Experimental and theoretical study on infinite dilution activity coefficients of various solutes in ionic liquid 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

J. Chem. Thermodynamics 140 (2020) 105894

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Experimental and theoretical study on infinite dilution activity coefficients of various solutes in ionic liquid 1-propyl-2,3dimethylimidazolium bis(trifluoromethylsulfonyl)imide Zhuang-Zhuang He a,b,c, Rui-Qi Li a, Ai-Lin Sun a, Zhao Mu a,c, Yan-Li Xiao a, Hui-Wen Chang a, Yu-Hai Jiao a,c, Ming-Lan Ge a,b,c,⇑ a

College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China Beijing Key Laboratory of Fuels Cleaning and Advanced Catalytic Emission Reduction Technology, Beijing 102617, China c Institute of Oilfield Applied Chemistry and Chemical Technology, Beijing 102617, China b

a r t i c l e

i n f o

Article history: Received 29 April 2019 Received in revised form 3 August 2019 Accepted 6 August 2019 Available online 7 August 2019 Keywords: Activity coefficient at infinite dilution [PMMIM][NTf2] Solubility parameter Selectivity Separation

a b s t r a c t The infinite dilution activity coefficients (c1 i Þ and gas–liquid partition coefficients (KL) of solutes in the ionic liquids (ILs) are important parameters of thermodynamic properties. They can evaluate the separation selectivity (S1 ij ) of ILs and screen out suitable extractants for difficult separation systems. The retention times of solutes in IL 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [PMMIM][NTf2] were measured by gas chromatography (GC) to calculate the c1 i and the ΚL values of organic solutes (eg. n-alkanes, alkyl benzenes, acetonitrile, tetrahydrofuran, ethyl acetate, etc.) in [PMMIM][NTf2] at 303.15 K-353.15 K. The density of [PMMIM][NTf2], as a function of temperature, was E;1 measured at p = 101.3 kPa. The values of the partial molar excess enthalpies at infinite dilution (Hi ) E;1 1 were derived from the T dependence of the lnci . The entropies (Tref Si ) and Gibbs free energies E;1 (Gi ) of organic solutes in [PMMIM][NTf2] at a reference temperature Τref = 298.15 K can be calculated from the c1 i values. The Hildebrand’s solubility parameters of the IL [PMMIM][NTf2] were obtained from the regular solution theory (RST) combined with Flory ‘‘combinatorial” equation. At 323.15 K, the selec1 tivity (S1 ij ) of n-hexane (i)/benzene (j) and cyclohexane (i)/benzene (j) and the capacity (kj ) at infinite dilution of benzene have been determined. The results were analyzed by comparing with literature data for alkylimidazolium-based ILs with [NTf2]. Ó 2019 Elsevier Ltd.

1. Introduction The ionic liquids (ILs) are liquids at or near room temperature. They can be described by room temperature ionic liquid (RTIL), non-aqueous ionic liquids, liquid organic salts, etc. [1]. Their unique properties, such as non-volatile, non-combustion, high stability, good solubility have made many chemists be aware of the huge application prospects of ILs [2,3]. Today, ILs are regarded as a new type of green solvent, which has attracted the attention of many researchers [4–6]. The thermodynamic data related to the ILs system is indispensable for the solvent selection in the chemical separation process. And it is also an important basis for engineering design. The selectivity of ILs can be calculated from the infinite dilution activity 1 coefficients (c1 values i Þ values of the solutes in the ILs. The ci

are employed to estimate different effects of different structural ILs on the selectivity [7]. Therefore, the parameter c1 i plays a key role in developing thermodynamic models and selecting an appropriate extractant in the field of extraction separation [8–13]. In this work, the thermodynamic parameters of 22 kinds of organic solutes (including n-alkanes, cycloalkanes, aromatic hydrocarbons, alcohols, acetone, esters, acetonitrile, tetrahydrofuran and chloromethanes) in the IL [PMMIM][NTf2] were measured. At T = 303.15 K-353.15 K, the retention time of the organic solutes in the packed column with IL 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [PMMIM][NTf2] as stationary phase was determined, and the c1 values of the solutes in the i [PMMIM][NTf2] were calculated. The influences of the structures and properties for organic solutes on the values c1 i were studied by thermodynamic parameters. The values of the partial molar E;1

