Measurement of activity coefficients at infinite dilution of organic solutes in the ionic liquid 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate at T = (308.15, 313.15, 323.15 and 333.15) K using gas + liquid chromatography

J. Chem. Thermodynamics 70 (2014) 245–252

Contents lists available at ScienceDirect

J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct

Measurement of activity coefficients at infinite dilution of organic solutes in the ionic liquid 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate at T = (308.15, 313.15, 323.15 and 333.15) K using gas + liquid chromatography Indra Bahadur, Byron Bradley Govender, Khalid Osman, Mark D. Williams-Wynn, Wayne Michael Nelson, Paramespri Naidoo, Deresh Ramjugernath ⇑ Thermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa

a r t i c l e

i n f o

Article history: Received 4 October 2013 Accepted 9 October 2013 Available online 22 October 2013 Keywords: Activity coefficient at infinite dilution 1-Ethyl-3-methylimidazolium 2-(2methoxyethoxy) ethylsulfate Gas–liquid chromatography Selectivity

a b s t r a c t In this study, the interactions between volatile organic compounds and the ionic liquid (IL) 1-ethyl-3methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [EMIM]+[MDEGSO4] were investigated using gas + liquid chromatography measurements. The activity coefficients at infinite dilution c1 13 were determined for 28 polar and non-polar organic solutes (alkanes, cycloalkanes, alkenes, alkynes, aromatic compounds, alcohols, and ketones) in 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate at T = (308.15, 313.15, 323.15 and 333.15) K. Packed columns with phase loadings of 0.27 and 0.37 mass fraction of the IL in the stationary phase were employed to obtain c1 13 values at each temperature. Density and refractive index values were also measured for the pure IL at P = 0.1 MPa and at the experimental were calculated for the solutes temperatures. Partial molar excess enthalpies at infinite dilution DHE;1 1 from the temperature dependency of the c1 13 values. The uncertainties in the activity coefficient at infinite dilution were critically evaluated and estimated on average to be ±5.3%. Selectivity values at infinite dilu1 tion S1 ij for the hexane/benzene separation were also calculated at T = 308.15 K. The selectivity Sij value of the IL investigated in this study is approximately 3.7 times greater than that for NMP, 2.6 times greater than that for NFM, and 2.3 times greater than that for sulfolane. These results indicate the potential use of this IL in extractive separation processes. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The knowledge of activity coefficients at infinite dilution of a solute in a solvent is important for the selection of suitable solvents for aliphatic/aromatic hydrocarbon separation [1] by liquid + liquid extraction and extractive distillation [1,2]. The data are also important for screening ILs in solvent enhanced separation processes [1] and provide information about the solute–solvent interactions necessary for developing correlative and predictive excess Gibbs free energy models. Typical solvents employed in industrial separations of aliphatic/ aromatic hydrocarbon mixtures are N-methyl-2-pyrrolidone (NMP) [3,4], N-formylmorpholine (NFM) and sulfolane [3]. The ILs have been recognised as a new class of solvents that could replace organic solvents, including sulfolane [5] and NMP [5], in many chemical processes [6–8]. Since ionic liquids have comparatively low vapour pressures, they can be exploited for extraction ⇑ Corresponding author. Tel.: +27 (0) 31 2603128; fax: +27 (0) 31 2601118. E-mail address: [email protected] (D. Ramjugernath). 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.10.017

and separation as suitable green solvents [9], thereby reducing possible atmospheric pollution and environmental hazards [10,11]. The low volatility of ILs may result in less complex processes and simpler regeneration of solvents, which reduce energy consumption in comparison to volatile solvents [10]. The recyclability of ILs [12] also makes them eminently useful in automated multi-stage extractions, needed for industrial processes. ILs are organic salts that consist of an organic cation and an organic or inorganic anion. Various cation and anion combinations can be synthesized [13] to adjust the liquid properties such as density, sound velocity, refractive index, viscosity and miscibility [14,15], as required for specific applications [16]. An extensive review of numerous anion and cation combinations has been undertaken by Marciniak [10]. The 2D and 3D structures of the IL under investigation, 1-ethyl3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [EMIM]+[MDEGSO4] are shown in figure 1. The potential applications of ILs with imidazolium cations have been studied by Stepnowski [17]. Studies of [EMIM]+[MDEGSO4] have been limited to density and refractive index [18], heat capacity and electrical

246

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252 TABLE 1 Comparison of experimental density, q, and refractive index, n, values of 1-ethyl-3methylimidazolium 2-(2-methoxyethoxy) ethylsulfate with the corresponding literature values at T = (298.15, 303.15, 308.15, 313.15, 323.15 and 333.15) K.

CH3 N

O O

N

S

O(CH2CH2O) 2CH3

T/K

O 308.2 313.2 323.2 333.2

CH3

(a) a

q/(g  cm3)

n

Exp

Lit.a

Exp

Lit.a

1.23040 1.22621 1.21960 1.21366

1.2300 1.2266 1.2198 1.2131

1.47929 1.47565 1.47281 1.46995

1.4793

Reference [18].

TABLE 2 Solute specification: supplier, CAS number and purity used in this study.

