Impact of Fluorine on the Phase Behavior of Bisphenol-Type

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Impact of Fluorine on the Phase Behavior of Bisphenol-Type Compounds in Supercritical CO2, 1,1-Difluoroethane, and 1,1,1,2-Tetrafluoroethane Jun Liu, Dan Li, Hun Soo Byun,† and Mark A. McHugh* Department of Chemical and Life Science Engineering, Virginia Commonwealth UniVersity, Richmond, Virginia 23284, and Department of Chemical System Engineering, Chonnam National UniVersity, Yeosu, Jeonnam 550-749, South Korea

Phase behavior data are reported for 2,2-bis(4-hydroxyphenyl) propane (BPA) and 2,2-bis(4-hydroxyphenyl) hexafluoropropane (BPAF) in CO2, difluoroethane (F152a), and tetrafluoroethane (F134a). BPA dissolves in F152a but not in CO2 or F134a. BPAF is most soluble in F152a, moderately soluble in F134a, and least soluble in CO2. Aromatic rotation in BPAF is hindered by the CF3 groups, which is expected to reduce configurationally dependent BPAF-BPAF hydrogen bonding. Although both freons have similar polarizabilities and dipole moments, F152a is a superior solvent with a larger effective dipole moment due to its smaller molar volume. Also, the electrostatic potential field of F152a projects slightly beyond the molecular dimension of the fluorine and hydrogen atoms more so than with F134a, suggesting that F152a exhibits stronger electrostatic interactions over greater separation distances than F134a. Hence, fluorine has a significant effect on the phase behavior whether fluorine is added to the solute or solvent. Introduction CO2 is an interesting supercritical fluid solvent that has hybrid polar/nonpolar character, neither of which is very large. Although the polarizability of CO2 is small, near that of methane, it is easy to increase the density of CO2 which increases nonpolar dispersion interactions that are proportional to the product of polarizability times density.1 CO2 does not have a dipole moment, but it does have a quadrupole moment that is a result of the strong electronegative character of oxygen relative to carbon. However, polar interactions fall off as the temperature is increased1 so that at modest-to-high temperatures CO2 behaves more like a weak nonpolar solvent than a polar solvent. Interestingly, numerous studies have shown that fluorinated compounds, relative to their hydrocarbon analogs, have higher solubility in CO2.2-5 Several explanations have been offered for the enhanced solubility of fluorinated compounds in CO2, some of which are presented here. Iezzi et al. suggested that perfluorohexane is more soluble than hexane in CO2 because perfluorohexane-perfluorohexane interactions are much smaller than hexane-hexane interactions.6 This comparison is quite interesting since perfluorohexane has a molecular weight that is almost four times greater than that of hexane, which by itself suggests that perfluorohexane would be less soluble, although the critical temperature of perfluorohexane, a measure of self-interactions, is more than 50 °C lower than that of hexane.6 Several researchers have used NMR to determine if specific interactions exist between CO2 and fluorinated molecules.7-11 While Yonker’s results show that there were no specific interactions between fluorine and CO2,10,11 other researchers suggest that specific solute-solvent interactions do exist between CO2 and fluorinated compounds.7-9 In a different approach, Yee and co-workers12 used infrared, solvatochromic shifts to quantify the interaction strength of hydrocarbon and perfluorinated alcohols with CO2 and ethane. The fluorinated alcohols were more soluble than the hydrocarbon * To whom correspondence should be addressed. Telephone: (804) 827-7031. Fax: (804) 828-3846. E-mail: [email protected]. † Chonnam National University.

