Inert-Gas-Stripping Method for Measuring Solubilities of Sparingly

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Inert-Gas-Stripping Method for Measuring Solubilities of Sparingly Soluble Gases in Liquids. Solubilities of Some Gases in Protic Ionic Liquid 1-Butyl, 3-Hydrogen-imidazolium Acetate Waheed Afzal, Brian Yoo, and John M. Prausnitz* Energy Biosciences Institute, Department of Chemical and Biomolecular Engineering and Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720-1462, United States ABSTRACT: The inert-gas stripping method is useful for rapidly measuring solubilities of moderately and sparingly soluble gases in liquids. Earlier versions of this method can give only solubilities of volatile-liquid solutes in low-volatile solvents. However, the modifications presented here enable measurement of very low solubilities of gases in liquids. Henry’s constants are reported for carbon dioxide, krypton, oxygen, air, and nitrogen in n-dodecane and n-pentadecane, and for carbon dioxide in ethylene glycol at near-ambient conditions. The new Henry’s constants compare well with those in the literature. Henry’s constants are reported for carbon dioxide, ethane, ethylene, krypton, oxygen, air, and nitrogen at near-ambient conditions in the protic ionic liquid 1-butyl, 3-hydrogen-imidazolium acetate; we select this protic ionic liquid because, relative to other ionic liquids, it has low viscosity and because solubility data in protic ionic liquids are rare.

1. INTRODUCTION Solubilities of gases in liquids have been studied for more than a century. Numerous experimental methods have been presented in the literature.1 Methods based on retention-volume measurements using gas−liquid chromatography are generally not suitable for measuring gas solubilities in liquids;2 however, synthetic methods or those based on compositional analysis are often useful.3−6 Regrettably, essentially all experimental methods suffer from increasing uncertainty as the gas solubility becomes very small. The solubility of gas i is often expressed in terms of the Henry’s constant Hi defined by Hi = lim xi → 0 fi /xi (1)

however, are not suitable for measuring solubilities of sparingly soluble gases. The conventional IGS method7,10,17 is based on continuously stripping a volatile solute in a well-mixed vessel from a very dilute solution of that volatile solute in a low-volatile liquid solvent. An inert gas (helium) flows through the solution to deplete the volatile solute from the solvent. Agitation of the liquid is required to ensure that the gas phase and the liquid phase are at equilibrium. The stripping gas is chemically inert and essentially insoluble in the liquid. To determine Henry’s constants, we need to know the mass of solvent, the flow rate of the inert gas, the volume of the vapor space above the liquid inside the stripper, and the time-dependent concentration profile (stripping curve) of the solute in the exiting gas. In previous studies,7,10,11,17 the stripping curve was obtained by periodically sampling the inert gas leaving the stripper and analyzing with a gas chromatograph (GC). The composition of the vapor phase (or the area corresponding to the solute peak obtained using the GC) as a function of time is required to construct a stripping curve. We require at least 15 data (si, t) to establish a stripping curve with reasonable accuracy. Here, t is time and si is the concentration of solute in the exiting stripping gas as determined by the peak area for the solute obtained from the GC. For a typical solute with moderate solubility, about 2 h is needed to construct a stripping curve with 15−20 data.10 For sparingly soluble gaseous solutes that are rapidly stripped from the liquid solvent, it is not possible to determine the stripping curve accurately using the conventional sampling method because we cannot obtain 15 samples in a very short time. Richon18 has recently suggested a rapid method for constructing a stripping curve. Instead of withdrawing samples, he proposed

where f i is the fugacity and xi is the liquid-phase mole fraction of solute i. At low pressure, f i is the partial pressure of solute i. The limiting activity coefficient γi∞ is related to Henry’s constant by Hi = γ∞ i f i°