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (M.-L. Ge). https://doi.org/10.1016/j.jct.2019.105894 0021-9614/Ó 2019 Elsevier Ltd.

excess enthalpies at infinite dilution (Hi ) at a reference temperature Τref = 298.15 K were obtained by fitting the values lnc1 to i 1/T. Based on the c1 i values of the solutes in the [PMMIM][NTf2],

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Z.-Z. He et al. / J. Chem. Thermodynamics 140 (2020) 105894

the other infinite dilution thermodynamic parameters including E;1

E;1

the entropies (Tref Si ) and Gibbs free energies (Gi ) of organic solutes in [PMMIM][NTf2] at a reference temperature Τref = 298.15 K were calculated. According to the formula, the KL values of organic solutes in [PMMIM][NTf2] were obtained. The infinite dilution selectivity (S1 ij ) values represents the ratio of the infinite dilution activity coefficients of different solutes in ILs. The S1 ij values for 1

n-hexane/benzene, cyclohexane/benzene, and capacity factor (kj ) for benzene at Τ = 323.15 K were calculated from the values c1 i . This is a relatively simple calculation method for S1 ij . The data of S1 ij can evaluate the separation performance of ILs. The influence of IL cation structure on S1 ij was analyzed by comparison with a series of ILs containing [NTf2]. 2. Experimental 2.1. Experimental materials and method The IL [PMMIM][NTf2] was purchased from Shanghai Chengjie Chemical Co., Ltd. and had a purity of greater than0.99 mass fraction. According to manufacturer’s specifications, with the following certified mass fraction of impurities: waterless than103. The chemical structure of [PMMIM][NTf2] is presented in Fig. 1. Due to the hydrophilicity of ILs, before the experiment, the selected IL was placed in a vacuum oven at T = (373–383) K about 24 h to remove possible traces of solvents and moisture. After completing the above step, the water content determined by Karl Fischer analysis was less 4  104. Chromosorb W/AW-DCMS 80/100 mesh was supplied by SUPELCO. The GC column (stainless steel) with a length of 2 m and an inner diameter of 0.2 cm was used. 22 kinds of organic solutes with analytical grade (mass fraction was about 99%) were provided by Beijing Chemical Reagents Company and were used as received without further purification. The sources and mass fraction purity of materials are listed in Table 1S in the Supplementary Materials. The values of c1 for the 22 organic i solutes in [PMMIM][NTf2] stationary phase were determined by GC method. 2.2. Density measurements The density of [PMMIM][NTf2] was determined by an Anton Paar DMA 4500 M digital vibrating-tube densimeter (Graz, Austria) in the temperature range from 303.15 K to 353.15 K at 10 K intervals at atmospheric pressure p = 101.3 kPa. The oscillating U-tube method for density measurement was invented at a research institute in Graz, Austria. Two integrated Pt 100 platinum thermometers together with Peltier elements provide an extremely precise thermostatting of the sample. The measuring range is (0–3) gcm3, repeatability density is 0.00001 gcm3 [14]. 2.3. Apparatus and procedure for the measurement of c1 i The SP-3420A gas chromatograph with a thermal conductivity detector was used to carry out this experiment. The related exper-

Fig. 1. The chemical structure of 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [PMMIM][NTf2].

