and very complex

r1 = 0.029281V1/cm 3·mol−1 q1 =

(z − 2)r1 2(1 − l1) + z z

(4)

where z is a coordination number of quasi-lattice, as usually set equal to 10. The symbol l1 stands for the bulkiness factor of IL: it was assumed that l1 = 0. 287



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The residual contribution accounts for differences in interactions between pairs of like and dislike molecules. Two binary interaction parameters per a pair of solutes, namely, τ12 and τ12, are introduced. To describe more effectively the experimental data reported over a wide range of temperature, the energetic parameters are allowed to be temperaturedependent, with accordance to the following formula: ⎡ ⎤ bij τij = exp⎢aij + + cij ln(T /K)⎥ T /K ⎣ ⎦

Table 1. Density (ρ), Viscosity (η), and Isobaric Heat Capacity (Cp0) as a Function of Temperature (T) for Pure IL [C8iQuin][SCN] at Pressure p = 0.1 MPaa T/K 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

(5)

The six coefficients a12, b12, c12, a21, b21, and c21 were obtained by fitting to experimental HE data by means of a standard nonlinear least-squares procedure.

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RESULTS AND DISCUSSION In this section the results of measurements of various physical properties of pure [C8iQuin][SCN] and its aqueous solutions are summarized and discussed. In particular, basic thermal characterization of pure IL, namely, DSC, TG/DTA, density (ρ), viscosity (η), and heat capacity of pure IL (Cp0), as well as the properties of aqueous solutions of [C8iQuin][SCN], such as liquid−liquid equilibrium (LLE), heat capacities (Cp), and excess enthalpies (HE), are presented over wide range of composition at different temperatures. Pure [C8iQuin][SCN]. The thermal analysis of [C8iQuin][SCN] is summarized in Figure 1. TGA/DTA measurements

ρ/g·cm−3

1.06727 1.06444 1.06169 1.05882 1.05596 1.05311 1.05026 1.04742 1.04457 1.04172 1.03894 1.03620 1.03347

η/mPa·s

2388.0 1612.1 1109.5 743.87 513.78 365.48 266.77 199.26 152.21 118.51 93.828

Cp/J·K−1·mol−1 510 513 516 519 522 525 528 531 533 535 537 538 540 540

a

Standard uncertainties u are as follows: u(ρ) = 0.01 %; u(η) = 5 %, u(Cp) = 3 %, and u(T) = 0.01 K.

and shown in the Figure 2 (panel a and b, respectively). The density of the IL ranges in values from 1.06727 g·cm−3 at T = 288.15 K to 1.03347 g·cm−3 at T = 348.15 K. The experimental data can be accurately represented with the following equation: ln ρ /(g·cm−3) = 0.059822 − 5.3861·10−4(T /K − 298.15) (6)

Figure 1. Results of thermal analysis of IL [C8iQuin][SCN]: (a) thermal decomposition, TG/DTA; (b) glass transition, DSC.

shown in Figure 1a revealed thermal decomposition of IL at Td = 528 K. The decomposition temperature is approximately the same as that obtained previously for [C6iQuin][SCN] (Td = 531 K).51 In turn, a glass transition at Tg = (216.1 ± 3.0) K (with heat capacity change due to this transition, ΔCp(g) = 136.8 J·K−1·mol−1) was evidenced by DSC measurements, as was shown in Figure 1b. Similar values of Tg and ΔCp(g) were observed for [C6iQuin][SCN] (216.4 K and ΔCp(g) = 136.8 J·K−1·mol−1, respectively).51 The experimental data of the density and viscosity of pure [C8iQuin][SCN] at different temperatures are listed in Table 1

Figure 2. Pure fluid properties for ILs [C8iQuin][SCN] (this work) and [C6iQuin][SCN]:52 (a) liquid density (ρ)−experimental data (markers) and predictions with GC models of Paduszyński and Domańska30 and GC equation of state of Shen et al.;31 (b) liquid viscosity (η)−experimental data (markers) and correlation using eq 7. 288



