J. Phys. Chem. B 1997, 101, 6707-6714
6707
Water Core within Perfluoropolyether-Based Microemulsions Formed in Supercritical Carbon Dioxide Mark P. Heitz,† Claude Carlier,‡ Janet deGrazia,‡ Kristi L. Harrison,§ Keith P. Johnston,*,§ Theodore W. Randolph,*,‡ and Frank V. Bright*,† Department of Chemistry, Natural Sciences Complex, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000, Department of Chemical Engineering, UniVersity of Colorado at Boulder, Boulder, Colorado 80309-0424, and Department of Chemical Engineering, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: July 23, 1996; In Final Form: June 11, 1997X
We report new experimental data on the ability of a perfluoropolyether-based surfactant (PFPE) to form stable reverse micelles in supercritical CO2. Previous work from our groups showed that PFPE reverse micelles formed in CO2 can host a wide variety of hydrophiles and even provide an environment capable of solubilizing large proteins [Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626]. In the current work we report cloud point data for PFPE in CO2, X-band EPR studies, and time-resolved anisotropy measurements. The cloud point data show that a one-phase water-in-CO2 microemulsion can be formed with a nominal molar water-to-surfactant ratio (R) of 20.7 at 45 °C and 158.1 bar. EPR experiments on PFPE (with 4-hydroxy-TEMPO) and Mn(PFPE)2 show that PFPE aggregates in CO2 at pressures below which a water pool can be formed. Stable Mn(PFPE)2 micelles can also be formed in supercritical CO2, and the internal water pool within these micelles is able to ionize manganese, demonstrating that the water within this pool differs significantly from water within the CO2 bulk phase. EPR results also suggest that these micelles exist in a nonspherical form. The rotational reorientation kinetics of two model fluorescent probes, rhodamine 6G and lissamine rhodamine B sulfonyl hydrazine, are described well by a biexponential decay law. The faster rotational reorientation time (φfast) is approximately 100 ps and remains constant regardless of CO2 continuous phase density or R. We interpret the fluorophore rotational dynamics using three established models: a wobbling-in-a-cone model in which the fluorophore precesses about its emission transition dipole, a lateral diffusion model wherein the probe diffuses along the reverse micelle headgroup/water core interface boundary, and an anisotropic rotor model where the micelle shape itself is nonspherical.
Introduction Supercritical fluids are an attractive alternative to conventional solvents for a variety of applications.1-10 Supercritical CO2, in particular, is generally viewed as an environmentally acceptable, benign alternative solvent because it is inexpensive, essentially nontoxic, exhibits relatively low critical conditions, is nonflammable, and can easily be recaptured and recycled after use. Unfortunately, the use of neat supercritical CO2 is limited because it is a poor solvent for hydrophiles. This problem can be addressed in part by the addition of small quantities of polar cosolvents (modifiers) in an attempt to improve hydrophile loadings in supercritical CO2.1-9 However, even in the presence of relatively strong cosolvents, the solubility of certain hydrophiles (e.g., proteins) is very limited in supercritical CO2. The formation of reverse micelles using supercritical fluid continuous phases is a well-studied process and has been extensively reported in the literature.11-16 However, over the past several years, most of the attention has focused on reverse micelles formed in near- and supercritical alkanes.11-16 In fact, most of the effort has centered on sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT, AOT) reverse micelles formed in alkanes or Xe.11-16 For example, Smith and co-workers used dynamic light scattering11a,f,g to show that the apparent micellar * To whom all correspondence should be directed. † State University of New York at Buffalo. ‡ University of Colorado at Boulder. § University of Texas at Austin. X Abstract published in AdVance ACS Abstracts, July 15, 1997.
S1089-5647(96)02224-9 CCC: $14.00
diameter depends on the continuous phase (i.e., fluid) density and decreases as density increases. This phenomena was attributed to micelle/micelle interactions at the lower continuous phase densities.11g Other reports12,13 have addressed the issue of how water loading (i.e., the molar ratio of water-to-surfactant, R) and/or continuous phase density affects model solutes or probe molecules sequestered within the micelle water core. From an environmental perspective, a supercritical hydrocarbon continuous phase is environmentally “unfriendly” and represents a less than ideal solvent medium. Unfortunately, the search for surfactants capable of forming thermodynamically stable reverse micelles in more environmentally benign supercritical fluids has not been rewarding.16 Thus, finding, designing, and characterizing surfactant systems that can form reverse micelles in supercritical CO2 remains at the forefront of surfactant research. In 1988, Randolph et al. demonstrated that cholesterol-based spin probes form molecular aggregates in supercritical CO2.17 However, water-in-CO2 emulsions did not appear to be formed. Several recent reports have aimed to identify and/or design CO2soluble surfactants capable of forming water-in-CO2 microemulsions.16,18-21 Unfortunately, early attempts to disperse water into a supercritical CO2 phase were unsuccessful despite the testing of over 150 surfactants.16,20 In 1994 Harrison et al.20 showed that a bifurcated surfactant with fluorocarbon/hydrocarbon hybrid tails (F7H7) can form reverse micelles in CO2 and reach water loadings of up to 32. Fulton et al.11h showed that a perfluorinated acrylate-g-poly(ethylene oxide) copolymer © 1997 American Chemical Society
6708 J. Phys. Chem. B, Vol. 101, No. 34, 1997
Figure 1. Chemical structure of the PFPE monomer.