imental data was collected using CT-22 USB Chromatography Data Acquisition Unit. In this work, the column preparation method was the same as those described previously by Ge [15–19]. And Chromosorb W/AW-DCMS 80/100 mesh was used as the solid support. Acetone was selected as solvent to coat the IL on a known volume of the Chromosorb. During coating, the IL coating on the solid support was carried out by evaporation of the solvent in a rotating evaporator. The selected IL and Chromosorb were weighed on an electronic balance of precision of ±0.0001 g before and after coating process. The column packing with 34.96% (1.1573 g) mass percent of the support material measured retention time of 22 solutes when column temperatures were 303.15 K to 353.15 K at 10 K intervals. Another column with 45.06% (1.6121 g) mass percent of [PMMIM][NTf2] with a precision of ±0.0001 g was used to measure retention time for 10 solutes. It was very important to fill uniformly the column with the ILs-coated solid support with the help of an ultrasound vibrator. The column aging process was completed by heating under nitrogen for 8 h at the column temperature of 160 °C. Dry hydrogen was used as the carrier gas and with the gas flow rate of (20–22) mLmin1, which was determined using a calibrated soap bubble flow meter placed at the outlet after the detector. The flow rates were corrected for water vapour pressure. The injector temperature was set higher than the boiling point of the selected solutes. The organic solutes were injected into the gas chromatograph using a micro-syringe. Sample injection volume ranged from 0.1 to 1.0 mL and can be considered to be at infinite dilution. Each solute injection was repeated three times to ensure the accuracy of the experimental data and the deviation of the retention time of the three measurements were within ±0.05 min. The retention time of methane was considered as the dead time tG . The retention time of each solute in the gas chromatographic column was recorded. Due to the change of experimental conditions, such as the possible elution of the stationary phase by the carrier gas stream, each injection operation was required to ensure experimental stability. The measurements of retention time were repeated systematically every (6–8) h for nhexane and benzene. The retention time observed did not change during 80 h of continuous operation. 3. Theory 3.1. Activity coefficients at infinite dilution (c1 i ) and gas–liquid partition coefficients (KL) In this experiment, the activity coefficients at infinite dilution (c1 i ) for solutes in [PMMIM][NTf2] were obtained by the Eq. (1) proposed by Cruickshank et al. [20] and Everett [21].



lnc1 i ¼ ln

n3 RT

V N Pi

0





Bii  v i 0 2Bi2  v i 1 3 pi þ J 2 p0 RT RT

ð1Þ

where c1 i is the activity coefficient of solute i at infinite dilution in the stationary phase (3), p0i is the vapour pressure of the pure liquid solute i, n3 is the number of moles of the stationary phase component on the column, po is the pressure at the column outlet and V N is the standardized retention volume obtained by Eq. (2).

   1 T col p0 V N ¼ J 32 U 0 ðt r  t G Þ 1 w Tf p0

ð2Þ

where tr denotes the retention time, t G the dead time, U 0 the flow rate of the carrier gas, T col column temperature, T f the flowmeter temperature, p0w the saturation vapour pressure of water at T f and po denotes the pressure at the column outlet. The second and third terms in Eq. (1) are correction terms arising from the nonideality of the mobile gaseous phase and the effect of pressure. Bii is the second virial coefficient of the solute, Bi2 is the

Z.-Z. He et al. / J. Chem. Thermodynamics 140 (2020) 105894

cross second virial coefficient of the solute (i) with the carrier gas (2), v i is the liquid molar volume of pure solute, and v 1 i is the partial molar volume of the solute in the stationary phase (3) at infinite dilution. For all solutes, values of p0i were calculated from the Antoine equation, with Antoine coefficients given by Boublik et al. [22]. Molar volumes of solutes v i were estimated by using their experimental densities [23]; Partial molar volumes of solutes at infinite dilution v i 1 have been assumed to be equal to v i . Bii and Bi2 have been estimated according to the equations suitable for nonpolar and polar liquids by Tsonopolous’s method [24] with an uncertainty of <±10 cm3∙mol1. The critical parameters and acentric factor x were available from the literature [23,24]. The cross critical propertiesP cij , T cij , v cij , Z cij , and acentric factor xij were calculated by using equations given in the literatures [24,25]. The vapour pressure of the solutes (i) at temperatures of (303.15–353.15) K, the critical constants T c , P c , Z c , V c , and acentric factors x of the solutes and the carrier gas used in calculation of the virial coefficients were presented in Tables 2S and 3S in the Supplementary Materials. The pressure correction term J 32 is given by [26].

 3 pi 1 p0 2 3 J 2 ¼  2 3 pi 1 p0

ð3Þ

Here pi and po are the inlet and outlet pressures of the GC column, respectively. The inlet column pressure pi was determined by inner manometer. Outlet pressure p0 was kept equal to atmospheric pressure. The representation of the thermodynamic properties is gas–liquid partition coefficient K L ¼ ðC Li =C Gi Þ, for a volatile solute (i) partitioning between an involatile solvent (3) and a carrier gas (2), K L can be obtained from the following equation [27].

