Energy & Fuels 1998, 12, 823-827
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Determination of the Local Environment Surrounding Pyrene in Supercritical Alkanes: A First Step toward Solvation in Supercritical Aviation Fuels Emily D. Niemeyer and Frank V. Bright* Department of Chemistry, Natural Sciences Complex, State University of New York at Buffalo, Buffalo, New York 14260-3000 Received February 25, 1998. Revised Manuscript Received April 14, 1998
We report quantitative data on the local microenvironment surrounding a model organic solute (pyrene) dissolved in several supercritical components of aviation fuels (n-pentane, n-hexane, n-heptane, n-octane). Toward this end, we use static fluorescence spectroscopy to determine whether these supercritical alkanes influence the local microenvironment surrounding pyrene in a way similar to other supercritical fluids (e.g., CO2, CF3H, C2H6, H2O). Steady-state fluorescence measurements indicate that, in all alkanes studied, there is an increase in local fluid density surrounding the pyrene molecule relative to the fluid bulk density (solute-fluid density augmentation). The maximum in this local density augmentation occurs at approximately one-half the fluid critical density and subsequently decreases at higher fluid densities. These results are fully consistent with observations for pyrene in other supercritical fluid systems (i.e., CO2, H2O). The maximum relative local density augmentation increases with increasing alkane chain length. These results correlate with changes in alkane polarizability. Another, somewhat more tenuous, correlation suggests that the maximum degree of local density augmentation may decrease as chain length increases in supercritical linear alkanes above n-octane.
Introduction One of the consequences associated with the continuing introduction of more high-performance, high-efficiency aircraft has been the development of jet fuels with increased thermal stability and improved performance at elevated temperatures.1-4 This is directly related to the need for aviation fuels to perform many noncombustion related tasks, including the cooling of subsystem components and the aircraft engine.1-4 Recently, the Air Force announced the development of an additives package to its current aircraft fuel (JP-8) which significantly increases the fuel heat-sink capacity and its thermal stability for use in proposed higher speed aircraft.1 However, even with such advanced fuels, it is likely that temperatures within the newer, faster aircraft fuselage may increase beyond the fuel critical temperature.1-4 This, in turn, may lead to the circulating fuel temporarily becoming supercritical within certain regions of aircraft plumbing. Given this, one must question how the fuel and its additives behave under such conditions. When a liquid is raised above its critical temperature (Tc) and pressure (Pc), it becomes a unique medium * To whom all correspondence should be addressed. (1) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E., III Trans. ASME 1996, 118, 170-9. (2) Yu, J.; Ester, S. Ind. Eng. Chem. Res. 1995, 34, 404-9. (3) Zabarnick, S.; Zelesnik, P.; Grinsted, R. R. J. Eng. Gas Turbines Power 1996, 118, 271-7. (4) Edwards, T.; Zabarnick, S. Ind. Eng. Chem. Res. 1993, 32, 311722.
known as a supercritical fluid. A supercritical fluid retains gas- and liquid-like properties, and its physicochemical properties (i.e., density, dielectric constant, refractive index), solvating power, and mass transport are strong functions of the system temperature and pressure.5 Therefore, if a fuel were raised above its critical temperature within an aircraft, this will likely lead to significant differences in solute-fluid (e.g., additive-primary fuel component) intermolecular interactions relative to the same system under “normal” subcritical, liquid operating conditions. It is now widely accepted that a phenomenon exists in supercritical fluid systems wherein there can be an increase in local fluid density surrounding a dissolved solute relative to the bulk fluid density.5-7 This phenomenon has been termed local density augmentation,5 solute-fluid clustering,6 or molecular charisma.7 The maximum extent of local density augmentation has been shown to occur at approximately one-half the fluid critical density.8 For example, in supercritical CO2, experimental results have shown that local densities up (5) For recent reviews on supercritical fluids, see: (a) Kauffman, J. F. Anal. Chem. 1996, 68, 248A-53A. (b) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. L. Nature 1996, 383, 313-8. (c) Brennecke, J. F. Chem. Ind. 1996, 21, 831-4. (d) 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. (6) Kim, S.; Johnston, K. P. AIChE J. 1987, 33, 1603-11. (7) Eckert, C. A.; Knutson, B. L. Fluid Phase Equil. 1993, 83, 93100. (8) O’Brien, J. A.; Randolph, T. W.; Carlier, C.; Ganapathy, S. AIChE J. 1993, 39, 1061-71.