can form very large micelles in CO2 with hydrated poly(ethylene oxide) cores. Most recently, our groups used a new fluoroether surfactant, ammonium carboxylic perfluoropolyether (PFPE, Figure 1), to form stable reverse micelles in supercritical CO2.22 In our original offering,22 a variety of spectroscopic techniques (e.g., UV-vis, FT-IR, static fluorescence, and EPR) were used to characterize PFPE reverse micelles/microemulsions. These techniques all confirmed the presence of water pools that “displayed characteristics similar to those of bulk water”.22 On comparing previous work on AOT reverse micelles formed in alkanes11-16 with the recently discovered CO2-soluble PFPE reverse micelles,22 one might anticipate that the water core regions of both micelles are similar in character. Thus, the central purpose of this paper is to develop a better understanding of the PFPE/CO2/H2O system and to provide a more detailed view of the interactions between a sequestered solute/probe and the water core region within PFPE reverse micelles formed in supercritical CO2. Specifically, we aim to address three issues: (1) the phase equilibria of these new microemulsions formed in CO2, (2) the form of the water pool within these PFPE-based microemulsions, and (3) the dynamics and mobility of model solutes/probes sequestered within these microemulsions all as a function of water loading and the continuous phase density. Toward these ends, we report new cloud point data for PFPE in CO2 over a wide range of water loadings, we present X-band EPR data for PFPE micelles with the probe 4-hydroxy-TEMPO and Mn(PFPE)2 in supercritical CO2, and we report new timeresolved fluorescence anisotropy measurements13,23 of two cationic probes, rhodamine 6G (R6G) and lissamine rhodamine B sulfonyl hydrazide (LRSH), located exclusively within the water core region of PFPE reverse micelles formed in supercritical CO2. Experimental Section Materials. PFPE in its carboxylate form was generously provided by J. Howell of Du Pont, Clearwater, NJ. PFPE in the ammonium carboxylate form was provided by A. Chittofrati of the Ausiemont Company, Milan, Italy. CO2 was purchased from Matheson (99%) or Air Products (instrument grade) and further purified by passage through an oxygen trap (Matheson or Labclear) before entering the high-pressure pumps. Air Products CO2 was dried over P2O5 (Aldrich) prior to entering the pumping system. Distilled and deionized water was used throughout the experiments. MnCl2 was purchased from Aldrich and used as received. R6G (99% dye content) was purchased from Aldrich and LRSH was from Molecular Probes; each was used as received. 4-Hydroxy-2,2,6,6-tetramethylpiperidino-1oxy (4-hydroxy-TEMPO) was from Sigma and used as received. Sample Preparation. Cloud Point Measurements. PFPE was stored in a desiccator prior to use. The required amount of PFPE needed to produce a 1.4 wt % solution was added directly to a variable volume view cell. Carbon dioxide was metered into the view cell using a high-pressure syringe pump at a constant pressure. Deionized water was added to the cell in increments with a 25.9 µL sample loop using a six-port rotary valve and a motor-driven pump.24 A temperature-controlled water bath was used to maintain a constant temperature for each experiment. The pressure and temperature were measured to (0.2 bar and (0.2 °C, respectively.
Heitz et al. EPR-Based Experiments. To prepare the Mn(PFPE)2 surfactant, typically 1.2 g (1 mmol) of the free acid carboxylate form of PFPE was mixed with 0.063 g (0.5 mmol) of manganese chloride and heated to 150 °C under vacuum for 24 h with constant stirring to remove HCl. The mixture was centrifuged to remove any excess manganese chloride, and the top liquid was decanted and placed in a vial within a desiccator until use. Mn(PFPE)2 Samples. Mn(PFPE)2, to make a 1.4 wt % solution, was weighed out in an injection loop on an analytical balance. The loop was then attached to a high-pressure system, and the sample was injected. The system was pressurized to 10-15 bar with CO2 and then depressurized to remove any air. This step was repeated twice. The system was then pressurized to the starting pressure (from 45 to 60 bar), and the system was allowed to mix for 6-24 h. To ensure adequate mixing, a highpressure recirculating pump was used to circulate the mixture through the system. The contents of the system were stirred throughout this mixing process, as well as after every pressure change, using a magnetic stir bar. The flow rate of the pump was approximately 100 mL/min, and care was taken to avoid two-phase flows. Two-phase flows consisting of a CO2-rich phase and a water-rich phase cause distinctive fluctuations in microwave absorbance in EPR; under our conditions no such fluctuations were observed, and EPR tuning parameters were always those characteristic of a CO2-rich phase. The system was kept isothermal at 35 °C for all EPR experiments. Two sets of EPR experiments were done either at constant manganese concentration or at constant manganese mole fraction. In the first, the system was pressurized to approximately 60 bar (0.159 g/mL) with CO2 and was allowed to stabilize, and an EPR spectrum was recorded. The pressure was then increased by adding fresh CO2 from the syringe pump (keeping the manganese concentration constant), and the process was repeated. In the second experiment, the system was pressurized to about 120 bar (0.768 g/mL). After the system was allowed to stabilize, an EPR spectrum was recorded. A new pressure was then set by opening a micrometer valve