Ind. Eng. Chem. Res. 2003, 42, 6511-6517
6511
Solvent Strength Characterization of Carbon Dioxide Expanded Fluorinated Solvents Yeh Wei Kho, Daniel C. Conrad, Reed A. Shick, and Barbara L. Knutson* Department of Chemical & Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046, Whirlpool Corp., 2000 N. M-63, Benton Harbor, Michigan 49022-2693, and Dow Chemical Co., 2030 Dow Center, Midland, Michigan 48642
CO2-expanded solvents have been investigated as media for chemical reactions, separation, and materials processing. The tunability of CO2-expanded fluorinated solvents was examined by two complementary approaches: solvatochromic characterization, which elucidates specific solute/ solvent interactions, and Hansen solubility parameter estimation, which describes bulk solvent properties. The π* (dipolarity/polarizability) of CO2-expanded ethoxynonafluorobutane (ENB), as determined from solvatochromic measurements, decreased with increasing CO2 expansion pressure, in agreement with its volume expansion behavior. Analogous to cosolvent tuning in supercritical fluids, the solvent strength of ENB was manipulated with the addition of strong hydrogen-bond-accepting and -donating cosolvents, 2,2,2-trifluoroethanol (TFE; R ) 1.51) and N-methyl-2-pyrrolidinone (NMP; β ) 0.78), respectively. The Hansen solubility parameters of ENB and CO2-expanded ENB (at 8-20 bar) were estimated from fluorous/organic miscibility studies. The experimental Hansen solubility parameters were used to guide the further selection of immiscible organofluorous pairs that become a homogeneous mixture under mild CO2 pressure. Fluorinated hydrocarbon chains have significant solubility in compressed and supercritical CO2,1 whose solvent interactions are neither hydrophilic nor lipophilic. In addition, perfluorinated compounds have very weak intermolecular interactions, low surface energies, and low refractive indexes.2 However, the C-F bond is polarized because of the extreme electronegativity of fluorine. Thus, the physiochemical properties of partially fluorinated materials can be significantly different from both their hydrocarbon and perfluorocarbon analogues.2 Partially fluorinated solvents such as hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs) are replacing chlorofluorocarbons (CFCs) in various applications3-6 because of their excellent chemical stability and reduced environmental impact relative to CFCs. However, their general applicability as process solvents is hindered by their limited solvent power relative to CFCs. Potential approaches to address the low solubility of hydrophilic and lipophilic species in fluorinated solvents include cosolvent addition, solvent blending, and microemulsion formation. Similar approaches have been employed in supercritical CO2 processing to enhance the solubility of polar species. In addition, the ability of CO2 to dissolve in and expand a broad range of solvents7 has been used to create solvent systems with enhanced solvent strength relative to pure CO2. These gasexpanded liquids (GELs) provide pressure-tunable solvent strength and enhanced mass-transfer characteristics relative to traditional liquids. GELs have been investigated as media for chemical reaction,8,9 separation,10 and materials processing. The increased solubility of gaseous reactants in CO2-expanded organic solvents can enhance chemical reaction rates and selectivity.8,9,11 In addition, downstream separation can be * To whom correspondence should be addressed. Tel.: (859) 257-5715. Fax: (859) 253-1221. E-mail: bknutson@ engr.uky.edu.
achieved by manipulating the system pressure. Temperature has recently been reported as a “phase switch” to create homogeneous reaction systems of miscible fluorinated + organic solvents from initially immiscible liquid mixtures.12 Similarly, dissolved CO2 has been demonstrated as a phase switch to render immiscible fluorinated + organic solvent systems a homogeneous solution for homogeneous catalysis.13 Expanding fluorinated solvent systems with moderate CO2 pressure has the potential to combine the tunability of GELs and the CO2-philic properties of fluorinated solvents.14 This investigation examines the ability to tune the solvent strength of CO2-expanded fluorinated solvents using pressure. In addition, the ability of cosolvents to alter the solvent characteristics of the CO2-expanded mixture was quantified using Kamlet-Taft parameters, which describe the following solvent properties: the dipolarity/polarizability (π*), hydrogen-bond-donor acidities (R), and hydrogen-bond-acceptor basicities (β). These solvent strength parameters form the basis of a linear free-energy relationship that has been used to correlate a broad range of thermodynamic and kinetic phenomena.15,16 Two cosolvents with widely varying hydrogenbonding abilities [2,2,2-trifluoroethanol (TFE), R ) 1.51; N-methyl-2-pyrrolidinone (NMP), β ) 0.78] were used to modify the solution properties of the CO2-expanded fluorinated solvents. The Hansen solubility parameters (HSPs)17 (δD, δP, and δH) have been used successfully as a screening tool to correlate solubility behavior in organic solvents and solvent blends.18-21 This study tests its applicability to identifying desirable phase behavior in organic and fluorous solvent mixtures. The HSPs of ethoxynonafluorobutane (ENB) and CO2-expanded ENB (at 8-20 bar) were determined at room temperature from miscibility studies of pairs of organic + fluorinated solvents. The resulting Hansen solubility spheres were used to
10.1021/ie030669p CCC: $25.00 © 2003 American Chemical Society Published on Web 11/14/2003
6512 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 1. Schematic (top view) of a UV-vis high-pressure system for solvatochromic measurements.