KL ¼

RT q3 0 c1 i pi M 3

ð4Þ

whereq3 and M 3 stand for specific density and molar mass of solvent (IL). p0i is the vapour pressure of the pure liquid solute i. 3.2. Solubility parameters The activity coefficients at infinite dilution of different solutes in a given solvent can be used to estimate solubility parameters of the solvents when the solubility parameters of these solutes are known. The Hildebrand-Scatchard regular solution theory (RST) combined with Flory ‘‘combinatorial” equation [28–30] is applied in this work to estimate the Hildebrand’s solubility parameter of the IL. 1comb lnc1 þ lnc1res i ¼ lnci i

¼ lnc1comb i lnc1res ¼ i

lnðci

c3Þ

v  i

RT

þ1

ci c3

ðdi  d3 Þ2

ð5Þ ð6Þ

ð7Þ

where ci and c3 are the van der Waals volumes of solute and solvent, v i is the solute molar volume; di andd3 are solubility parameters of solute and solvent, respectively. Information on v i and di in Eqs. (5–8) was obtained from the literature [31]. The size parameters of solute (ci ) and IL (c3 ) are assumed to be proportional to molar volume. Thus, ci =c3 ¼ v i =v 3 ¼ ðMi q3 =M3 qi Þ, In this equation, qi and Mi stand for specific density and molar mass of solute i, q3 and M3 for specific density and molar mass of solvent (IL).

3

The solubility parameter of ILs can be correlated from the experimental lnc1 data. A residual function Y i can be rearranged from i Eq. (8) according to literature [28–30].

Yi ¼ 

lnc1res i

vi

þ

d2i 2d3 d2 ¼ di  3 RT RT RT

ð8Þ

This equation shows that there is a linear relation between Y i and the solute solubility parameters di for a given solvent and temperature T. The values of the solvent solubility parameter d3 can be obtained from the slope of this line. The values of lnc1comb were i calculated by Eq. (6). According to Eq. (5), with the lnci 1 known (by experimental data), the value of lnci 1res can be calculated, and finally the values of Y i for different solutes in an IL were calculated according to Eq. (8).

4. Results and discussion 4.1. Activity coefficients at infinite dilution The ci 1 values of the column packing 34.96% for the 22 solutes in [PMMIM][NTf2] at temperature range from 303.15 K to 353.15 K at 10 K intervals are presented in Table 1. The ci 1 measurement results for 10 selected solutes in the column packing 45.06% at 303.15 K and 313.15 K are listed in Table 4S in the Supplementary Materials. Two packed columns basically give the same experimental results. The deviation of c1 i values obtained from the two columns was within5%. The difference between the two sets of c1 i values is due to the order of the experimental uncertainty. The c1 is one of the thermodynamic parameters of organic i solutes in ILs. According to the data from Table 1, at the same temperature, for the same type of organic solutes, the c1 values i increase as the carbon atom numbers of solutes increase. The alkane c1 values are larger than other types of organic solutes. i The reason is that alkanes are weakly polar solutes and have weak interaction with polar ILs. For organic solutes with the same carbon number, the c1 i values of alkenes are smaller than alkanes such as cyclohexene and cyclohexane. The introduction of the double bond causes a reduction of c1 i , because the double bond in alkenes can enhance interactions with the polar IL. For solutes with the same number carbon, cycloalkane reduces the c1 i values in comparison to the corresponding linear alkane such as n-hexane and cyclohexane. The cycloalkane has stronger interaction with the polar IL. The high c1 i values of n-alkanes indicate low solubility and weak interactions between solute-IL. The smaller c1 value of cycloalkane, i alkenes indicate more strong interaction between solute molecules with the IL. Aromatic compounds have smaller c1 i values than alkanes and naphthenes. Because of the stronger interaction between the six p-delocalized electrons in the aromatic hydrocarbon structure with polar IL. And the values of c1 i increase with increasing size of the alkyl group. For other polar solutes (eg. halogenated hydrocarbons, esters, ketones, etc.), the c1 values are smaller than that of alkanes, i cycloalkanes, and aromatic hydrocarbons. The c1 values are i slightly larger than 1 or less than 1, indicating that the interaction between the solute–solute and the solute-IL is close. The reason is that these organic solutes contain atoms such as O and N, and have strong interaction with IL. The values of dichloromethane and trichloromethane are similar, and clearly lower than tetrachloromethane. The reason of this phenomenon is that dichloromethane and trichloromethane are polar molecules, and tetrachloromethane is a non-polar molecule. The c1 i values of polar solutes are lower than non-polar solutes, which mean that more polar solutes have better solubility in the IL, because of the