to allow both manganese and CO2 to be vented, thus maintaining constant mole fractions. Both methods gave equivalent results. PFPE Ammonium Carboxylate Samples. A sample of 10-5 M 4-hydroxy-TEMPO was added to a CO2 solution containing 1.4 wt % PFPE ammonium carboxylate at 45 bar (0.102 g/mL) and 35 °C. Water was added to give an uncorrected water loading (R) of 14. Pressure was adjusted by adding additional CO2 via a screw piston. Fluorescence-Based Experiments. PFPE was stored in a desiccator and the following procedure used to prepare a sample. The high-pressure optical cell is charged with a small aliquot of the fluorophore stock solution, and the liquid solvent (ethanol) is removed by flowing a gentle stream of N2 into the cell. The fluorophore analytical concentration is 1 µM for all experiments. The required amount of PFPE needed to produce a 1.4 wt % solution was weighed on an analytical balance and added directly into the high-pressure optical cell. The appropriate amount of water, depending on the desired R value, is next loaded into the cell with a micropipet, and the cell is sealed and heated to the desired experimental temperature (35 ( 0.1 °C). CO2 from a microflow syringe pump (Isco, Model 260D) is pumped into the high-pressure optical cell and the pressure maintained to within (0.05 bar. The contents of the cell are stirred throughout an experiment. The sample is allowed to equilibrate for 20-30 min between pressure changes. In these fluorescence experiments, we investigated uncorrected water loadings of R ) 5 and 9. The general details of the highpressure apparatus and high-pressure optical cells have been
Water Core in Microemulsions described previously.25 The CO2 continuous phase density is computed using SFSolver (Isco Inc.) or an accurate equation of state.24 Relatively low water loadings (R ) 5 and 9) were studied in the fluorescence experiments in order to follow micelle dynamics and micelle/fluid, probe/water, and probe/micelle interactions simultaneously. Instrumentation and Methodology. Phase behavior experiments were carried out in a stainless steel variable-volume view cell with a sapphire window mounted on the front end as described previously.26 The cell had a maximum volume of 28 mL with the piston inserted. The movable piston was used to adjust the volume (and consequently the pressure) of the sample portion of the cell using CO2 as the pressurizing fluid. System cloud points were determined visually. At a given temperature, the pressure was increased to at least 275 bar where a single-phase solution was obtained and the solution was stirred using a Teflon stir bar for 15 min. Stirring was stopped, and the pressure was slowly decreased until the system became cloudy; this pressure was defined as the cloud point pressure. The pressure was increased slightly to obtain a clear solution, and the pressure was then decreased until the solution became cloudy again. This procedure was repeated again to obtain an average value for the reported cloud point pressure at each temperature. The uncertainty in the reported cloud points was (3 bar. X-band EPR spectra were recorded using a Bruker ESP300 EPR spectrometer with a modulation amplitude of 1 G, a modulation frequency of 100 kHz, a time constant of 0.16 ms, and microwave power of 10-20 mW. The center field was set at 3475-3480 G and the sweep width was 60-800 G depending on the particular experiment. Each recorded spectrum was the average of 32 scans. The PEEK cells and entire high-pressure system have been described previously.27 All dynamic fluorescence measurements were carried out using a multiharmonic frequency-domain fluorometer (SLM Aminco Model 48000 MHF).25 Data were acquired using an argon ion laser (Coherent; Innova Model 400-10) operating at 351.1 or 514.5 nm as the excitation source. The appropriate interference filter was placed in the excitation beam path to eliminate extraneous plasma discharge from reaching the detector. Fluorescence was monitored through a 550 nm longpass filter. Contributions from the blank were always less than 0.5% of the total fluorescence. All samples were checked against the appropriate blanks for Raleigh and/or Raman scatter and none was detectable. The high-pressure optical cells were tested with several dilute fluorophore solutions in liquids with known rotational reorientation times and excited-state lifetimes, and the recovered values were within experimental uncertainties of the literature values. We also investigated the reverse micelle system studied previously by Eastoe et al.14 in supercritical fluids and recovered fluorescence intensity and anisotropy decay kinetics within 6% of the values reported previously. For all our phase and modulation measurements, magic angle polarization was used to eliminate polarization biases.28 Dilute R6G in water was used as the reference fluorescence lifetime standard; its lifetime was assigned a value of 3.85 ns.13c The Pockels cell modulator was operated at 5 MHz, and data were acquired from 5 to 180 MHz (36 total frequencies). The theory of frequency-domain fluorescence spectroscopy has been described elsewhere.29-31 It is often difficult to resolve fluorophore rotational motions, particularly when the probe is associated with a complex matrix and/or when the probe motion itself is anisotropic.32,33 Two methods have been developed to help deconvolve and quantify
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6709
Figure 2. Cloud point data for PFPE in CO2 at several R values: (b) 0; (9) 5.2; ([) 7.0; (2) 10.3; (1) 15.5; (O) 20.7.