alcohols in both CO2 and ethane, which was attributed to a reduction in the hydrogen-bonding energy of the fluorinated alcohol and not necessarily to any specific interaction of the fluorine atoms with the carbon of CO2. This result is not surprising since hydrogen bonding interactions are as much as an order of magnitude stronger than physical dispersion and dipolar interactions. Therefore, a reduction of the strength of alcohol-alcohol hydrogen bonding interactions will dominate fluorine-carbon polar and dispersion interactions. It is important to recognize the limitations of using solvatochromic techniques to quantify solvent behavior when supercritical fluids are involved. The research groups of Liotta and Eckert have used this technique in a variety of studies (see, for example, ref 13); however, they also recommend caution when using this technique with CO2-expanded liquids.14 Several research groups have performed experimental studies to measure the effect of fluorine on the solubility of benzoic acid15,16 and triphenylphosphine (TPP)17 compounds. Higashi et al. show that the solubility of 2- and 3-trifluoromethyl benzoic acid is 2 and 10 times higher, respectively, than the solubility of methyl benzoic acid in CO2. However, the solubility of 4-trifluoromethyl benzoic acid is approximately the same as 4-methyl benzoic acid likely due to the very high heat of fusion of the 4-trifluoromethyl benzoic acid. In a very informative study, Wagner and co-workers compare the CO2 solubility of four different triphenylphosphines (TPP) in which 0, 5, 9, and 15 hydrogen atoms are replaced with fluorine atoms. They demonstrate conclusively that the CO2 solubility of the fluorinated solid TPPs are 1 to 2 orders of magnitude more soluble on a weight basis than the nonfluorinated solid TPP depending on the operating temperature and pressure.17 Molecular modeling has also been used to provide some insight to the observation that fluorinating a compound increases its solubility in CO2, although this approach is not without its contradictions.18-25 Stone et al.22 demonstrate that CO2 and perfluorocarbons do not exhibit any substantial specific interaction in line with the results of other modeling studies18,19,21,23,26 and phase behavior studies of fluorinated polymer-CO2 mixtures.2,27 Fried and Hu20 performed ab initio molecular orbital calculations for CO2 with methane, ethane, and propane and

10.1021/ie070387f CCC: $40.75 © 2008 American Chemical Society Published on Web 06/28/2007

Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 525 Table 1. Chemical Structure and Physical Properties for 2,2-Bis(4-hydroxyphenyl)propane (BPA) and 2,2-Bis(4-hydroxyphenyl)hexafluoropropane (BPAF) molecular weight

melting temperature (°C)

heat of fusion (kJ/mol)a

bisphenol A (BPA)

228

158

30.6

bisphenol AF (BPAF)

368

162

35.1

compound

a

structure

Measured in our laboratories with a TA Instruments model Q200 differential scanning calorimeter.

with CF4, CH3CF3, and CH3CH2CF3 and showed that there are favorable polar interactions between CO2 and fluoroalkyl groups. Their conclusion that a favorable polar interaction exists for the CO2-CF4 system is contrary to the results of the previously mentioned studies. Two other modeling studies, performed with aromatic-based polar solutes, concluded in one case25 that there is a possibility for a specific interaction between CO2 and a fluorinated methyl group on the aromatic and in the other case24 that there is no enhanced interaction between fluorine and CO2 but enhanced interactions resulted from the increased solvent accessible surface of the fluorinated aromatic. All of the molecular modeling studies mentioned here agree that fluorinating a material has a significant effect on both the stereoelectronics and geometric characteristics of the material. The modeling results of Stone et al.,22 Raveendran and Wallen,21 and Baradie et al.18 demonstrate that enhanced interactions, leading to enhanced solubility, can be realized with semifluorinated materials having both Lewis acid and base sites configured in a manner to allow both sites to interact simultaneously. The results from these three studies are in agreement with the interpretation of fluoropolymer-CO2 phase behavior studies.2,27 Successful equation of state modeling26,28 of fluorocarbon-CO2 phase behavior has paid particular attention to dispersion interactions and geometric packing for these type of mixtures. The interpretation of the solubility results presented here for bisphenol-type compounds-supercritical solvent mixtures also invokes the importance of interactions and geometry of both the solvents and the solutes. The primary goal of the work presented here is to contrast and compare the solubility level of two different bisphenol compounds, 2,2-bis(4-hydroxyphenyl) propane (bisphenol A, BPA) and 2,2-bis(4-hydroxyphenyl) hexafluoropropane (bisphenol AF, BPAF), in three supercritical fluid (SCF) solvents, CO2, CH3CHF2 (F152a), and CH2FCF3 (F134a). No attempt is made to determine detailed features of the phase diagrams for these binary mixtures. Rather, the objective is to ascertain the conditions needed to dissolve 2-10 wt % bisphenol in the SCF solvent of interest and to compare how those conditions change with the fluorine content of the solute or solvent. Table 1 gives the structure and physical properties of the bisphenol compounds used in this study. Both bisphenol compounds have approximately the same melting temperature although the BPAF has a much higher molecular weight due to the trifluoromethyl groups on the isopropylidene group separating the two aromatics. The heat of fusion of BPA is lower than that of BPAF which implies that the ideal mole fraction solubility of BPA is greater than that of BPAF. However, the ideal weight fraction solubilities for these two compounds are essentially equal after correcting for the differences in molecular weight. Therefore, we do not expect the solubility behavior of these two compounds to be dominated by differences in the heats of fusion. Bisphenol