(b) FIGURE 1. (a) 2D and (b) 3D structures of IL[EMIM]+[MDEGSO4] used in this work.

conductivity [19], interfacial tension of a two-phase system with compressed carbon dioxide [20], diffusion coefficients in water at infinite dilution [21], and carbon dioxide solubility [22]. Experimental data for activity coefficients at infinite dilution for ILs based on the [MDEGSO4] anion are only available in the literature for the ILs 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [BMIM]+[MDEGSO4] [23] and 1-octyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [OMIM]+[MDEGSO4] [24]. In this study, we explore the interactions between organic compounds and the IL 1-ethyl-3-methylimidazolium 2-(2methoxyethoxy) ethylsulfate [EMIM]+[MDEGSO4] by systematic gas–liquid chromatography (glc) retention measurements. The activity coefficients at infinite dilution were determined for 28 organic solutes (alkanes, alkenes, alkynes, cycloalkanes, alcohols, aromatics and ketones) in [EMIM]+[MDEGSO4] using the glc technique at T = (308.15, 313.15, 323.15 and 333.15) K. This study further investigates the influence of the alkyl chain length, for [MDEGSO4] anion based ILs on hexane/benzene separations. This study is a continuation of our research group’s investigation on activity coefficients at infinite dilution of organic solutes in ILs [25–34].

2. Experimental

Solute

Supplier

CAS No.

Mass fraction purity

Pentane Hexane Heptane Octane Nonane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Methylcyclohexane Pent-1-ene Hex-1-ene Hept-1-ene Oct-1-ene Non-1-ene Pent-1-yne Hex-1-yne Hept-1-yne Oct-1-yne Non-1-yne Benzene Toluene Ethylbenzene Methanol Ethanol 1-Propanol Acetone 2-Butanone

Sigma–Aldrich Merck Fluka Merck Fluka Sigma–Aldrich Merck Sigma–Aldrich Merck Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Fluka Merck Fluka Sigma–Aldrich Sigma–Aldrich Fluka Fluka Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Fluka Sigma–Aldrich Merck Sigma Merck Sigma–Aldrich

109-66-0 110-54-3 142-82-5 111-65-9 111-84-2 287-92-3 110-82-7 291-64-5 292-64-8 108-87-2 109-67-1 592-41-6 592-76-7 111-66-0 124-11-8 627-19-0 693-02-7 628-71-7 629-05-0 3452-09-3 71-43-2 108-88-3 100-41-4 67-56-1 64-17-5 71-23-8 67-64-1 78-93-3

P0.99 P0.99 P0.99 P0.99 P0.99 P0.99 P0.99 P0.98 P0.99 P0.99 P0.98 P0.99 >0.90 >0.97 P0.99 P0.99 P0.97 >0.98 >0.98 P0.99 P0.99 P0.99 P0.99 P0.99 P0.99 P0.99 P0.99 P0.99

purity of the solutes. Chromosorb W-HP (80/100 mesh) supplied by Supelco (USA) was used for the solvent phase as a support material. The Chromosorb was dried by vacuum purification at an elevated temperature for several hours to remove the absorbed moisture. Helium with a supplier stated molar purity of greater than 0.99999, which was purchased from Afrox (South Africa), was used as a GC carrier gas.

2.1. Materials The IL [EMIM]+[MDEGSO4] (M = 310.37 g  mol1), commonly known as EcoEng, has a certified mass fraction purity of at least 0.98 and was supplied by Solvent Innovation. In order to remove any volatile chemicals and water from the IL, the IL was further purified through the use of vacuum evaporation coupled with ultrasonic heating at T = 353.15 K for approximately (10 to 12) h. The water content in the degassed IL was determined by KarlFischer titration [MKS 500] and was found to be 0.0005 mass fraction. The purity of the IL was further assessed by the comparison of the experimental density and refractive index with literature [18]; the values are listed in table 1. The list of solutes, including details by suppliers and the purities are listed in table 2. Purification of the solutes was not necessary since the glc technique separates any impurities during measurements on the column. The absence of significant impurity peaks, detected during the runs, verified the

2.2. Experimental procedure 2.2.1. Density and refractive index measurements The density of the pure IL was measured using a digital vibrating-tube densimeter (Anton Paar DMA 5000) with an accuracy of ±0.02 K in temperature. Measurement of the refractive index of the pure IL was obtained with a digital automatic refractometer (ATAGO, model RX-7000a, Japan) with an accuracy of ±0.03 K in temperature. Calibration of the DMA 5000 and RX-7000a was performed by measuring the density and refractive index of ultrapure water and dry air at the experimental temperatures. The density and refractive index values of [EMIM]+[MDEGSO4] at P = 0.1 MPa and T = (308.15, 313.15, 323.15 and 333.15) K are listed in table 1. The estimated uncertainties in the reported density and refractive index are ±0.00003 g  cm3and ±0.00002, respectively.