identify additional immiscible fluorous + organic solvent pairs that become miscible with the addition of CO2. Materials and Methods Materials. HFEs are commercially available from 3M: methoxynonafluorobutane (HFE-7100; C4F9OCH3; MNB; >99.0%) and ENB (HFE-7200; C4F9OC2H5; >99.0% purity). MNB is an approximately equimolar mixture of the isomers methoxynonafluoroisobutane and MNB, with essentially identical properties according to product literature. 1,1,1,2,3,4,4,5,5,5-Decafluoropentane, a highly fluorinated HFC (Vertrel XF; C5F10H2; DFP; ∼99% purity), is a product of Dupont Chemicals. 4-Nitroanisole (97%), 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (∼97%), 4-nitroaniline (98%), 2-nitroaniline (98%), N-methyl-2-nitroaniline (98%), NMP (99%), and TFE (99+%) were obtained from Aldrich (Milwaukee, WI). N,N-Diethyl-4-nitroaniline (∼98%) was purchased from Frinton Laboratories (South Vineland, NJ). The following organic solvents, used in the HSP study, were purchased from Aldrich (Milwaukee, WI): 1,1,1-trichloroethane (99.5%), propylene glycol methyl ether acetate (99%), cyclohexanone (99.8%), n-hexane (95+%), ethylene glycol butyl ether (99+%), ethanol (95%), propylene glycol phenyl ether (93+%), propylene carbonate (99.7%), dimethyl sulfoxide (99.9%), benzyl alcohol (99.8%), 1,2-dichlorobenzene (99%), N,Ndimethylformamide (99.9+%), styrene (99+%), propionitrile (98%), and benzaldehyde (99+%). Distilled water was used throughout the study. High-purity carbon dioxide (99.99%) was obtained from Scott Gross Co. (Lexington, KY). All purchased chemicals were used without further purification. High-Pressure Spectroscopic Cell. The solvatochromic measurements were performed in a customdesigned, high-pressure, variable-volume, spectroscopic cell (Thar Designs Inc., Pittsburgh, PA) (Figure 1). The cell had a stainless steel cylinder (total volume ∼25 mL) fitted with sapphire windows (one 2.54 cm × 0.953 cm and two 1.59 cm × 0.48 cm windows; path length ) 2.5 cm). Temperature control of the cell was achieved by a heating tape, a type K thermocouple ((0.1 °C), and an Omega CN9000A series proportional-integral-derivative controller. The contents of the spectroscopic cell was mixed using a Teflon-coated magnetic stir bar driven by an outside magnetic stirrer (Vari Mag 90407U). A hand-operated, high-pressure screw pump (High Pressure Equipment, Erie, PA) was used to adjust the volume of the pressurizing fluid, sealed from the sample fluid by a double-O-ring piston. The pressure generated
by the pump was measured using a Druck digital pressure gauge (model DPI 280). The maxima in the absorption spectra of the probes in the CO2-expanded fluorinated solutions were determined using a diode-array UV-vis spectrophotometer (Hewlett-Packard 8450). The compressed CO2 was introduced into the spectroscopic cell thermostated at 35 °C, using a high-pressure syringe pump (ISCO 500D; Lincoln, NE). A two-position switching valve (Valco Cheminert), equipped with a sample loop, was used to introduce fluorinated solutions of solvatochromic probes at pressure and to vent excess expanded liquid from the view cell. HSP Miscibility Studies. The HSPs of ENB and CO2-expanded ENB were determined from solvent miscibility studies conducted at room temperature. The organic and fluorous solvents were mixed in equal volumetric proportions (10 mL each) in a pressure view cell (Jerguson cell L1-T-20; total volume ∼50 mL). The miscibility behavior of the atmospheric solvent mixture was noted. The mixture was then expanded with moderate CO2 pressure (8-20 bar) from a syringe pump (ISCO 500D, Lincoln, NE). An excess CO2 phase was maintained during the expansion process, and the mixture was shaken vigorously to promote saturation of CO2 in the liquid mixture. The miscibility of the organic and fluorous mixtures was noted following system equilibration (>30 min). The process was duplicated for each pair of organofluorous solvent mixtures. Measurement and Analysis of Kamlet-Taft Parameters. The Kamlet-Taft parameters of the CO2expanded fluorinated solvents were measured using solvatochromic probes. Prior to CO2 expansion, approximately 10-15 mL of the fluorinated solvents was introduced into the cell. The concentration