TABLE 1: Cloud Point Pressure for a Given Temperature for 1.4 wt % PFPE Water-in-CO2 Microemulsions T (°C)
P (bar)
CO2 density (g/mL)
R
Rcorr
35 35 45 55 65 35 45 55 65 45 55 65
89.6 98.5 125.3 145.1 163.9 117.0 140.7 160.7 177.8 158.1 176.6 194.6
0.66 0.71 0.68 0.64 0.60 0.76 0.72 0.68 0.64 0.76 0.72 0.68
5.2 10.3 10.3 10.3 10.3 15.5 15.5 15.5 15.5 20.7 20.7 20.7
0.8 5.8 5.3 2.6 0.2 10.8 9.3 7.3 4.9 14.1 12.0 9.6
complex rotational motions.34-40 We use both methods in this work and recover equivalent results. All multifrequency phase and modulation data were fit to various test models using a commercially available software package (Globals Unlimited).34 Unless otherwise stated, all mention of statistical significance are asserted at the 95% confidence level. Results and Discussion Cloud Point Experiments. The cloud points for a mixture of 1.4 wt % surfactant in ∼10 g CO2 are shown in Figure 2 for various amounts of added water. The CO2 density was obtained from an accurate equation of state for pure CO2.24 For each isopleth, the system is one phase at densities (or pressures) above the isopleth. The water-to-surfactant ratios (R) given in Figure 2 are based on the total water present in the system. These values represent an upper bound on the amount of water that can be solubilized in the microemulsion at the cloud point pressure. Water is much more soluble in CO2 than in ethane or propane.41 Therefore, water partitions between the micelle pseudophase and the bulk CO2. The corrected (or lower bound) amount of water, Rcorr, solubilized in the micelles can be obtained by subtracting the amount of water soluble in the bulk CO2 from the total amount of water. Table 1 lists R and Rcorr for various temperatures and densities from the data in Figure 2. The proximity of the cloud point curves of the low water loadings to the cloud point curve for the pure surfactant can be explained to some extent by careful inspection of the corrected R values. At low water loadings a significant amount of the water may be solubilized by the bulk CO2 and the reverse micelles would be expected to have behavior much closer to that of the pure surfactant. The values of R are much larger than for AOT reverse micelles in
6710 J. Phys. Chem. B, Vol. 101, No. 34, 1997 ethane where R is less than 5 at 37 °C and 260 bar (0.424 g/mL) for 0.1 M AOT.11,12a For 0.07 M AOT in liquid propane, R reaches more than 50 at 25 °C and 250 bar (0.340 g/mL). The phase behavior of the microemulsion is controlled by the natural curvature of the interface governed by intramicellar interactions and micelle/micelle interactions. Theoretical models have been developed to understand and predict the effect of these two factors on the phase behavior of microemulsions formed in compressed fluids.11g,42 For systems with small waterto-oil (or CO2) ratios, attractive micelle/micelle interactions tend to control the droplet size and the R. At pressures above the phase transition, the surfactant tails are well solvated by the CO2. As the solvent pressure is decreased, there is an enthalpic and an entropic contribution to cause phase separation. As the density is decreased, the unfavorable contribution arises because the cohesive energy density of the solvent becomes significantly smaller than that of the surfactant tails. The entropic contribution to phase separation increases the interactions between the micelles because solvent is expelled from the high-density interfacial region as the bulk fluid density is decreased. Therefore, the solvent/tail interactions must be sufficiently strong in order to stabilize reverse micelles in compressible fluids. The structure of the PFPE surfactant is conducive to the formation of water-in-CO2 microemulsions because of several factors: (1) the surfactant contains a very “CO2-philic”43 tail group, (2) the headgroup of the surfactant has favorable interactions with water, and (3) the pendent fluoromethyl groups increase the volume on the CO2 side of the interface and favor curvature around the water. The most CO2 soluble polymer materials include silicone, fluoropolymers, and fluoroethers.43-46 Thus, the PFPE surfactant fluoroether tail is very compatible with the CO2 phase. The high solubility of siloxane, fluorocarbon, and fluoroether materials in CO2 can be related to the weak van der Waals forces in CO2. The ammonium carboxylate ionic headgroup of the surfactant interacts strongly with water and very weakly with CO2 because CO2 has a very low dielectric constant. Thus, the driving force for the ionic group to leave the bulk CO2 phase is enormous. EPR-Based Experiments. EPR Studies of PFPE Ammonium Carboxylate Surfactant. The spin/probe method has been frequently used to characterize micellar solutions and has been recently used to characterize normal micelles of PFPE ammonium carboxylates.47,48 Briefly, the technique involves addition of an EPR-active probe (such as a nitroxide free radical) to the micellar solution. The probe molecule is chosen so that it partitions into the micellar phase, where it serves to report information regarding molecular motion, polarity, etc. In this case, we chose 4-hydroxy-TEMPO, a nitroxide that is soluble in water but only sparingly soluble in supercritical CO2. Thus, we expect that 4-hydroxy-TEMPO should partition into the hydrophilic headgroup area of PFPE micelles formed in CO2. 4-Hydroxy-TEMPO interacts strongly with PFPE ammonium carboxylate, as evidenced by the large increase in probe solubility (ca. 3 orders of magnitude) in CO2 upon addition of the surfactant. In the presence of PFPE ammonium carboxylate, EPR spectra of 4-hydroxy-TEMPO in dry or water-saturated CO2 at densities from 0.18 to 0.57 g/mL show a signal characteristic of highly anisotropic rotational motion (Figure 3). For rotational anisotropies of a small probe molecule such as 4-hydroxy-TEMPO in a low-viscosity fluid such as supercritical CO2 to be detectable on the EPR time scale of about 10-10 s, the probe must be present as part of an organized molecular assembly. EPR spectra of 4-hydroxy-TEMPO in CO2 without surfactant show only isotropic motion. This suggests that the PFPE ammonium carboxylate surfactant is solubilized as an “empty” micelle even at CO2 densities as low as 0.18
Heitz et al.