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where R2 = 0.99998, RMSD(ρ) < 10−4 g·cm−3, and AARD(ρ) < 0.01 %. The slope in eq 6 corresponds to isobaric thermal expansion coefficient of IL, that is, αp = 5.3861·10−4 K−1. According to eq 6 the value of αp is constant over the studied temperature range. In Figure 2a predictions of density of [C8iQuin][SCN] and [C6iQuin][SCN] with Paduszyński− Domańska30 and Shen et al.31 GC models are presented. The performance of the former method is much better than the latter one, which is clearly evidenced by the values of AARD(ρ) (< 1 % and ≈ 10 %, respectively). It should be noted that isoquinolinium thiocyanates were not included into the models development process and hence, the presented calculations are pure predictions. The viscosity of [C8iQuin][SCN] varies from 2388 mPa·s at T = 298.15 K to 93.828 mPa·s at T = 348.15 K, see Figure 2b. The experimental data given in Table 1 were described with the well-known Vogel−Fulcher−Tammann equation: ln η /(mPa·s) = − 4.80774 +

1791.44 T /K − 156.1

(7)

where the coefficient in the denominator was fixed as (Tg − 60 K). The resulting statistics are as follows: R2 = 0.99969, RMSD(η) = 25.6 mPa·s, and AARD(η) = 1.6 %. A summary of heat capacity data treatment is given in Figure 3. Experimental Cp0 data for [C8iQuin][SCN] was correlated by using the quadratic function: C p0/(J·K−1·mol−1) = − 8.4159 + 2.9768(T /K) − 0.00401(T /K)2

(8)

where R2 = 0.99781, RMSD(Cp0) = 0.5 J·K−1·mol−1, and AARD(Cp0) = 0.08 %. The resulting fit is demonstrated in Figure 3a. In turn, calculations of Cp0 for the tested IL in terms of different predictive models are shown in Figure 3b. It can be seen that correlation proposed by Valderrama and Rojas33 provides the most accurate description of experimental data. However, that method is actually a correlation for temperature dependence of Cp, and it takes experimental Cp at T = 298.15 K as a reference value. The other, entirely predictive, models fail and overestimate the experimental values within about 10 % and 25 % in the case of ANN model. It should be stressed that the adopted models are based on empirical correlations, and they were developed based on experimental data only, whereas isoquinolinium ILs were not included in their development as they are completely new and still weakly characterized compounds. As a final comment to results for pure IL we will briefly discuss the impact of the structure of isoquinolinium thiocyanates on their physical properties. The density of [C8iQuin][SCN] is quite low and comparable to density of other [SCN]-based ILs published thus far.44−51 In particular, the comparison with [C8iQuin][NTf2]27 [where [NTf2] = bis(trifluoromethyl-sulfonyl)imide] shows that the IL tested in this work has a lower density but more than six times higher viscosity. In particular, the comparison with [C8iQuin][NTf2]27 (where [NTf2] denotes bis{trifluoromethyl}sulfonylimide) shows that the tested IL has lower density but more than six times higher viscosity. Moreover, with the increase of the alkyl chain length in the quinolinium cation the density decreases from (1.08134 to 1.06169) g·cm−3 and viscosity increases from (745.1 to 2388) mPa·s for [C6iQuin][SCN]51 and [C8iQuin][SCN], respectively (at T = 298.15). This tendency is confirmed in the case of imidazolium-based ILs where the

Figure 3. Experimental and predicted heat capacity (Cp) of pure IL [C8iQuin][SCN] and its aqueous solutions: (a) experimental data and correlation; (b) prediction of Cp in terms of empirical correlations of Valderrama and Rojas,33 Valderrama et al.,34 Paulechka et al.,32 Gardas and Coutinho,35 and artificial neural network (ANN) developed by Lashkarbolooki et al.39

density decreases with the increase in length of the alkyl substituent in the imidazolium ring from (1.11684 to 1.05692) g·cm−3 for 1-ethyl-3-methylimidazolium44 and 1-hexyl-3-methylimidazolium48 thiocyanate, respectively. Apart from that, an increase in viscosity of those ILs with increase of the alkyl chain length in the imidazolium ring was also observed [from (22.15 to 93.62) mPa·s] at 298.15 K. The viscosities of thiocyanatebased ILs with isoquinolinium cations are much higher than that for ILs with the imidazolium cation. For example, the viscosity of [HiQuin][SCN]48 at 298.15 K was 93.62 mPa·s, while the viscosity measured for [HiQuin][SCN]51 was equal to 745.1 mPa·s under the same conditions. Finally, the decrement in density due to the length of the alkyl substituent in the isoquinolinium ring corresponds to an increment in molar volume equal to 15.5 cm3·mol−1 per single CH2 group, which is in good agreement with the imidazolium series of ILs based on the [SCN] anion (15.4 cm3·mol−1).44,48 The heat capacity of [C8iQuin][SCN] is higher than in the case of [C6iQuin][SCN].52 The difference at T = 298.15 K is 71 J·K−1·mol−1, which corresponds to ca. 30 J·K−1·mol−1 per addition of the CH2 group to alkyl side chain of the cation. For a comparison, one can refer to a paper published recently by Rocha et al.53 The authors measured and reported Cp0 at T = 298.15 K for the extended series of imidazolium ILs based on 289



