Ionic Liquids IV Not Just Solvents Anymore - American Chemical Society

Chapter 25

Hydrophobic n-Alkyl-N-isoquinolinium Salts: Ionic Liquids and Low Melting Solids 1,2

1,3

1,4

Ann E. Visser , Jonathan G. Huddleston , John D. Holbrey , W. Matthew Reichert , Richard P. Swatloski, and Robin D. Rogers* 1

1

Downloaded by UNIV OF SYDNEY on May 4, 2015 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0975.ch025

1,

1

Department of Chemistry and Center for Green Manufacturing, The University of Alabama, Tuscaloosa, A L 35487 Current address: Savannah River National Laboratory, Aiken, SC 29808 Current address: Millipore Bioprocessing Ltd., Medomsley Road, Consett, County Durham DH8 6SZ, United Kingdom Current address: QUILL, The Queen's University of Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdom 2

3

4

A series o f hydrophobic n-alkyl-N-isoquinolinium ionic liquids (ILs) with a linear alkyl-chain substituent containing from 4-18 carbon atoms in combination with hexa -fluorophosphate, bis(trifluoromethylsulfonyl)amide, and bis(perfluoroethylsulfonyl)amide have been synthesized and characterized (water content, density, DSC, T G A , and LSER). The crystal structures o f [C isoq][PF ] (prepared and isolated only for the comparative X-ray diffraction study), [C isoq][PF ], and [C isoq] [PF ] illustrate the underlying interactions in the higher melting salts. The isoquinolinium-based ILs studied here are interesting due to their highly aromatic nature and physical and solvent properties. 2

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362

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© 2007 American Chemical Society

In Ionic Liquids IV; Brennecke, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

363

Introduction The search for new ionic liquids (ILs) with the potential to interact with aromatic hydrocarbons for separation purposes, led us to the study of a new class of cations based on the isoquinoline molecule (/). Our previous study focused on bis(perfluoroethylsulfonyl)amide ([N(S0 CF CF ) ;T or [BETA]") salts o f the w-alkyl-N-isoquinolinium ([C isoq] ) cation with alkyl chain lengths ranging from 4-18. We found that these ILs have an affinity over [C mim][PF ] for aromatic solutes as demonstrated by their increased distribution ratios in IL/water biphasic systems. Here we expand this work with additional data for[C isoq] salts of hexafluorophosphate ([PF ]*) and bis(trifluoromethylsulfonyl)amide) ([NTf ]"). Partitioning data was collected to study solvent properties based on interpretation of Abraham's linear solvation energy relationships (LSER) (2) and provides insight into how subtle changes in the cation, not only the anion, can affect the solvent properties. Solid-state analyses of the [PF ]" salts of [C isoq] , [C isoq] , and [Ci isoq] were carried out to gain insight into the interactions responsible for the physical and solvent properties of these ILs (5). 2

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n

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Experimental A l l aqueous solutions were prepared with deionized water that was purified with a Barnstead NANOpure water system (Dubuque, IA) and polished to 18.3 ΜΩ/cm. A l l salt and acid solutions were prepared as molar concentrations by transferring a known amount of material to a volumetric flask and diluting to the specified volume with deionized water. When needed, pH adjustments of the aqueous phase were made using H N 0 or NaOH. H P F was supplied by Ozark Mahoning (Tulsa, O K ) and was used as received. L i [ N ( S 0 C F ) ] (Li[NTf ]) and L i [ N ( S 0 C F C F ) ] (Li[BETA]) were obtained from 3 M (Minneapolis, M N ) and used as received. A l l other chemicals were of reagent grade, obtained from Aldrich (Milwaukee, WI), and used without further purification. 3

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Synthesis of lC isoq]Cl Salts n

The w-alkyl-TV-isoquinolinium chloride salts were prepared by alkylation of isoquinoline with an appropriate chloroalkane using a procedure developed for the alkylation of imidazoles to form 1-alky 1-3-methylimidazolium ILs described in the literature (4). Approximately 100 g batches were prepared by reaction of a 1:1 molar ratio of chloroalkane and isoquinoline with magnetic stirring at 70 °C for 72 h. After 72 h, two phases had formed with the top layer being the


364 unreacted starting material and the bottom layer being the isoquinolinium IL. The top layer was decanted and the bottom layer washed with ethyl acetate. The wash was repeated two more times and the IL dried under vacuum with heating at 70 °C to remove any remaining ethyl acetate.

