Applications of Correlation Gas Chromatography and Transpiration

Jul 12, 2012 - Sublimation enthalpies for the following compounds were measured by transpiration (in kJ·mol–1; T = 298.15 K): perfluorododecane (85...
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Applications of Correlation Gas Chromatography and Transpiration Studies for the Evaluation of the Vaporization and Sublimation Enthalpies of Some Perfluorinated Hydrocarbons Darrell Hasty, Joshua Drapekin,† Tanvi Subramanian,‡ Thomas C. Winter,§ and James S. Chickos* Department of Chemistry and Biochemistry, University of Missouri-St. Louis, 1 University Boulevard, Saint Louis, Missouri 63121, United States

Artemiy A. Samarov,∥ Andrei V. Yermalayeu, and Sergey P. Verevkin Department of Physical Chemistry, University of Rostock, 18059 Rostock, Germany S Supporting Information *

ABSTRACT: The fusion, vaporization, and sublimation enthalpies of a series of perfluorinated alkanes have been measured by combining measurements obtained by differential scanning calorimetry (DSC), transpiration, and correlation-gas chromatography and comparing the results to available data in the literature. Additionally, experiments are reported to provide a guide in identifying appropriate structural features of fluorinated compounds suitable for use as standards in correlation gas chromatography measurements. Fusion enthalpies and fusion temperatures by DSC for the following compounds were measured (in kJ·mol−1; K): decafluorobiphenyl (20.5 ± 0.6, 339.6), perfluorododecane (24.2 ± 0.6, 346.6), perfluorotridecane (27.9 ± 0.4, 361.7), perfluorotetradecane (31.5 ± 0.3, 375.6), perfluoropentadecane (35.1 ± 0.2, 388.1), perfluorohexadecane (38.7 ± 0.1, 399.7), perfluoroeicosane (50.3 ± 0.3, 436.2), and perfluorotetracosane (63.2 ± 0.6, 461.1). Sublimation enthalpies for the following compounds were measured by transpiration (in kJ·mol−1; T = 298.15 K): perfluorododecane (85.8 ± 0.6), perfluorotridecane (94.3 ± 0.5), perfluorotetradecane (102.4 ± 1.0), and perfluoropentadecane (109.4 ± 0.4). Vaporization and sublimation enthalpies, respectively, were also evaluated for the following compounds (kJ·mol−1; T/K = 298.15): perfluorohexadecane (88.6 ± 4.0, 117.6 ± 4.9), perfluoroeicosane (113.7 ± 7.4, 148 ± 8.8), and perfluorotetracosane (141.4 ± 2.2, 168.3 ± 11.1). The measured vaporization enthalpies of the perfluorinated alkanes behave linearly as a function of the number of CF2 groups similar to what is observed with n-alkanes. Correlation-gas chromatography experiments confirmed previous findings that hydrocarbons can be used as standards for compounds containing a few fluorine atoms but otherwise standards need to be chosen with similar fluorine substitution and functionality.

■

INTRODUCTION

standards is of fundamental importance for the determination of reliable property data. Since in the case of the perfluorinated alkanes very little data are available in the literature, the vapor pressures and sublimation enthalpies of some of the larger perfluorinated alkanes were also evaluated by transpiration studies. Additionally, their fusion enthalpies were also determined. This report summarizes the results of these studies. In the case of hydrocarbons, structure does not play a significant role as far as the selection of standards in c-gc experiments is concerned. Any hydrocarbon, aliphatic or aromatic, branched, or cyclic or acyclic can be used as a standard for another hydrocarbon of unknown vaporization enthalpy. When other functional groups are also present, in

Interest in fluorinated compounds ranges from their use as lubricants to artificial blood.1−3 The size similarity of fluorine to hydrogen coupled with the stronger carbon fluorine bond strength makes partially fluorinated compounds excellent candidates as enzyme inhibitors. Despite the presence of fluorine in many commercially available compounds, there are remarkably little thermochemical data available in the literature. In some cases, the data are either old or what is available is in poor agreement.4,5 The aim of this work was to determine whether correlation-gas chromatography (c-gc) experiments could be used successfully to evaluate the vaporization enthalpies of partially and completely fluorinated hydrocarbons and to provide additional thermochemical information about their properties. The evaluation of vapor pressure and vaporization enthalpy using this technique relies on the use of appropriate standards. The identification of appropriate © 2012 American Chemical Society

Received: May 29, 2012 Accepted: June 26, 2012 Published: July 12, 2012 2350

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Table 1. Chemical Sources

a

CAS No.

formula

name

source

% purity

392-56-3 355-42-0 355-02-2 355-57-9 98-08-8 459-60-9 108-88-3 661-11-0 307-34-6 335-65-9 703-87-7 95-47-6 375-96-2 108-67-8 313-72-4 306-94-5 307-45-9 91-20-3 434-90-2 307-59-5 376-03-4 307-62-0 2264-03-1 355-49-7 37589-57-4 1766-41-2

C6 F 6 C6F14 C7F14 C7F16 C7H5F3 C7H7FO C7H8 C7H15F C8F18 C8HF18 C8H6F4 C8H10 C9F20 C9H12 C10F8 C10F18 C10F22 C10H8 C12F10 C12F26 C13F28 C14F30 C15F32 C16F34 C20F42 C24F50

hexafluorobenzene perfluorohexane perfluoro(methylcyclohexane) perfluoroheptane α,α,α-trifluorotoluene p-fluoroanisole toluene 1-fluoroheptane perfluorooctane 1H-perfluorooctane 2,3,5,6-tetrafluoro-p-xylene p-xylene perfluorononane mesitylene octafluoronaphthalene perfluorodecalin (cis/trans) perfluorodecane naphthalene decafluorobiphenyl perfluorododecane perfluorotridecane perfluorotetradecane perfluoropentadecane perfluorohexadecane perfluoroeicosane perfluorotetracosane

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Baxtera Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Eastman Kodak Sigma-Aldrich MCB Sigma-Aldrich Sigma-Aldrich SynQuestb Sigma-Aldrich Sigma-Aldrich SynQuest SynQuest SynQuest Sigma-Aldrich SynQuest SynQuest Sigma-Aldrich

≥99 ≥98 ≥90 ≥85 ≥99 99 ≥98 ≥98 ≥98 ≥99 ≥99 ≥95 ≥97 ≥95 ≥96 ≥95 ≥95 ≥99 ≥99 ≥97 ≥98 ≥97 ≥95 ≥97 ≥97 ≥99

Baxter Healthcare Corp., McGraw Park, IL. bSynQuest, P.O. Box 309, Alachua, FL.