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Table 1 The experimental activity coefficients at infinite dilution c1 i for various solutes in the IL [PMMIM][NTf2] at different temperatures (solute standard state is a hypothetical liquid at zero pressure).a T/K

Solutes (iÞ

n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane Cyclohexane Methylcyclohexane Cyclohexene Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Acetonitrile Acetone Tetrahydrofuran Ethyl acetate Dichloromethane Trichloromethane Tetrachloromethane a

303.15

313.15

323.15

333.15

343.15

353.15

33.65 40.07 41.52 47.10 57.68 72.78 15.75 16.58 7.66 1.22 1.55 2.39 2.03 2.25 2.15 0.52 0.59 1.04 1.23 1.13 1.10 3.70

32.567 37.56 39.04 44.06 52.39 67.00 15.43 16.21 7.41 1.23 1.60 2.56 2.11 2.40 2.27 0.53 0.61 1.06 1.25 1.19 1.15 3.75

31.48 34.91 36.78 41.24 47.58 61.65 15.05 15.85 7.18 1.25 1.66 2.73 2.19 2.56 2.38 0.56 0.63 1.09 1.27 1.26 1.21 3.80

30.36 32.87 34.70 38.62 43.14 56.47 14.78 15.50 6.93 1.27 1.72 2.91 2.26 2.73 2.49 0.56 0.65 1.12 1.29 1.33 1.27 3.85

29.41 31.27 32.81 36.08 39.27 52.31 14.46 15.16 6.71 1.29 1.78 3.11 2.35 2.92 2.62 0.57 0.67 1.15 1.31 1.40 1.33 3.90

28.48 29.53 30.83 33.75 35.64 48.03 14.15 14.83 6.49 1.31 1.85 3.32 2.43 3.11 2.76 0.58 0.69 1.17 1.33 1.47 1.40 3.96

Standard uncertainties (u) are as follows: u(c1 i ) = 5%, u(T) = 0.05 K.

preferred attractive interaction of polar molecules with the charged ions of the IL. And they were prone to strongly keep in the IL stationary phase. The c1 i values of organic solutes are not consistent with temperature, which is related to the dissociation energy of the solute itself and the energy breaking the interaction between the solute-IL. 1 From the temperature dependence of the lnc1 i , lnci can be split to its respective enthalpy and entropy terms. E;1

lnc1 i ¼

lnc1 i ¼ aþ

ð10Þ

K E;1

Thus the partial molar excess enthalpy, Hi

¼ Rb and

E;1 Si

¼ Ra at infinite dilution can be calculated from its slope and intercept, respectively. The coefficients a and b, the standard deviation r of the fitted equations, and the values of c1 i at 298.15 K are listed in Table 2. The plots of measured lnc1 i versus 1/T values and the linear fit of their data are given in Fig. 1S to 5S respectively, which show a

E;1

Hi S  i RT R

b
T

ð9Þ

E;1

fairly good fitting quality of Eq. (10). The values of Hi

whereR is the gas constant. The temperature dependence of the activity coefficients can be calculated from Eq. (10)

E;1

solutes studied are listed in Table 2. The Hi

for the

values are positive

Table 2 E;1 Coefficients a and b of Eq. (11), standard deviation r, c1 ), i atT ref = 298.15 K calculated using Eq. (11), values of the partial molar excess enthalpies at infinite dilution (H i E;1 E;1 entropies(T ref Si ), and Gibbs energies (Gi ) of organic solutes in [PMMIM][NTf2] at a reference temperatureT ref = 298.15 K.a Solutes (iÞ

b

c1 i298:15K

E;1

Hi

/(kJmol1)

