Limiting Partition Coefficients of Solutes in Biphasic

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J. Phys. Chem. B 2007, 111, 7620-7625

Limiting Partition Coefficients of Solutes in Biphasic Trihexyltetradecylphosphonium Chloride Ionic Liquid-Supercritical CO2 System: Measurement and LSER-Based Correlation Josef Planeta, Pavel Kara´ sek, and Michal Roth* Institute of Analytical Chemistry, Academy of Sciences of the Czech Republic, VeVeı´ U+0159 97, 60200 Brno, Czech Republic ReceiVed: February 23, 2007; In Final Form: April 28, 2007

Limiting partition coefficients of a selection of low-to-medium volatility solutes between both phases in a biphasic trihexyltetradecylphosphonium chloride ([thtdp][Cl]) ionic liquid (IL)-supercritical carbon dioxide (scCO2) system were obtained by capillary column chromatography with [thtdp][Cl] as the stationary liquid and scCO2 as the carrier fluid. It is shown that supercritical fluid chromatography can be used to probe the partitioning behavior of solutes in biphasic IL-scCO2 systems even when the IL has nonzero solubility in scCO2. Relative partition coefficients of solutes at a particular temperature and density of CO2 can be correlated within Abraham’s linear solvation energy relationships. Compared with our previous results in a [bmim][BF4]-scCO2 system, the solute partition coefficients in the [thtdp][Cl]-scCO2 system are more sensitive to the solute hydrogen bond acidity descriptor.

Introduction Organic salts with melting points near room temperature, or ionic liquids (ILs), have received increasing attention during the past decade, with an almost exponential year-to-year increase in the number of original papers on the subject. To date, a large portion of the efforts concerning ILs have been spent on imidazolium-based ILs. Recently, after the development of proper synthetic routes,1 phosphonium-based ILs have also become frequented, offering some specific properties when used as reaction media,2-8 phase transfer catalysts,9 cocatalysts,10 carriers of reactive gases,11 conductive additives to nonpolar solvents,12 antielectrostatic agents,13 biocompatible solvents,14 or sensing materials in gas sensors.15 Some phosphonium-based ILs, notably those containing cations with long alkyl chains, are less dense than water;16-18 this property can be important in applications. Recently, several valuable sources of thermodynamic and thermophysical information on phosphonium ILs have appeared, covering pure IL properties,16,17 limiting activity coefficients of a range of organic solvents in several phosphonium ILs,19-21 and gas solubilities and diffusivities in ionic liquids containing trihexyltetradecylphosphonium cation.22-24 Because of substantial contribution of electrostatic interactions to the cohesive energy, ILs have extremely low vapor pressures,25 which is the main reason for the label of “green solvents” occasionally attributed to ILs. Recently, combinations of ILs with supercritical carbon dioxide (scCO2), a solvent with density-tunable solvent power, have become important. Typically, ILs can dissolve large amounts of CO2, but ILs themselves are usually insoluble in scCO2. The biphasic IL-scCO2 systems have gained importance in biphasic catalysis26-29 and in extractions of organic nonelectrolytes from an IL environment with scCO2.30,31 Pressurized CO2 can also be used as a phase * Corresponding author. Phone: +420 532 290 171. Fax: +420 541 212 113. E-mail: [email protected].

separation switch in IL-organic solvent28,32,33 or IL-water34,35 systems. Consequently, since the pioneering studies by Blanchard et al.,30,31 there have been a number of experimental investigations of CO2 solubilities in ILs at low (99.9 mol %), and methylene chloride (99.9 mol %) were purchased from Sigma-Aldrich. Ionic liquid [thtdp][Cl] (brand name CyPhos IL 101) was supplied by Cytec Industries, Inc. (West Paterson, NJ). Fused-silica capillary tubing for preparation of the column and the outlet restrictor was purchased from CACO (Bratislava, Slovak Republic). Open-Tubular Capillary Column. Before the use for column preparation, the inner surface of fused-silica tubing was etched with a saturated solution of ammonium hydrogen difluoride in methanol at room temperature for 48 h. After the surface-roughening procedure, the column was filled with a coating solution (30 mg of [thtdp][Cl] in 4 mL of CH2Cl2), one end of the column was tightly sealed, and methylene chloride was carefully evaporated by applying vacuum to the open end. The resultant column was 3.5 m long, had an 85 µm i.d., and contained 2.85 × 10-7 mol [thtdp][Cl]. The equivalent film thickness of [thtdp][Cl] in the column at 298 K and at ambient pressure was 0.18 µm. Apparatus and Procedure. The experimental setup was described elsewhere,72 and the procedure was similar to that used in our previous SFC study with [hmim][Tf2N].74 To maximize the usable lifetime of the column, the measurements were performed in the sequence of increasing density of CO2. The injection solvent was n-hexane, and the concentrations of individual solutes ranged within 1.5-3 mg/mL, with each injection solution containing naphthalene as a reference. Throughout the measurement procedure, the current amount of [thtdp][Cl] in the column was frequently determined from regular checks of the retention factor of naphthalene at 333 K and 10 MPa, as described before.74 All retention factors reported below were corrected to the initial amount of [thtdp][Cl] in the column. At the end of the whole series of measurements, the column only contained 68.8% of the initial amount of [thtdp][Cl].