perfluorooctane as the solvent was due to the limited solubility of the perfluorinated alkanes in typical organic solvents. Whether the solvent was also used as a standard depended on its retention time. Methylene chloride was used as the solvent for the perfluorinated aromatic compounds, runs 21−2. Adjusted retention times, ta, were calculated by subtracting the measured retention time of the nonretained reference from the retention time of each analyte as a function of temperature, at T = 5 K intervals over a 30 K range. Further details are provided in the Supporting Information. Column temperatures above ambient were controlled by the gas chromatograph. All temperatures were monitored using a Vernier temperature probe. Temperatures maintained by the gas chromatograph above ambient temperatures were constant to ± 0.1 K. Subambient temperatures were achieved by immersion of the column into a VWR Scientific Products Chiller’s tank (model 1166) filled with a 50:50 ethylene glycol−water mixture. This setup enabled temperatures to be controlled within ± 0.3 K. Enthalpies of transfer were calculated as the product of the slope of the line obtained by plotting ln(to/ta) versus 1/T and the gas constant, R. All plots of ln(to/ta) versus 1/T, where to = 1 min, were characterized by correlation coefficients, r2, > 0.99. The retention times measured for all analytes are reported in the Supporting Information. The uncertainties associated with the correlations (± σ) were calculated from the uncertainty in the slope and intercept of the equations listed at the bottom of each respective table. Transpiration Studies. The method of transpiration using a nitrogen stream was used to measure vapor pressures of the perfluoroalkanes, and vaporization enthalpies were calculated from their temperature dependence as previously described.7 Glass beads (10 g) with a diameter of 1 mm were used to

many instances, both the nature and the number of the functional groups must be considered in the selection of appropriate standards. A goal of this work was to evaluate whether fluorine substitution needed to be treated as a functional group, taking into account both the number of fluorine atoms present and their location or whether fluorine substitution could be treated much like hydrogen. A previous study demonstrated that a single fluorine atom could be substituted for hydrogen successfully and its location on the molecule did not appear to be significant.6 As is described briefly below, perfluorinated compounds behave in a more complex manner than the corresponding hydrocarbons of similar structure.

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EXPERIMENTAL SECTION Correlation-Gas Chromatography Experiments. All compounds used in this study were obtained from commercial sources and used as purchased. Their origin and composition are listed in Table 1. All were analyzed as mixtures separated by chromatography. Correlation gas chromatography experiments were performed on an HP 5890 gas chromatograph equipped with a flame ionization detector and run at a split ratio of approximately 100:1. Retention times were recorded on an HP Chemstation. The compounds were run isothermally on a 0.25 mm, 30 m Restek RTX-200 crossbonded trifluoropropyl methyl column using helium as the carrier gas, except for runs 21 and 22 for which a 30 m DB-5 column was used. Below T = 373 K, the retention time of the methane was used as the nonretained reference; above this temperature perfluorooctane, also used as the solvent in most runs, was used as the nonretained reference. The retention times of methane or perfluorooctane were used to determine the dead volume of the column. The use of 2351

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Table 2. Vapor Pressure Results and Equations and Sublimation Enthalpies Measured for Some Perfluorinated Alkanes Ta K

mb mg

V(N2)c dm

3

gas flow dm ·h 3

Perfluorododecane; 397.53 112230.82 88.5 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ R R ·(T /K) R ⎝ 298.15 ⎠

pd

−1

(pexp − pcalc)

σe

ΔgcrHm

Pa

%

kJ·mol−1

Pa

ΔgcrHm(298.15

K) = (85.84 ± 0.63) kJ·mol

−1

313.3 318.2 323.2 328.2 333.2 338.2 343.2 348.2

15.30 6.50 7.40 28.70 19.50 31.20 45.00 35.40

0.979 1.96 67.2 0.8 0.249 0.99 107.4 −1.9 0.166 0.99 178.1 −0.3 0.398 0.99 283.2 −3.2 0.166 0.99 457.1 5.1 0.166 0.99 726.9 24.6 0.167 1.00 1038.8 −35.8 0.083 1.00 1630.2 9.3 Perfluorotridecane; ΔgcrHm(298.15 K) = (94.31 ± 0.48) kJ·mol−1 418.51 122693.70 95.2 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ R R ·(T /K) R ⎝ 298.15 ⎠

0.51 0.50 0.50 0.50 0.50 0.50 0.50 0.50

84.51 84.07 83.63 83.19 82.75 82.30 81.86 81.42

313.2 318.4 323.4 329.3 333.5 339.3 343.3 349.2 354.1 358.2

16.00 19.00 22.70 21.40 18.90 18.80 20.20 23.70 38.00 26.10

0.53 0.52 0.51 0.51 0.50 0.50 0.50 0.50 0.50 0.50

92.88 92.39 91.91 91.35 90.95 90.40 90.02 89.45 88.99 88.60

313.2 318.2 328.2 333.2 338.2 343.2 348.2 353.1 358.2 358.2

15.00 16.40 14.90 14.20 14.10 15.00 16.60 18.00 21.30 21.50

0.64 0.58 0.53 0.52 0.51 0.51 0.50 0.50 0.50 0.50

100.84 100.32 99.31 98.79 98.29 97.78 97.27 96.76 96.24 96.24

312.1 323.2 333.2 338.2 343.3 348.2 353.1 358.2 363.2 368.2

8.70 16.20 15.50 10.00 13.20 16.40 20.00 16.10 15.50 25.10

1.33 0.70 0.56 0.54 0.52 0.51 0.51 0.50 0.50 0.50

107.90 106.70 105.61 105.07 104.51 103.98 103.45 102.90 102.35 101.81

4.36 3.74 14.3 0.2 2.81 3.74 25.3 0.0 1.87 3.74 44.3 0.9 0.968 0.97 79.6 −0.2 0.565 0.97 119.9 −1.6 0.323 0.97 207.7 −4.8 0.242 0.97 297.0 −11.5 0.161 0.97 521.7 −3.2 0.161 0.97 835.7 32.5 0.081 0.97 1147.4 13.1 Perfluorotetradecane; ΔgcrHm(298.15 K) = (102.36 ± 0.99) kJ·mol−1 439.06 132745.12 101.9 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R R ·(T /K) R 15.32 1.06 3.49 0.10 8.49 3.92 6.64 0.37 2.61 3.92 19.1 −0.6 1.50 3.92 31.5 −2.6 0.883 1.06 53.0 −4.3 0.512 1.02 97.1 1.8 0.341 1.02 161.0 5.3 0.239 1.02 249.3 0.2 0.171 1.02 412.8 15.0 0.177 1.06 402.5 4.0 Perfluoropentadecane; ΔgcrHm(298.15 K) = (109.41 ± 0.40) kJ·mol−1 454.96 141791.21 108.6 ⎛ T /K ⎞ ⎟ ln(p /Pa) = − − ln⎜ ⎝ 298.15 ⎠ R R ·(T /K) R 48.08 20.43 6.22 2.23 1.72 1.25 0.934 0.445 0.267 0.267

3.74 5.33 5.33 5.15 5.15 3.74 3.74 1.07 1.07 1.07

0.60 2.50 7.76 13.9 23.8 40.8 66.2 111.9 179.6 290.8

0.01 0.05 −0.26 −0.2 −0.6 −0.1 −1.0 1.0 1.3 8.6

a Temperature of saturation. bMass of transferred sample, condensed at T = 283 K. cVolume of nitrogen used to transfer mass m of sample. dVapor pressure at temperature T, calculated from m and the residual vapor pressure at T = 283 K. eThe uncertainty in the pressure measurements estimated by the expression: σ(p/Pa)) = 0.005(p/Pa) + 0.05 recommended in ref 26.