247

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252

2.2.2. Activity coefficient at infinite dilution measurements The experimental procedures for the preparation of the glc packed column and c1 13 determination via the glc analytical technique used in this study have been previously described [23,28,29]. The column was stainless steel tubing (304 grade) which was 1.0 m in length and a 4.1 mm internal diameter and was supplied by Swagelok. The columns were cleaned before use by rinsing them several times in sequence with hot soapy water, hot water, and acetone, and thereafter flushed with distilled water. A vacuum flask containing IL was immersed into an ultrasonic bath and degassed for at least 8 h to remove dissolved air, water, and other volatile impurities from the IL. A two stage Edwards vacuum pump was used to draw the vacuum. Chromosorb W-HP was used as the support material for the IL. A rotary evaporator was used to dry the Chromosorb for at least 4 h (at 313.15 K) prior to preparation of a pre-weighed amount of a mixture of IL and Chromosorb W-HP. For the preparation of the stationary phase, HPLC-grade dichloromethane was added to a pre-weighed mixture of IL and Chromosorb W-HP support material. The dichloromethane facilitated uniform coating of the IL onto the Chromosorb W-HP. The mixing of IL and Chromosorb was undertaken in a rotary evaporator at a moderate rotation speed for at least 12 h to ensure complete mixing. The temperature of the rotary evaporator bath was gradually raised from T = (308.15 to 318.15) K to prevent entrainment of the IL with the more volatile dichloromethane. The prepared packing material was inserted gradually into the column to ensure tight packing and elimination of dead volumes. The newly prepared column was connected to a Shimadzu GC-2014 GC and conditioned for at least 12 h by passing dry helium gas through the column at flow rates between (15 and 30) cm3  min1 and at a temperature of 343.15 K, in order to remove traces of volatile materials remaining in the packing and to pre-saturate the packing with the carrier gas. The columns were used immediately after preparation and conditioning. Two columns were prepared for two independent sets of c1 13 measurements with different mass fraction phase loadings of 0.27 (4.9 mmol) and 0.37 (9.9 mmol) of the IL. The details regarding the importance of two different mass fraction phase loadings for the measurements of activity coefficients at infinite dilution c1 13 have been described previously [25,35–37]. The retention measurements were performed on a Shimadzu GC-2014 GC equipped with a thermal conductivity detector. The injector and detector were both maintained at T = 523.15 K for all measurements. To minimize any contamination in the column, through deterioration of the septum, Thermogreen LB-2 septa (Supelco, USA) were used in the injection port. Solute samples (0.2 lL) were injected individually by the Shimadzu AOC-20i/s Auto Injector/Auto Sampler that ensured excellent reproducibility. At the experimental temperatures, multiple injections of the same solute were undertaken (at least three times) in a sequence to check reproducibility. The holdup or dead-time for a non-retained component was obtained by making injections of dry air into the column. The flow rate and inlet pressure of the helium carrier gas were controlled by the automatic flow controller on the GC. The carrier gas flow rates were measured using a 100 cm3 soap bubble flow-meter placed at the outlet of the detector. The volumetric flow rate of the carrier gas was corrected for the vapour pressure yielded by the water existing in the soap solution. The outlet pressure was equal to atmospheric pressure which was measured using a WIKA CPH 6000 digital sensor. The pressure drop measured by the chromatograph, varied between (7 and 45) kPa depending on the carrier gas flow rate, temperature and length of the column. Twenty eight solutes, listed in table 2, were analysed at different column temperatures of T = (308.15, 313.15, 323.15 and 333.15) K.

2.3. Theory The equation of Everett [38] and Cruickshank et al. [39] (equation (1)) was used to obtain activity coefficients at infinite dilution c1 13 of the solutes in the IL solvent:

ln c1 13 ¼ ln



 n3 RT P ðB11  V 1 Þ Po J 32 ð2B12  V 1 1 Þ  1 þ ;  V N P1 RT RT

ð1Þ

where VN denotes the net retention volume of the solute, Po the outlet pressure, J32 the pressure correction term, n3 the number of moles of solvent in the column packing, T the column temperature, P 1 the saturated vapour pressure of the solute at temperature T, B11 the second virial coefficient of the pure solute, V 1 the molar volume of the solute, V 1 1 the partial molar volume of the solute at infinite dilution in the solvent, and B12 (where 2 refers to the carrier gas) the mixed second virial coefficient of the solute and the carrier gas. The net retention volume of the solute VN is calculated via:

V N ¼ J 32 ðtR  tG Þqov ;

ð2Þ

where tR is the retention time of the given solute, tG is the gas retention time of the non-retainable component (taken to be air), and qov is the carrier gas flow at the column temperature and column outlet pressure. The carrier gas flow rate was corrected for the effects of water vapour pressure with the following equation:

qov ¼ qo ðT=T f Þ½1  ðP w =P o Þ;

ð3Þ

where qo is the flow rate measured with the bubble flow-meter at the column outlet, Tf is the flow-meter temperature measured with a thermometer, and P w is the saturated vapour pressure of water at temperature Tf. The term J 32 appearing in equations (1) and (2) corrects for the influence of the pressure drop along the column and is given in equation (4):

J 32 ¼

2 ðPi =Po Þ3  1 ð Þ; 3 ðPi =Po Þ2  1

ð4Þ

where Pi is the column inlet pressure. The second virial coefficients were calculated using the equation proposed by McGlashan and Potter [40], and is given below:

B=V c ¼ 0:430  0:886ðT c =TÞ  0:694ðT c =TÞ2  0:0375ðn  1ÞðT c =TÞ4:5 ; ð5Þ where n is the number of carbon atoms present in the solute molecule. For the B12 values, the Hudson and McCoubrey combining rules [41,42] allowed the calculation of T c12 and volumes V c12 from the critical properties [43] and ionisation energies [44,45] of the pure components. The saturated vapour pressures for the solutes were determined using vapour pressure correlations [43,46,47]. The virial coefficients of the alcohols and ketones were calculated using correlations by Tsonopoulos [48]. 2.4. Estimation of the experimental uncertainty The expression and estimation of uncertainty has been described in detail by NIST [49,50]. In this study, the estimation of the combined expanded uncertainty for the activity coefficient at infinite dilution is governed by the standard uncertainty inherent to the equation proposed by Everett [38] and Cruickshank et al. [39] uev e ðc1 i Þ and the standard uncertainty due to the repeatability of the experimental measurements (multiple solute injections and column loadings) urep ðc1 i Þ: 1 2 1 2 uðc1 i Þ ¼ ð½urep ðci Þ þ ½uev e ðci Þ Þ

1=2

:

