Predictions of LLE of Ionic Liquid Systems

Recent experimental work suggests that ILs can be considered as fully dissociated cations and anions. Further, the mechanism of hydrogen bonding (Scheiner, 1997) in the ILs is also not fully available. This fact was used to study the coordination chemistry extraction mechanisms of metal ions, particularly actinide (Cocalia, Gutowski, & Rogers, 2006; Jensen, Neuefeind, Beitz, Skanthakumar, & Soderholm, 2003; MacFarlane & Seddon, 2007). Experimentally, these prove that ILs are completely dissociated into cations and anions when in solution. Also, the definition of IL states it to be a liquid with melting point below 100°C and containing ions exhibiting ionic conductivity. The fact that the ionic conductivity can be measured experimentally proves that ILs consist of ions in solutions. Ionic conductivity studies have been carried out by Carda-Broch et al., (2003), Berthod and Armstrong (2003) on fluorinated anions such as [PF6].

Further, in molecular dynamic simulations, a cubical box is taken, which contains equal number of cations and anions, instead the whole IL molecule. For example, in the work of Morrow and Maginn (2002), the authors studied the thermo-physical properties of ionic liquids by using 300 cations and 300 anions. Similarly, in the work of Lee, Jung and Han (2005), the ionic conductivity of ILs were found using a cubical box of 100 cations and 100 anions. On the contrary, Earle et al. (2006) recently distilled IL without decomposition, at very low pressures. In their pioneering work, they were successful in distilling the [(CF3SO2)2N]-based ILs at 100 Pa and 573 K, without decomposition. However, in LLE experiments, ILs are assumed to be in isobaric condition of 1 atm, whereas the distillation is observed with pressures ~0.000009 atm. Since the operating pressures of LLE are 1 atm, our approach holds true.

Dissociation was also observed during vapor-liquid equilibria (VLE) predictions on reported systems (Kato & Gmehling, 2005; Kato, Krummen, & Gmehling, 2004) in ILs, where we obtained better results than obtained using a single composite molecule (Diedenhofen, Eckert, & Klamt, 2003; Table 3.1). This prompted us to propose that the IL be considered as a dissociated pair of cation and anion for both the VLE and LLE predictions. This novel approach can be applied to ILs containing a limitless combination of

TABLE 3.1

VLE Comparison of RMSD with Different Models

COSMO-RS

No.

Reference

System

Single

Molecule

Cation and Anion

1

(Doker & Gmehling, 2005)

[Emim][(CF3SO2)2N] + Acetone

2.70

1.81

2

(Doker & Gmehling, 2005)

[Emim][(CF3SO2)2N] + 2-Propanol

3.80

2.60

3

(Doker & Gmehling, 2005)

[Emim][(CF3SO2)2N] + Water

9.80

5.26

4

(Doker & Gmehling, 2005)

[Bmim][(CF3SO2)2N] + Acetone

3.50

3.08

5

(Doker & Gmehling, 2005)

[Bmim][(CF3SO2)2N] + 2-Propanol

4.00

2.89

6

(Doker & Gmehling, 2005)

[Bmim][(CF3SO2)2N] + Water

9.80

4.56

7

(Kato & Gmehling, 2005)

[Mmim][(CH3)2PO4] + Acetone

5.10

2.50

8

(Kato & Gmehling, 2005)

[Mmim][(CH3)2PO4] + Tetrahydrofuran

4.60

2.28

9

(Kato & Gmehling, 2005)

[Mmim][(CH3)2PO4] + Water

10.60

6.84

10

(Kato & Gmehling, 2005)

[Mmim][(CF3SO2)2N] + Benzene

8.92

3.66

11

(Kato & Gmehling, 2005)

[Mmim][(CF3SO2)2N] + Cyclohexane

6.26

4.23

12

(Kato et al., 2004)

[Emim][EtSO4] + Benzene

9.75

5.62

13

(Kato et al., 2004)

[Emim][EtSO4] + Cyclohexane

6.94

2.36

Average

6.59

3.66

cation and anion (~1018). Therefore, one has to do separate quantum mechanical calculations for cations and anions. Thereafter, one can test an IL for a particular application by addition of sigma profiles and sigma potential of cation and anion only.

Moreover, till date, an application concerned with the assumption of complete dissociation of ILs into cations and anions with equimolar concentrations has been carried out earlier by Klamt and Schuurmann (1993) has assumed a complete dissociation of cations and anions for their prediction of infinite dilution activity coefficients (IDAC) of solutes in ILs by COSMO-RS implementation. In their work, the ILs have been described by an equimolar mixture of two distinct ions, the cation and the anion, which finally contribute to the sigma profile of the mixture as two different compounds.

Hence, for the prediction of LLE-based system involving ILs, two modes of approach were assumed: (1) IL as a pure solvent and (2) IL consisting of a pair of cations and anions (Table 3.2). The latter approach assumes a solvent of a mixture of two components: cation and anion. The latter approach has proved to give better phase split with lower root-mean-square deviation. Hence, we proposed a complete dissociation of ILs into cations and anions with equimolar quantities, which is in line with the linear addition of the sigma profiles of the cation and anion.

Where, pcation(a) and panion(a) are the sigma profiles for cation and anion, respectively. This is based on the assumption that the IL in a solution is fully dissociated into its respective cations and anions with equimolar concentrations. It should be noted that the entropy of mixing will not exist, since we are dealing with a sigma profile of a single compound/mixture. Thereafter, the linear additions of COSMO area and volume are done, so as to obtain the profile of single mixture/compound.