Figure 3. EPR spectra of 4-hydroxy-TEMPO nitroxides (10-4 M) in CO2 at 35 °C. Left-hand side is with PFPE surfactant and water at R ) 14. Water-to-4-hydroxy-TEMPO molar ratio is approximately 1700: 1. Right-hand side is with water-saturated CO2 and no added surfactant. Pressures are (a) 50 bar (0.118 g/mL), (b) 70 bar (0.220 g/mL), and (c) 80 bar (0.426 g/mL). Center field is 3480 G, and the sweep width is 60 G.
g/mL. At CO2 densities above 0.57 g/mL, the EPR signal changes rapidly with time, losing intensity and reflecting decreasing rotational anisotropy. The rapid change in the signal occurs at CO2 densities lower than that at the visually observed cloud point. However, the rapid change does occur at densities approaching that of the cloud point of 1.4 wt % surfactant in CO2 without added water. The loss of signal is due to the rapid reduction of the nitroxide once water is solubilized within the micelle.22 The apparent decrease in anisotropy of the probe’s motion is due to consumption of the probe in the micelle interior where motion is most anisotropic, leaving the probe only in the bulk CO2 phase where motion is much more rapid. EPR Studies of Mn(PFPE)2 Surfactant. A two-tailed manganese perfluoropolyether (Mn(PFPE)2) surfactant with an average molecular weight of 2400 Da was studied in near-critical and critical CO2 solutions. Manganese produces an EPR spectrum with six peaks, indicative of its nuclear spin quantum number of 5/2. Manganese has the additional advantage of being ionizable in water, and previous work has shown that ionized manganese has a characteristically different EPR spectrum from manganese that is not ionized.49 We performed a series of EPR measurements in dry and water-saturated solutions. Manganese surfactant alone at room temperature and atmospheric pressure produces a spectrum that is indicative of unhydrated manganese49 (Figure 4a). When the manganese is hydrated, the signal exhibits six peaks as in Figure 4b, which is a spectrum typical of low concentrations (less than 0.1 mM) of MnCl2 in liquid water. We performed a series of EPR measurements on the manganese surfactant at water contents of R ) 0 and 40. Pressure was adjusted from 44 to 160 bar (0.098-0.828 g/mL). Experiments carried out in a quartz EPR tube indicated that the manganese surfactant did not go into solution without water, even at 160 bar (0.828 g/mL). However, once water was added to the system, the surfactant went into solution at subcritical pressures. The same experiments were performed in a continuous loop system, and the results were the same. With no water
Water Core in Microemulsions
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6711
Figure 4. EPR spectra of manganese species: (a) manganese surfactant without water; (b) MnCl2 in liquid water. Temperature was controlled at 35 °C.
in the system (i.e., R ) 0), no manganese could be detected in solution. In water-saturated solutions, the EPR spectrum is composed of six peaks, indicating that the manganese surfactant is in solution and the manganese is hydrated. Figure 5 presents representative spectra of the manganese surfactant in CO2 at pressures of 65, 74, and 95 bar (0.185, 0.260, and 0.693 g/mL) with R ) 40. These EPR spectra are further evidence that the PFPE surfactant forms stable reverse micelles in CO2 and that the water pool contained within the interior of the micelle differs from water in the CO2 phase. The spectra also indicate that the manganese is ionized, which requires water with “bulk” characteristics. Such an interpretation is consistent with that of Kitahara et al.,49 who showed similar effects for AOT/alkane systems. The resulting EPR spectra show no indication of the line broadening or shifting of the outer lines toward the center of the spectrum that accompanies spin exchange.50 Because spin exchange typically occurs when the radius of interaction is less than 5-15 Å, we calculated whether a micelle of R ) 40 could exist in a spherical form without exhibiting significant spin exchange. Spherical micelles with aggregation numbers between 40 and 100 would have water core diameters between 44 and 60 Å, and Mn2+ ions (presumably located near the surface of the water core) would be separated by less than 10 Å, a distance where spin exchange should be appreciable. A spherical geometry would minimize the surface available for surfactant molecules, thus maximizing the likelihood of spin exchange. This suggests that a more likely shape for these particular micelles is lamellar or ellipsoidal. Fluorescence-Based Data. Excited-State Intensity Decay Kinetics. We carried out a series of excited-state intensity decay
Figure 5. EPR spectra of Mn(PFPE)2 and water in CO2 at (a) 65 bar (0.185 g/mL), (b) 74 bar (0.260 g/mL), and (c) 95 bar (0.692 g/mL). Temperature was controlled at 35 °C.