ð6Þ

248

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252

The standard uncertainty due to the measurement repeatability was treated as a Gaussian type distribution. The equation developed by Everett [38] and Cruickshank et al. [39] in its proposed form is a   1 3 function of ten variables, c1 i ¼ f ðn3 ; T; V N ; P 1 ; B11 ; B12 ; P o ; V 1 ; V 1 ; J 2 Þ, thus any fluctuation or inaccuracy in these independent variables contribute to an uncertainty in the calculated value of the activity coefficient at infinite dilution. By the law of propagation of errors, we can quantify the influence of these variables with respect to the magnitude of the activity coefficient at infinite dilution: 0 h B B B uev e ðc1 i Þ¼B B @

@ c1 i @n3

 i2 h@ c1  i2 h@c1  i2 h@c1  i2 11=2 uðn3 Þ þ @Ti uðTÞ þ @ViN uðV N Þ þ @Pi uðP 1 Þ 1 C C h@ c1  i2 h @ c1  i2 h@c1  i2 C i C : uðB12 Þ þ @Pi0 uðPo Þ þ @Bi uðB11 Þ þ @B12 C 1 A h@c1  i2 h @ c1  i2 h@c1  i2 3 i uðJ þ @Vi uðV 1 Þ þ @Vi1 uðV 1 Þ þ Þ 1 2 @J3 1

1

2

ð7Þ

However, both the net retention volume VN and the factor in fact dependent variables, therefore:

J32

are also

0

h  i2   2  N  2 11=2 @V N uðJ 32 Þ þ @V@tN uðtÞ þ @V uðTÞ @T @J32 B C uðV N Þ ¼ @ h  i2 h  i2 h  iA  @V N @V N @V N þ @T uðT f Þ þ @P uðP w Þ þ @Po uðP o Þ f

w

ð8Þ and

0" uðJ 32 Þ

¼@

! #2 " ! #2 11=2 @J 32 @J 32 uðPi Þ þ uðPo Þ A : @Pi @Po

ð9Þ

It must be noted that the expressions above, used to estimate the experimental uncertainty in this study, by no means gauge the accuracy of the equation proposed by Everett [38] and Cruickshank et al. [39] in actually producing the true value of the activity coefficient at infinite dilution. Table 3 lists the estimated standard uncertainties (treated as rectangular distributions) and their respective influences. It is clear that the value of c1 i is majorly influenced by the retention volume, VN, the number of moles of ionic liquid in the packing n3 and the saturated vapour pressure of the solute P1 . The contributions of these aforementioned variables largely swamp the contributions  3 of the remaining variables (B11, B12, V 1 1 , V 1 , J 2 , Po). It is interesting to note that the uncertainty in temperature is more influential on c1 indirectly via the Antoine equation, as P1 ¼ f ðTÞ, rather than i through the temperature directly input into the equation proposed by Everett [38] and Cruickshank et al. [39]. The uncertainty for the

net retention volume VN was estimated at around 2.12% and its value is predominantly influenced by the net retention time, the corrected carrier gas flow rate qov and the temperature of the carrier gas Tf. The uncertainty attributed to c1 through uev e ðc1 i i Þ alone for all the experimental measurements was averaged at roughly 2.9% (coverage factor of k = 2 applied). The fluctuation in the final value of the uncertainty was largely attributed to the repeatability of the results. Overall, the average uncertainty including repeatability for all the experimental results was estimated at ±5.3% (k = 2). 3. Results and discussion The average c1 13 values obtained using both column loadings together with the expanded uncertainties at different temperatures are given in table 4. Figures 2–8 show the natural logarithm of the c1 13 as a function of the inverse absolute temperature for all solutes. From table 4 it can be seen that alkanes, alkenes and cycloalkanes have higher c1 13 values as compared to other solutes, which indicate weaker solute–solvent attractive interactions. It is evident that the more aliphatic the compound, the higher the c1 13 values. Cycloalkanes have lower values of c1 13 than the linear alkanes due to stronger attractive interactions between the cyclic structure and the imidazolium ring. Furthermore, the values of the molar volume of the cycloalkanes (1.089  104 m3  mol1 for cyclohexane) [51] are lower than that of the linear alkanes (1.314  104 m3  mol1 for hexane) [51], indicating a greater packing effect of cycloalkanes which causes additional interactions with the IL. Alkenes have lower c1 13 values than alkanes with the same carbon number due to the interaction of the double bond in alkenes with the polar IL. The alkynes have lower c1 13 values than the corresponding alkanes, alkenes and cycloalkanes due to stronger attractive interactions caused by the hydrogen atoms and p electrons of the triple bond in alkynes. The c1 13 values for aromatic hydrocarbons are lower than that for alkanes, cycloalkanes and alkenes due to stronger attractive interactions between the six p-delocalized electrons in the benzene structure and the polar IL. The lowest values (below unity) of c1 13 for methanol and ethanol indicate the strongest interactions with [EMIM]+[MDEGSO4] due + to hydrogen bonding. The c1 13 values for all solutes in [EMIM] [MDEGSO4] increases with an increase in the length of the solute functional group as shown in figure 9. This trend is consistent with that observed for imidazolium based ILs such as [BMIM]+[MDEGSO4] [23] and [OMIM]+[MDEGSO4] [24]. From table 4, it can be seen that the c1 13 values for alkanes, alkenes, cycloalkanes

TABLE 3 Estimates and contributions of the uncertainty in the measured variables. Variables influencing c1 i

a

3

u(VN) /m u(n3)/mol uðP 1 Þ/kPa u(T)/K u(B11)/m3  mol1 uðV 1 Þ/m3  mol1 u(B12)/m3  mol1 3 1 uðV 1 1 Þ/m  mol uðJ32 Þ u(Po)/kPa a b c d e f