(2)

where f i° is the reference fugacity for the solute. When the system temperature is well below the critical temperature of the solute, f i° is the vapor pressure of solute i at system temperature. When the system temperature is above the critical temperature of the solute, f i° is hypothetical. Much attention has been given to measuring limiting activity coefficients (or Henry’s constants) for volatile-liquid solutes in solvents that have low volatility.1,7−12 For such systems, gas− liquid chromatography (GLC) and inert-gas stripping (IGS) are widely used experimental methods, although other techniques are also available.8,9 The GLC method is especially useful for obtaining solubilities in nonvolatile solvents and liquid polymers,13,14 whereas IGS is advantageous for moderately volatile and/or mixed solvents.7,12,15,16 Eckert and co-workers8,12 successfully extended the IGS method for measuring large infinite-dilution activity coefficients of volatile solutes in water. Both methods, © 2012 American Chemical Society

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Figure 1. Schematic diagram of the experimental apparatus. (a) Stripper made of Pyrex glass. (b) Complete apparatus. The stripper is placed inside the oven of a commercial GC (Bruker-430 GC). DAS is the data-acquisition system; FM is the flow meter; FC is the flow controller; TCD is the thermal-conductivity detector. Makeup flow is adjusted to achieve constant flow rates in the TCD.

either moderately or sparingly soluble gases in low-volatile liquids with good accuracy. We can rapidly obtain si-vs-t data to construct a continuous stripping curve using an online data-acquisition system. For the design of the stripper, we considered the relevant factors identified in previous studies2,12,16,17 to ensure equilibration between gas and liquid. The gas phase volume inside the stripper must be minimized. Figure 1 shows our apparatus. The TCD uses two gas lines (main and reference) to generate a signal proportional to the gas-phase composition. The exiting stripping gas passes through the main line, while the pure stripping gas is the reference. Our TCD also uses a makeup line of helium to the main line to achieve a constant flow rate. We operate the TCD at a constant flow rate of helium. The makeup flow rate is adjusted according to the difference between the flow rate of the exiting gas from the

to pass the exiting gas through a GC and to record the signal from the GC detector as a function of time. However, Richon18 did not report any solubility data for gases in liquids. Following the suggestion of Richon,18 this work describes simple and inexpensive experimental equipment and a rapid procedure for obtaining the solubilities of moderately or sparingly soluble gases in low-volatile liquids. In our IGS method, we record the real-time concentration of the solute in the exiting stripping gas using an online thermal-conductivity detector (TCD) as proposed by Richon;18 however, our apparatus is miniaturized such that we can put our stripper inside a conventional gas−liquid chromatograph, replacing the GC analytical column. The stripping gas (helium) connected to the GC injector passes from the stripper and exits after the detector. Real-time monitoring of the gas-phase composition gives a stripping curve for 4434

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stripper and that in the reference. This adjustment is necessary to attain a stable baseline for the TCD. Other GC detectors sensitive to the solute could be used. If the solvent is volatile, the entering inert gas must be saturated with solvent to keep the quantity of solvent constant in the stripper. In that case, because the exiting stripping gas may contain a substantial amount of solvent, the reference must also be saturated with solvent at the temperature of study. The flow rates in the main line and in the reference line must be equal. To avoid possible solvent condensation, the TCD is operated at a temperature slightly above the boiling point of the volatile solvent. For highly viscous or foaming solvents, special attention must be given to achieve equilibrium and to avoid entrainment.16 At present, our IGC method is limited to solvents that have high boiling points (>473 K), e.g., higher hydrocarbons, polymers, and ionic liquids with low to moderate viscosity. For volatile solvents, solutes must be injected using a septum port. To demonstrate experimental reliability, we selected sparingly soluble solutes carbon dioxide, krypton, oxygen, air, and nitrogen and solvents n-dodecane and n-pentadecane. Henry’s constants were obtained at near-ambient conditions. These systems were chosen because the conventional IGS method cannot be used to measure solubilities of these sparingly soluble solutes in these liquids, and because independent solubility data are available for comparison. We also measured the Henry’s constant of carbon dioxide in ethylene glycol at near-ambient conditions because the viscosity of ethylene glycol is 1 order of magnitude larger than those of n-dodecane and n-pentadecane. To contribute to the sparse database19,22,23 for solubilities in protic ionic liquids, we also report Henry’s constants for carbon dioxide, ethane, ethylene, krypton, oxygen, air, and nitrogen at near-ambient conditions in the protic ionic liquid 1-butyl, 3-hydrogen-imidazolium acetate [BHIM][Ac]. We selected this ionic liquid due to increasing interest20 in the use of protic ionic liquids for separation processes or for use as the electrolyte in fuel cells or solar cells. This ionic liquid has a relatively low density and low viscosity. At ambient temperature, the density of [BHIM][Ac] is 0.988 g·cm−3. For a fixed temperature, a decrease in solvent density tends to raise gas solubility.