measurements on R6G and LRSH as a function of water loading and CO2 continuous phase density. In all cases the excitedstate intensity decay kinetics were best described by a single exponential rate law. For R6G we observed a statistically significant (95% confidence interval) decrease in the fluorescence lifetime as the continuous phase density increases (4.85 ( 0.05 ns at 0.74 g/mL vs 4.52 ( 0.05 ns at 0.90 g/mL). The excited-state lifetime of LRSH in PFPE reverse micelles is independent of the continuous phase density and water loading (3.08 ( 0.05 ns). Rotational Reorientation Kinetics. Recent work by Heitz and Bright13c on the AOT/propane system has shown that timeresolved fluorescence anisotropy decay techniques can be used to investigate micelle/micelle dynamics. Given the envisaged similarities in the AOT and PFPE water pool environments, one might hypothesize that the probe rotational dynamics, within the water pools, may exhibit similar behavior. Multifrequency differential phase and polarized modulation data (not shown) were fit to several test models using a series of exponential functions29-31 and various established “linking”34 schemes. Figure 6 summarizes the effects of water loading and CO2 density on the rotational reorientation times for R6G and LRSH in the PFPE reverse micelle system. The solid lines connecting the symbols are for ease of visualization only and do not represent any form of model. Inspection of these results
6712 J. Phys. Chem. B, Vol. 101, No. 34, 1997
Heitz et al. TABLE 2: Predicted PFPE Micelle Rotational Reorientation Time (Omicelle) in CO2 at 35 °Ca φmicelle (ns)c
pressure (bars)/ density (g/mL)
η (µP)
R)5
R)9
107/0.736 120/0.768 155/0.822 208/0.873
615 655 720 825
2.8 3.0 3.3 3.8
4.7 5.0 5.6 6.3
b
a See text for details on calculations. b Continuous phase viscosity calculated from ref 52. c Predicted rotational reorientation time for an individual PFPE reverse micelle.
Figure 6. Effects of CO2 density and water loading on the rotational reorientation times for R6G and LRSH in PFPE reverse micelles (T ) 35 °C): φfast, for R6G R ) 5 (b) and 9 (O), for LRSH R ) 5 (9) and 9 (0); φslow, R ) 5 (2) and 9 (4). Error bars are from replicate measurements. Arrows denote the dynamics for LRSH and R6G alone in neat liquid water at 21 °C.
show several key points. The fluorescence anisotropy decay kinetics are always best described by two rotational reorientation times. The faster rotational reorientation times (φfast) for R6G and LRSH are similar to each another (ca., 100 ps) and independent of water loading and CO2 density. The slower rotational reorientation time (φslow) is probe independent at either water loading (R ) 5 (2) or R ) 9 (4)). This result suggests that both probes effectively “sense” the same slow motion at a given R. As we increase R, at any given continuous phase density, φslow increases. Finally, φslow decreases on increasing the continuous phase density. Interpretation of the Slower Rotational Reorientation Time. Quitevis et al.40 have developed a lateral diffusion model to describe the anisotropy decay of probe molecules sequestered within spherical reverse micelles. In this approach, the probe is viewed as diffusing along the reverse micelle headgroup/ water core interfacial region. To apply such a model, one requires two quantities: (1) information on the slower of the two rotational reorientation times for the probe “within” the reverse micelle (vide infra) and (2) an estimate of the true reorientation time for the entire PFPE reverse micelle. Unfortunately, there are no experimental data on the dynamics of the PFPE reverse micelles or their dimensions in supercritical CO2. To estimate the intrinsic rotational reorientation time for the PFPE reverse micelles in supercritical CO2, we begin with the Debye-Stokes-Einstein (DSE) equation51 and assume for the moment that PFPE forms spherical reverse micelles:
φ)
ηV kT
3ν R+δ as
(estimated δ ) 25 Å). We next assume that the headgroup area, as, for PFPE is similar to that of AOT (-COO- vs -SO3-) and estimate the PFPE as from the corresponding AOT expression:55
as ) 0.596 - 0.468 e-0.401xR
(3)
where the constants arise from numerical integration of the Poisson-Boltzmann equation.56 By use of eqs 2 and 3, the AOT micelle radius can be predicted to within 0.5% over a range R ) 0-50.55 Our estimate is clearly not as accurate, but we suspect it is not off by more than 20-25%. Further, because we have assumed the PFPE molecule within the micelle to be completely linear (unlikely), our estimates must be considered to be an upper limit on the actual size of an individual PFPE reverse micelle. Table 2 collects the results of these calculations on PFPE reverse micelles. On the basis of these estimates, we can proceed to estimate the probe lateral diffusion by using eqs 4 and 5:40
(1)
In this expression, η is the pressure-dependent viscosity of the CO2 continuous phase (calculated using the Chuang method),52 V is the volume of an individual reverse micelle, k is the Boltzmann constant, and T is the absolute temperature. We proceed to estimate the spherical reverse micelle volume (V ) 4/ πr3) by calculating first the hydrodynamic micelle radius 3 (rh):53
rh )
Figure 7. Effects of CO2 density and water loading on the apparent lateral diffusion coefficient (DLD) for R6G and LRSH in PFPE reverse micelles (T ) 35 °C): (b) R ) 5; (O) R ) 9.