Variables influencing VN Estimateb

Variable

2.12% 1.0% 1.0% 0.5 1.0% 1.0% 2.0% 1.0% 1.0% 0.1

Influencec (%)

Influenced (%)

49.39 23.30 22.22 3.61 0.97 0.11 0.23 0.14 0.03

49.14 23.18 22.25 3.34 0.86 0.76 0.19 0.08 0.20

0.00

0.00

Variable e

u(t) /s u(qov)/m3  s1 u(Tf)/K u(T)/K uðP w Þ f/kPa u(Pi)/kPa u(Po)/kPa

Estimateb

Influencec (%)

Influenced (%)

1.5% 1.0% 1.0 0.5 1.0% 0.5 0.1

49.57 33.04 11.16 5.28 0.95 0.008 0.002

49.74 33.16 11.16 4.98 0.96 0.008 0.002

VN is calculated through Equation (2). This is the estimated standard uncertainty (treated as rectangular distributions). Example of the percentage influence upon the combined uncertainty, for measurements with n-pentane at T = 308.15 K. Example of the percentage influence upon the combined uncertainty, for measurements with acetone at T = 333.15 K. t ¼ t R  tG . Vapour pressures calculated via the Antoine constants from the DDB [43].

249

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252 TABLE 4 Average activity coefficients at infinite dilution and uncertainties of organic solutes in [EMIM]+[MDEGSO4]- at T = (308.15, 313.15, 323.15 and 333.15) K. Solutes

308.15

U(c1 13 )

313.15

U(c1 13 )

323.15

U(c1 13 )

333.15

U(c1 13 )

Pentane Hexane Heptane Octane Nonane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Methylcyclohexane Pent-1-ene Hex-1-ene Hept-1-ene Oct-1-ene Non-1-ene Pent-1-yne Hex-1-yne Hept-1-yne Octy-1-ne Non-1-yne Benzene Toluene Ethylbenzene Methanol Ethanol 1-Propanol Acetone 2-Butanone

65.8 103.0 158.1 215.2 291.7 27.1 45.4 60.3 82.6 71.9 26.1 45.6 71.3 113.5 167.8 3.62 6.05 9.61 16.4 23.9 2.31 4.43 8.04 0.390 0.719 1.006 1.42 2.33

3.9 3.5 4.8 6.8 12.7 1.0 1.5 2.1 2.5 2.2 2.2 1.9 4.1 4.1 5.1 0.15 0.25 0.36 0.5 0.7 0.09 0.16 0.27 0.012 0.025 0.047 0.06 0.10

65.6 102.0 157.6 216.2 302.6 26.0 45.0 58.7 79.8 70.6 26.5 44.7 71.7 113.3 167.9 3.74 6.22 9.76 16.7 24.7 2.33 4.45 8.04 0.392 0.705 0.989 1.46 2.34

3.4 3.6 6.0 7.8 9.3 1.2 2.5 2.5 2.5 2.4 1.2 1.6 2.2 4.8 7.2 0.22 0.32 0.46 0.7 0.8 0.13 0.23 0.39 0.012 0.025 0.053 0.06 0.11

60.8 94.0 146.6 212.2 288.4 24.1 41.5 54.1 73.4 65.8 26.3 43.4 68.6 107.4 159.9 3.98 6.49 10.05 17.1 25.7 2.41 4.51 8.06 0.374 0.664 0.942 1.47 2.37

2.1 4.1 4.7 7.5 17.7 1.8 2.9 3.4 3.6 3.5 1.8 3.8 4.2 4.3 4.7 0.38 0.52 0.76 1.2 1.7 0.16 0.30 0.55 0.020 0.021 0.052 0.10 0.16

59.0 85.7 138.8 201.5 280.9 22.8 38.8 50.6 68.2 62.3 25.4 40.3 65.6 101.0 149.4 4.13 6.73 10.35 17.4 26.4 2.48 4.62 8.17 0.369 0.666 0.913 1.51 2.42

4.7 6.8 4.1 7.9 8.5 1.2 3.9 4.7 5.8 5.3 1.8 2.9 5.7 7.3 7.4 0.38 0.61 0.95 1.6 2.2 0.20 0.42 0.69 0.011 0.020 0.060 0.12 0.20

ð10Þ

The DHE;1 values, describing the temperature dependence of 1 c are shown in table 5. From figures 2–8, it can be seen that the ln c1 13 values do not vary prominently with temperature. The values of DHE;1 are positive for all solutes except for alkynes, 1 aromatic hydrocarbons, and ketones. The positive DHE;1 values 1 indicate that the c1 13 values of these solutes decrease with an increase in temperature. The negative DHE;1 values indicate the 1 opposite effect and also denotes strong associative interactions between [EMIM]+[MDEGSO4] and alkynes, or aromatic hydrocarbons, or ketones. A purpose of this study is to assess the suitability of [EMIM]+[MDEGSO4] for use in solvent-enhanced separation processes. This can be achieved through the use of selectivity S1 ij values at infinite dilution, calculated using equation (11):

+  FIGURE 2. Plot of ln c1 versus 1/T for alkanes: ( ) 13 for [EMIM] [MDEGSO4] pentane; ( ) hexane; ( ) heptane; ( ) octane; ( ) nonane.