length of 15 cm and a 0.5 cm diameter. A nitrogen flow at constant temperature (± 0.1 K) was used to transport the vapor into a cold trap using a soap bubble flow meter to

provide a suitable surface area to which approximately 0.5 g of sample was added and melted to coat the beads, then cooled. The beads were added to a thermostatted U-shaped tube with a 2352

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Table 3. Summary of the Fusion Enthalpy Measurements sample mg

ΔlcrHm(Tfus) −1

J·g

Tfus K

ΔlcrHm(Tfus) kJ·mol

−1

Tfus

ΔlcrHm(Tfus)

Tfus

K

kJ·mol−1 (lit)

K (lit)

38.2e

348.5e

Decafluorobiphenylb 11.14 9.92 5.00

61.715 61.954 60.513

339.79 339.54 339.45

1.690 1.690 1.912 37.42b,f

38.40 38.05 37.52 39.10

346.41 346.51 346.65

3.104 3.104 3.658

40.75 40.25 40.62

361.63 361.56 361.78

20.5 ± 0.6

339.6a

Perfluorododecanec 24.2 ± 0.6

346.6d

24.9 Perfluorotridecanec

346.5a

27.9 ± 0.4

361.7d

Perfluorotetradecanec 2.842 2.842 1.812

42.86 42.41 42.76

375.56 375.48 375.68

2.284c 2.284c 1.664c 20.72b,f

44.79 44.62 44.39 44.68

388.10 388.13 387.97 388.10

16.74 27.26 13.11

46.18 46.08 46.25

399.6 399.9 399.6

16.15 20.39 20.89

48.72 48.26 48.27

436.27 436.17 436.17

21.49 19.81 16.20

51.52 50.84 50.71

461.6 460.9 460.8

31.5 ± 0.3

375.6d

Perfluoropentadecane 35.1 ± 0.2

388.1d

35.1 Perfluorohexadecaneb

388.1a

38.7 ± 0.1

399.7a

61.1e

402.2e 398.2g

436.2a

80.3e 44.8g

437.9e 443.2g

461.1a

100.8e

465.2e

Perfluoroeicosaneb 50.3 ± 0.3 Perfluorotetracosaneb 63.2 ± 0.6

a

Mean onset temperature. bMeasured in St. Louis. cMeasured in Rostock. determination. gReferences 9 and 18.

measure the flow rate. This rate was optimized to ensure complete saturation equilibrium. The quantity of condensed vapor in the trap was measured by weight gain. The vapor pressure was calculated from the amount of mass transported over a measured period of time and flow rate using the ideal gas law. The total mass collected was adjusted for the residual mass loss occurring at the temperature of the cold trap due to the flow by an iterative process using the vapor pressure extrapolated from the observed temperature dependence. Applying Dalton's law of partial pressures to the nitrogen stream saturated with the substance of interest and assuming its applicability, values of pi were calculated: pi = mi ·R ·Ta/V ·Mi (VN2 ≫ Vi )

d

Mean temperature measured on peak. eReference 9. fSingle

bubble meter. Vapor pressures were obtained as a function of temperature and were fit to the following equation: R ·ln pi = a +

⎛T ⎞ b + Δcrg Cp·ln⎜ ⎟ T ⎝ T0 ⎠

(2)

ΔgcrCp

where a and b are adjustable parameters, is the difference in the molar heat capacities of the gas and solid phase, respectively, and T0 is the reference temperature, 298.15 K. The experimental results of parameters a and b are listed in Table 2. The sublimation enthalpy at temperature T is derived from eq 2 as: Δcrg Hm(T ) = −b + Δcrg Cp·(T )

V = VN2 + Vi

(3)

ΔgcrCp

Values of have been calculated from the isobaric molar heat capacities of the solid perfluoroalkanes, Cp(cr) according to the procedure described by Chickos and Acree.8 DSC Studies. The fusion enthalpies measured in St. Louis were measured on a Perkin-Elmer DSC 7 instrument using the Pyris Series Thermal Analysis software. The experiments were conducted in hermetically sealed aluminum pans under an atmosphere of nitrogen at a flow rate of 20 mL·min−1 at a ramp

(1)

where R = 8.314472 J·K−1·mol−1; mi is the mass of transported compound, Mi is the molar mass of the compound, and Vi is its volume contribution to the gaseous phase. VN2 is the volume of transporting gas, and Ta is the temperature of the gas at the 2353

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Table 4. Literature Vaporization Enthalpies of Some Fluorocarbons and Hydrocarbons and Their Adjustment to T = 298.15 K Δgl Hm(Tm) kJ·mol perfluorobenzene perfluorohexane perfluorotoluene perfluoro(methylcyclohexane) perfluoroheptane α,α,α-trifluorotoluene 4-fluoroanisole toluene perfluorooctane perfluorooctane perfluoro(1,3-dimethylcyclohexane p-xylene mesitylene perfluorononane trans-perfluorodecalin cis-perfluorodecalin perfluorodecane naphthalene

Tm

−1

ΔCpΔTa

Cp(l) −1

−1

J·mol ·K

K

kJ·mol

Δgl Hm(298 K)

−1

kJ·mol−1

31.7 ± 0.1a,b

311

310.8

1.2 ± 0.2

32.2

345

348.8

4.7 ± 0.8

39.2a,b

311

403.6

1.4 ± 0.2

44.6 ± 0.1a,b

310

450

1.6 ± 0.2

34.0

420

496

17.0 ± 1.9

36.1 ± 32.9 ± 40.5 ± 37.0 ± 36.3 ± 37.6 ± 48.7 ± 38.1 ± 40.6 ± 41.2c 38.6 ± 42.3 ± 47.5 ± 46.2 ± 45.7 ± 46.5 ± 51.0 ± 55.4 ±

0.1 0.2 0.2 0.8 0.3 0.4 1.2 0.1 0.2c 0.1 0.1 0.1 0.2 0.5 0.5 1.9 1.4

ref 5 5 5 14d 27 17 20 21 22 19 19 17 17 5 19 19 14,d 15 21

a

Calculated using the second term in eq 5. bCalculated from the vapor pressures reported in ref 5 in the temperature range T = (288 to 333) K. cAn average of 40.9 ± 0.3 was used in subsequent correlations. dCalculated from the Antoine eq at Tm = 420 K (Δgl H(Tm) = 2.303RB(Tm/(Tm + C))2 where B = 1132.493 and C = −62.084 for perfluoro(methylcyclohexane) and B = 3345.34 and C = 156.09 for perfluorodecane, and R is the gas constant. The applicable temperature range of these constants for perfluoromethylcyclohexane is T = (305 to 385) K and from T = (387 to 524) K for perfluorodecane. The uncertainty reported only reflects the uncertainty associated with the temperature adjustment.