(2)
where ν is the molecular volume of water (3.0 × 10-29 m3),53 as is the area occupied by a single surfactant headgroup, and δ is the surfactant tail length. To calculate δ, we assume that PFPE possesses a linear surfactant tail and use the average C-C and C-O bond lengths54 by summing over the number of bonds
1 1 1 ) φLD φslow φmicelle
(4)
where φLD is the rotational reorientation time associated with the probe lateral diffusion within the reverse micelle, φslow is the slower of the measured rotational reorientation times, and φmicelle is the rotational reorientation time predicted for the entire reverse micelle (eqs 1-3, Table 2). The probe lateral diffusion coefficient along the headgroup/water pool interface, DLD, is then written:40
DLD )
rh2 4φLD
(5)
where rh is the radius of the entire micelle (eq 2). The results of these calculations are presented in Figure 7 for R6G and LRSH in PFPE at R ) 5 (b) and R ) 9 (O). As
Water Core in Microemulsions we increase the R (b f O) at a given CO2 density, the apparent lateral diffusion coefficient increases by 20-40%. This result is consistent with the probes becoming better hydrated and more mobile within the PFPE micelle water core. Interestingly, on increasing the CO2 continuous phase density at a particular R, we see an increase in the lateral diffusion coefficient. Three possible explanations are consistent with this behavior. First, at lower CO2 densities, the water pool may not be fully formed and the water core/headgroup region is such that lateral diffusion is hindered partially by the headgroups not being evenly aligned about the water core. As a result, probe molecules would not, on average, be diffusing along a uniform core/headgroup interface but would trek along a more roughened or corrugated interface. If this scenario were in operation, lateral diffusion would be slower at lower CO2 densities. Second, as one increases the CO2 density, CO2 may partition into the water core and form carbonic acid. A more acidic water core could alter the intermolecular interactions between the probes and the surfactant headgroups. Unfortunately, previous work on AOT reverse micelles formed in liquid alkanes has shown that it is difficult to estimate the pKa changes in the water pool.57-59 (Experiments are currently in progress to determine the acidity of the PFPE reverse micelle water core as a function of continuous phase density.) Third, the presence of CO2 in the water core could simply lower the local microviscosity surrounding the probe, allowing the probe to diffuse more rapidly. Within the Quitevis framework,40 these results show that the CO2 continuous phase density and water loading can control the probe dynamics within the PFPE reverse micelles. Interpretation of the Faster Rotational Reorientation Time. The time-resolved fluorescence anisotropy results (Figure 6) show that both fluorophores experience the same fast motion within the PFPE water core and water loading does not appear to affect the local probe motion per se. We aim to interpret this behavior by considering the changes that occur in the water core as water is added. With reference to AOT reverse micelles formed in liquids, at low water loadings (R < 5), added water serves to hydrate the micellar headgroups.57-59 At higher water loadings, beyond that required for complete headgroup hydration (5-6 for AOT), further addition of water hydrates the probe. Thus, probe hydration at higher water loadings should result in dynamics that are more similar to those in bulk water.40 The arrows on the right-hand side of Figure 6 represent the measured rotational reorientation times for R6G (lower arrow) and LRSH (upper arrow) in neat liquid water at 21 °C. The rotational reorientation times for each probe in the PFPE water core are approximately one-half the value measured in neat liquid water. A portion of the difference in rotational reorientation times in neat liquid water and PFPE reverse micelles is due to differences in the experimental temperatures (21 vs 35 °C). However, the decrease in the faster rotational reorientation time relative to that in neat liquid water cannot be completely ascribed to a temperature difference. There are at least two interpretations for the observed φfast being less than the rotational dynamics for the same probes in neat liquid water. First, if CO2 partitions into the micelle core, it would lower the water core “microviscosity”. Using this hypothesis, we can estimate the PFPE water core microviscosity using eq 1 and the experimental φfast values. The results of this calculation show that the PFPE water core microviscosity is independent of the continuous phase density. Water loading influences the internal microviscosity; at R ) 9, the microviscosity is statistically less than for R ) 5 (0.42 ( 0.08 vs 0.58 ( 0.08 cP) at all densities investigated. This is consistent with preferential hydration of the reverse micelle headgroups at lower water loadings.60,61 Finally, comparison of the PFPE water core
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6713 microviscosity to the viscosity of neat water at 35 °C (η ) 0.7194)62 suggests that the microviscosity within the PFPE water core is about 30-40% less than that in neat liquid water. To further test this hypothesis, we measured the rotational reorientation time of R6G in liquid water subjected to 125 bar of CO2. The measured R6G rotational reorientation time was 2030% lower than the value in neat liquid water at 35 °C. An alternative explanation for the φfast results involves a rapid probe wobbling motion at the micelle headgroup/water core interface. Quitevis et al.40 developed a wobbling-in-a-cone model to explain such motion in terms of rapid precession of the fluorescent probe within a spherical reverse micelle. Analysis of our data within the Quitevis framework shows that the precession semiangle (θ) for R6G is always greater than LRSH under the same conditions (〈θ〉 ) 70 ( 10 vs 45 ( 8). This suggests that R6G is more mobile compared to LRSH and that some aspect of the LRSH motion is hindered compared to that of R6G. Water loading does not affect the semiangle, suggesting that the actual wobbling process is restricted to probe molecules associated with the PFPE reverse micelle headgroups. Anisotropic Micelles. The previous models were predicated on PFPE