+  FIGURE 3. Plot of ln c1 13 for [EMIM] [MDEGSO4] versus 1/T for cycloalkanes: ( ) cyclopentane; ( ) cyclohexane; ( ) cycloheptane; ( ) cyclooctane; ( ) cyclononane.

and alcohols generally decreases with an increase in temperature. The opposite trend is observed for alkynes, ketones and aromatic hydrocarbons. For ILs with the common [MDEGSO4] anion, the c1 13 values of the same solute at the same temperature decreases with an increase of the alkyl chain length on the imidazolium ring. This behaviour is also observed for other imidazolium based ILs such as [EMIM]+[NTf2] [52], [BMIM]+[NTf2] [52] and [OMIM]+[NTf2] [53]. The partial molar excess enthalpies at infinite dilution of the organic solutes in the IL have been determined from the temperature dependency of the ln c1 13 versus temperature, according to the Gibbs–Helmholtz equation given below:

Dðln c1 DHE;1 13 Þ 1 : ¼ Dð1=TÞ R

1 13 ,

250

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252

+  FIGURE 4. Plot of ln c1 13 for [EMIM] [MDEGSO4] versus 1/T for alkenes: ( ) pent1-ene; ( ) hex-1-ene; ( ) hept-1-ene; ( ) oct-1-ene; ( ) non-1-ene.

+  FIGURE 7. Plot of ln c1 versus 1/T for alcohols: ( ) 13 for [EMIM] [MDEGSO4] methanol; ( ) ethanol; ( ) 1-propanol.

+  FIGURE 5. Plot of ln c1 13 for[EMIM] [MDEGSO4] versus 1/T for alkynes: ( ) pent-1yne; ( ) hex-1-yne; ( ) hept-1-yne; ( ) oct-1-yne; ( ) non-1-yne.

+  FIGURE 8. Plot of ln c1 versus 1/T for ketones: ( ) 13 for [EMIM] [MDEGSO4] acetone; ( ) 2-butanone.

+  FIGURE 6. Plot of ln c1 versus 1/T for aromatic hydro13 for [EMIM] [MDEGSO4] carbons: ( ) benzene; ( ) toluene; ( ) ethylbenzene.

+  FIGURE 9. Plot of ln c1 versus carbon number at 13 for [EMIM] [MDEGSO4] T = 333.15 K for organic solutes: ( ) alkanes; ( ) alkenes; ( ) cycloalkanes; ( ) alkynes; ( ) aromatic hydrocarbons; ( ) ketones; ( ) alcohols.

251

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252 TABLE 5 Partial molar excess enthalpies at infinite dilution of selected organic solutes in [EMIM]+[MDEGSO4]. Solutes

DH1E;1 /kJ  mol1

Pentane Hexane Heptane Octane Nonane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Methylcyclohexane Pent-1-ene Hex-1-ene Hept-1-ene Oct-1-ene Non-1-ene Pent-1-yne Hex-1-yne Hept-1-yne Octy-1-ne Non-1-yne Benzene Toluene Ethylbenzene Methanol Ethanol 1-Propanol Acetone 2-Butanone

4.10 6.48 4.74 2.26 1.81 5.98 5.64 6.10 6.64 5.07 0.94 4.03 3.06 4.14 4.09 4.53 3.57 2.54 1.97 3.38 2.46 1.42 0.55 2.20 2.85 3.40 1.86 1.26

solvents have been investigated for the separation of the hexane(i)/ benzene(j) mixture at T = 308.15 K. The selectivity values S1 ij listed in table 6, for ethyl, butyl and octyl imidazolium ILs with the common anion [MDEGSO4], is in + the order: [EMIM]+[MDEGSO4] (S1 ij = 44.6), > [BMIM] [MDEGSO4]1 1  +  [23] (Sij = 35.0) > [OMIM] [MDEGSO4] [24] (Sij = 8.8), indicating a significant decrease in the S1 ij values with an increase in the alkyl chain length on the imidazolium ring. These results also indicate that [EMIM]+[MDEGSO4] has a higher efficiency for the separation of the hexane(i)/benzene(j) mixture than [BMIM]+[MDEGSO4] and [OMIM]+[MDEGSO4]. The selectivity value for [EMIM]+[MDEGSO4] is also higher than several other ILs from literature which contain the common cation [EMIM]+ and different anions [52,56–59], except for the anions [SCN] [60] and [BF4] [61]. The range of selectivity values shows the appreciable effect of the anion and cation on hexane(i)/benzene(j) separation. Furthermore, the selectivity values of [EMIM]+[MDEGSO4] are about 3.7 times greater than that for NMP [62], 2.6 times greater than that for NFM [62], and 2.3 times greater than that for sulfolane [63]. These results suggest the potential of the IL investigated for use in industrial separation processes.

4. Conclusions

1 1 S1 ij ¼ ci3 =cj3

ð11Þ

where i refers to hexane and j to benzene in this study [54]. A common separation challenge in the petrochemical and chemical industry is the separation of an aliphatic/aromatic hydrocarbon mixture [55]. In this study the effectiveness of imidazolium based ILs with different anion and cation combinations together with conventional

In this study, we investigated the interactions of organic compounds with 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [EMIM]+[MDEGSO4]. The gas–liquid chromatography technique was used to determine activity coefficients at infinite dilution c1 13 for 28 polar and non-polar organic solutes (alkanes, cycloalkanes, alkenes, alkynes, aromatic compounds, alcohols and ketones) in [EMIM]+[MDEGSO4] at T = (308.15, 313.15, 323.15 and 333.15) K. The expanded uncertainties in measured infinite dilution activity coefficient values were estimated on average to be approximately 5.3%. The partial molar excess enthalpies were calculated from the temperature dependency of ln c1 13 . Selectivity values at infinite dilution S1 were also calculated from the ij