Regrettably, we do not know the molecular composition of our ionic liquid. 2.2. IGS Method. The stripper shown in Figure 1 consists of a glass vessel containing a magnetic stirrer and two vertical baffles to facilitate mixing. The glass vessel houses 10 stainlesssteel capillaries having 100 μm i.d. connected to a common manifold with a preheated helium supply. These capillaries are used to bubble helium through the liquid phase. The purpose of the exit cap of the stripper is to avoid liquid entrainment. In separate experiments, we used two strippers with different volumes: 131.0 and 81.9 cm3, including transfer lines. Two sizes were used to study the influence of stripper size on experimental results. The inlet of the stripper is connected to the injector of the GC with 2-m-long tubing to serve as a preheater to allow helium to attain the desired temperature. The injector is heated to about 10 K above the desired temperature. The exit of the stripper is connected to the GC detector through a tube about 10 cm long. A 10-cm3 gas syringe is used to inject about 5 cm3 of solute gas into the injector. A continuous and constant flow of helium is set. The detector is set at 500 K to avoid condensation of the solvent. The stripper is placed inside the column oven of a Bruker430 GC with a low-volume TCD (300 μL). The temperature of the column oven is controlled with an air bath. The temperature, pressure, and flow rate are set and the detector signal is recorded using an online data-acquisition system (DAS) provided with the GC. A calibrated gas-flow meter (Agilent, ADM 2000) measures the flow rate at the exit of the detector. A magnetic stirrer (Thermo Scientific, Variomag Micro, maximum allowable temperature 373 K) provides agitation. 2.3. Data Reduction. Earlier studies7,16,17 have described data reduction in detail. We use the data-reduction method of Duhem and Vidal.17 Henry’s constant Hi is given by Hi = m

m=−

a

supplier

min purity/%

Sigma-Aldrich Sigma-Aldrich Io-li-tech Praxair Praxair Praxair − Praxair Praxair Praxair

99.5 99.5 98 99.9 99.9 99.9 99 99.99 99.9 99.9

(3)

⎛s ⎞ 1 ln⎜ i ⎟ t − t0 ⎝ si 0 ⎠

(4)

Henry’s constant Hi is for volatile solute i in a particular solvent; t is time and si is any property of the gas phase directly proportional to the gas-phase concentration of solute; n is the number of moles of solvent in the stripper; subscript zero denotes si and t at the start of the stripping curve; R is the gas constant; T is temperature; F is the flow rate of inert gas in the stripper and Vg is the vapor-space volume above the liquid in the stripper. Equation 4 defines slope m. Because the concentration of solute is decreasing in the liquid, slope m is always negative. In this work, si/si0 is provided by the signal from the TCD operating in its linear range as shown in Figure 2. Slope m is determined by plotting the logarithm of the TCD signal versus time as shown in Figure 3. About 90% of the stripper volume is filled with the known mass of solvent. The known total volume of the stripper, coupled with the density and mass of solvent, is used to determine the volume of liquid and the volume of vapor space inside the stripper. After placing the stripper inside the oven of the GC and connecting it with the GC injector, helium flows at a constant rate while keeping good agitation in the stripper. The exit line of the stripper is connected to the main line of the GC detector. After the detector’s baseline is stable at si = 0, the solute gas is

Table 1. Materials Used in This Work n-dodecane ethylene glycol [BHIM][Ac] krypton oxygen nitrogen aira carbon dioxide ethane ethylene