Table 5. Vaporization Enthalpies of Some Perfluorinated Hydrocarbons Evaluated at T = 298.15 K Using Equations 2 to 4

naphthalene-F8 biphenyl-F10 biphenyl-F10 dodecane-F26 dodecane-F26 tridecane-F28 tetradecane-F30 pentadecane-F30 hexadecane-F34 eicosane-F42 tetracosane-F50

ΔlcrHm(Tfus)a

Tfus

ΔgcrHm(Tm)

Tm

Cp(l)/Cp(cr)

ΔlcrH(298 K)

ΔgcrH(298 K)

Δgl H(298 K)

kJ·mol−1

K

kJ·mol−1

K

J·mol−1·K

kJ·mol−1

kJ·mol−1

kJ·mol−1

358.8 339.6 339.6 346.6 348.5e 361.7 375.6 388.1 399.7 436.2 461.1

79.6 ± 0.5 85.7 ± 0.8c 87.8d

308 310 310

283/283 346/350 346/350 589.2/584.8 589.2/584.8 635.6/629.4 682/674 728.4/718.6 774.8/763.2 960.4/941.6 1936/1120

b

17.55 20.5 ± 20.5 ± 24.2 ± 38.2e 27.9 ± 31.5 ± 35.1 ± 38.7 ± 50.3 ± 63.2 ±

0.6 0.6 0.6 0.4 0.3 0.3 0.1 0.3 0.6

c

15.1 18.5 18.5 20.6 34.2 22.8 24.9 27.0 29.0 34.4 26.9

± ± ± ± ± ± ± ± ± ± ±

0.7 0.9 0.9 1.1 1.1 1.5 2.0 2.4 2.9 4.8 10.9

79.8 86.3 88.4 85.8 85.8 94.3 102.4 109.4

± ± ± ± ± ± ± ±

0.6 0.8 0.2 0.6 0.6 0.5 1.0 1.0

64.8 67.8 69.9 65.3 51.6 71.5 77.4 82.4

± ± ± ± ± ± ± ±

0.9 1.2 0.9 1.3 1.3 1.6 2.2 2.6

a

This work unless noted otherwise. bReference 14. cReference 13; calculated from the experimental vapor pressures reported. dReference 14. Reference 9.

e

rate of 5 °C·min−1 from T/K = (298 to Tfus). Indium metal, 99.999 %, was used as the standard. Triplicate determinations of the fusion enthalpy are reported in Table 3. The fusion enthalpies evaluated in Rostock were measured on a Mettler Toledo 822 DSC instrument equipped with STARe software. It was calibrated with indium (99.999 %) at different scanning rates. Samples were sealed hermetically in 40 μL aluminum pans and heated at a ramp of 10 K·min−1. These enthalpies and melting temperatures are also listed in Table 3. The fusion enthalpy measurements in this work are also compared to literature values reported by Starkweather in Table 3.9 In view of the significant discrepancy between our results and those reported by Starkweather, this prompted the measurements of several of these compounds to be performed both in St. Louis and in Rostock using different samples and different instruments. The results from both laboratories were completely reproducible. It is significant to point out that the

ratio of Starkweather’s values to the values reported in this work is nearly constant for all the compounds measured by Starkweather: 1.58, 1.58. 1.60, and 1.59, respectively. This discrepancy is discussed further in the section on results. Temperature Adjustments. Unless available otherwise, all vaporization, sublimation, and fusion enthalpies were adjusted to T/K = 298.15 using eqs 4 to 7, respectively. Equations 4 to 6 have been used previously and shown to provide reasonable adjustments for temperature for sublimation, vaporization, and fusion enthalpies.10 The heat capacity terms required for these temperature adjustments, Cp(cr) and Cp(l), are reported in Tables 4 and 5 and were evaluated by group additivity.8,11,12 Equation 7 describes the thermochemical cycle relating sublimation, vaporization, and fusion enthalpies. Results of the correlations are summarized in Table 6. 2354

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that it is the π region of the aromatic ring that is mainly responsible for the retention times observed for C6F6 and C7F8. Perfluorinated hexane was not retained sufficiently on the column at ambient temperatures to allow accurate measurement of its retention times over 30 K. These measurements required the use of subambient temperatures. The results of a series of correlations using alkanes n-C6F14 through to C16F34 and up to n-C24F50 are reported in Table 6. All correlations were performed in duplicate. The details of only one of the two correlations are reported in Table 6 although the results of both correlations are summarized by the correlation equations reported below each run, eqs 8 to 17. Additional details regarding each correlation can be found in the Supporting Information. The first four runs in Table 6 were performed to determine whether enthalpies of transfer would correlate with the corresponding vaporization enthalpies available in the literature. Good linear correlations were obtained. Runs 3 and 4 treated perfluorododecane as an unknown to identify the most probable of the two discordant vaporization enthalpies calculated for this material in Table 5. Values of (64.3 ± 2.2) kJ·mol−1 and (64.0 ± 2.2) kJ·mol−1 were obtained from run 3 and run 4, respectively, from good linear correlations. These results compare favorably with the experimental vaporization enthalpy of (65.3 ± 1.3) kJ·mol−1 evaluated from the sublimation enthalpy and the DSC results reported in Table 3, confirming that the fusion enthalpy reported by Starkweather9 for perfluorododecane is in error. The constant ratio in the fusion enthalpy results observed between our work and that reported by Starkweather for the other perfluoralkanes suggests the presence of a systematic error of unknown origin in Starkweather’s work for both perfluorododecane and the other perfluorinated alkanes reported by this author. The experimental value of (65.3 ± 1.3) kJ·mol−1 for perfluorododecane was used as a standard in subsequent runs. The remaining six correlations evaluated the vaporization enthalpies of crystalline perfluorohexadecane, perfluoroeicosane, and perfluorotetracosane. The results of all 10 correlations are summarized in Table 7. The results are also illustrated in Figure 2 in which vaporization enthalpies are plotted as a function of the number of perfluoromethylene groups, nCF2. The equation of the line obtained by linear regression is given by eq 18. The correlation coefficient, r2 = 0.9970, confirms that a good linear relationship exists between vaporization enthalpy and the number of CF2 groups. In comparison, the corresponding relationship between the vaporization enthalpies of the corresponding hydrocarbons and the number of methylene groups using recommended values for pentane to eicosane16 is given by eq 19.