TABLE 6 +  Selectivity S1 ij at infinite dilution of various solvents for n-hexane/benzene separation for ILs in literature and the IL[EMIM] [MDEGSO4] at T = 308.15 K. S1 ij

Solvent +

b

1-Ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [EMIM] [MDEGSO4] 1-Butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [BMIM]+[MDEGSO4]c 1-Octyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [OMIM]+[MDEGSO4]d 1-Ethyl-3-methylimidazolium trifluoroacetate [EMIM]+[TFA]e 1-Ethyl-3-methylimidazolium tetracyanoborate [EMIM]+[TCB]f 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [EMIM]+[NTf2]g 1-Ethyl-3-methylimidazolium diethylphosphate [EMIM]+[DEP]h 1-Ethyl-3-methylimidazolium tetrafluoroborate [EMIM]+[BF4]i 1-Ethyl-3-methylimidazolium thiocyanate [EMIM]+[SCN]j 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [EMIM]+[NTf2]k 1-Ethyl-3-methylimidazolium ethylsulfate [EMIM]+[C2H5OSO3]k N-methyl-2-pyrrolidone [NMP]l N-formylmorpholine[NFM]l Sulfolanem a b c d e f g h i j k l m

Interpolated value. This work. Reference [23]. Reference [24]. Reference [56]. Reference [57]. Reference [58]. Reference [59]. Reference [60]. Reference [61]. Reference [52]. Reference [62]. Reference.[63].

44.6 35.0 8.8a 28.3 26.7 35.8a 29.9a 46.5a 84.3 22.5a 36.2a 12.2a 17.3a 19.1a

252

I. Bahadur et al. / J. Chem. Thermodynamics 70 (2014) 245–252

experimental c1 13 values for hexane/benzene separation. The IL [EMIM]+[MDEGSO4] shows higher S1 values than [BMIM]+[ij MDEGSO4] and [OMIM]+[MDEGSO4] and other conventional solvents such as NMP, NFM, and sulfolane for hexane/benzene separation. These results suggest the potential of [EMIM]+[MDEGSO4] for use in industrial separation processes. Acknowledgements The authors acknowledge University of KwaZulu-Natal for a postdoctoral scholarship for Dr. I. Bahadur. This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation. References [1] K. Weissermel, H.-J. Arpe, Industrial Organic Chemistry, fourth Completely Revised edition., Wiley-VCH, Weinheim, 2003. pp. 313–336. [2] G.W. Meindersma, A.R. Hansmeier, A.B. de Haan, Ind. Eng. Chem. Res. 49 (2010) 7530–7540. [3] A. Marciniak, J. Chem. Thermodyn. 43 (2011) 1446–1452. [4] T.M. Letcher, B. Soko, D. Ramjugernath, N. Deenadayalu, A. Nevines, P.K. Naicker, J. Chem. Eng. Data 48 (2003) 708–711. [5] R. Krishna, A.N. Goswami, S.M. Nanoti, B.S. Rawat, M.K. Khana, J. Dobhal, Ind. J. Technol. 25 (1987) 602–606. [6] S. Tang, G.A. Baker, H. Zhao, Chem. Soc. Rev. 41 (2012) 4030–4066. [7] J.H. Davis, Chem. Lett. 33 (2004) 1072–1077. [8] H. Rodríguez, J.F. Brennecke, J. Chem. Eng. Data 51 (2006) 2145–2155. [9] Z. Li, X. Liu, Y. Pei, J. Wang, M. Heb, Green Chem. 14 (2012) 2941–2950. [10] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. [11] S. Werner, M. Haumann, P. Wasserscheid, Annu. Rev. Chem. Biomol. Eng. 1 (2010) 203–230. [12] C.F. Poole, S.K. Poole, J. Chromatogr. A 1217 (2010) 2268–2286. [13] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Fuel Process. Technol. 87 (2005) 59–70. [14] H. Olivier-Bourbigou, L. Magna, J. Mol. Catal. A: Chem. 182–183 (2002) 419– 437. [15] J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002) 3667–3692. [16] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, vol. 40, Willey-VCH, Weinheim, 2003. pp. 1233–1248. [17] P. Stepnowski, Int. J. Mol. Sci. 7 (2006) 497–509. [18] A.N. Soriano, B.T. Doma Jr, M.-H. Li, J. Taiwan Inst. Chem. Eng. 41 (2010) 115– 121. [19] Y.-H. Yu, A.N. Soriano, M.-H. Li, J. Chem. Thermodyn. 41 (2009) 103–108. [20] A. Hebach, A. Oberhof, N. Dahmen, P. Griesheimer, J. Chem. Eng. Data 54 (2009) 1249–1253. [21] C.-L. Wong, A.N. Soriano, M.-H. Li, Fluid Phase Equilib. 271 (2008) 43–52. [22] A.N. Soriano, B.T. Doma Jr, M.-H. Li, J. Chem. Thermodyn. 40 (2008) 1654–1660. [23] T.M. Letcher, U. Domanska, M. Marciniak, A. Marciniak, J. Chem. Thermodyn. 37 (2005) 587–593. [24] N. Deenadayalu, S.H. Thango, T.M. Letcher, D. Ramjugernath, J. Chem. Thermodyn. 38 (2006) 542–546. [25] P. Reddy, M.A. Siddiqi, B. Atakan, M. Diedenhofen, D. Ramjugernath, J. Chem. Thermodyn. 58 (2013) 322–329. [26] K. Tumba, T.M. Letcher, P. Naidoo, D. Ramjugernath, J. Chem. Thermodyn. 49 (2012) 46–53. [27] U. Domanska, M. Zawadzki, M. Królikowska, M.M. Tshibangu, D. Ramjugernath, T.M. Letcher, J. Chem. Thermodyn. 43 (2011) 499–504.