F − mVg

where

2. MATERIALS AND METHODS 2.1. Materials. Table 1 identifies all fluids used in this work. All solvents were degassed using helium as stripping agent for

solvent or solute

njRT

Ambient air from the laboratory with low relative humidity.

about 1 h. To remove traces of moisture and oxygen, helium was passed over a commercial molecular sieve (Varian CP 17973 Gas Clean Filter). All other fluids were used as received. Protic ionic liquids like [BHIM][Ac] may have some unreacted molecular species from which they are synthesized. 4435

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Figure 2. Typical stripping profile for a gas−liquid system using helium for stripping. Segment A shows the stable baseline (si = 0) at a stable flow; segment B shows injection of gaseous solute; segment C (dashed line) is the exponential stripping curve used to determine slope m. Line D shows the final part of the stripping curve as si → 0.

injected into the stripper through the GC injector. For sparingly soluble gases such as oxygen and nitrogen, about 5 cm3 of gas is sufficient; however, for more soluble gases such as carbon dioxide, we use a smaller amount. Helium, mixed with the solute gas, passes through the solvent making a very dilute solution. Then, the continuous and constant flow of helium strips the solute away; the exit flow from the stripper passes through the TCD providing a real-time profile of the gas-phase composition in terms of signal as a function of time. The detector must operate in the linear range, usually 3−5 orders of magnitude higher than the detection limit. Agitation ensures equilibrium between helium bubbles and the liquid phase. The small vapor space inside the stripper must be carefully calculated as indicated above. The solubilities of gases studied here are substantially higher than that of helium in a particular solvent. We assumed that helium has no appreciable influence on the gas solubility beyond the labeled uncertainty of data. At constant helium flow rate, detector signal si depends on time t according to ln si = mt + ln p

(5)

Here, signal si is directly proportional to the concentration of solute in the gas phase; t is time; m is the slope of the stripping curve and p is an insignificant constant which depends upon the system, the detector, the time of injection of solute gas while recording the curve, etc. Figure 2 indicates segments of the stripping profile. Part A shows a stable baseline signal; part B shows a sharp peak due to injection of solute; parts C and D show an exponential decrease in signal due to depletion and, finally, complete removal of solute from the solvent. Slope m is determined from the data given in part C shown as a dotted line. Upon fitting the data, a coefficient of regression (R2 > 0.999) provides one indication of good measurements. Figure 3 shows an exponential stripping profile for oxygen in n-dodecane. The procedure to obtain slope m and its influence on Henry’s constant is illustrated by a sample calculation in the Appendix. The sensitivity of Henry’s constants to slope m is within experimental uncertainty. For gases having Henry’s constants of 10 cP) or low-soluble gases are used, the experiment should be repeated four to five times at slightly different flow rates. The Henry’s constant must be independent of flow rate while working in the optimized flow-rate range. An unusually high Henry’s constant and/ or a poor coefficient of regression (R2 < 0.99) indicates an unsteady flow rate or a failure to attain equilibrium inside the stripper. 2.4. Volumetric Method. A well-known alternative technique for measuring solubilities is the volumetric method. This method requires careful determination of molar quantities of solvents and solute charged into the equilibrium cell and molar volumes for pure components. Measured quantities at equilibrium are temperature, pressure, moles of solvents and solutes, and volumes of the gas and liquid phases in the equilibrium cell. These measurements enable calculation of equilibrium compositions using mass balances. No sampling and chemical 4436

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3.3. Stripper Size. We used two strippers with different internal diameters but the same length; the volumes are 131.0 and 81.9 cm3. We obtained Henry’s constants for various gases in dodecane using both strippers; the results are reproducible and identical for both strippers.

analysis are required. We used the volumetric method to obtain data for a few systems for comparison. In this work, emphasis is given to the IGS method. Principles of the volumetric apparatus are given elsewhere.4−6

3. OPTIMIZATION OF THE EXPERIMENTAL PROCEDURE FOR THE IGS 3.1. Flow Rate. The optimum flow rate depends upon several factors including the cell structure. Figure 4 shows the