Δcrg Hm(298.15 K)/(kJ ·mol−1) = Δcrg Hm(Tm)/kJ·mol−1 + [0.75 + 0.15Cp(cr) /(J·mol−1·K−1)][Tm /K − 298.15]/1000

(4)

Δgl Hm(298.15 K)/(kJ ·mol−1) = Δgl Hm(Tm)/kJ·mol−1 + [(10.58 + 0.26Cp(l)/(J ·mol−1· K−1)) (Tm/K − 298.15)]/1000

(5)

Δcrl Hm(298.15 K)/(kJ ·mol−1) = Δcrl Hm(Tfus)/kJ·mol−1 + [(0.15Cp(cr) − 0.26Cp(l)) /(J·mol−1·K−1) − 9.83)][Tfus/K − 298.15]/1000 (6)

Δgl Hm(298.15 K) = Δcrg Hm(298.15 K) − Δcrl Hm(298.15 K)

■

(7)

RESULTS The GC chromatographs of the n-perfluoroalkanes are remarkable. Despite boiling temperatures similar to their hydrocarbon counterparts, their retention times are surprisingly short in comparison. The chromatographic separation of the perfluorinated alkanes is considerably more problematic than with the corresponding hydrocarbons. Peak shape and tailing were considerably more sensitive to the temperature, flow rate, and concentration on the column used for the measurements, trifluoropropylmethylsiloxane. The use of dilute mixtures and a column pressure set to ca. 21 kPa was found to achieve the best separation and enabled the sharpest peak definition. Figure 1 illustrates the retention times of various perfluorinated alkanes from C8 to C15 in relation to hexafluorobenzene and octafluorotoluene. The perfluoroalkanes that have retention times comparable to hexafluorobenzene and octafluorotoluene are compounds that have roughly twice as many carbon atoms and three times as many fluorine atoms. These results suggest

n‐perfluoroalkanes: Δgl Hm(298.15 K)/(kJ ·mol−1) = (6.08 ± 0.10)nCF2 + (4.63 ± 1.3) r 2 = 0.9970

(18)

n‐alkanes: Δgl Hm(298.15 K)/(kJ ·mol−1) = (5.01 ± 0.01) nCH2 + (11.50 ± 0.09) Figure 1. Retention time data for mix 6, T/K = 338.2. From left to right, the retention times of CH4, 3.3 min; C8F18 (solvent), 3.43 min; C9F20, appears as shoulder, retention time not reported, C12F26, 3.8 min; C13F28, 4.1 min; hexafluorobenzene, 5.0 min, C15F13, 5.2 min; octafluorotoluene, 5.8; n-C16F34, 6.2 min.

r 2 = 0.9999

(19)

The slope of eq 19 suggests that a methylene group makes a slightly smaller contribution to the vaporization enthalpy than a perfluoromethylene group. The contributions of a methyl and perfluoromethyl group appear to be reversed. On the basis of 2355

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Table 6. Summary of the Correlation between the Enthalpies of Transfer and Vaporization Enthalpy of the n-Perfluoroalkanes ΔgslnHm(282 K)

slopea T/K

Run 1

intercept

Δgl Hm(298 K)

−1

a

−1

kJ·mol

kJ·mol

perfluorohexane −2108.3 9.108 17.53 perfluoroheptane −2401.9 9.449 19.97 perfluorooctane −2927.1 10.621 24.33 perfluorononaneb −3445.6 11.755 28.65 Run 1: Δgl Hm(298.15 K)/kJ·mol−1 = (1.17 ± 0.04)ΔgslnHm(282 K) + (12.5 ± 0.84); r2 = 0.9981 (8) Run 2: Δgl Hm(298.15 K)/kJ·mol−1 = (1.17 ± 0.03)ΔgslnHm(282 K) + (12.5 ± 0.7); r2 = 0.9985 (9) slope ΔgslnHm(318 K) Δgl Hm(298 K) T/K

Run 3

kJ·mol−1

intercept

kJ·mol−1 (lit)

−2594.3 9.246 21.57 40.9 ± perfluorooctane perfluorononane −2914.9 9.741 24.23 46.2 ± perfluorodecane −3252.0 10.294 27.04 51.0 ± perfluorododecane −3968.1 11.536 32.99 perfluorotridecane −4339.4 12.202 36.08 71.5 ± perfluorotetradecane −4747.0 12.978 39.46 77.4 ± Run 3: Δgl Hm(298.15 K)/kJ·mol−1 = (2.08 ± 0.05)ΔgslnHm(318 K) − (4.36 ± 1.5); r2 = 0.9983 (10) Run 4: Δgl Hm(298.15 K)/kJ·mol−1 = (2.10 ± 0.05)ΔgslnHm(318 K) − (5.42 ± 1.5); r2 = 0.9983 (11) slope ΔgslnHm(338 K) b

Run 5 perfluorododecane perfluorotridecane perfluorotetradecane perfluoropentadecane perfluorohexadecane Run 5: Δgl Hm(298.15 K)/kJ·mol−1 Run 6: Δgl Hm(298.15 K)/kJ·mol−1 Run 7 perfluorotetradecane perfluoropentadecane perfluorohexadecane perfluoroeicosane Run 7: Δgl Hm(298.15 K)/kJ·mol−1 Run 8: Δgl Hm(298.15 K)/kJ·mol−1 Run 9

T/K

intercept

kJ·mol

intercept

kJ·mol

T/K

intercept

kJ·mol

−1

a

40.5 46.1 51.9 64.3 70.7 77.8

± ± ± ± ± ±

Δgl Hm(298 K) 65.3 71.5 77.4 82.4

−1

± ± ± ±

(lit) 1.3 1.6 2.2 2.6

Δgl Hm(298 K) −1

(lit)

77.4 ± 2.2 82.4 ± 2.6 88.6 ± 4.1

Δgl Hm(298 K) kJ·mol

perfluoropentadecane −4490 12.325 37.33 perfluorohexadecane −4746 12.651 39.46 perfluoroeicosane −5877.2 14.3 48.86 perfluorotetracosane −7122.7 16.283 59.22 Run 9: Δgl Hm(298.15 K)/kJ·mol−1 = (2.70 ± 0.04)ΔgslnHm(399 K) − (18.2 ± 1.6); r2 = 0.9998 (16) Run 10: Δgl Hm(298.15 K)/kJ·mol−1 = (2.64 ± 0.02)ΔgslnHm(399 K) − (20.3 ± 1.0); r2 = 0.9999 (17)

± ± ± ±

1.1 1.1 1.2 1.3

kJ·mol−1 (lit)

kJ·mol

−4088.1 11.607 33.99 −4333.5 11.889 36.03 −4592.9 12.223 38.18 −5758.4 13.957 47.87 = (2.67 ± 0.12)ΔgslnHm(389 K) − (13.5 ± 4.4); r2 = 0.9979 (14) = (2.46 ± 0.12)ΔgslnHm(389 K) − (5.02 ± 4.4); r2 = 0.9975 (15) slope ΔgslnHm(399 K)

33.0 36.0 41.1 46.2

kJ·mol−1 (calc)

kJ·mol

−1

0.2 1.3 0.3 0.2

Δgl Hm(298 K)

1.6 2.2

−1

kJ·mol−1 (calc)

(lit)

Δgl Hm(298 K)

0.3 0.2 1.9

−3648.4 11.018 30.33 −4005.9 11.644 33.30 −4369.0 12.287 36.32 −4734.5 12.937 39.36 −5107.0 13.606 42.46 = (1.90 ± 0.07)ΔgslnHm(338 K) + (8.0 ± 2.5); r2 = 0.9972 (12) = (1.93 ± 0.07)ΔgslnHm(338 K) + (6.43 ± 2.6); r2 = 0.9971 (13) slope ΔgslnHm(389 K) T/K