Table 2. Physical Properties of the Solvents Used in This Studya

CO2 CH2FCF3 F134a CH3CHF2 F152a a

molecular weight (g/mol)

Tc (°C)

Pc (bar)

polarizability (Å3)

dipole moment (D)

44.0 102.0

31.0 101.1

73.8 40.6

2.65 4.38

0 2.1

66.1

113.1

45.2

4.15

2.3

CO2 has a quadrupole moment of 4.3 × 10-26 erg1/2 cm5/2.

compounds have been used as synthetic vulcanizing agents.29-32 Hence, the experimental results reported here also provide the process engineer with data on the conditions that might be used for vulcanization reactions in CO2. Table 2 gives physical property information on the SCF solvents used in this study. Note that both F134a and F152 have relatively large dipole moments and polarizabilities that are much larger than CO2. In fact, the polarizability of CO2 is close to that of methane and perfluoromethane and the polarizabilities of these two freons are similar to that of propane. Hence, the two freons are expected to be very good solvents for BPAF since BPAF contains both fluorinated and hydrocarbon groups. It is interesting that F134a has a lower critical temperature than F152a even though F134a has a larger molecular weight. The larger number of fluorine atoms in F134a confer a larger molecular weight to this fluorinated ethane, but it also confers weaker self-interactions, which are a hallmark of fluorocarbons. Although Raveendran and Wallen only performed quantum chemical calculations on partially fluorinated methanes, they note the importance of the orientation of the fluorine atoms on partially fluorinated higher order alkanes and the charge separation and partial charges on the fluorine and hydrogen atoms which affect the strength of interactions even when these charges are weak.21 A comparison of the solubility of BPA and BPAF in F134a and F152a provides insight into how solvent power is affected by the separation of the hydrogen and fluorine atoms and the resulting electrostatic potential field surrounding the solvent molecules. Experimental Section Described in detail elsewhere are the apparatus and techniques used to obtain SCF-solute phase behavior data.33 A highpressure, variable-volume cell (7.0 cm OD × 1.6 cm ID, ∼30 cm3 working volume) is loaded with a measured amount of solid to within (0.001 g and then purged three times with gaseous solvent, at pressures less than three bars, to remove entrapped air. The SCF solvent is then transferred into the cell gravimetrically to within (0.05 g using a high-pressure bomb. The cell contents are viewed with a borescope (Olympus Corporation, model F100-024-000-55) placed against a sapphire window secured at one end of the cell. A stir bar activated by a magnet