micelles being spherical; however, the EPR results suggest that these micelles are nonspherical in supercritical CO2. The fluorophores themselves are isotropic rotors in neat liquids, but if they were part of a nonspherical micelle, one would anticipate the decay of fluorescence anisotropy to reveal at least two decay terms.29-33,37 If we assume the experimentally observed rotational reorientation times (Figure 6) to arise from the probe associated with a micelle having an intrinsic ellipsoidal shape, we can estimate D|, the rate of rotation about the symmetry axis, D⊥, the rate of rotation about the axis perpendicular to the symmetry axis, and the axial ratio.37 By use of this analysis, we find that D| is (2.3 ( 0.1) × 109 s-1 at all densities and water loadings tested. D⊥ systematically increases with fluid density and decreases as water loading increases (range ) (1.6-3.5) × 108 s-1). This suggests that the asymmetry of a PFPE micelle is such that the ratio of the micelle’s in-plane and out-of-plane rotation D|/D⊥ is on the order of 7-14. This result is consistent with micelles having axial ratios (thickness/diameter) between 0.3 and 0.5. Conclusions Based on new cloud point data for water-in-CO2 microemulsions formed using an ammonium carboxylate perfluoropolyether surfactant, the water-to-surfactant ratio is much larger than in previous studies of reverse micelles in CO2 for pressures up to 300 bar. The water partitions between the micelle pseudophase and the bulk CO2. For low water loadings a significant portion of the water may be solubilized in the CO2; whereas at high water loadings (R > 5) the CO2 phase is saturated and the reverse micelles solubilize water, which has been shown to have “bulk-like” properties.22 EPR results show that PFPE ammonium carboxylate is solubilized as aggregates in CO2 even at pressures below which no water pool can be formed. Stable reverse micelles of Mn(PFPE)2 can also be formed in supercritical CO2. The interior of the Mn(PFPE)2-based micelles consists of a water pool that is able to ionize manganese and therefore differs from any water contained in the CO2 continuous phase. The EPR data also suggest that these reverse micelles may exist in a form that is other than spherical. Time-resolved fluorescence anisotropy decay measurements are used to quantify the dynamics of two fluorophores, R6G and LRSH, within the water core region of PFPE reverse micelles formed in supercritical CO2. The anisotropy decay for each probe is always best described by two rotational
6714 J. Phys. Chem. B, Vol. 101, No. 34, 1997 reorientation times. The dynamics associated with these rotational reorientation times are explained in terms of probe lateral diffusion and CO2-mediated changes in the water core microviscosity and/or a wobbling-in-a-cone model wherein the probe precesses about its emission transition moment. An alternative to the aforementioned view, supported by the EPR and fluorescence data, is that the PFPE micelle is itself intrinsically anisotropic and that the observed rotational reorientation times are a result of motion about at least two principle axes. Acknowledgment. This work was generously supported by funding from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (DE-FG02-90ER14143-A002) to F.V.B., an NSF Presidential Young Investigator Award and NSF (CTS-9414759) to T.W.R., a Department of Defense NDSEG Award to J.dG., and the NSF (CHE-9315429), Unilever Research and the Separations Research Program at the University of Texas to K.P.J. We thank A. Chittofarti for supplying the ammonium carboxylate PFPE surfactant. References and Notes (1) Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. (2) Supercritical Fluid Technology - ReViews in Modern Theory and Applications; Bruno, T. J., Ely, J. F., Eds.; CRC Press: Boca Raton, FL, 1991. (3) Supercritical Fluid Technology - Theoretical and Applied Approaches in Analytical Chemistry; Bright, F. V., McNally, M. E. P., Eds.; ACS Symposium Series 488; American Chemical Society: Washington, DC, 1992. (4) Supercritical Fluid Engineering Science - Fundamentals and Applications; Kiran, E., Brennecke, J. F., Eds.; ACS Symposium Series 514; American Chemical Society: Washington, DC, 1993. (5) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction Principles and Practice; Butterworths: Boston, MA, 1993. (6) Kim, S.; Johnston, K. P. Ind. Eng. Chem. Res. 1990, 29, 1682. (7) (a) Kim, S.; Johnston, K. P. AIChE J. 1987, 33, 1603. (b) Kim, S.; Johnston, K. P. Ind. Eng. Chem. Res. 1987, 26, 1206. (8) Hawthorne, S. B. Anal. Chem. 1990, 62, 633A. (9) Tomasko, D. L.; Knutson, B. L.; Pouillot, F.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. 1993, 97, 11823. (10) Smith, R. D.; Fulton, J. L.; Jones, H. K.; Gale, R. W.; Wright, B. W. J. Chromatogr. Sci. 1989, 27, 309. (11) (a) Gale, R. W.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987, 109, 920. (b) Gale, R. W.; Fulton, J. L.; Smith, R. D. Anal. Chem. 1987, 59, 1977. (c) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903. (d) Smith, R. D.; Fulton, J. L.; Jones, H. K. Sep. Sci. Technol. 1988, 23, 2015. (e) Fulton, J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J. Phys. Chem. 1989, 93, 4198. (f) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J. Phys. Chem. 1990, 94, 781. (g) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1990, 94, 1997. (h) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241. (12) (a) Yadzi, P.; McFann, G. J.; Fox, M. A.; Johnston, K. P. J. Phys. Chem. 1990, 94, 7224. (b) McFann, G. J.; Johnston, K. P. J. Phys. Chem. 1991, 95, 4889. (13) (a) Zhang, J.; Bright, F. V. J. Phys. Chem. 1992, 96, 5633. (b) Zhang, J.; Bright, F. V. J. Phys. Chem. 1992, 96, 9068. (c) Heitz, M. P.; Bright, F. V. Appl. Spectrosc. 1996, 50, 732. (14) Eastoe, J.; Robinson, B. R.; Visser, A. J. W. G.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1991, 87, 1899. (15) Bartscherer, K. A.; Minier, M.; Renon, H. Fluid Phase Equilib. 1995, 107, 93 (16) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (17) Randolph, T. W.; Clark, D. S.; Blanch, H. W.; Prausnitz, J. M. Science 1988, 239, 387. (18) Ritter, J. M.; Paulaitis, M. E. Langmuir 1990, 6, 935. (19) Iezzi, A.; Enick, R.; Brady, J. In Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989.
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