Because [thtdp][Cl] is soluble in hexane,18 venting of excess hexane from the 60 nL sampling loop prior to injection was more important here than in our previous studies with imidazolium ILs.72-74 Results and Discussion Below, the solute will be identified by subscript 1, the principal component of the stationary phase ([thtdp][Cl]) by subscript 2, and the mobile-phase fluid (CO2) by subscript 3. The quantities pertaining to the stationary and the mobile phases will be denoted by subscripts s and m, respectively. Partition Coefficients. Tables 1-3 list the relative partition coefficients (relative retention factors) of the solutes with respect to naphthalene at the particular temperature and pressure. The retention factor is given by

k1 ) (tR - t0)/t0

(1)

where tR is the solute retention time and t0 is the column holdup time. The solute partition coefficient is defined by

Kc ) c1s/c1m

(2)

where c1s and c1m are the molar concentrations of the solute in the stationary and mobile phases, respectively. The relationship between k1 and Kc is

Kc ) k1Vm(1 - x3s)/(n2sVs)

(3)

where Vm is the geometric volume of the mobile phase in the column, x3s is the solubility (mole fraction) of CO2 in [thtdp][Cl], n2s is the number of moles of [thtdp][Cl] in the column, and Vs is the molar volume of CO2-saturated [thtdp][Cl]. Table 4 shows the Kc data of naphthalene, and it also contains K factor values of naphthalene. The K factor is defined by

K ) x1m/x1s

(4)

where x1m and x1s are the mole fractions of the solute in the mobile and stationary phases, respectively. The relationship between k1 and K is

K ) M3n2s/[k1VmFm(1 - x3s)]

(5)

where M3 is the molar mass of CO2 and Fm is the density of CO2 at the temperature and mean pressure in the column. The

7622 J. Phys. Chem. B, Vol. 111, No. 26, 2007

Planeta et al.

TABLE 2: Relative Values of Solute Partition Coefficients with Respect to Naphthalene at 333 K P/MPa, Fm/kg‚m-3 solute A

8.7, 221.3

10.0, 290.0

10.9, 350.4

12.1, 442.0

13.3, 523.8

7.46 0.268 2.05 8.11 0.348 7.77 8.96 0.473 4.69 0.644 5.47 22.4 6.93 32.5 1.27 0.575

9.46 0.303 1.96 6.16 0.371 6.36 6.74 0.495 4.69 0.585 5.30 20.4 5.14 23.9 1.28 0.579

KcA/KcBa aniline anisole azulene benzil camphor coumarin dibenzothiophene N,N-dimethylaniline 1-hexanol R-ionone N-methylaniline phenethyl alcohol phenoxathiin pyrene thianaphthene veratrole a

0.178 2.42

0.207 2.31

0.394

0.381

0.505

0.425 4.32 0.975 5.82

0.236 2.16 11.8 0.374 10.2 12.1 0.458 4.58 0.791 5.76

1.38 0.601

1.32 0.583

1.25

1.35 0.623

B ) naphthalene.