of the probe molecules in the fluorinated solvents were adjusted with small aliquots of a concentrated probe solution (dissolved in fluorinated solvents), such that the absorbance unit fell between 0.2 and 1.0. Next, compressed CO2 was introduced into the cell using the syringe pump. The volume of the stirred cell was adjusted throughout the expansion process to ensure the presence of a gas headspace22 above the saturated, expanded fluorinated solvent. Duplicate spectra were collected in the equilibrated system at each pressure interval, which were achieved by the successive additions of CO2. Identical procedures were employed for cosolvent-modified fluorinated solvents, which were introduced into the spectroscopic cell as 10 mol % cosolvent mixtures. The maximum absorption wavelengths (λmax) of the solvatochromic probes and probe pairs were used to determine the Kamlet-Taft parameters of the expanded fluorinated solvents. λmax was determined using the “9/ 10” method of Kamlet and Taft,23,24 from smoothed spectra in TableCurve.25 The π* (dipolarity/polarizability) values of the fluorinated solvents as a function of CO2 expansion were determined from two different, single-probe measurements (2-nitroaniline and N-methyl-2-nitroaniline). The π* values are calculated using the following equation:
νmax ) νo + sπ*
(1)
where νmax is the experimentally observed maximum absorption wavelength in 103 cm-1. νo and s are probespecific constants26 (Table 1).
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6513 Table 1. Solvatochromic Probe-Specific Constants Used in Calculating π*26
Table 2. Determination of the HSPs (MPa0.5) of ENB and CO2-Expanded ENB (8 bar CO2) at Room Temperature
probe
νo, ×103 cm-1
s
solvent
miscbilitya
δD
δP
δH
RED
2-nitroaniline N-methyl-2-nitroaniline
26.55 24.59
1.536 1.593
1,1,1-trichloroethane (C111) propylene glycol methyl ether acetate (PMA) cyclohexanone (CHO) n-hexane (HEX) ethylene glycol butyl ether (EB) ethanol (ETOH) NMP propylene glycol phenyl ether (PPH) propylene carbonate (PC) dimethyl sulfoxide (DMSO) benzyl alcohol (BZOH) water (H2O) ENB (room temperature, ambient pressure) 8 bar CO2/ENB
1 1
16.8 16.1
4.3 6.1
2 6.6
0.8 0.32
1 1* 1
17.8 14.9 16
6.3 0 5.1
5.1 0 12.3
0.71 1.08 0.45
1* 0* 0*
15.8 18 18.7
8.8 12.3 5.7
19.4 7.2 11.3
1.23 1 0.84
0 0 0* 0
20 18.4 18.4 19.5 15.0
18 16.4 6.3 17.8 5.3
4.1 10.2 13.7 17.6 8.5
1.79 1.4 0.92 1.91 9.3b
15.3
6.2
8.1
9.4b
The values of R’s (hydrogen-bond-donor acidities) and β’s (hydrogen-bond-acceptor basicities) of the cosolventmodified, CO2-expanded fluorinated solvents were determined from paired probe measurements.27,28 For the determination of R (hydrogen-bond-donor acidities), the paired probes are 4-nitroanisole (1) and 2,6-diphenyl4-(2,4,6-triphenyl-1-pyridinio)phenolate [Reichardt’s dye (2)], while the probe pairs used for β measurements are 4-nitroaniline (3) and N,N-diethyl-4-nitroaniline (4). For the paired probes of 1 and 2, R is defined as the enhanced solvatochromic shift for probe 2 relative to probe 1:27
R)
ν(2)max + 1.87ν(1)max - 74.58 6.24
(2)
The hydrogen-bond-acceptor basicity (HBA), β, is determined from the solvatochromic shift for 4-nitroaniline (3) relative to N,N-diethyl-4-nitroaniline (4):28
β ) 0.370ν(4)max - 0.357ν(3)max + 0.943
(3)
Estimation of HSPs. HSPs are an extension of the Hildebrand solubility parameter (δ). Additional molecular interactions such as permanent dipole-permanent dipole interactions and hydrogen-bonding interactions are taken into account by the HSPs.29 HSP was formulated from the assumption that cohesive energy (E) arises from dispersive (ED), permanent dipole-dipole interactions (EP), and hydrogen bonding (EH):17
E ) ED + EP + EH
(4)
Upon division of the energies by the molar volume, the square of the total (Hildebrand) solubility parameter, as the sum of the squares of the HSPs’ dispersive, polar, and hydrogen-bonding components, is obtained:
δ2 ) δD2 + δP2 + δH2
(5)