± ± ± ±

32.9 36.3 40.9 46.2

Δgl Hm(298 K)

−1

(lit)

82.4 ± 2.6 88.6 ± 4.0 113.7 ± 7.4

1.8 1.9 2.0 2.2 2.3 2.4

65.3/51.6

Δgl Hm(298 K) kJ·mol−1 (calc) 65.6 71.3 77.0 82.8 88.6

± ± ± ± ±

3.3 3.4 3.6 3.7 3.9

Δgl Hm(298 K) kJ·mol−1 (calc) 77.2 82.7 88.5 114.3

± ± ± ±

6.1 6.3 6.4 7.4

Δgl Hm(298 K) kJ·mol−1 (calc) 82.6 88.4 113.7 141.7

± ± ± ±

2.1 2.2 2.4 2.7

The slope and intercept of the line obtained by plotting ln(to/ta) against 1/T. bAlso used as the solvent.

the relative accessible surface area of these two groups, the contribution to the vaporization enthalpy of both groups might be expected to be larger in the perfluoroalkanes. A number of the perfluoroalkanes investigated are solids at room temperature. In addition to the sublimation enthalpies measured directly, it is also possible to calculate the sublimation enthalpies of perfluorinated hexadecane, eicosane, and tetracosane at T/K = 298.15 by combining their respective fusion and vaporization enthalpies measured in this work according to eq 7. Table 8 summarizes all of these sublimation enthalpies. All of the perfluoroalkanes investigated thus far by correlation−gas chromatography have been the linear ones. To determine whether branched or cyclic alkanes can be

included in these correlations, several cyclic/branched alkanes were also investigated using the linear perfluoroalkanes as standards. The compounds chosen include perfluoro(methylcyclohexane), a cis/trans mixture of perfluoro(1,3dimethylcyclohexane) and cis- and trans-perfluorodecalin. The nature of their retention times required individual analyses. The vaporization enthalpy of cis/trans-perfluoro(1,3-dimethylcyclohexane) has been measured as a mixture since the two stereoisomers were not separated by chromatography column used. This mixture was judged as a suitable test material since the vaporization enthalpy in the literature was also a combined value for the two diasteriomers.19 cis/trans-Perfluorodecalin was purchased as a mixture but was separated by the chromatography. Runs 11 through 16 in Table 9 summarize the results of 2356

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Table 7. Summary of the Vaporization Enthalpies, Δgl Hm(298.15 K)/kJ·mol−1, of the n-Perfluoroalkanes Run 1 perfluorohexane perfluoroheptane perfluorooctane perfluorononane perfluorodecane perfluorododecane perfluorotridecane perfluorotetradecane perfluoropentadecane

33.0 36.0 41.1 46.2

± ± ± ±

Run 5 perfluorododecane perfluorotridecane perfluorotetradecane perfluoropentadecane perfluorohexadecane perfluoroeicosane perfluorotetracosane

65.6 71.3 77.0 82.8 88.6

± ± ± ± ±

3.3 3.4 3.6 3.7 3.9

Run 2

1.1 1.1 1.2 1.3

33.1 36.0 41.0 46.2

± ± ± ±

Run 3

0.9 1.0 1.1 1.2

Run 6 65.6 71.3 77.0 82.8 88.6

± ± ± ± ±

40.5 46.1 51.9 64.3 70.7 77.8 Run 7

3.4 3.6 3.7 3.9 4.1

77.2 82.7 88.5 114.3

± ± ± ±

± ± ± ± ± ±

Run 4

1.8 1.9 2.0 2.2 2.3 2.4

40.7 46.1 51.6 64.0 70.5 78.0

Run 8

6.1 6.3 6.4 7.4

77.2 82.7 88.5 113.0

± ± ± ±

6.1 6.2 6.5 7.4

± ± ± ± ± ±

average 33.1 36.0 40.8 46.2 51.8 64.2 70.6 77.4 82.7

1.9 1.9 2.0 2.2 2.4 2.5 Run 9

82.6 88.4 114.0 141.7

± ± ± ±

2.1 2.2 2.4 2.7

lit.

± 1.0 ± 1.1 ± 1.4a ± 1.6 ± 2.0 ± 2.2b ± 2.4 ± 4.1a ± 3.9a Run 10

82.5 88.4 113.7 141.0

± ± ± ±

1.4 1.4 1.6 1.7

32.5 ± 0.1 36.3 ± 0.3 40.9 ± 0.3 46.2 ± 0.2 51.0 ± 1.9 65.3 ± 1.3 71.5 ± 1.6 77.4 ± 2.2 82.4 ± 2.6 average

88.6 ± 4.0b 113.7 ± 7.4b 141.4 ± 2.2

a The average of all runs. bThe value and uncertainty reported are the average obtained from the two correlations in which the material was treated as an unknown. All subsequent correlations used the available literature value or this average value as a standard.

It was previously shown that, if a single fluorine was substituted for hydrogen in hydrocarbons and carboxylic acids, these compounds successfully correlated with the corresponding unsubstituted hydrocarbons and carboxylic acids.6 To test whether the analogous substitution with the perfluoroalkanes would likewise be successful, 1H-perfluorooctane was analyzed using C8F18, C9F20 C10F22, and C13F28 as standards. 1HPerfluorooctane was highly retained by the column relative to the other perfluorinated compounds similar to that observed with the aromatic compounds in Figure 1 eluting between C10F22 and C13F28. As indicated in Runs 17 and 18 of Table 9, the vaporization enthalpy calculated for this material was significantly greater than the literature value, clearly indicating that the use of these perfluoralkanes as standards would lead to an erroneous value. It is remarkable that the replacement of a single fluorine atom with a hydrogen atom can have such a large effect on retention time and its temperature dependence. Another set of correlations, Runs 19 and 20, were performed to address whether perfluorinated alkanes can be used as standards for perfluorinated aromatic compounds. As noted previously, the retention times of the perfluoroaromatics were likewise longer than the perfluoroalkanes with a similar number of carbon atoms. Not surprisingly and similar to 1Hperfluorooctane, vaporization enthalpies obtained for perfluorobenzene and perfluorotoluene are also calculated to be greater than the literature values, again emphasizing the need to use more appropriate standards. The last set of correlations reported test the behavior of perfluorinated aromatic compounds with the corresponding aromatic hydrocarbons. The results of one of the correlations are provided in Table 10, and both sets of correlations are summarized in Table 11, resulting in eqs 30 and 31. The elution order that was observed is indicated in Table 10 starting from top to bottom. Despite the fact that decafluorobiphenyl has a larger vaporization enthalpy than octafluoronaphthalene, the former elutes off the column first. Naphthalene is the last compound to elute off the column. The vaporization enthalpies of two of the partially fluorinated aromatics, α,α,α-trifluorotoluene and p-fluoroanisole, are well reproduced by using aromatic hydrocarbons as observed previously.6 The results for

Figure 2. A plot of the average vaporization enthalpy of the nperfluoroalkanes obtained by correlation-gas chromatography (average values, Table 7) as a function of the number of perfluoromethylene groups.