The results of all the reported IL ternary systems are given in Table 3.2. Using the sigma profile for the single composite molecule, it is observed that for nearly half of the systems, the values of the activity coefficients were not able to predict the split between the extract (IL phase) and raffinate phase for the given tie line. Predictions giving an average RMSD of 36.5% and a maximum root-mean-square deviation (RMSD) of 90% ([Omim][Cl]-Benzene- Heptane) were observed. NO SPLIT (NS) condition for 15 systems was encountered. For 13 systems, an RMSD much greater than 10% was observed.

The data sets contain 12 different ILs comprising seven different cations and eight anions. Apart from imidazolium-based ILs, pyridinium-based IL (1-butyl-3-methylpyridinium tetrafluorob orate [Bmpy][BF4]) has also been studied. Switching from composite to additive sigma profile, we notice that a 'NO SPLIT' situation has been converted to a 'SPLIT' between the two phases for all the 15 cases. It is clear that the improvement is drastic for the IL containing system namely [Bmim][CF3SO3], [Omim][MDeg] and [Bmpy][BF4]

TABLE 3.2

LLE Comparison of Single and Cation-Anion Pair by Using COSMO-RS

System No.

T/K

System

Root-Mean-Square

Deviation

Experiment

Single

Molecule

Cation + Anion

Error

1

298.15

[Bmim][(CF3SO3] - Ethanol- ethyl-ferf-butyl ether

NS

0.095

NA

2

298.15

[Bmim][(CF3SO3] - Ethanol-ferf-amyl ethyl ether

NS

0.032

0.004

3

298.15

[Hmim][BF4] - Benzene-heptane

0.711

0.043

0.004

4

298.15

[Hmim][BF4] - Benzene-dodecane

0.087

0.098

0.004

5

298.15

[Hmim][BF4] - Benzene-hexadecane

0.481

0.072

0.004

6

298.15

[Hmim][BF4] - Ethanol-hexene

NS

0.147

0.004

7

298.15

[Hmim][BF4] - Ethanol-heptene

NS

0.125

0.004

8

298.15

[Hmim][PF6] - Benzene-heptane

0.856

0.042

0.004

9

298.15

[Hmim][PF6] - Benzene-dodecane

0.385

0.047

0.004

10

298.15

[Hmim][PF6] - Benzene-hexadecane

0.457

0.059

0.004

11

298.15

[Hmim][PF6] - Ethanol-hexene

NS

0.315

0.004

12

298.15

[Hmim][PF6] - Ethanol-heptene

NS

0.306

0.004

13

298.15

[Omim][Cl] - Methanol-hexadecane

0.029

0.005

NA

14

298.15

[Omim][Cl] -

Ethanol-hexadecane

0.03

0.008

NA

15

298.15

[Omim][Cl] -

Ethanol-ferf-amyl ethyl ether

0.382

0.067

NA

16

298.15

[Omim][Cl] -

Benzene-heptane

0.896

0.096

0.006

17

298.15

[Omim][Cl] -

Benzene-dodecane

0.058

0.096

0.006

18

298.15

[Omim][Cl] -

Benzene-hexadecane

0.146

0.07

0.006

19

298.15

[Emim][C8H17SO4] - Benzene-heptane

0.362

0.07

0.006

20

298.15

[Emim][C8H17SO4] - Benzene-hexadecane

0.456

0.14

0.006

(Continued)

TABLE 3.2 (Continued)

LLE Comparison of Single and Cation-Anion Pair by Using COSMO-RS

System No.

T/K

System

Root-Mean-Square

Deviation

Experiment

Single

Molecule

Cation + Anion

Error

21

298.15

[Omim][MDEG]a - Benzene-heptane

NS

0.103

0.006

22

298.15

[Omim][MDEG]a - Benzene-hexadecane

NS

0.043

0.006

23

313.15

[BmpyKBFJ - Xylene-octane

NS

0.03

0.0025

24

348.15

[Bmpy][BF4] - Xylene-Octane

NS

0.024

0.0025

25

313.15

[BmpyKBFJ - Ethylbenzene-octane

NS

0.041

0.0025

26

348.15

[Bmpy][BF4] - Ethylbenzene-octane

NS

0.029

0.0025

27

313.15

[BmpyKBFJ - Benzene-hexane

NS

0.048

0.0025

28

333.15

[Bmpy][BF4] - Benzene-hexane

NS

0.036

0.0025

29

298.15

[BmpyKBFJ - Toluene-heptane

NS

0.075

0.0025

30

298.15

[Mmim][CH3SO4] - Toluene-heptane

0.455

0.008

0.0025

31

298.15

[Emim][C2H5SO4] - Toluene-heptane

0.116

0.031

0.0025

32

298.15

[Bmim][[CH3SO4] - Toluene-heptane

0.235

0.032

0.0025

33

313.15

[Emim][EtSO4] - Ethanol-hexene

NS

0.124

0.005

34

313.15

[Emim][EtSO4] - Ethanol-heptene

NS

0.023

0.005

35

313.15

[E-2,3-dmim][EtSO4] - Ethanol-hexene

NS

0.032

0.005

36

313.15

[E-2,3-dmim][EtSO4] - Ethanol-heptene

NS

0.065

0.005

Note: NS: No splitting of the phase; NA: Not available a MDEG: monomethyldiethyleneglycol

systems and also for the alkene-based systems [System No. 6, 7, 11, 12], for which no prediction had been possible when considering the IL as a single molecule. For the [Bmim][CF3SO3], [Omim][MDeg] and [Bmpy][BF4] systems (System No. 1, 2, 21, 22 and 23-29), the RMSD is less than 10%. Out of the 36 systems studied, most of the predictions are excellent, considering the fact that only six systems gave RMSD greater than 10%.

 
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