Synthesis of [C isoq][X] ( X = [PF ]", [NTf l", and [BETA]") Salts n

6

2

|C isoq][PF ]: The salts below were prepared by metathesis from the appropriate w-alkyl-iV-isoquinolinium chlorides following the procedure described by Huddleston, et al. (4) for the preparation of 1,3-dialkylimidazolium salts. Approximately 100 g of [C isoq][Cl] was transferred to a 2 L container lined with a perfluorinated material followed by the addition of 500 mL deionized water. A n aqueous solution of 60% H P F in a 1.1:1 molar ratio of HPF :[C isoq]Cl was added. Caution: the addition of HPF^should be done slowly to minimize the amount of heat generated. As H P F was added, two phases formed, where [C isoq][PF ] formed the brown bottom phase and aqueous HC1 the top phase. The upper phase was decanted and 500 mL of water was added followed by vigorous shaking and mixing. After the mixture settled, the upper aqueous phase was decanted and the lower ionic liquid phase was washed again with deionized water. This procedure was repeated until the p H of the upper phase was ~7. [C„isoqJ[NTf J: Approximately 100 g of [C isoq]Cl was transferred to a 250 mL plastic bottle and a 1.1:1 molar ratio of LipsfTf ]:[C isoq]Cl was added, followed by 50 mL of deionized water. After mixing, two phases formed where the brown bottom phase was [C isoq][NTf ] and the colorless top phase was aqueous L i C l . After decanting the top phase, 50 mL of fresh deionized water was added and the solution was thoroughly mixed. This washing step was repeated twice. [C isoq][BETA]: Approximately 100 g of [C isoq]Cl was transferred to a 250 mL plastic bottle and a 1.1:1 molar ratio of Li[BETA]:[C isoq]Cl was added, followed by 50 mL of deionized water. After mixing, two phases formed where the brown bottom phase was [C isoq][BETA] and the colorless top phase was aqueous L i C l . After decanting the top phase, 50 mL of fresh deionized water was added and the solution was thoroughly mixed. This washing step was repeated twice. n

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Physical Property Measurements After initial synthesis, the ILs were purified and dried. Since these ILs were prepared to study liquid/liquid partitioning from water, the physical properties of the hydrophobic ILs were studied after equilibration with an equal volume of water over a 24 h period.



365 The water content of each IL (Table I) was determined by Karl Fischer titration using a volumetric Aquastar Karl Fischer titrator ( E M Science, Gibbstown, NJ) with Composite 5 solution as the titrant and anhydrous methanol as the solvent. Each sample was water ' equilibrated before measurements were taken. Sample weights were at least 1 g and duplicate measurements were performed on each sample with results agreeing to within 5%. The density o f each water-equilibrated IL was determined by gravimetric analysis. After calibrating a 1 m L pipet (Rainin, Woburn, M A ) to dispense 1.0 g of water, that pipet was used to transfer 1.0 mL of each IL to determine the mass of that volume of liquid. Each measurement was repeated 10 times and the average value is reported. A l l measurements were taken at room temperature (25 =fc 1 °C). Melting point and glass transition temperatures were determined by differential scanning calorimetry (DSC) using a T A Instruments (New Castle, DE) model 2920 differential scanning calorimeter. Temperature calibration was performed on a sample of indium. Each sample was approximately 10 mg and analyzed in a hermetically-sealed aluminum pan. For each experiment, an empty hermetically-sealed pan was referenced as the blank. A ramp temperature of 10 °C/min was employed over the temperature range of -150 to 100 °C. To ensure properly equilibrated transition temperatures, the samples were cycled through this cooling/heating method once before temperatures were assigned to transition peaks. Transition temperatures were recorded at the peak maximum o f the thermal transition.

X-ray Diffraction Studies Crystals suitable for single crystal X-ray diffraction of [C isoq][PF ], [C isoq][PF ], and [C isoq][PF ] were obtained by recrystallization of the salts by heating above the melting point and allowing the ILs to slowly cool to room temperature. The [C isoq][PF ] salt was prepared and crystallized only for the comparative X-ray diffraction study. Single crystals of each salt were mounted on a glass fiber and transferred to the goniometer of a Siemens S M A R T diffractometer equipped with a C C D area detector and graphite monochromated Mo-Κα radiation. The crystals were cooled to -100 °C with a stream of nitrogen gas. The data were corrected for absorption using S A D A B S (5) and S H E L X T L Version 5 was used for structure solution and refinement (6). In each structure, the atoms were readily located and the positions of all non-hydrogen atoms were refined anisotropically. In the [C isoq] salt, the [PF ]" anion was found to be disordered. Four fluorine positions in one plane were modeled with alternate positions of 70%/30% occupancy. The two fluorine atoms (axial to the disordered atoms) are ordered. 2