To describe the solvation properties of the CO2expanded fluorinated solvent using HSPs, three-dimensional solubility spheres were constructed.17 The axes of the solubility sphere are represented by each of the subcomponents of HSPs. The HSP of the fluorinated solvent in question is located at the center of the solubility sphere, while the radius of the sphere, Ro, demarcates the solvent’s miscibility boundary with tested organic solvents on the basis of their known HSPs. The radius of the solubility sphere is obtained by minimizing a data-fitting function.17,30 The input data to the data-fitting function are obtained from experimental observations of the solubility behavior of the fluorinated solvent in various organic solvents. A “good” solvent, which was miscible with ENB or CO2expanded ENB (in the presence of 8 bar of CO2), was assigned a value of “1” in its miscibility (Table 2). An immiscible solvent with ENB was assigned a value of “0”. The solubility sphere constructed for each solvent system (ENB and CO2-expanded ENB at 8 bar) was used to correlate the HSP and the radius of the
a Miscibility behavior assessed from visual observations of 50/ 50 (v/v) ENB/organic solvent mixtures: “1” indicates a good solvent that is miscible with ENB; “0” is assigned to an immiscible solvent with ENB; “*” is assigned to a solvent that is miscible with ENB but resides outside the correlated solubility sphere and vice versa. b R ) radius of a correlated solubility sphere (MPa0.5). o
solubility sphere (Ro) of that system. An asterisk was assigned to a solvent that was miscible with ENB but resided outside the correlated solubility sphere and vice versa (Table 2). A simple normalization technique, which ratios the distance of the potential test solvent (i) with respect to the fluorinated solvent (Ra) against the radius of its solubility sphere, Ro, was used to select further solvents that might become miscible with ENB in the presence of gaseous CO2. Ra is described by the following Hansen relation:
Ra ) [4(δda - δdi)2 + (δpa - δpi)2 + (δha - δhi)2]1/2 (6) The normalization technique yields the relative energy number (RED):
RED ) Ra/Ro
(7)
Using the large database of documented HSPs, the following solvents, which lie at the edge of the solubility sphere of ENB (RED ∼ 1), were identified (with the resulting REDs in parentheses): 1,2-dichlorobenzene (0.97), dimethylformamide (0.97), styrene (0.99), propionitrile (0.90), and benzaldehyde (1.03). Their miscibility behaviors with ENB in the absence and presence of CO2 addition were validated experimentally. Results and Discussion Fluorinated solvents undergo significant changes in their thermophysical properties (density and viscosity) as they are expanded by CO2 (P ) 1-72 bar, T ) 2535 °C).14 For example, over the range of 1-72 bar at 298 K, the concentration of dissolved CO2 in ENB increased to >95 mol %.14 The liquid also experienced a 49% reduction in density and an 80% reduction in viscosity at 57.9 bar and 25 °C in the presence of CO2 relative to atmospheric pressure.14 However, the corresponding change in solvent strength of expanded fluorinated solvents has not been determined. Kamlet-Taft parameters provide a solvent strength scale that facilitates comparisons between CO2-ex-
6514 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 2. Kamlet-Taft parameters (π* ) dipolarity/polarizability; β ) hydrogen-bond-acceptor basicities; R ) hydrogen-bonddonor acidities) for pure MNB, ENB, and DFP at room temperature compared with those of NMP and TFE. R values for MNB and ENB were compiled from work by Lagalante and co-workers.32
panded fluorinated solvents and other conventional process fluids. The measured Kamlet-Taft parameters of the pure fluorinated solvents at 35 °C are provided in Figure 2. The π* values measured for HFEs (π* ) 0.23 for MNB and 0.185 for ENB) are much higher than their perfluorinated anologues. For example, perfluorohexane has a π* of -0.40.31 The highly fluorinated hydrocarbon DFP has the highest π* values of the three fluorinated solvents (π* ) 0.55). The β values for the three fluorinated solvents tested (MNB, ENB, and DFP) are -0.095, -0.07, and -0.12, respectively. The measured π* and β values for MNB and ENB compare well with results obtained by Lagalante and co-workers for MNB (π* ) 0.188 and β ) -0.025) and ENB (π* ) 0.182 and β ) -0.022) at 35 °C.32 The R values for the pure fluorinated solvents were not obtained in this study because of the low solubility of the probe