Phase Equilibria with Gases and Liquids of 1-n-Butyl-3

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Downloaded by PENNSYLVANIA STATE UNIV on June 2, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch022

Phase Equilibria with Gases and Liquids of 1-nButyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

Jacob M . Crosthwaite, Laurie J. Ropel, Jennifer L . Anthony, Sudhir Ν . V . K. A k i , Edward J. Maginn, and Joan F. Brennecke

Department of Chemical and Biomolecular Engineering, University of Notre Dame, South Bend, IN 46556

The solubility of carbon dioxide in 1-n-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf N]) at 25 °C is reported and compared with CO solubility in 1-n-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate ([bmim][PF ] and [bmim][BF ]). In addition, we report liquid-liquid equilibrium data for [bmim][Tf N] with 1-butanol and 1-hexanol. By comparison with literature data, we show the influence of anion and alkyl chain length on the imidazolium ring on phase behavior with alcohols. Finally, we present an estimate for the octanol/water partition coefficient, which is a measure of potential bioaccumulation, for [bmim][Tf N]. 2

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© 2005 American Chemical Society

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

293 Here we present preliminary results for the solubility of carbon dioxide in l-w-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([bmim] [Tf N]). Previously, we have measured the solubilities of a wide variety of gases in l-w-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF ]) (1), as well as the solubility of carbon dioxide and methane in l-«-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF ]) (2). The solubility of these two gases in [bmim][PF ] and [bmim][BF J are relatively similar. The goal of the current work with [bmim][Tf N] is to explore further whether there is any influence of the nature of the anion on gas solubilities. We know of no other published values of the solubility of gases in [bmim][Tf N]. We are particularly interested in understanding the solubility of gases in ionic liquids due to the potential of using these non-volatile solvents for gas separations (3-5). Second, we present the liquid-liquid equilibrium (LLE) of 1-butanol and 1hexanol with [bmim][Tf N]. These results are part of a systematic study that we are conducting of the LLE of alcohols with imidazolium-based ILs as a means of understanding the characteristics of the IL (e.g., cation, choice of anion, substituents on the cation) that are important in determining the phase behavior. LLE is of practical importance for understanding the solubility of reactants and products in ILs, as well as the amount of IL that will contaminate organic or aqueous phases that are brought into contact with the IL. While a number of researchers have presented LLE results for various dialkylimidazolium salts with alcohols (6-9), we know of only one study of L L E of any dialkylimidazolium Tf N compounds with alcohols (10). All of these systems show upper critical solution temperature (UCST) behavior; i.e. the mutual solubilities increase with increasing temperature. Finally, we report an estimate for the octanol/water partition coefficient, Kow, of [bmim][Tf N]. Ko is defined as the ratio of the concentration of a solute in the octanol phase to the concentration of the same solute in the water phase in a two phase mixture of octanol, water, and solute. This definition is valid when the solute concentration is very dilute. K values provide estimates of the partitioning of a solute between water and the lipid layers of aquatic organisms, as well as the distribution of a solute between water and sediments (11). Thus, Kow gives a first estimate of bioaccumulation potential. Moreover, K values have been correlated with the toxicity to aquatic organisms (11). Partitioning of alkylmethylimidazolium chlorides, where the alkyl chain ranged from butyl to dodecyl, between octanol and water have been reported (9). However, these measurements were made for highly concentrated solutions and cannot be compared directly with the dilute K value reported here. In addition, Choua et al. (12) reported K values for [bmim][PF ] and ethyl-3-methylimdazoIium hexafluorophosphate ([emim][PF ]) that ranged from 0.005 to 0.020 using a 2

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294 shake-flask method without centrifugation. This method can lead to inaccurate results if the emulsion is not fully separated.


Experimental A gravimetric microbalance (IGA 003, Hiden Analytical) was used to measure the low pressure (less than 14 bar) gas solubilities reported in this work. This microbalance and the technique used to measure gas solubilities in ILs has been described in detail elsewhere (1,13). Briefly, a small IL sample is placed on the balance and the system is thoroughly evacuated to remove any volatile impurities, including water. The gas is introduced and the gas solubility is determined from the mass uptake. The high pressure (up to about 65 bar) C 0 solubility in [bmim][Tf N] was measured using a static high-pressure apparatus that has been described previously (14). Known amounts of C 0 are metered into a cell containing IL. Using the known volume of the vapor space and an equation of state to determine the number of moles in the vapor, the solubility of C 0 in the IL is determined by the difference of these two amounts of substance. The liquid phase behavior of [bmim][Tf N] with 1-butanol and 1-hexanol was measured using a cloud point technique described previously (2). The slow-stirring method was used for determining the octanol/water partition coefficient of [bmim][Tf N]. The experimental apparatus consisted of a sealed vial, equipped with a dip-tube to prevent contamination of the syringe needle when a sample of the lower phase is taken. The deionized water and 1octanol were initially mixed together so they were in equilibrium prior to use. The pre-equilibrated deionized water was added to the 40 mL clear glass vial containing a Teflon coated magnetic stir bar. The lid consisted of a cap, septum, and a dip-tube that penetrates through the septum. This lid was placed on the vial so that the dip-tube extended below the surface of the water-rich phase. A known (dilute) concentration of [bmimJ[Tf N] was added to the preequilibrated 1-octanol separately. This solution was carefully added to the vial containing the water so that the solution did not emulsify. Then the vial was sealed. Five vials using different dilute octanol-[bmim][Tf N] "stock" solutions were made to determine the concentration dependence. The vials were stirred slowly to prevent emulsification and maintained at room temperature, which was 24 ± 2 °C, Three syringe samples were taken of the octanol-rich and water-rich phases over the course of ten days, at which time the concentrations in each phase had stabilized. Concentrations of [bmim][Tf N] in each phase were measured using UV-vis spectroscopy (Cary 3, Varian), which has a sensitivity of ± 0.01 absorption units. 2