4. RESULTS Table 2 gives Henry’s constants for carbon dioxide, krypton, oxygen, air, and nitrogen in n-dodecane, n-pentadecane, ethylene Table 2. Experimental Henry’s Constants (Hi) for Gases in Liquids Hi/MPa solvent n-dodecane

ethylene glycol

Figure 4. Influence of flow rate on Henry’s constant for air in n-dodecane at 300 K. The best flow rate is in the range 5−10 cm3 min−1. The error bars are ±3% (Hi = 69 MPa).

pentadecane

influence of flow rate on the Henry’s constant for air in n-dodecane at room temperature. Flow rates near 10 cm3 min−1 give the best results for n-dodecane and n-pentadecane. Higher flow rates (>15 cm3 min−1) give experimental scatter. For solvents with higher viscosity like ethylene glycol, the flow rate should be 6−8 cm3 min−1. 3.2. Agitation. The optimum agitation level depends upon several factors including the cell structure. Figure 5 shows the

[BHIM][Ac]

[BHIM][Ac]

solute

T/ K

this work

RSDa/%

IGS Method (This Work) 299 7.8 3 CO2 Kr 299 14 6 O2 299 44 5 N2 302 84 2 air 302 69 6 CO2 309 46 3 CO2 313 49 2 Kr 305 13 1 O2 305 44 5 O2 323 48 2 N2 305 84 2 CO2 308 6.4 3 C2H4 309 8.8 4 C2H6 308 9.3 5 Kr 308 35 6 O2 307 108 6 O2 310 120 7 Air 308 170 7 N2 308 190 8 Volumetric Method (This Work) Kr 303 42 3 Kr 313 43 2 Kr 323 44 1 Kr 333 45 2 O2 303 121 3 O2 308 118 3 O2 313 123 2 N2 304 179 3 N2 313 170 3

literature (at T/K) 9.4 (313)21 13 (298)22 44 (298)22 77 (298)22 49 53 12 42 50 78

(309)23 (313)23 (298)22 (298)22 (298)22 (298)22

a

Relative standard deviation RSD = standard deviation/average; the average is taken from a minimum of three measurements.

Figure 5. Influence of agitation on Henry’s constant for O2 in n-dodecane at 300 K. The best agitation is in the range 500−1000 rpm. The error bars are ±3% (Hi = 44 MPa).

glycol, or [BHIM][Ac]. Comparison between our data and those from the literature21−23 shows good agreement with average deviation less than 5%. For some systems, we also compare experimental solubilities obtained using our IGS method with those obtained from the volumetric method. Agreement is good. Table 2 also gives the relative standard deviation (RSD) associated with each measurement repeated four or five times for the IGS method and three times for the volumetric method. We used this apparatus to measure Henry’s constants of these sparingly soluble gases in a few more ionic liquids having low viscosity; new data are consistent with those from the literature when available.

influence of agitation on the Henry’s constant for oxygen in n-dodecane at room temperature. Agitation varies from 100 to 1500 rpm. Each experiment is conducted at least twice to obtain Henry’s constants and to note data scatter. The data show appreciable scatter at very low and very high agitation. Best results are obtained in the range 500−1000 rpm indicated by the vertical dotted lines in Figure 5. For viscous solvents and for very low-soluble gases, to ensure equilibrium, it is necessary to give attention to good mixing of the solute-rich stripping gas and liquid.12 4437

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5. CONCLUSIONS An extension of Richon’s18 exponential-stripping method is useful for obtaining the solubilities of noncondensable gases that have small solubilities (high Henry’s constants) in lowvolatile liquids. The design of the apparatus must consider solvent viscosity, level of agitation, and helium flow rate in the stripper; it is necessary to optimize these parameters for a particular solute−solvent system. Henry’s constants are reported for carbon dioxide, krypton, oxygen, air, and nitrogen in n-dodecane and n-pentadecane at near-ambient conditions. We also report the Henry’s constant for carbon dioxide in ethylene glycol at near-ambient conditions. Henry’s constants vary from 8 to 85 MPa; they are in good agreement with those in the literature using other experimental methods. We report Henry’s constants for carbon dioxide, ethane, ethylene, krypton, oxygen, air, and nitrogen in [BHIM][Ac] at near-ambient conditions. For [BHIM][Ac], Henry’s constants vary from 6 to 180 MPa. These results are in good agreement with those obtained using the volumetric method.