Table 8. Sublimation Enthalpy Summary of Several Perfluoroalkanes

perfluorododecane perfluorotridecane perfluorotetradecane perfluoropentadecane perfluorohexadecane perfluoroeicosane perfluorotetracosane

ΔlcrHm(298 K)

Δgl Hm(298 K)

ΔgcrHm(298 K)

kJ·mol−1

kJ·mol−1

kJ·mol−1

29.0 ± 2.9 34.4 ± 4.8 26.9 ± 10.9

88.6 ± 4.0 113.7 ± 7.4 141.4 ± 2.2

85.8 94.3 102.4 109.4 117.6 148 168.3

± ± ± ± ± ± ±

0.6 0.5 1.0 1.0 4.9 8.8 11.1

duplicate runs obtained by treating these materials as unknowns, resulting in eqs 20 to 29. The experimental details of these experiments are available in the Supporting Information. The results suggest that the linear perfluoroalkanes can probably be used as standards although branched or cyclic perfluoralkanes would probably give the best results. 2357

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Table 9. Vaporization Enthalpy Results at 298.15 K in kJ·mol−1 of Some Cyclic and Branched Perfluoroalkanes perfluoro(methyl-cyclohexane) perfluoro(1,3-dimethylcyclohexane trans-perfluorodecalin cis-perfluorodecalin 1H-perfluorooctane perfluorobenzene perfluorotoluene

Run 11a

Run 12b

37.4 ± 2.4

37.8 ± 1.9

Run 13c

Run 14d

39.1 ± 3.5

Run 17g

Run 18h

59.7 ± 3.5

59.4 ± 2.9

Run 15e

Run 16f

average

39.1 ± 3.0

Run 19i

50.0 ± 4.1 50.9 ± 4.1 Run 20j

58.7 ± 2.8 66.2 ± 2.9

59.5 ± 1.9 66.9 ± 2.0

50.8 ± 3.7 51.7 ± 3.7 averagek

36.6 39.1 50.4 51.3

± ± ± ±

lit.

2.2 3.3 3.9 3.9

37.0 38.6 45.7 46.5 lit.

59.6 ± 3.2 59.1 ± 2.4 66.6 ± 2.5

± ± ± ±

0.8 0.1 0.5 0.5

44.1 ± 0.2 36.1 ± 0.1 40.5 ± 0.2

a Run 11: Δgl Hm(298.15 K)/kJ·mol−1 = (1.44 ± 0.08)ΔgslnHm(292 K) + (7.84 ± 1.8); r2 = 0.9944 (20). bRun 12: Δgl Hm(298.15 K)/kJ·mol−1 = (1.37 ± 0.06)ΔgslnHm(292 K) + (9.64 ± 1.4); r2 = 0.9962 (21). cRun 13: Δgl Hm(298.15 K)/kJ·mol−1 = (2.23 ± 0.09)ΔgslnHm(322 K) − (8.09 ± 2.8); r2 = 0.9964 (22). dRun 14: Δgl Hm(298.15 K)/kJ·mol−1 = (2.18 ± 0.08)ΔgslnHm(322 K) − (6.82 ± 2.5); r2 = 0.9971 (23). eRun 15: Δgl Hm(298.15 K)/ kJ·mol−1 = (2.19 ± 0.10)ΔgslnHm(318 K) − (8.14 ± 3.0); r2 = 0.9978 (24). fRun 16: Δgl Hm(298.15 K)/kJ·mol−1 = (2.11 ± 0.09)ΔgslnHm(318 K) − (5.0 ± 2.8); r2 = 0.9978 (25). gRun 17: Δgl Hm(298.15 K)/kJ·mol−1 = (2.17 ± 0.09)ΔgslnHm(318 K) − (4.66 ± 2.3); r2 = 0.9968 (26). hRun 18: Δgl Hm(298.15 K)/kJ·mol−1 = (2.12 ± 0.07)ΔgslnHm(318 K) − (5.51 ± 1.9); r2 = 0.9979 (27). iRun 19: Δgl Hm(298.15 K)/kJ·mol−1 = (2.21 ± 0.06)ΔgslnHm(333 K) − (5.19 ± 2.0); r2 = 0.9983 (28). jRun 20: Δgl Hm(298.15 K)/kJ·mol−1 = (2.22 ± 0.04)ΔgslnHm(333 K) − (4.53 ± 1.4); r2 = 0.9992 (29). kValues in italics were obtained using inappropriate standards and should not be used.

Table 10. Enthalpies of Transfer and Vaporization of Some Aromatic Fluorinated Compounds slopea 1,1,1-trifluorotoluene toluene p-xylene 4-fluoroanisole mesitylene perfluorobiphenyl perfluoronaphthalene naphthalene

ΔgslnHm(367 K)

Δgl Hm(298 K)

Δgl Hm(298 K)

Δgl Hm(298 K)

kJ·mol−1 (lit)

kJ·mol−1 (calc)

kJ·mol−1 (lit)

± ± ± ± ± ± ± ±

37.6 ± 0.4

T/K

intercepta

kJ·mol−1

−3723.7 −3711.5 −4091.6 −4481.9 −4533.4 −5708.6 −5350.0 −5188.5

10.836 10.294 10.616 11.313 11.159 13.505 12.459 11.486

30.96 30.86 34.02 37.26 37.69 47.46 44.48 43.13

38.1 38.0 42.5 47.0 47.6 61.4 57.2 55.3

38.1 ± 0.1 42.3 ± 0.1 47.5 ± 0.1

55.4 ± 1.4

0.7 0.7 0.8 0.8 0.8 0.9 0.9 0.9

48.7 ± 1.2 67.8, 69.9 64.8 ± 0.9

See footnote a in Table 6. Run 21: Δgl Hm(298.15 K)/kJ·mol−1 = (1.41 ± 0.02)ΔgslnHm(367 K) − (5.50 ± 0.6); r2 = 0.9997 (30). Run 22: Δgl Hm(298.15 K)/kJ·mol−1 = (1.28 ± 0.05)ΔgslnHm(373 K) − (0.28 ± 1.9); r2 = 0.9966 (31). a

Table 11. Summary of the Correlation between Aromatic Hydrocarbons and Related Fluorinated Compounds Run 21 α,α,α-trifluorotoluene toluene p-xylene p-fluoroanisole mesitylene perfluorobiphenyl perfluoronaphthalene naphthalene a

38.1 38.0 42.5 47.0 47.6 61.4 57.2 55.3

± ± ± ± ± ± ± ±

0.7 0.7 0.8 0.8 0.8 0.9 0.9 0.9

Run 22 36.5 37.7 42.7 46.8 47.8 60.4 56.5 55.1

± ± ± ± ± ± ± ±

2.4 2.5 2.6 2.7 2.8 3.1 3.0 3.0

Δgl Hm(298 K)a

Δgl Hm(298 K)

−1

kJ·mol−1 (lit)

kJ·mol 37.3 37.9 42.6 46.9 47.7 60.9 56.9 55.2

± ± ± ± ± ± ± ±

(avg) 1.6 1.6 1.7 1.8 1.8 1.9 2.0 2.0

37.6 38.1 42.3 48.7 47.5 68.9 64.8 55.4

± ± ± ± ± ± ± ±

0.4 0.1 0.1 1.2 0.1 1.1 0.9 1.4

Values in italics are unreliable.