TABLE 3: Relative Values of Solute Partition Coefficients with Respect to Naphthalene at 353 K P/MPa, Fm/kg‚m-3 solute A

10.0, 221.6 11.8, 288.6 13.3, 352.4 15.3, 439.9

aniline anisole azulene benzil camphor coumarin dibenzothiophene N,N-dimethylaniline 1-hexanol R-ionone N-methylaniline phenethyl alcohol phenoxathiin pyrene thianaphthene veratrole a

5.21 0.202 2.29 0.446 13.6 0.455 2.80 1.23 4.26

KcA/KcBa 5.46 0.240 2.14 14.7 0.443 10.5 13.0 0.470 2.72 1.00 3.87 15.8 10.8

1.33 0.634

1.29 0.628

5.12 0.272 2.09 11.0 0.441 8.44 9.86 1.29 2.58 0.871 4.08 13.9 8.04 38.7 1.29 0.633

5.13 0.311 1.91 6.93 0.436 6.84 7.29 1.27 2.83 0.684 3.69 13.0 5.81 25.1 1.27 0.624

B ) naphthalene.

TABLE 4: Retention Factors, Partition Coefficients, and K-Factors of Naphthalene T/K

P/MPa

Fm/kg‚m-3

k1

Kc IL w/o CO2

K IL w/o CO2

313

8.1 8.5 8.8 9.2 10.5 8.7 10.0 10.9 12.1 13.3 10.0 11.8 13.3 15.3

289.8 353.9 429.1 532.0 660.1 221.3 290.0 350.4 442.0 523.8 221.6 288.6 352.4 439.9

5.15 2.70 1.38 0.552 0.253 6.04 2.66 1.34 0.670 0.364 3.21 1.55 0.876 0.453

608 318 163 65.2 30.0 703 310 156 78.3 42.6 370 178 101 52.3

0.000 426 0.000 667 0.001 08 0.002 17 0.003 81 0.000 477 0.000 825 0.001 36 0.002 15 0.003 34 0.000 895 0.001 43 0.002 06 0.003 20

333

353

densities of CO2 were calculated from the equation of state of Span and Wagner.76 We did not find any published values of solubility x3s of CO2 in [thtdp][Cl] within the temperature and pressure range of this work. Therefore, the Kc and K data of naphthalene in Table 4 are not thermodynamically rigorous parameters. Instead, they were calculated from eqs 3 and 5 subject to approximations x3s ) 0 and Vs ) V02L, where V02L is the molar volume of pure

[thtdp][Cl] at the particular temperature and pressure. The V02L values used to obtain the Kc and K data in Table 4 were interpolated from the densities reported by Esperanc¸ a et al.17 Unlike the previous work with [bmim][BF4], we observed that some acidic solutes (benzoic acid, phenol, p-cresol) and indole do not elute from the [thtdp][Cl] column within a reasonable time. LSER Interpretation of Partition Coefficients. LSERs provide an effective tool to analyze or predict free energies of partition in two-phase systems, and they have frequently been used in IL-gas systems77,78 as well as in chromatography.79,80 As in the previous work with [bmim][BF4],73 we have employed here the general solvation parameter model of Abraham.81 At 333 K and 12.1 MPa, linear regression of retention data on solute molecular descriptors82 yields

log(KcA/KcB) ) 2.326EA - 1.863SA + 4.545AA + 1.575BA - 1.395VA (6) where KcA/KcB is the relative partition coefficient of solute A with respect to naphthalene, and the solute descriptors82,83 EA, SA, AA, BA, and VA of solute A describe the solute’s excess molar refractivity, dipolarity/polarizability, dipolarity, hydrogen bond acidity (hydrogen bond donating ability), hydrogen bond basicity (hydrogen bond accepting ability), and the McGowan’s characteristic volume,84 respectively. Figure 1 shows a comparison between experimental and back-calculated values for a selection of solutes. At 313 K and 10.5 MPa, with a slightly different set of solutes (absence of dibenzothiophene), the resultant fit is

log(KcA/KcB) ) 2.243EA - 1.507SA + 4.591AA + 2.466BA - 1.909VA (7) Figure 2 gives an illustration of the predictive abilities of LSER correlations as the data points for 2-methyl-1-butanol, acetophenone, and benzothiazole were not included in the solute set used to obtain the parameters of eq 7. Comparison of eqs 6 and 7 with LSER relationships obtained previously for a [bmim][BF4]-containing system73 is somewhat difficult because of different solute sets in both ILs. However, the most distinctive feature of eqs 6 and 7 is an increased sensitivity of relative retention in the [thtdp][Cl]-scCO2 system to the solute hydrogen bond acidity descriptor A. This finding is consistent with the long retention times of the acidic solutes,

Solute Partitioning in [thtdp][Cl]-scCO2 System

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7623

Figure 1. LSER fit of relative partition coefficients at 333 K and 12.1 MPa (eq 6): O, aniline; b, anisole; 0, azulene; 9, 1-hexanol; *, dibenzothiophene; 4, N,N-dimethylaniline; 2, naphthalene; 3, Nmethylaniline; 1, phenethyl alcohol; ], pyrene; and [, thianaphthene.