co-workers for general assistance. We much appreciate fruitful discussions with Dr. Sasisanker Padmanabhan and Dr. Maria Francisco. We thank Mr. Jim Breen and Mr. Eric Granlund (University of California, Berkeley, College of Chemistry Workshops) for technical advice.

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(1) Measurement of the Thermodynamic Properties of Multiple Phases; Weir, R. D., de Loos, T. W., Eds.; Elsevier: New York, 2005. (2) Rosenboom, J.-G.; Afzal, W.; Prausnitz, J. M. Solubilities of some organic solutes in 1-ethyl-3-methylimidazolium acetate. Chromatographic measurements and predictions from COSMO-RS. J. Chem. Thermodyn. 2012, 47, 320−327. (3) Maurer, G.; Tuma, D. Gas Solubility (and Related High-Pressure Phenomena) in Systems with Ionic Liquids. In Ionic Liquids: From Knowledge to Application; Plechkova1, N. V., Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 1030; American Chemical Society: Washington, DC, 2009; pp 1−20. (4) Jacquemin, J.; Costa Gomes, M. F.; Husson, P.; Majer, V. Solubility of carbon dioxide, ethane, methane, oxygen, nitrogen, hydrogen, argon, and carbon monoxide in 1-butyl-3-methylimidazolium tetrafluoroborate between temperatures 283 and 343 K and at pressures close to atmospheric. J. Chem. Thermodyn. 2006, 38 (4), 490−502. (5) Afzal, W. Phase equilibria of glycol-natural gas systems. Doctoral Dissertation, Ecole des Mines de Paris/MINES ParisTech, 2009. (6) Afzal, W.; Breil, M. P.; Ioannis, T.; Mohammadi, A. H.; Kontogeorgis, G. M.; Richon, D. Experimental Study and Phase Equilibrium Modeling of Systems Containing Acid Gas and Glycol. Fluid Phase Equilib. 2012, 318, 40−50. (7) Leroi, J. C.; Masson, J. C.; Renon, H.; Fabries, J. F.; Sannier, H. Accurate Measurement of Activity Coefficient at Infinite Dilution by Inert Gas Stripping and Gas Chromatography. Ind. Eng. Chem. Process Des. Dev. 1977, 16 (1), 139−144. (8) Eckert, C. A.; Sherman, S. R. Measurement and prediction of limiting activity coefficients. Fluid Phase Equilib. 1996, 116 (1−2), 333−342. (9) Kojima, K.; Zhang, S.; Hiaki, T. Measuring methods of infinite dilution activity coefficients and a database for systems including water. Fluid Phase Equilib. 1997, 131 (1−2), 145−179. (10) Coquelet, C.; Richon, D. Measurement of Henry’s Law Constants and Infinite Dilution Activity Coefficients of Propyl Mercaptan, Butyl Mercaptan, and Dimethyl Sulfide in Methyldiethanolamine (1) + Water (2) with w1 = 0.50 Using a Gas Stripping Technique. J. Chem. Eng. Data 2005, 50 (6), 2053−2057. (11) Krummen, M.; Gruber, D.; Gmehling, J. Measurement of activity coefficients at infinite dilution in solvent mixtures using the dilutor technique. Ind. Eng. Chem. Res. 2000, 39 (6), 2114−2123. (12) Li, J.; Dallas, J.; Eikens, D. I.; Carr, P. W.; Bergmann, D. L.; Hait, M. J.; Eckert, C. A. Measurement of large infinite dilution activity coefficients of nonelectrolytes in water by inert gas stripping and gas chromatography. Anal. Chem. 1993, 65 (22), 3212−3218. (13) Newman, R. D.; Prausnitz, J. M. Polymer-solvent interactions from gas-liquid partition chromatography. J. Phys. Chem. 1972, 76 (10), 1492−1496. (14) Liu, D. D.; Prausnitz, J. M. Solubilities of Gases and Volatile Liquids In Polyethylene and in Ethylene-Vinyl Acetate Copolymers in the Region 125−225 °C. Ind. Eng. Chem. Fundam. 1976, 15 (4), 330− 335. (15) Burnett, M. G. Determination of Partition Coefficients at Infinite Dilution by the Gas Chromatographic Analysis of the Vapor above Dilute Solutions. Anal. Chem. 1963, 35 (11), 1567−1570. (16) Richon, D.; Sorrentino, F.; Voilley, A. Infinite dilution activity coefficients by the inert gas stripping method: extension to the study of viscous and foaming mixtures. Ind. Eng. Chem. Process Des. Dev. 1985, 24 (4), 1160−1165.