α,α,α-trifluorotoluene is encouraging since it suggests that several fluorine substitutions can be tolerated using hydrocarbons as standards. The result for p-fluoroanisole is reassuring since it suggests that the combination of two functional groups, an ether oxygen and a fluorine substituent, both previously demonstrated to correlate individually with hydrocarbons, continue to due so when used in combination.6,23,24 The agreement observed between these results and the literature values suggests that it may be possible to obtain good correlations with other partially fluorinated aromatic ethers and hydrocarbons. Results for the fully fluorinated aromatic compounds investigated clearly indicate that these compounds do not correlate either with hydrocarbons or with themselves. Whether the enthalpies of transfer of completely planar

perfluorinated aromatics will correlate with their experimental vaporization enthalpies remains to be determined. It is clear that extreme caution is necessary when performing correlations to evaluate vaporization enthalpies with perfluorinated compounds.

■

SUMMARY The results of this work suggests that correlation-gas chromatography experiments can be successful in measuring the vaporization enthalpies of perfluorinated alkanes provided similar compounds are used as standards. The perfluorinated alkanes appear to behave linearly with the number of CF2 groups similar to what is observed for n-alkanes of similar size.25 Correlations of enthalpies of transfer with vaporization 2358

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(12) Chickos, J. S.; Hesse, D. G.; Liebman, J. F. A Group Additivity Approach for the Estimation of Heat Capacities of Organic Liquids and Solids. Struct. Chem. 1993, 4, 261−269. (13) Radchenko, L. G.; Kitaigorodskii, A. I. The Vapor Pressures and Heats of Sublimation of Naphthalene, Biphenyl, Octafluoronaphthalene, Decafluorobiphenyl, Acenaphthene and α-Nitronaphthalene. Russ. J. Phys. Chem. 1974, 48, 1595−97. (14) Stephenson, R. M.; Malanowski, S. Handbook of the Thermodynamics of Organic Compounds; Elsevier: New York, 1987. (15) Ermakov, N. V.; Skripov, V. P. Zh. Fiz. Khim. 1967, 41, 77−81. As reported in Dykyj, J.; Svoboda, J.; Wilhoit, R. C.; Frenkel, M. L.; Hall, K. R. Vapor Pressure of Chemicals: Part A. Vapor Pressure and Antoine Constants for Hydrocarbons and Sulfur, Selenium, Tellurium and Hydrogen Containing Organic Compounds; Springer: Berlin, 1999. (16) Ruzicka, K.; Majer, V. Simultaneous treatment of vapor pressures and related thermal data between the triple point and normal boiling temperatures for n-alkanes C5-C20. J. Phys. Chem. Ref. Data 1994, 23, 1−39. (17) Majer, V.; Svoboda, V. Enthalpies of Vaporization of Organic Compounds. A Critical Review and Data Compilation; IUPAC Chemical Data Series No. 32; Blackwell Scientific Publications: Oxford, U.K., 1985. (18) Schwickert, H. Dissertation. Johannes Gutenberg University, Mainz, Germany, 1984. (19) Varushchenko, R. M.; Druzhinina, A. I.; Pashchenko, L. L. Thermodynamics of vaporization of some cyclic perfluorocarbons. Fluid Phase Equilib. 1996, 126, 93−104. (20) Ribeiro da Silva, M. A. V.; Ferreira, A. I. M. C. L. Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers. J. Chem. Thermodyn. 2009, 41, 361−6. (21) Roux, M. V.; Temprado, M.; Chickos, J. S.; Nagano, Y. Critically Evaluated Thermochemical Properties of Polycyclic Aromatic Hydrocarbons. J. Phys. Chem. Ref. Data 2008, 37, 1855−1996. (22) Dias, A. M. A.; Caco, A. I.; Coutinho, J. A. P.; Santos, L. M. N. B. F.; Pineiro, M. M.; Vega, L. F.; Costa Gomes, M. F.; Marrucho, I. M. Thermodynamic properties of perfluoro-n-octane. Fluid Phase Equilib. 2004, 225, 39−47. (23) Maxwell, R.; Chickos, J. An Examination of the Thermodynamics of Fusion, Vaporization and Sublimation of Ibuprofen and Naproxen by Correlation Gas Chromatography. J. Pharm. Sci. 2012, 101, 805−14. (24) Umnahanant, P.; J. S. Chickos, J. S. An Examination of the Thermodynamics of Fusion, Vaporization, and Sublimation of Several Parabens by Correlation Gas Chromatography. J. Pharm. Sci. 2011, 100, 1847−55. (25) Chickos, J. S.; Lipkind, D. Hypothetical Thermodynamic Properties: Vapor pressures and Vaporization Enthalpies of the nAlkanes from C78 to C92 at T = 298.15 K by Correlation−Gas Chromatography. J. Chem. Eng. Data 2008, 53, 2432−2440. (26) Ruzicka, K.; Fulem, M.; Ruzicka, V. Recommended vapor pressure of solid naphthalene. J. Chem. Eng. Data 2005, 50, 1956− 1970. (27) Steele, W. V.; Chirico, R. D.; Knipmeyer, S. E.; Nguyen, A. Vapor Pressure, Heat Capacity, and Density along the Saturation Line, Measurements for Cyclohexanol, 2-Cyclohexen-1-one, 1,2-Dichloropropane, 1,4-Di-tert-butylbenzene, (±)-2-Ethylhexanoic Acid, 2(Methylamino)ethanol, Perfluoro-n-heptane, and Sulfolane. J. Chem. Eng. Data 1997, 42, 1021−36.

enthalpies also appear to be successful using hydrocarbons as standards when only a few fluorine substituents are present on a molecule.6 On the other hand, polyfluorinated compounds do not appear to correlate well with perfluorinated compounds. The importance of experimentally verifying the appropriateness of the standards to be used in these correlations is clearly evident.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables including the experimental retention times described in the text and literature references of the standards used. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 314 516 5377; fax: 314 516 5342; e-mail: [email protected]. Notes

The authors declare no competing financial interest. † 2006 Summer STARS student from Whitfield School, St. Louis, MO 63141. ‡ 2009 Summer STARS student from John Burroughs School, St. Louis, MO 63124. § 2009 Summer STARS student from Westminster Christian Academy, St. Louis, MO 63141. ∥ On leave from Samara State Technical University, 443100 Samara, Russia.

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REFERENCES

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