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located below the cell mixes the contents of the cell. The solution temperature is measured to within (0.1 °C with a type K thermocouple calibrated against an NIST certified thermometer. The solution is compressed by moving a floating piston in the cell using water in a high-pressure generator (HIP Inc., model 37-5.75-60). The pressure is measured on the waterside of the piston and a small correction (∼1 bar) is added to account for the pressure required to move the piston. A pressure transducer (Viatran model 245 accurate to (3.4 bar) is used to measure solution pressures higher than 690 bar, a Heise pressure gauge (0-690 bar, accurate to (0.7 bar) is used to measure solution pressures between 690 and 170 bar, and a second Heise pressure gauge (accurate to (0.2 bar) is used to measure solution pressures up to 170 bar. The solution remains at the temperature of interest for at least 30 min to ensure thermal equilibrium. The mixture in the cell is compressed to a single phase, and the pressure is then slowly decreased isothermally until a second phase appears. A bubble point is obtained if small bubbles appear in the cell, a dew point is obtained if a fine mist appears in the cell, and a solid solubility point is obtained if very small crystals appear. In all three cases, the composition of the predominant phase is equal to the overall solution composition as the amount of mass present in the second phase is negligible. A liquid-liquid-vapor point is obtained when a bubble appears in the presence of two liquid phases as the pressure is slowly decreased isothermally and a solid-liquid-vapor point is obtained when a solid crystal appears in the presence of a liquid and a vapor as the pressure is slowly decreased isothermally. For these three phase points, no attempt is made to determine the composition of any of the phases. Materials Medical grade CO2 (99.8% minimum purity) was purchased from Roberts Oxygen Inc. CH3CHF2 (F152a, 98+% purity) and CH2FCF3 (F134a, 99+% purity) were purchased from Aldrich (Milwaukee, WI). Bisphenol AF (97% purity) and bisphenol A (99+% purity) were both purchased from Aldrich. All of these materials were used as received. Results and Discussion CO2 + Bisphenol A (CO2 + BPA). Bisphenol A, at ∼7.0 wt %, did not dissolve in CO2 even at temperatures of 200 °C and pressures to 2000 bar. The results from this experiment are consistent with those of Margon et al. who reported dew point and bubble point pressures for the CO2-BPA system at ∼130200 °C.34 Margon and co-workers report that the solubility of BPA at 300 bar ranges from 0.1 to 0.4 wt % for temperatures of ∼130-200 °C, respectively, and bubble point data at 99.096.7 wt % BPA for the same temperature range and pressures of ∼25-100 bar, respectively. The low solubility of BPA in CO2 is likely a result of hydrogen bonding, self-interactions of BPA that far outweigh those of CO2-bisphenol A cross interactions. No further experiments were performed with BPA and CO2 since solute solubility levels are apparently extremely low. CO2 + Bisphenol AF (CO2 + BPAF). Figure 1 shows isopleths of CO2 + BPAF mixtures obtained at 30-140 °C and pressures to 2000 bar. (Data tables for the mixtures investigated in this study are provided in the Supporting Information.) The liquid + liquid f fluid data shown in this figure are obtained at ∼50 °C below the normal melting point of BPAF, which indicates that CO2 is very soluble in the BPAF-rich liquid phase.

Figure 1. Experimental phase behavior of CO2 + BPAF mixtures with 1.0 (triangles), 1.8 (diamonds), 2.8 (circles), and 3.6 wt % (squares) BPAF. The open symbols are fluid f solid + liquid transitions, and the filled circles are fluid f liquid + liquid transitions.

Figure 2. Experimental phase behavior of F134a (CH2FCF3) + BPAF mixtures with 3.4 (filled circles), 5.6 (open squares), and 7.0 wt % (open circles) BPAF. The dashed line represents a solidification boundary.

At temperatures below ∼115 °C, BPAF precipitates as needle crystals. Nevertheless, replacing the isopropylidene methyl groups with trifluoromethyl groups makes this bisphenol dissolve in CO2 which is a bit surprising since both bisphenol compounds have essentially the same normal melting points. From a strictly energetic viewpoint, it can be argued that the favorable interaction between fluorine and the carbon of CO2 is sufficiently strong to overcome solid-solid or CO2-CO2 interactions. However, as the temperature is reduced, the solid + liquid f fluid isopleths in Figure 1 exhibit very steep, negative slopes indicating that CO2 density has a very modest effect on BPAF solubility and CO2-BPAF interactions are now much less than BPAF-BPAF hydrogen bonding interactions. Note that although the partially fluorinated BPAF dissolves in CO2 at levels to 3.6 wt %, fairly high pressures are still needed to obtain a single phase. CH2FCF3 + Bisphenol A (F134a + BPA). It was surprising that BPA, at 4.7 wt %, did not dissolve in F134a even at temperatures to 200 °C and pressures in excess of 650 bar. Apparently, BPA hydrogen-bonding self-interactions far outweigh F134a-BPA cross interactions including F134a dipolararomatic quadrupolar interactions. Also, it is important to recognize that aliphatic fluorocarbon-aliphatic hydrocarbon mixtures are not compatible energetically.35 Therefore, the hydrocarbon isopropylidene group is not expected to be compatible with the semifluorinated F134a, which again suggests that BPA should not dissolve to a large extent in a semifluorinated alkane. No further experiments were performed with BPA and F134a since BPA exhibits very low solubility in F134a. CH2FCF3 + Bisphenol AF (F134a + BPAF). In contrast to BPA, BPAF is very soluble in F134a when the temperature