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366 +

The hydrogen atoms in the disordered [C isoq] salt were added in geometrically appropriate positions and refined constrained in a riding model. The hydrogen atoms in the two other salts were added in geometrically appropriate positions and then fully refined isotropically. 2

Partitioning Studies


l4

C-labeled solutes were obtained from Sigma (St. Louis, M O ) . Liquid scintillation analyses were performed using Ultima Gold scintillation cocktail (Packard Instrument, Downers Grove, IL) and a Packard Tri-Carb 1900 T R Liquid Scintillation Analyzer. For each study, 1 mL of IL and 1 m L of an aqueous phase were mixed followed by vortexing (2 min) and centrifuging (2000 g, 2 min) to equilibrate the phases. When using ionizable solutes, the aqueous phase p H was adjusted (using either NaOH or H N 0 ) to a pH where the solutes would be neutral. Addition of the organic tracer (ca. 0.005 μ Ο , 5 μL) was followed by two intervals of vortexing (2 min) and centrifuging (2000 g, 2 min) to ensure that the phases were fully separated. 100 μΐ, of each phase was removed for radiometric analysis. Since equal volumes of both phases were removed for analysis, the distribution ratios for the organic solutes were determined as in eq. 1 : 3

Ρ

=

Activity in the IL lower phase

^^^

Activity in the aqueous upper phase Each experiment was carried out in duplicate and the results agreed to ± 5 % .

Results and Discussion Physical Properties We have synthesized a series of IL salts by straight chain N-alkylation ( C = 4-18, all even) of isoquinoline followed by metathesis of the chloride salts resulting in pairing with [PF ]\ fNTf ]", and [BETA]" anions (Figure 1). A l l [PF ]' salts were obtained as low melting solids, whereas the salts containing either of the fluorinated sulfonylamide anions ([NTf]" or [BETA]") were liquid at room temperature. Since these salts were prepared for possible use in I/I extraction, the reported water contents and densities (Table I) are for the salts after equilibration with water. The water contents of the saturated [C isoq] salts are in general higher than those reported for the same Ca-substituted methylimidazolium ([C„mim] ) derivatives (4). A s observed with other cation classes, the trend in water content n

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367

cr

χ

Figure 1. The general synthesis and metathesis of the n-alkyl-N-isoquinolinium salts (n = 4-18 all even, X = [PFJ, [NTfJ, or [BETA]-): (a) CJi ,Cl; (b) HPF , LifNTfJ, orLifBETA]. 2n+


6

Table I. Water Content and Density of Water-Equilibrated [C„isoq] ILs +

η

Anion

4 4 4 6 6 6 8 8 8 10 10 10 12 12 12 14 14 14 16 16 16 18 18 18

[PF6Ï [NTfJ[BETA]' [PF r [NTf ]" [BETA]' 6

2

[PF y [NTf ]" [BETA]' [PF r [NTf ]" [BETA]" [PF ][NTf ]" [BETA] [PF«][NTf ]" [BETA]6

2

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2

[PF ]" [NTf ]' [BETA][PF ][NTf r [ΒΕΤΑ]· 6

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Water content (ppm)

Density (x/mL)