Figure 3. Retention factors vs CO2 density at 333 K: O, naphthalene; b, anisole; 0, veratrole; 9, R-ionone; 4, azulene; 2, camphor; 3, 1-hexanol; 1, thianaphthene; [, N,N-dimethylaniline; ], N-methylaniline; circle with lines, aniline; +, benzil; s, coumarin; ×, dibenzothiophene; triangle with lines, phenethyl alcohol; *, phenoxathiin; square with lines, pyrene.

TABLE 5: Characteristic Volumes and Molar Masses of CO2, ILs, and IL Cations

Figure 2. LSER fit of relative partition coefficients at 313 K and 10.5 MPa (eq 7). Symbol keys are the same as in Figure 1. Additional solutes (square with lines, acetophenone; triangle with lines, benzothiazole; circle with lines, 2-methyl-1-butanol) were not used to obtain the fit parameters.

as mentioned above. The explanation of this observation is not obvious, but it can perhaps be related to the charge distribution in the present IL. In [bmim][BF4], the charges of both cation and anion are delocalized over several atoms, whereas in [thtdp][Cl], the charges of both cation and anion are confined to a single atom. Unlike the LSER relationships observed with [bmim][BF4],73 eqs 6 and 7 also show negative coefficients at the S descriptor. This probably reflects a lower polarizability of the thtdp+ cation in the present IL as compared with the imidazolium ring in [bmim][BF4]. Together with the previous results with [bmim][BF4],73 the arbitrary test of predictive ability of eq 7 (see Figure 2) suggests that the accuracy of the predictions may be sufficient for initial design of pilot-scale processes involving biphasic IL-scCO2 solvent systems. Variation of Retention Factors with CO2 Density. In SFC, the isothermal plots of ln k1 vs ln Fm are often nearly linear,85 with the slope given by86

( ) ∂ ln k1 ∂ ln Fm

T

)

[

( ) ( )]

∂µ∞1s 1 vj∞1m - vj∞1s RTβmT ∂x3s

T,P,n2s

∂x3s ∂P

-

T,σ

Vs βsTσ (8) Vm βmT

substance/ion

formula

carbon dioxide thtdp+ [thtdp][Cl] bmim+ [bmim][PF6] [bmim][BF4] hmim+ [hmim][Tf2N]

CO2 C32H68P C32H68PCl C8H15N2 C8H15N2PF6 C8H15N2BF4 C10H19N2 C12H19N3SO4F6

VMcGowan molar mass VvdW Å3‚molecule-1 cm3‚mol-1 g‚mol-1 38.16 583.17 599.70 141.71 204.53 176.30 331.79