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Figure 3. Experimental phase behavior of F152a (CH3CHF2) + BPA mixtures with 2.9 (open circles), 4.7 (squares), and 7.9 wt % (filled circles) BPA. The dashed line represents a solidification boundary.

is higher than ∼130 °C, as shown in Figure 2. In this case, the liquid + liquid f fluid curves are in the 100-200 bar range, whereas they were in the 200-2000 bar range for the BPAFCO2 system. Also, the BPAF-F134a transition curves do not exhibit a sharp increase in pressure as the temperature is lowered. It is interesting that the melting point depression of BPAF in F134a is similar to that observed in CO2. The steric character of the two bulky CF3 groups on BPAF hinders the free rotation of the aromatic groups to a greater extent than does two methyl groups,36 which likely reduces the BPAF-BPAF hydrogen-bonding that is strongly dependent on molecular configuration. Liu et al.37 recently performed semiempirical quantum mechanics calculations to determine the energy barrier for rotation about the dihedral angle for isopropylidiene diphenyl and hexafluoroisopropylidene diphenyl, two compounds very similar to BPA and BPAF. For these calculations, Liu et al. used the MOPAC PM5 method with CAChe 4.9 software since the PM5 Hamiltonian is reported to be four times more accurate than the AM1 or PM3 Hamiltonians.38 The maximum value of the rotational barrier was ∼4.35 kcal/mol when the isopropylidene group contained two CF3 groups (C(CF3)2) compared with ∼1.33 kcal/mol, with two methyl groups (C(CH3)2). Espeso et al.39 found the same relative differences in rotational energy barriers for C(CF3)2 and C(CH3)2 although they used the AM1 method and found that the difference was approximately a factor of 2,39 whereas we find a factor of 3. In addition to an expected reduction in BPAF-BPAF self-interactions, it is also reasonable to assume that the hydrogens in polar F134a hydrogen bond with the hydroxyl groups in BPAF. Both the reduced selfinteractions of BPAF and the favorable cross-interactions of BPAF and F134a explain why it takes very low pressures to dissolve BPAF in F134a. CH3CHF2 + Bisphenol A (F152a + BPA). Figure 3 shows isopleths of BPA + F152a mixtures obtained at 105-180 °C and pressures to 650 bar. The isopleth curves in Figure 3 increase in pressure as the temperature is reduced, and they eventually intersect a crystallization boundary. It is somewhat surprising that BPA dissolves in F152a while it does not dissolve to any significant degree in F134a given that both F134a and F152a have essentially the same polarizabilities and dipole moments. One reason for the difference in solvent strength is that F152a has a smaller molar volume than F134a which endows F152a with a higher effective polarity, µeffective ) µ2/ σ3kT, where µ is the permanent dipole moment, σ is the collision diameter, k is Boltzmann’s constant, and T is the absolute temperature.40 As mentioned earlier, we speculate that another reason for the difference in solvent power between F134a and F152a is the impact of the separation of the hydrogen and fluorine atoms and the resulting electrostatic potential field

Figure 4. Different views of the molecular structures and electrostatic potential isosurfaces of F134a (CH2FCF3, left) and F152a (CH3CHF2, right). The size of the atoms is shown as the van der Waals radius. The grey is for hydrogen, the lime for fluorine, and the green for carbon. The red color represents the +0.03 au electrostatic potential isosurface, and the blue color represents the -0.03 au electrostatic potential isosurface.