E;1

T ref Si

/(kJmol1)

359.94 34.45 2.99 5.78 653.78 41.55 5.44 3.80 632.48 43.19 5.26 4.08 712.07 49.33 5.92 3.74 1029.62 61.63 8.56 1.66 888.82 77.06 7.39 3.38 228.96 15.98 1.90 4.97 238.54 16.83 1.98 5.02 355.92 7.84 2.96 2.15 167.96 1.20 1.40 1.85 378.69 1.51 3.15 4.17 696.23 2.29 5.79 7.84 380.97 1.99 3.17 4.87 696.9 2.15 5.79 7.69 525.35 2.08 4.37 6.19 227.12 0.52 1.89 0.25 347.25 0.58 2.89 1.53 265.88 1.02 2.21 2.26 157.32 1.22 1.31 1.80 566.65 1.09 4.71 4.93 516.81 1.06 4.30 4.45 145.64 3.66 1.21 4.43  E;1  E;1 E;1 1 Standard uncertainties (u) are as follows: u Hi ¼ 0:5kJ  mol  1, uðGi Þ ¼ 0:5kJ  mol , and uðT ref Si Þ ¼ 0:5kJ  mol1.

n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane Cyclohexane Methylcyclohexane Cyclohexene Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Acetonitrile Acetone Tetrahydrofuran Ethyl acetate Dichloromethane Trichloromethane Tetrachloromethane a

a 2.3322 1.5340 1.6443 1.5102 0.6679 1.3634 2.0035 2.0233 0.8658 0.7475 1.6834 3.1633 1.9637 3.1026 2.4958 0.0990 0.6153 0.9108 0.7276 1.9877 1.7932 1.7866

E;1

Gi

/(kJmol1)

8.77 9.24 9.33 9.66 10.22 10.77 6.87 7.00 5.11 0.46 1.02 2.05 1.70 1.90 1.82 1.64 1.36 0.05 0.50 0.22 0.15 3.22

r 0.0025 0.0036 0.0045 0.0059 0.0078 0.0066 0.0018 0.0019 0.0028 0.0013 0.0032 0.0056 0.0030 0.0054 0.0048 0.0025 0.0027 0.0022 0.0013 0.0043 0.0041 0.0013

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Z.-Z. He et al. / J. Chem. Thermodynamics 140 (2020) 105894

for alkanes, cyclohexane, cyclohexene, which indicate that the dissociation energy of the solutes is greater than the interaction between the solute-IL. And it means that the interaction between solute-IL (such as alkane-IL or cycloalkane-IL) is smaller than the E;1

interaction between the solute molecules. The Hi values are negative for aromatic hydrocarbons and other solutes, due to induction between the anion-cation of IL with solutes. The partial E;1

molar excess Gibbs energies Gi of solutes in [PMMIM][NTf2] at a reference temperature T = 298.15 K are given in Table 2. The most E;1

values of Gi

for solutes are positive. And compared with other E;1

organic solutes, the values of Gi

for aliphatic hydrocarbon are lar-

0.0975%. At T = 299.15 K, the density data is 1.4503 gcm3 in this work and it was 1.481 gcm3 in the literature [36]. The deviation was about 2.12% and maybe due to the different measurement method. The experimental results for K L were given in Table 3. For the same type of solutes, the K L values increase with carbon chain growth, due to the intermolecular force. The introduction of double bonds and benzene rings increases the K L value, because of the greater interaction between these groups with ILs. The K L values gradually decrease with the temperature increasing. When the temperature increases, the solutes are more volatile, the K L values are lower. The greater the K L values are, the better the solubility of the solutes in the IL.

E;1

ger. The Gi values of aliphatic hydrocarbons increase with increasing of the carbon atom number of the n-alkanes. The largest E;1

positive value of Gi

observed was for decane 10.77 kJmol1. The

E;1 Gi

larger values of indicate that the interaction between aliphatic hydrocarbons with ILs has a weaker effect on aliphatic hydrocarbons.