28.09 484.35 498.74 126.23 168.00 153.62 154.41 261.46

44.01 483.86 519.32 139.22 284.18 226.02 167.27 447.42

where R is the molar gas constant, Vs is the geometric volume of the CO2-saturated [thtdp][Cl] in the column, βmT is the isothermal compressibility of CO2, βsTσ is the isothermal compressibility of CO2-saturated [thtdp][Cl], µ∞1s is the infinitedilution chemical potential of the solute in CO2-saturated [thtdp][Cl], and vj∞1m and vj∞1s are the infinite-dilution partial molar volumes of the solute in CO2 and in CO2-saturated [thtdp][Cl], respectively. Figure 3 shows an example of the retention factor vs density plot; linearity is comparable to our earlier studies with [bmim][PF6] and [bmim][BF4], suggesting that SFC can provide information on limiting partition coefficients of solutes, even with the ILs showing nonzero solubility in scCO2. An overview of the slopes (∂ ln k1/∂ ln Fm)T obtained with [bmim][PF6],72 [bmim][BF4],73 [hmim][Tf2N],74 and [thtdp][Cl] is available as Supporting Information. A possible source of the slope differences between [thtdp][Cl] and imidazolium ILs is related to molecular size considerations and to combinatorial entropy effect. The van der Waals (VvdW) and McGowan (VMcGowan) volumes of CO2 and ILs can be obtained from the summation formulas and spreadsheet calculator developed by Zhao et al.87 Table 5 shows the resultant VvdW, VMcGowan, and molar mass data for CO2 and several cations and anions. The mass and the bulkiness of thtdp+ cation, respectively, have two effects that bear on (∂ ln k1/∂ ln Fm)T by affecting the individual terms in eq 8: (a) An elevated value of (∂x3s/∂P)T,σ in [thtdp][Cl] as compared with [bmim][PF6] or [hmim][Tf2N]. This is indicated by comparison of the data on imidazolium IL-scCO2 systems with the results of Morgan et al.22 on the [thtdp][Cl]scCO2 system. (b) A more significant contribution of combinatorial entropy to (∂µ∞1s/∂x3s)T,P in thtdp+-containing systems as compared with the systems containing less bulky alkylmethylimidazolium cations.

7624 J. Phys. Chem. B, Vol. 111, No. 26, 2007

Planeta et al. of the Abraham’s A descriptor) showed prohibitively long retention times. Acknowledgment. We thank Wilfred van Wijk of Cytec Industries, Inc. for a generous gift of the [thtdp][Cl] sample (brand name CyPhos IL 101) used in this study. We gratefully acknowledge the financial support of this work by the Grant Agency of the Academy of Sciences of the Czech Republic (Project No. B400310504), by the Czech Science Foundation (Project No. GA203/05/2106 and Project No. GA203/07/0886), and by the Academy of Sciences of the Czech Republic through Institutional Research Plan No. AV0Z40310501.

Figure 4. Solubility parameter of CO2-saturated [thtdp][Cl] calculated from relative retention factors of anisole (An) and R-ionone (Io) with respect to naphthalene.

Obviously, the effects a and b above can combine to make the product (∂µ∞1s/∂x3s)T,P(∂x3s/∂P)T,σ in eq 8 more important in [thtdp][Cl]-containing systems as compared with the systems containing alkylmethylimidazolium ILs. Solubility Parameter of CO2-Saturated [thtdp][Cl]. Regular solution theory makes it possible to estimate the solubility parameter, δs, of CO2-saturated [thtdp][Cl] from the quadratic equation72,73

(V0BL - V0AL)δs2 + 2(V0ALδA - V0BLδB)δs + (V0AL - V0BL)δm2 + 2(V0BLδB - V0ALδA)δm - RTln(KcA/KcB) ) 0 (9) Except for the relative partition coefficient of solute A with respect to naphthalene (B), the quantities in eq 9 are purecomponent properties. Symbols V0AL and V0BL are the subcooled liquid molar volumes of solute A and naphthalene, respectively, and δA and δB are the respective solubility parameters. The solubility parameter of CO2 at the particular temperature and pressure, δm, can be calculated from the equation of state of Span and Wagner.76 To calculate the molar volumes of subcooled liquid solutes, one can use a modified form88 of the Rackett equation employing a reference density89 and the critical properties estimated from the Joback correlation.90 The solubility parameters of the subcooled liquid solutes91 then result from combining the molar volumes with the cohesive energies obtained from vaporization enthalpy data92 or from vapor pressure equations.93 Figure 4 shows the estimations of δs obtained with anisole and R-ionone as the test solutes. In the Fm f 0 limit, Figure 4 seems to indicate a lower value of the solubility parameter of pure [thtdp][Cl] as compared with the values for [bmim][PF6]72 and [bmim][BF4].73 This finding is qualitatively consistent with the Reichardt’s ETN polarity scale that shows a lower relative polarity of phosphonium-based ILs with respect to imidazolium-based ILs.94 Conclusion The results of this work indicate that SFC can provide information on partitioning of a small amount of solute between both phases in an IL-scCO2 system, even when the IL has a nonnegligible solubility in scCO2. Retention data for a selection of solutes of varying volatility and polarity between both phases in the [thtdp][Cl]-scCO2 system were measured and corrected for the loss of [thtdp][Cl] from the column. The resultant relative partition coefficients could be reasonably correlated with the LSER relationships. Acidic solutes (solutes with high values

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