molecules in the fluorinated solvents. However, Lagalante and co-workers determined R for MNB and ENB using a custom-designed measuring cell that provided a longer path length for the measurement of dilute solute spectra.32 The relatively high R values (MNB ) 0.547 and ENB ) 0.554) suggest that the HFEs are strong hydrogen-bond donors, a solvent property that was not observed in previous studies of liquid and supercritical fluorinated ethanes.33-35 In addition, HFC134a (1,1,1,2-tetrafluoroethane) was shown to have negligible R and β values.35 These high R’s are consistent with MNB and ENB’s miscibilities with highly polar, short-chain aliphatic alcohols (Rmethanol ) 0.98, Rethanol ) 0.86, and R2-propanol ) 0.78). In addition, the HFEs are also miscible with a majority of ethers, glycol ethers, and ketones.36 A decrease in the solvent strength of CO2-expanded organic solvents is expected because of the reduction in density of the expanded liquid.22 A significant reduction in the density of CO2-expanded fluorinated solvents14 suggests a concomitant reduction in their solvent strength. However, the similar nature of CO2 and fluorinated hydrocarbon interactions, which are neither hydrophilic nor lipophilic, suggests that the changes in the solvent strength with CO2 expansion may be minimal. The solvent strength (π*) of CO2-expanded ENB was measured with 2-nitroaniline and N-methyl-2nitroaniline at 35 °C as a function of CO2 expansion pressure (Figure 3). For up to 30 bar of CO2 pressure,
Figure 3. π* values of ENB, as a function of the CO2 pressure measured by two solvatochromic probes: 2-nitroaniline (O) and N-methyl-2-nitroaniline ([) at T ) 35 °C. Also shown on the secondary axis is the volume expansion behavior of ENB as a function of pressure (2), measured at 25 °C. Error bars not shown are smaller than the symbols.
the measured π* values of the solvent system stayed relatively constant (-9% change relative to the pure solvent). A significant decrease in the solvent strength occurred at pressures higher than 50 bar. This decrease in the solvent strength corresponds roughly to the onset of exponential volume expansion of ENB with increasing pressure and a corresponding increase in CO2 dissolution (>80 mol % CO2 in ENB at 25 °C at 49.3 bar)14 (Figure 3). The decrease in π* of the expanded fluorinated solvents can be explained by the increasing contribution of dissolved CO2 to the fluid properties. The π* values of liquid CO2 (FCO2 ) 0.8 g/cm3 and T ) 26.528 °C) and supercritical CO2 (FCO2 ) 0.86 g/cm3 and T ) 36-42 °C) are -0.146 and -0.138, respectively.37 Both solvatochromic probes captured the reduction in π* with an increasing amount of dissolved CO2. An approach to enhance the low solvent strength of CO2-expanded ENB (as determined by its relatively low π* values) is the addition of cosolvents. Cosolvents are commonly employed in supercritical fluid technology to improve or modify the bulk solvent power of compressed or supercritical CO2.38 The effect of adding 10% TFE, a strong hydrogen-bond donor (π* ) 0.73 and R ) 1.51), to CO2-expanded ENB is shown in Figure 4. At ambient pressure, the addition of 10 mol % TFE to ENB increased the R from 0.55 (for pure ENB)32 to 1.05 at 35 °C (Figure 4). As the TFE/ENB cosolvent mixture was expanded by CO2, a similar R was measured in the upper and lower pressure range (approximately 1.05 at 1 and 59.6 bar), although a slight maximum of R was observed in the middle pressure range (Figure 4). The relatively constant R of the TFE/ENB/CO2 system suggests a strong hydrogen-bond or solute-solvent interaction occurring in the vicinity of the probe molecule, which was unaffected by the increased dissolution of CO2. Preferential solvation around probe molecules in solvent mixtures has been observed previously in binary solvent mixtures,39 alcohol/water mixtures,40 and near-critical and supercritical solvents.41 Thus, the constant R observed may be caused by the preferential solvation of the probe molecules by TFE. On the other hand, previous X-ray diffraction studies suggested
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6515
Figure 4. R values as a function of CO2 expansion at T ) 35 °C, for the solution of 10 mol % TFE in ENB.