4.3. Solubility parameters The solubility parameter provides a numerical estimate of solute–solvent interaction. And it is a measure of the solvating power for given solvent. In general, when di = d3 , the solute i is

4.2. Gas-liquid partition coefficients The calculation of the gas–liquid partition coefficients (K L ) requires the density q3 data of [PMMIM][NTf2]. The experimental data of [PMMIM][NTf2] density q3 at atmospheric pressure p = 101.3 kPa was listed in Table 5S in the Supplementary Materials. Comparing density data in this work with literature values for [PMMIM][NTf2] were listed in Fig. 6S. The deviation is defined as qexp  qlit =qexp . Compared with the literature [32], the deviation was less than 0.74%. The Anton Paar DMA digital vibrating-tube densimeter was used in this work and 1 mL pycnometer was used in the literature [32]. The density data in this work are very similar to the literature [33]. The deviation was less than 0.342%. At T = 333.15 K, the density data is 1.4178 gcm3 in this work and it was 1.4205 gcm3 in the literature [34]. The deviation was about 0.19%. Using the extrapolation method, the density of [PMMIM] [NTf2] at T = 298.15 K is 1.4513 gcm3 in this work. It was 1.4527 gcm3 in the literature [35], and the deviation was about

Fig. 2. Correlation of residual function Y i with di of solutes for [PMMIM][NTf2] atT = 303.15 K.

Table 3 Experimental gas–liquid partition coefficients K L for various solutes in the IL 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [PMMIM][NTf2] at different temperatures.a Solutes (iÞ

n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane Cyclohexane Methylcyclohexane Cyclohexene Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Acetonitrile Acetone Tetrahydrofuran Ethyl acetate Dichloromethane Trichloromethane Tetrachloromethane a

T/K 303.15

313.15

323.15

333.15

343.15

353.15

3.15 8.69 26.87 75.02 193.20 477.71 33.97 66.96 78.07 450.63 1146.36 2160.84 3619.60 2721.87 2637.40 1069.65 387.42 312.61 439.56 108.33 245.28 124.37

2.37 6.37 18.52 48.90 120.76 283.27 23.47 45.07 54.29 296.71 705.50 1215.90 2073.33 1458.36 1733.71 740.30 258.19 208.64 283.89 73.13 161.31 83.68

1.82 4.84 13.15 33.14 79.75 170.50 16.76 31.29 38.89 201.62 447.25 714.84 1230.67 853.75 885.96 494.83 177.13 143.24 189.19 50.28 109.16 57.94

1.44 3.73 9.62 23.14 53.87 109.82 12.21 22.35 28.65 140.72 292.92 435.16 760.15 516.55 542.00 340.27 124.48 100.78 129.90 35.51 75.74 41.06

1.15 2.90 7.21 16.74 38.66 72.66 9.14 16.36 21.58 100.69 197.34 272.56 484.24 322.87 347.23 238.94 89.42 72.56 91.58 25.61 53.76 29.80

0.94 2.33 5.56 12.56 27.99 50.18 6.99 12.25 16.60 73.64 136.11 175.78 317.38 207.28 227.00 171.81 65.49 53.29 66.10 18.81 38.93 22.04

Standard uncertainties (u) are as follows: uðK L Þ ¼ 5%, u(T)=0:05K.

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Z.-Z. He et al. / J. Chem. Thermodynamics 140 (2020) 105894

Table 4 Solubility parameters d3 /(MPa1/2) of [PMMIM][NTf2].a

a

T/K

a

b

d3 from Slope

d3 from Intercept

R2

303.15 313.15 323.15 333.15 343.15 353.15

0.0186 0.0185 0.0183 0.0182 0.0180 0.0178

0.2181 0.2170 0.2137 0.2105 0.2074 0.2043

23.48 24.08 24.62 25.15 25.66 26.18

23.45 23.77 23.96 24.15 24.32 24.49

0.9927 0.9811 0.9806 0.9801 0.9794 0.9786

R2: correlation coefficient.