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comparison with ILs based on the [NTf2] anion where the immiscibility gap started from x1 ≈ 0.75.51 As can be noticed, the water solubility decreases with increase of the alkyl chain length in the cation. A different type of phase behavior was detected for ILs based on the thiocyanate anion and imidazolium, pyrrolidinium, and piperidinium cations. For aqueous solutions of those ILs complete miscibility in the liquid phase in the whole range of IL mole fraction was detected.48−50,55 The experimental heat capacities, Cp, were measured for {[C8iQuin][SCN] (1) + water (2)} binary systems in the temperature range from T = 278.15 K to T = 343.15 K and IL mole fractions from x1 ≈ 0.4 to pure fluid. The experimental data versus x1 at different temperatures are collected in Table 3 and shown in Figure 3. As it can be observed, the Cp increases slightly with increasing temperature at the same mole fraction. Moreover, the additivity principle

the [NTf2] anion. For those ILs, the increment in heat capacity per methyl group was 47.3 J·K−1·mol−1. We can see that in spite of totally different anion’s chemical nature the trend for isoquinolinium thiocyanates seems to be very similar. Aqueous Solutions. Table 2 includes the direct experimental results of the LLE temperatures versus IL mole fraction Table 2. Experimental Liquid−Liquid Equilibrium (LLE) IL Mole Fractions (x1) versus Temperature (T) Data for the Binary System {[C8iQuin][SCN] (1) + Water (2)} at Pressure p = 0.1 MPaa x1

T/K

0.3206 0.3119 0.3070 0.2987 0.2921 0.2855 0.2796 0.2673 0.2585 0.2543 0.2487

298.5 305.0 312.2 318.9 326.7 332.7 336.2 344.1 347.6 350.0 352.7

Cp = x1C p0,1 + (1 − x1)C p0,2

(9)

provides a good approximation of experimental data over the whole range of concentration and temperature: RMSD(Cp) = 4.5 J·K−1·mol−1, AARD(Cp) = 1.0 %. Figure 5 shows the results of the excess enthalpies for the {[C8iQuin][SCN] (1) + water (2)} binary mixtures versus x1 at different temperatures from (298.15 to 308.15) K. A detailed list of experimental data can be found in Supporting Information, Table S1. The system investigated in this work exhibits the “S-shaped” behavior; that is, partially positive and partially negative excess enthalpies were observed depending on the range of IL concentration. The change of sign of the HE is clearly seen both in Figure 5a, where raw power-time data from TAM III calorimeter are shown, and in Figure 5b, where the final HE dependence on IL mole fraction and temperature is presented. Exothermic and endothermic mixing effects were also observed for other ILs based on the [SCN] anion.13,52 Moreover, the presence of the miscibility gap in the system is confirmed by calorimetric measurements, and the agreement between solubility of water in IL determined from HE and direct LLE measurements is satisfying. The comparison with [C6iQuin][SCN]52 shows that the HE increases with the increase of the alkyl chain length in the isoquinolinium cation. In fact, for aqueous solutions of [C6iQuin][SCN] the excess enthalpies were between (−290 and 355) J·mol−1, while for [C8iQuin][SCN] between (−476 and 129) J·mol−1 at T = 298.15 K. Furthermore, we observed that HE at high IL concentrations becomes more negative as temperature increases, and HE at low IL concentration (i.e., close to LLE composition) increases with temperature. The values for RMSD and AARD are reported including the data point at all three temperatures. UNIQUAC correlation in immiscibility region is a straight line connecting (x1, HE) data points at LLE solubility limits (for further details, see, for example, papers of Gmehling et al.). Finally, it can be seen that the UNIQUAC equation with the regressed parameters shown in Table 4 describes the experimental data with satisfactory accuracy: RMSD(HE) = 23.0 J·mol−1, AARD(HE) = 26.1 % (including only the data from complete miscibility region). In particular, LLE is captured by the model and hence, we attempted to calculate LLE phase diagram for the studied system by using the parameters optimized on the basis of HE. The predictions are shown in Figure 4 and the resulting RMSD (x1) = 0.14, AARD(x1) = 49 %. Although the calculated and

a

Standard uncertainties u are as follows: u(x1) = 0.0001 and u(T) = 0.5 K.