Figure 5. π* (0) and β (4) values as a function of CO2 expansion at T ) 35 °C, for the cosolvent mixture of 10 mol % NMP in ENB.
cluster formation of TFE molecules in neat TFE42 and in organic solvent mixtures.43,44 Therefore, it is also possible that clusters of TFE aggregates are crowding the cybotactic region of the probe molecules, resulting in high R values even at high CO2 concentration. The hydrogen-bond-accepting basicities of CO2-expanded fluorinated solvents can also be manipulated with cosolvent addition. The addition of 10% NMP, a strong hydrogen-bond acceptor (π* ) 0.92 and β ) 0.78), to atmospheric ENB at 35 °C increases the π* and β values of the cosolvent mixture to 0.37 and 0.80, respectively. As the mixture is expanded by CO2 (Figure 5), both the π* and β values decrease slightly (π* ) 10% and β ) 8.1%) under moderate CO2 pressures (up to ∼30 bar; corresponding to ENB volume expansion of approximately 33%).14 At 56 bar of CO2 pressure, the β value decreased as much as ∼39% to a value of 0.49. The β value of a CO2-expanded NMP/ENB mixture at 56 bar is similar to a variety of organic process solvents, ranging from ethers (diethyl ether ) 0.47 and disopropyl ether ) 0.49), ketone (acetone ) 0.48), aldehyde (benzaldehyde ) 0.44), and ester (ethyl acetate ) 0.45).26 HSPs of CO2-Expanded ENB. The estimation of HSPs from ENB/organic and CO2-expanded ENB/ organic miscibility studies provides direct insight into the solubility behavior of these systems. HSPs have been used successfully to correlate the solubility behavior of commercial resins,45 polymers,30 pharmaceuticals,46 and refrigerant mixtures47 in organic solvents and solvent mixtures.
Figure 6. Two-dimensional Hansen solubility sphere of ENB at room temperature (dark dashed line): (black [) test solvents that are miscible with ENB; (gray [) immiscible solvents that reside within the solubility sphere; (]) immiscible solvents that either lie at the edge of the solubility sphere or fall outside of the sphere; (O) miscible solvents with ENB but are outside of the sphere. Included in the figure is the solubility sphere (light dashed line) for a ENB/8 bar CO2 system. The + marks the correlated HSP for ENB/8 bar CO2.