Table 5 1 Selectivity (S1 ij ) and capacity (kj ) for different ILs with anion [NTf2] at 323.15 K. Cation

[EMIM] [EMIM] [EMIM] [EMMIM] [AMIM] [BMIM] [BMIM] [BMIM] [HMIM] [HMIM] [HMIM] [HMMIM] [OMIM] [OMMIM] [C6OCMIM] [(C6OC)2IM] [CN-C3MIM] [CN-C3MMIM] [PMMIM]

S1 ij

c1 i

1

n-Hexane

Cyclohexane

Benzene

n-Hexane/benzene

Cyclohexane/benzene

Benzene

23.5 24.2 24.3 23.8 20.8 12.7 12.8 12.8 7.31 7.48 7.52 7.41 4.95 6.52 7.03 3.01 113 54.2 34.91

8.46 13.5 13.4 13.7 11.8 7.74 6.70 7.90 4.88 4.84 4.94 5.14 3.39 4.16 4.75 2.11 45.8 28.2 15.05

0.702 1.21 1.18 1.10 1.22 0.902 0.800 0.850 0.775 0.693 0.777 0.72 0.652 0.74 0.814 0.607 1.70 1.71 1.25

33.4 20.0 20.6 21.6 17.0 14.1 16.0 15.1 9.4 10.8 9.7 10.29 7.6 8.8 9.1 5.0 66.3 31.7 27.93

12.1 11.2 11.4 12.5 9.7 8.6 8.4 9.3 6.3 7.0 6.3 7.1 5.2 5.6 5.8 3.5 27.0 16.5 12.04

1.42 0.83 0.0.85 0.91 0.82 1.11 1.25 1.18 1.29 1.44 1.29 1.39 1.53 1.35 1.23 1.65 0.59 0.59 0.8

completely miscible with the IL. Therefore, the values of di and d3 contributes to the screening of solvents. If the d3 values of the IL are very close to the solubility parameter of a solvent of the mixed system, the solvent can be extracted from the mixed system. The linear dependence between Y i and di at 303.15 K for [PMMIM] [NTf2] was displayed in Fig. 2 and the average solubility parameter d3 at 303.15 K is 23.47 MPa1/2. The solubility parameters of [PMMIM][NTf2] from (303.15 to 353.15) K are given in Table 4, which show that d3 approaches to a constant in the range of temperature studied. 4.4. Analysis for n-hexane (i)/benzene (j) and cyclohexane (i)/benzene (j)

Refs

kj

[37] [38] [39] [39] [40] [38] [41] [39] [40] [8] [42] [43] [42] [44] [44] [45] [41] [41] This work

solutes in [PMMIM][NTf2] are determined using GC. The infinite E;1

dilution thermodynamic parameters including Hi

E;1

; T ref Si

, and

E;1

Gi are calculated atT ref = 298.15 K. According to the HildebrandScatchard theory and the Flory equation, the solubility parameters d3 of [PMMIM][NTf2] are obtained from the c1 i values. The K L values have been calculated at the temperature range from 303.15 K of [PMMIM][NTf2] for n-hexane/benzene, to 353.15 K. The S1 ij 1

cyclohexane/benzene, and the kj of benzene have been calculated at T = 323.15 K. Comparing with literature values, [PMMIM][NTf2] has good selectivity for n-hexane/benzene, cyclohexane/benzene binary systems. Acknowledgment

1 1 1 The selectivity, S1 ij , is defined as Sij ¼ ci;IL /cj;IL , (where i and j 1

refers to the solutes to be separated) and the capacity (kj ) is defined as follow:

1 kj

¼ 1=c

1 j;IL .

Table 5 lists

S1 ij

and

1 kj

for ILs based

on [NTf2]— for n-hexane (i)/benzene (j) and cyclohexane (i)/ benzene (j) atT = 323.15 K. The Experimental results show that [PMMIM][NTf2] is an ideal extraction solvent for separation of n-hexane (i)/benzene (j) and cyclohexane (i)/benzene (j) binary systems. Compared with the literature values, it can be found that the S1 ij values of [PMMIM][NTf2] are larger than most values listed in the Table 5. It indicates that [PMMIM][NTf2] is an extraction solvent and has separation ability for difficult separation system of n-hexane/benzene, cyclohexane/benzene. 5. Conclusions In this paper, the c1 i values of various organic solutes including alkanes, cycloalkanes, aromatic hydrocarbons and some polar

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JCT 2019-359