Figure 5. Experimental phase behavior of F152a (CH3CHF2) + BPAF mixtures with 2.1 (triangles) and 7.9 wt % (filled circles) BPAF.

surrounding each of these solvent molecules. Figure 4 shows the electrostatic potential fields for F134a and F152a determined from semiempirical quantum mechanics calculations, using the MOPAC PM5 method with CAChe 4.9 software. Figure 4 shows that the isosurface with a potential of -0.03 au for F134a is located within the van der Waals radius of the fluorine. However, for F152a, a large portion of the two isosurfaces with potentials of +0.03 and -0.03 au project slightly beyond the molecular dimension of the respective atoms. In other words, F152a has stronger electrostatic interactions with other molecules over somewhat greater separation distances than does F134a, which translates into stronger solvent power for F152a. CH3CHF2 + Bisphenol AF (F152a + BPAF). Figure 5 shows that BPAF is soluble in F152a even at low temperatures. In fact, BPAF remains a liquid to temperatures as low as 70 °C, which is more than an 80 °C melting point depression. At temperatures below the critical point of F152a, the phase boundary virtually superposes with the saturated vapor pressure curve for F152a indicating that BPAF is soluble in liquid F152a. Figure 6 shows more detail on the phase behavior of BPAF in F152a at conditions near the critical point of F152a. The BPAF-F152 system exhibits liquid-liquid-vapor (LLV) behavior at temperatures between 108 and 123 °C. At high temperatures, the LLV line intersects the low-temperature portion of the BPAF-F152a mixture critical curve. At temperatures below ∼110 °C, the LLV line intersects the solidliquid-vapor (SLV) line at a solid-liquid-liquid-vapor quadruple point. At still lower temperatures, the LLV line now becomes an SLV line. The high-temperature portion of the SLV

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Figure 6. Phase behavior of the BPAF + F152a (CH3CHF2) system at temperatures near the critical point of F152a: (filled square) melting point of BPAF; (open circles) liquid-liquid-vapor curve; (open square) critical point of F152a; (filled circles) solid-liquid-vapor curve.

line is not measured in this study and, therefore, is shown as a dashed curve ending at the normal melting point of BPAF. Conclusions The phase behavior data reported here for two different bisphenol compounds demonstrate that even a modest amount of fluorine substitution for hydrogen has a significant impact on the solubility levels of these compounds. Fluorine substitution on the bisphenol compounds affects the energetics of bisphenolbisphenol and bisphenol-solvent intermolecular interactions and the energy barrier for aromatic rotation which is expected to reduce bisphenol-bisphenol and bisphenol-solvent hydrogen bonding. CO2 is a poor solvent for BPA and a modest solvent for BPAF. The impact of fluorine substitution for hydrogen on the resultant phase behavior is further demonstrated by comparison of the two fluorocarbon solvents used in this study, F134a and F152a. Once again, fluorine substitution has a significant impact on the strength of these two solvents that have the same polarizability and dipole moment. However, F152a is a superior solvent compared to F134a due to the differences in molar volumes and the distances of fluorine and hydrogen separation on these two solvents. It is our hope that the study reported here provides further insight for the interpretation of the interesting fluorous surfactant/reaction chemistry studies utilizing supercritical CO2 that are emanating from the Liotta/Eckert group (see, for example, refs 41 and 42). Acknowledgment H.S.B. acknowledges the financial support from LG Yonam Foundation for this project. Supporting Information Available: Solute-supercritical fluids isopleth data tables. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 1999. (2) Mertdogan, C. A.; DiNoia, T. P.; McHugh, M. A. Impact of Backbone Architecture on the Solubility of Fluorocopolymers in Supercritical CO2 and Halogenated Supercritical Solvents: Comparison of Poly(Vinylidene Fluoride-co-22 mol % Hexafluoropropylene) and Poly(Tetrafluoroethylene-co-19 mol % Hexafluoropropylene). Macromolecules 1997, 30, 7511.

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ReceiVed for reView March 14, 2007 ReVised manuscript receiVed May 11, 2007 Accepted May 17, 2007 IE070387F