18600 17700

1.26 1.23

-

-

16900 16200 18500 15600 14900

1.23 1.20 1.19 1.19 1.17

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7300 6900

1.13 1.09

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6800 6600

1.11 1.08

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6100 6100

1.09 1.07

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4800 4700

1.06 1.05

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4200 4100

1.05 1.05


368 follows the order [PF ]* > [NTf ]" » [BETA]'. Table I also indicates that, for a series of cations with a particular anion, increasing the length of the alkyl chain appended to the cation decreases the water content of the IL phase (also as previously observed (4)). For the series [C„isoq][BETA], a noticeable change in water content occurs at η = 10, suggesting a change in internal liquid structure due to the intérdigitation of the alkyl chains, as has been observed for imidazolium cations (7,8). Such intérdigitation allows for the closer packing of the ions and establishes a charge-rich region and a hydrophobic region in the liquid, which increases the over all hydrophobicity. The DSC-determined glass transition temperatures and melting points are summarized in Figure 2. While cooling, no crystallization events were observed, and the ILs cooled directly to a glassy state. The glass transition temperatures generally increase progressively with increasing alkyl chain length with the [BETA]" salts uniformly having the lowest T . Melting points also vary with the alkyl chain length as shown in Figure 2b. The trends of decreasing melting points with increasing alkyl chain length to a minimum at [C isoq][BETA], [C isoq][NTf ], and [C isoq][PF ], followed by increasing melting points for longer alkyl chain derivatives has been observed as a general feature of ILs containing an alkyl-chain substituent on quaternized imidazolium cations (9). Increasing chain lengths can lead to the formation of layered structures, often with the observation of thermotropic liquid crystalline phases on melting (10). Thermal decomposition of the salts was determined by T G A (Figure 3). Figure 3a illustrates the change in thermal stability of the [PF ]" salts as the alkyl chain length is varied. In all cases there was a 2-3% weight loss prior to 100 °C, which is attributed to the loss of water in the sample. After the initial loss of water, the weight of the sample remains relatively constant until a catastrophic weight loss between 325-375 °C corresponding to the decomposition of the cation. The effects of the anion on the decomposition of the [Cj isoq] salts is shown in Figure 3b. The CI" salt has the lowest decomposition temperature, which is consistent with the literature where halide-containing ILs have been shown to exhibit low decomposition temperatures (4,12) due to nucleophilic S 2 attack of CI" on the N - C bond (//). The increase in anion size and nucleophilicity from CI" to [PF ]' to [NTf ]" and [BETA]", results in an increase in the decomposition temperature, which produces more thermally stable ILs. The T G A results are similar to those of [1-alky 1-3-methylimidazolium] salts (4,12J 4).


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Single Crystal X-ray Diffraction Studies Details regarding the collection and treatment of the data and the structure solution and refinement are included in Table II. ORTEP illustrations of the three salts examined, [C isoq][PF ], [C isoq][PF ], and [C isoq][PF ] are 2

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10 12 14 16 18 20

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Alkyl Chain Length

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Figure 2. Glass transition temperatures (a) and melting point temperatures (b) for [C„isoqJ ILs as a function of increasing alkyl chain length. +

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Figure 3. TGA for [C„isoq][PF ] (a) and [C isoqJ 6

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(b) ILs.


600

370

Table II. Summary of Crystallographic Data (15) [C isoq][PF ] fCisoqjrPFJ fC isoq]fPF ] C H F NP C H F NP Formula C„H F NP 331.24 415.39 Formula Weight 303.19 Crystal Dimensions 1.10x0.50x0.30 0.32x0.22x0.10 0.42x0.20x0.18 Γ(Κ) 173 173 173 Triclinic Crystal System Triclinic Triclinic P-l Space group P-l P-l a (A) 6.821(3) 9.2078(10) 7.1750(9) b(A) 7.923(3) 9.3151(12) 9.2084(10) c(A) 10.3028(12) 18.985(7) 10.4638(14) α Ο 90.733(7) 104.173(2) 75.276(2) 94.716(5) 70.407(2) 111.701(2) β Ο 97.942(6) 87.602(2) 103.753(2) γ Ο ν (A ) 636.50(14) 732.84(14) 1012.3(6) ζ 2 2 2 1.582 1.501 1.363 Dcaic (g cm' ) μ(ΜοΚα) (mm- ) 0.274 0.245 0.193 F(000) 340 436 308 2.2, 23.3 2.3,23.2 2.3,23.3 Omin-max ( ) Reflections collected 2867 4457 3405 Unique reflections, R , 1815, 0.0997 2882,0.0262 2103,0.0170 [I > 2a(l)] data 1221 1838 1666 Transmission factors 0.1355, 0.9703 0.6664,0.9702 0.6466, 0.9703 Parameters varied 229 255 349 GOF 1.313 0.949 0.9 19 R, wR [I > 2σ(Ι)] 0.0334, 0.0843 0.0470, 0.1158 0.1279, 0.3048 R, wR (all data) 0.0447, 0.0894 0.1509, 0.3 161 0.0799, 0.1266 "GOF = {£[w(F -F ) ]l(n-p)} where η is the number of data and ρ is the number of parameters refined. *R = Z||F |-|F ||/X|F |; v/R = {E[w(F -F ) ]/L(w(F ) ]} 2