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295 For all the experiments, the [bmim][Tf N] ionic liquid was purchased from Covalent Associates. The 1-butanol and 1-hexanol for the L L E measurements were both 99+% anhydrous grade from Aldrich. The 99+% HPLC grade 1octanol was also purchased from Sigma-Aldrich. The C 0 was Coleman Instrument grade, obtained from Mittler Supply Company. A l l compounds were used without further purification. However, the IL was thoroughly dried at 80 °C under high vacuum and the water content was measured using KarlFisher titration (EM Science Aquastar V-200 Titrator) prior to use. Water content for the IL used in the high pressure gas solubility measurements was less than 500 ppm. For the LLE measurements, the IL water content was 460 ppm. In addition, the water content of the 1-butanol and 1-hexanol were 250 ppm and 590 ppm, respectively. In the low pressure gas solubility measurements with the gravimentric microbalance, the IL was dried further in situ with a 10* bar vacuum until the sample mass stabilized. 2


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Results and Discussion Gas Solubility. The solubility isotherm at 25 °C for C 0 in [bmim][Tf N] is shown in Figure 1, along with our previously reported isotherms for C 0 in [bmim][PF ] (1) and [bmim][BF ] (2), also at 25 °C. As can be seen in the graph, the solubility in [bmim][Tf N] is significantly larger than was seen with the other two ionic liquids. Changing the anion from [BF ] to [PF ] did not result in a significant change in affinity for C 0 , whereas the [Tf N] anion does increase this affinity. A Henry's law constant (H) can be calculated from the isotherms as the inverse slope at low pressures (Η = P/x) to yield the Henry's constant in units of pressure. At 25 °C, this Henry's constant is 33.0 + 3 bar for C 0 absorbed in [bmim][Tf N]. By comparison, the values for C 0 in [bmim][PF ] and [bmim][BF ] are 53.4 + 0.3 bar (1) and 56.5 + 5 bar (2), respectively. Again, the higher solubility of C 0 with the [Tf N] anion compared to either the [PF ] or [BF ] anions is seen by the smaller Henry's constant. We have also measured the solubility of C 0 in [bmim][Tf N] at higher pressures and the results are shown in Figure 2. The solubility measurements were repeated to ensure reproducibility and the results from both sets of data are shown in the figure. As expected, the solubility increases with an increase in pressure. For example, the solubility increased from 0.268 mole fraction at 11.4 bar to 0.717 mole fraction at the highest pressure measured (60.4 bar). Furthermore, we find excellent agreement between the low pressure data taken with the gravimetric microbalance and the higher pressure data taken with the static high pressure apparatus, as shown in the figure. 2

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0.35 [bmim][Tf N] 2

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§ 0.20 Φ 0.15

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Pressure (bar) Figure 1: Solubility of carbon dioxide in [bmim][Tf N], [bmim][PF ] (1) and [bmim][BF ] (2) at 25 °C. 2

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High pressure measurements-1 High pressure measurements - 2 Low pressure measurements

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Figure 2: Solubility of carbon dioxide in [bmim][Tf N] at 25 °C using both the low pressure and high pressure apparatuses. 2


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Mol% IL Figure 3: Τ-χ diagram for [bmim][Tf N] with 1-butanol ( · ) and 1-hexanol (•). 2

Liquid Liquid Equilibrium. The liquid phase behavior of [bmim][Tf N] with 1-butanol and 1-hexanol is shown in Figure 3, From the figure, it is clear that these systems exhibit upper critical solution temperature behavior (UCST). In addition, the alcohol-rich phase of both systems contains a small amount of ionic liquid, while the ionic liquid-rich phase contains a large concentration of alcohol. The effect of alcohol chain length is clear by comparing the two curves: increasing the chain length of the alcohol (i.e., making the alcohol more aliphatic) increases the UCST of the system. The effect of cation alkyl chain length is shown in Figure 4. By comparing our data for the l-butanol/[bmim][Tf N] system with data for 1-butanol/1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf N]) from Heintz et al. (10), one observes that increasing the alkyl chain on the cation decreases the UCST of the system. This trend is likely due to increased interaction between the alkyl chains on the cation and alcohol via van der Waals forces. Also shown in Figure 4 is the dramatic effect that the anion has on the liquid phase behavior. A comparison of our data for the 1butanol/[bmim][Tf N] system with data for the l-butanol/[bmim][PF ] system from Wu et al. (6) shows that changing the anion from [Tf N] to [PF ] increases the UCST from about 26 °C to over 100 °C. Thus, the choice of anion can have a dramatic effect on LLE behavior. 2

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Mol% IL Figure 4: Τ-χ diagram for [bmim][Tf N] ( · ) with 1-butanol. Comparison is made to [emim][Tf N] (•) from Heintz et al. (10) and [bmim][PF ] ( A ) from Wueta/. (6). 2

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Octanol/Water Partition Coefficient. The octanol/water partition coefficient of [bmim][Tf N] at 24 ± 2 °C was determined to be between 0.11 and 0.62. Even at dilute concentrations (1.5 χ 10" - 2.2 χ 10 M in the aqueous phase) the K

Phase Equilibria with Gases and Liquids of 1-n-Butyl-3

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