(x1) at the equilibrium temperatures for the investigated system. Figure 4 represents the data graphically. The LLE

Figure 4. Experimental liquid−liquid equilibrium (LLE) mole fraction (x1) versus temperature (T) in the binary system {[C8iQuin][SCN] (1) + water (2)} and comparison with literature data for aqueous systems with [C6iQuin][SCN]51 and [C8iQuin][NTf2].54 The shaded area designates immiscibility region predicted by UNIQUAC equation with parameters given in Table 4.

diagram with upper critical solution temperature (UCST) was observed in the ILs mole fraction from x1 ≈ 10−4 to 0.32. The maximum of the curve was not detected because of the boiling point of the solvent was lower. A comparison of the water solubility in [C6iQuin][SCN]51 and [C8iQuin][NTf2]54 shows that the best solubility decreases in the following series: [C6iQuin][SCN] > [C8iQuin][SCN] > [C8iQuin][NTf2]. The strong interaction between thiocyanate-based IL and water has been observed as evidenced by a small immiscibility gap in 290



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Table 3. Experimental Heat Capacity (Cp) for the Binary System {[C8iQuin][SCN] (1) + Water (2)} as a Function of IL Mole Fraction (x1) and Temperature (T) at Pressure p = 0.1 MPaa Cp/J·K−1·mol−1

a

T/K

x1 = 0.8935

x1 = 0.8097

x1 = 0.7214

x1 = 0.5945

x1 = 0.5237

x1 = 0.3977

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15

467 470 474 476 479 482 484 487 490 492 495 497 500

423 426 429 432 434 437 439 441 441 442 443 443 442

386 388 391 393 396 398 400 402 404 405 406 408 408

334 337 340 342 344 347 349 351 353 355 357 358 360

295 304 306 309 311 313 315 316 318 319 320 321 322

248 250 251 253 255 257 258 260 261 261 262 263 263

Standard uncertainties u are as follows: u(x1) = 0.0001, u(Cp) = 3 %, and u(T) = 0.01 K.

Figure 5. Excess enthalpies of mixing (HE) in binary systems {[C8iQuin][SCN] (1) + water (2)}: (a) raw power-time data from TAM III calorimeter (titration of water into IL at T = 303.15 K); (b) experimental HE data and UNIQUAC correlation with parameters given in Table 4.

design and operation. Moreover, they may play the key role in evaluating IL and water binary systems for absorption refrigeration applications. Apart from that these data are a source of purely scientific information about the mutual affinity between IL and water in aqueous solutions.

Table 4. UNIQUAC Parameters for the Excess Enthalpies of {[C8iQuin][SCN] (1) + Water (2)} Binary Systems, See eq 5 i

j

aij

bij

cij

1 2

2 1

51.0916 33.3151

−1252.2 −2233.5

−8.3296 −4.5052

■

experimental solubilities of water in [C8iQuin][SCN] are similar and the type of LLE phase behavior are captured properly, the value of UCST is significantly lowered.

ASSOCIATED CONTENT

S Supporting Information *

NMR spectra for [C8iQuin][SCN], Figures S1 to S5. Experimental data on HE as a function of IL concentration and temperature, Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

CONCLUSION In this work, the basic physical and thermodynamic properties for a new synthesized IL, [C8iQuin][SCN], have been determined at atmospheric pressure using various experimental techniques. The density, viscosity, and heat capacity of pure IL were determined at a wide range of temperature. Properties of aqueous solutions of the IL such as liquid−liquid phase equilibria, isobaric molar heat capacities, and excess enthalpies of mixing, were determined in different IL compositions and at a wide range of the temperature at ambient pressure. On the basis on the results obtained some influence of both cation and anion structures on general trends governing the properties were established and discussed. We believe that the data and correlations presented in this work can be very useful from the point of view of process

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Fax: +48-22-628 27 41. Tel.: +48-22-234 56 40. Funding

Funding for this research was provided by the National Science Centre in years 2011 to 2014 (Grant No. 2011/01/D/ST5/ 02760). Notes

The authors declare no competing financial interest. 291



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