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371 provided in Figure 4. Even though the majority of the [C isoq][PF ] salts melt above room temperature, finding suitable crystals was a challenge, complicated by formation of microcrystals, multiple crystals, etc. Nonetheless, in the three structures studied, all of the ions are ordered except for the anions in the [C isoq] salt, which showed typical disorder for the spherical [PF ]" anion. The close contacts and packing diagrams of the three structures are shown in Figure 5. These interactions illustrate the dominant nature of the Coulombic interactions in these salts and the increasing hydrophobic effect as the alkyl chain length increases; observations previously reported for a larger series of dialkyl-imidazolium [PF ]" salts (3,76). n

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c)

Figure 4. Asymmetric units of (a) [C isoq][PF ] (c)[C isoq][PF ]. 2

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(b) [C isoq][PF ] 4

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and

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372

Figure 5. Close contact and packing diagrams of the cation environments top views (a), the cation environments side views (b), the π- π stacking (c), the anion environments (d), and packing diagrams (e,f).


373 With cation-anion interactions centered near the charge rich region of the cation (i.e., the carbon atoms nearest the nitrogen), the cations can and do pack with π-π interactions, something much less common in the imidazolium salts. In [C isoq][PF ], the π-π interactions occur on the back side between C5*» C6 at a distance of 3.54 Â (Figure 5c), while [C isoq][PF ] exhibits a more eclipsed interaction with partial overlap of the cations between C5 **C9 and C3 *C7 at 3.53 À and with the C6 carbon residing over the nitrogen containing ring of the isoquinolinium cation at a distance of 3.74 Â. In [Ci isoq][PF ], the overlap is along the long edge between the €6···€9 and C7«"C8 positions at a distance of 3.47 A . e

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Solvent Properties Liquid/liquid partitioning of a series of organic solutes between water and the series of ILs consisting of [C isoq][BETA] (where η = 6, 8, or 14) and, separately, [ Q i s o q f combined with [PF ]\ [NTf ]", or [BETA]" (Figure 8) was carried out in order to determine the free energy of transfer of a methylene group (17,18) (-AG H2> a measure of the relative hydrophobicity of the phases) and linear solvation energy relationships (19,21) (LSERs, to describe the intermolecular forces in the form of solvent-solute interactions that control solute partitioning in various liquid/liquid systems). Previously, we (22,23) and others (24,25) have reported that solute partitioning in IL/aqueous systems generally follows the 1 -octanol/water log Ρ value with the most hydrophobic solutes having the highest affinity for an IL, and that in a series of [l-alkyl-3methyimidazolium][PF ] ([C mim][PF ]) IL, the most hydrophobic IL will exhibit the highest distribution ratios for hydrophobic solutes. n

6

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C

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n

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Distribution ratio data can also be used to further evaluate the solvent properties, the underlying interactions, and the polarity of ILs. [C mim][X] salts have been investigated with solvatochromatic dyes which give insight into specific interactions such as polarity and hydrogen bond character (14,26,27,28,29). Others have used the L S E R method to model IL behavior using G C and H P L C (24,30,31). Literature reports suggest a range of polarity for the [C mim] IL from more polar than acetonitrile (28) to similar to that of short chain alcohols (26). Variations in the [C mim] cation and choice of anion also affect the polarity where, as observed by Carmichael, et al. (26), ILs with short alkyl chains (n < 6 or 8) are most affected by the anion, whereas the cation has more of an influence for longer alkyl chains where hydrophobic interactions tend to dominate. The free energy of transfer of a methylene group (-AG m), as developed by Zaslavsky (17,18,32), has also been proposed as a means to describe the relative hydrophobicity of biphasic systems and describes the cohesiveness of the solvent. It is a measure of the energy requirement involved in making a cavity in the non-aqueous phase. Traditionally, the partitioning of a series of dinitron

+

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C



n

2

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io-

10"

10'

\0

2

- 1 0

• I

• 1

3

t • 14

•

• 15

1 2

3

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l4

a)

L o g Ρ (1 -octanol/water)

j

4

1

1 2

13 f

•

Φ t"

5

17. Acetic acid 18.4-Hydroxybenzoic acid 19. Benzamide 20.f-Chloroaniline 6 21. Nitrobenzene 22. Kthylacetate 23. Benzoyl alcohol I4.Chtorohenzene IS. Dichlorohenzene 24. Dichloroethane lit. Trtchlorobenzene 25. Anisole

I. Methanol 2. Mhanol 3. Isopropanof 4. Propanol 5. Butanol 6. Pentanol 7. Octanol N. Phthalic acid 9. Benzoic acid 10. Salicylic acid II. Phenol 25 12. Benzene *2

Ionic Liquids IV Not Just Solvents Anymore - American Chemical Society

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