On the basis of the observations on solubility behavior with selected (12) organic test solvents, the HSPs for pure ENB and CO2-expaned ENB (8 bar) at room temperature were correlated using established procedures.17 The estimated HSPs for ENB at room temperature are δd ) 15.0 MPa0.5, δp ) 5.3 MPa0.5, and δh ) 8.5 MPa0.5, respectively. The radius of the solubility sphere is 9.3 MPa0.5. A visual representation of ENB’s solubility space in a two-dimensional plot is presented in Figure 6. Note that several immiscible test solvents (gray diamonds: NMP, propylene glycol phenyl ether, and benzyl alcohol) were inside the correlated solubility sphere, while the opposite is also true (e.g., the test solvents ethanol and hexane). Improved procedures have been suggested to overcome this limitation of the correlation.48 A new set of HSPs were estimated for ENB in the presence of 8 bar of CO2 at room temperature based on the miscibility of the expanded liquid with the test organic solvents (Table 2). HSPs for CO2-expanded ENB at 8 bar are δd ) 15.3 MPa0.5, δp ) 6.2 MPa0.5, and δh ) 8.1 MPa0.5, respectively. The radius of the solubility sphere is 9.4 MPa0.5. CO2 expansion had a negligible effect on the radius of the solubility sphere (∼1%). However, noticeable changes were observed in δp (+18%), as well as δh (-6%), showing the sensitivity of HSPs to the change in the bulk properties due to the presence of CO2. The usefulness of this change in bulk properties with CO2 addition is highlighted by the ability to render the immiscible NMP/ENB solvent system miscible with the addition of CO2 (Table 2). The potential use of moderate CO2 pressure as a “switch” to manipulate the number of liquid phases in partially immiscible organic and fluorinated solvent systems was further explored using the estimated HSPs. We hypothesized that organic solvents close to the limit of ENB’s solubility sphere (RED ∼ 1) would be most susceptible to moderate CO2 pressure as a phase switch, rendering immiscible organic/ENB systems miscible and vice versa. Using the criterion of RED ∼ 1, several other organic solvents were chosen to test this hypothesis (Table 3). In two of the instances (o-dichlorobenzene and benzaldehyde, as well as in the original screening solvent, N-methylpyrrolidinone), the addition of CO2 acted as a phase switch to render the immiscible ENB/organic solvent pairs a
6516 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 Table 3. Phase Behavior of Organic Solvents with Low-Pressure CO2/ENB at Room Temperature solvent
RED
phase observation(s)
NMP 1,2-dichlorobenzene dimethylformamide styrene propionitrile benzaldehyde
1.0 0.97 0.97 0.99 0.90 1.03
2 f 1 phase at 8 bar 2 f 1 phase at 8 bar miscible miscible miscible 2 f 1 phase at 17.7 bar
homogeneous mixture. This HSP-based approach could be used to select reaction media that could be made homogeneous in the presence of CO2. Expansion by CO2 provides the enhanced mass transfer associated with GELs and simplifies product/reactant recovery through depressurization of the system, which would return to a two-phase organic/fluorous solvent system. Conclusions The expansion of fluorinated solvents with CO2 at moderate pressure has the potential to tune the liquid solvent strength, manipulate the number of phases of the solvent mixture, and alter the mass-transfer characteristics of the liquid fluorinated solvents. The effects of adding CO2, and cosolvents, to selectively modify the specific solution properties of the fluorinated solvents were studied and quantified using Kamlet-Taft parameters. Pure solvent measurements and literature comparisons indicate that HFEs (MNB and ENB) are relatively inert (π* ∼ 0.2) but possess higher R values relative to perfluorinated hydrocarbons. Cosolvent addition is a viable approach to increasing the relatively weak hydrogen-bonding abilities of these solvents. In addition, this investigation demonstrates the value of the HSP approach to correlate the solubility behavior of organics with fluorinated solvents, screen organic/ fluorous systems for phase switch behavior in the presence of moderate CO2 pressure, and identify further organic/fluorous solvent pairs that undergo phase change with the addition of CO2. Acknowledgment The authors thank Tremitchell Wright, Kristina Underly, Brian May, and Joel Luckman from Whirlpool Corp. for their assistance and Kevin Jared Tatum for his help in data collection. Literature Cited (1) O’Neill, M. L.; Cao, Q.; Fang, M.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Solubility of Homopolymers and Copolymers in Carbon Dioxide. Ind. Eng. Chem. Res. 1998, 37, 3067. (2) Smart, B. E. Characteristics of C-F Systems. In Organofluorine Chemistry: Principles and Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum Press: New York, 1994; p 57. (3) Sekiya, A.; Misaki, S. The Potential of Hydrofluoroethers to Replace CFCs, HCFCs and PFCs. J. Fluorine Chem. 2000, 101, 215. (4) Takada, N.; Tamai, R.; Yamamoto, H.; Sekiya, A.; Tsukida, N.; Takeyasu, H. Fundamental Study of Fluorinated Ethers as New Generation Blowing Agents. J. Cell. Plast. 1999, 35, 389. (5) Averill, A. F.; Ingram, J. M.; Nolan, P. F. Replacing TCA and CFC-113 with HFE and HFC Based Azeotropes and N-Propyl Bromide Based Solvents for Wipe Cleaning Metal Componentss Source Evaporation Rates and Models. Trans. Inst. Met. Finish. 1999, 77, 16. (6) Christensen, L. K.; Sehested, J.; Nielsen, O. J.; Bilde, M.; Wallington, T. J.; Guschin, A.; Molina, L. T.; Molina, M. J.
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Received for review August 15, 2003 Revised manuscript received October 7, 2003 Accepted October 7, 2003 IE030669P