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APPENDIX: SAMPLE CALCULATION We show a calculation for Hi for oxygen in n-dodecane at ambient temperature. If the flow rate is measured at a temperature different from that of the stripper, the temperature difference should be taken into account by F = FFM

T TFM

where “FM” stands for “flow meter”. The flow rate at the exit of the stripper−detector is FFM = 10.4 cm3 min−1 at T = TFM = 299 K. The volume of the vapor phase Vg in the stripper is calculated by subtracting the volume of solvent Vj from the total (t) volume of the glass vessel Vt. Vg = Vt − Vj = 131 cm3 −

86.65 g 0.7493

g

= 15.4 cm3

cm3

Figure 3 shows data for determining slope m of the stripping curve for oxygen in n-dodecane; m = −0.240 (±0.005) min−1. The number of moles of solvent n = 0.5087, obtained from 86.65 g of solvent in the stripper and molecular mass 170.34 g mol−1 for n-dodecane. R is the universal gas constant, 8 314 000 Pa cm3 mol−1 K−1. Henry’s constant at 299 K is calculated from

Here, the uncertainty of Hi ≤ 5% is due to the small uncertainty in slope m.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (510) 642-3592. Fax: (510) 642-4778.

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ACKNOWLEDGMENTS The authors are grateful to the Energy Biosciences Institute (University of California, Berkeley) and to the Environmental Energy Technologies Division (Lawrence Berkeley National Laboratory) for financial support, and to Prof. Alexis Bell and 4438

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(17) Duhem, P.; Vidal, J. Extension of the dilutor method to measurement of high activity coefficients at infinite dilution. Fluid Phase Equilib. 1978, 2 (3), 231−235. (18) Richon, D. New equipment and new technique for measuring activity coefficients and Henry’s constants at infinite dilution. Rev. Sci. Instrum. 2011, 82 (2), 025108. (19) Verevkin, S. P.; Zaitsau, D. H.; Tong, B.; Welz-Biermann, U. New for old. Password to the thermodynamics of the protic ionic liquids. Phys. Chem. Chem. Phys. 2011, 13 (28), 12708. (20) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108 (1), 206−237. (21) Henni, A.; Jaffer, S.; Mather, A. E. Solubility of N2O and CO2 in n-Dodecane. Can. J. Chem. Eng. 1996, 74 (4), 554−557. (22) Hesse, P. J.; Battino, R.; Scharlin, P.; Wilhelm, E. Solubility of Gases in Liquids. 20. Solubility of He, Ne, Ar, Kr, N2, O2, CH4, CF4, and SF6 in n-Alkanes n-ClH2l+2 (6 ≤ l ≤ 16) at 298.15 K. J. Chem. Eng. Data 1996, 41 (2), 195−201. (23) Zheng, D.-Q.; Ma, W.-D.; Wei, R.; Guo, T.-M. Solubility study of methane, carbon dioxide and nitrogen in ethylene glycol at elevated temperatures and pressures. Fluid Phase Equilib. 1999, 155 (2), 277− 286.

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