[28] E. Olivier, T.M. Letcher, P. Naidoo, D. Ramjugernath, J. Chem. Thermodyn. 43 (2011) 829–833. [29] K. Tumba, P. Reddy, P. Naidoo, D. Ramjugernath, J. Chem. Thermodyn. 43 (2011) 670–676. [30] P. Reddy, K.J. Chiyen, N. Deenadayalu, D. Ramjugernath, J. Chem. Thermodyn. 43 (2011) 1178–1184. [31] P. Reddy, N.V. Gwala, N. Deenadayalu, D. Ramjugernath, J. Chem. Thermodyn. 43 (2011) 754–758. [32] E. Olivier, T.M. Letcher, P. Naidoo, D. Ramjugernath, J. Chem. Thermodyn. 42 (2010) 78–83. [33] E. Olivier, T.M. Letcher, P. Naidoo, D. Ramjugernath, J. Chem. Thermodyn. 42 (2010) 646–650. [34] N.V. Gwala, N. Deenadayalu, K. Tumba, D. Ramjugernath, J. Chem. Thermodyn. 42 (2010) 256–261. [35] B.R. Kersten, C.F. Poole, J. Chromatogr. A 399 (1987) 1–31. [36] F. Mutelet, J.-N. Jaubert, J. Chem. Thermodyn. 39 (2007) 1144–1150. [37] S.A. Kozlova, S.P. Verevkin, A. Heintz, T. Peppel, M. Köckerling, J. Chem. Thermodyn. 41 (2009) 330–333. [38] D.H. Everett, Trans. Faraday Soc. 61 (1965) 1637–1645. [39] A.J.B. Cruickshank, B.W. Gainey, C.P. Hicks, T.M. Letcher, R.W. Moody, C.L. Young, Trans. Faraday Soc. 65 (1969) 1014–1031. [40] M.L. McGlashan, D.J.B. Potter, Proc. R. Soc. 267 (1951) 448–456. [41] G.H. Hudson, J.C. McCoubrey, Trans. Faraday Soc. 56 (1960) 761–766. [42] A.J.B. Cruickshank, M.L. Windsor, C.L. Young, Trans. Faraday Soc. 62 (1966) 2341–2347. [43] Dortmund Data Bank (DDB), Dortmund Data Bank Software and Separation Technology, Oldenburg, Germany, 2011. [44] NIST Chemistry WebBook, 2013, http://webbook.nist.gov/chemistry/. [45] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 86 ed., Taylor and Francis, Boca Raton, FL, 2005. [46] B.E. Poling, J.M. Prausnitz, J.P. O’Connell, The Properties of Gases and Liquids, fifth ed., McGraw-Hill, New York, 2001. [47] DIPPR Project 801 – Full Version, Design Institute for Physical Property Data/ AIChE, New York, USA, 2010. [48] C. Tsonopoulos, AIChE J. 20 (2) (1974) 263–272. [49] B.N. Taylor, P.J. Mohr, M. Douma, The NIST Reference on Constants, Units, and Uncertainty, 2007, Available from: www.physics.nist.gov/cuu/index.html. [50] B.N. Taylor, C.E. Kuyatt, Guidelines for evaluating and expressing the uncertainty of NIST measurement results; NIST Technical Note 1297, National Institute of Standards and Technology, Gaithersburg, 1994. [51] Design Institute for Physical Properties, Sponsored by AIChE (2005; 2008; 2009; 2010). DIPPR Project 801 – full Version, Design Institute for Physical Property Research/AIChE. [52] M. Krummen, P. Wasserscheid, J. Gmehling, J. Chem. Eng. Data 47 (2002) 1411–1417. [53] R. Kato, J. Gmehling, J. Chem. Thermodyn. 37 (2005) 603–619. [54] D. Tiegs, J. Gmehling, A. Medina, M. Soares, J. Bastos, P. Alessi, I. Kikic, DECHEMA Chemistry Data Series IX, Part 1, DECHEMA, Frankfurt/Main, 1986. [55] P. Reddy, T.M. Letcher, in: T.M. Letcher (Ed.), Thermodynamics Solubility and Environmental Issues, Elsevier, Amsterdam, 2007, pp. 85–107. [56] U. Domanska, A. Marciniak, J. Phys. Chem. B 111 (2007) (1988) 11984–11988. [57] U. Domanska, M. Królikowska, W.E. Acree Jr., G.A. Baker, J. Chem. Thermodyn. 43 (2011) 1050–1057. [58] N. Deenadayalu, T.M. Letcher, P. Reddy, J. Chem. Eng. Data 50 (2005) 105–108. [59] M.-L. Ge, J.-B. Chen, J. Chem. Eng. Data 56 (2011) 3183–3187. [60] M.-L. Ge, L.-S. Wang, J.-S. Wu, Q. Zhou, J. Chem. Eng. Data 53 (2008) 1970– 1974. [61] U. Domanska, A. Marciniak, J. Chem. Thermodyn. 40 (2008) 860–866. [62] M. Krummen, J. Gmehling, Fluid Phase Equilib. 215 (2004) 283–294. [63] C. Mollmann, J. Gmehling, J. Chem. Eng. Data 42 (1997) 35–40.

JCT 13-584