S0887-0624(98)00039-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/27/1998
824 Energy & Fuels, Vol. 12, No. 4, 1998
Niemeyer and Bright
Table 1. Critical Temperatures (Tc),19 Pressures (Pc),19 Densities (Gc),19 and Electronic Polarizabilities (r)24 as Well as the Dielectric Constant (E) and Refractive Index (n) Range of the Alkanes Used in This Study
a
fluid
Tc (°C)
Pc (psia)
Fc (g/mL)
R (×10-24 cm3)
rangea
n rangea
n-pentane n-hexane n-heptane n-octane
196.6 234.4 267.3 295.8
489 436 397 361
0.230 0.230 0.232 0.232
10.05 11.90 13.74 15.56
1.12-1.45 1.06-1.48 1.07-1.64 1.07-1.60
1.06-1.20 1.03-1.22 1.03-1.28 1.03-1.26
Over the reduced density range from approximately Fr ) 0.3 to Fr ) 1.8 at Tr ) 1.01.
to 2 or 3 times the bulk fluid density are observed, and this augmentation decreases as the bulk fluid density increases toward more liquid-like values.9 Although this local density augmentation phenomenon is known to affect solute solvation, chemical reaction kinetics, and chemical equilibria in many milder supercritical fluid systems,5-9 there have not been any published studies aimed toward determining whether solute-fluid density augmentation occurs within high temperature supercritical aviation fuels. Clearly, if such a process were to occur in supercritical jet fuels it will likely affect the immediate microenvironment surrounding any dissolved fuel additives. Moreover, because the properties of a supercritical fluid are strongly dependent on changes in the fluid temperature and pressure, such subtle changes could strongly influence an additive’s behavior/performance, its solubility, and the solubility of dissolved gases. In fact, Heneghan and Zabarnick have already determined that O2 dissolved in fuels at elevated temperatures and pressures is directly correlated with deposit formation on 304 stainless steel.10 Thus, local phenomena, even if they occur temporarily and in a defined domain, may govern not only fuel performance and lifetime but also the design and overall lifetime of aircraft components that come into contact with the fuel. In this paper, our aim is to determine experimentally the local fluid density surrounding a model solute/ additive that is dissolved in several neat fuel components above their critical temperatures. Toward this end, we use steady-state fluorescence spectroscopy to quantify the degree of solute-fluid density augmentation in four neat, straight-chain alkanes that are present in typical jet fuels (i.e., n-pentane, n-hexane, n-heptane, and n-octane).11 We have chosen pyrene as our model solute because it is among the most common probes used to study solvation in the condensed phase including supercritical fluids,9,12-17 it is strongly fluorescent,16 and fluorescence-based measurements allow us to work under conditions where solute-solute intermolecular interactions are minimal.9 Pyrene is also inherently present in some jet fuels, including JP-8.11 The emis(9) Rice, J. K.; Niemeyer, E. D.; Dunbar, R. A.; Bright, F. V. J. Am. Chem. Soc. 1995, 117, 5832-9. (10) Heneghan, S. P.; Zabarnick, S. Fuel 1994, 73, 35-43. (11) H. T. Mayfield JP-8 Composition and Variability; Air Force Materiel Command: Tyndall Air Force Base, Aug 1994 - Feb 1995. (12) Sun, Y.-P.; Bunker, C. E.; Hamilton, N. B. Chem. Phys. Lett. 1993, 210, 111-7. (13) Niemeyer, E. D.; Bright, F. V. Appl. Spectrosc. 1997, 51, 154753. (14) Johnston, K. P.; Balbuena, P. B.; Xiang, T.; Rossky, P. J. In Innovations in Supercritical Fluids Science and Technology; Hutchenson, K. W., Foster, N. R., Eds.; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; pp 77-92. (15) Zagrobelny, J.; Bright, F. V. J. Am. Chem. Soc. 1993, 115, 7017. (16) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 1721. (17) Winnik, F. M. Chem. Rev. 1993, 93, 587-614.
sion intensity from the pyrene 0-0 (I1) transition is sensitive to its local solvent environment while the intensity associated with the 0-3 (I3) transition is insensitive to solvent properties.16,18 Thus, by simply measuring the pyrene I1/I3 ratio, one can conveniently monitor changes in the local solvent microenvironment surrounding the average pyrene molecule.16,18 There are three primary factors that must be considered when attempting measurements in these supercritical alkanes. First, because of the high fluid critical temperatures (see Table 1),19 the solute must be stable at elevated temperatures. Second, the degree of temperature-induced spectral broadening must be small enough so that one can obtain useful spectroscopic data at elevated temperatures. Previous work has shown that pyrene is stable up to 400 °C in supercritical water13,14 and one can obtain useful I1/I3 data with pyrene up to 400 °C. Finally, it is critical to remove O2 from these solvents to avoid alkane combustion and/or accelerated pyrene decomposition. Simple Ar(g) purging and a continuous flow protocol13 allow one to address this issue. Experimental Section Reagents and Sample Preparation. Pyrene (99.9%), npentane, n-hexane, n-heptane, and n-octane (99+%; spectrophotometric grade) were purchased from Aldrich and used as received. Samples of pyrene in each alkane solvent were prepared as follows. The appropriate volume of a pyrene/ ethanol stock solution was pipetted into a flask. The ethanol was evaporated by heating the flask in a hot oven (∼100 °C) for approximately 1 h. After cooling, the appropriate volume of alkane was added directly to the flask to generate a 10 µM pyrene solution. Under the conditions investigated, there were no indications of aggregate or ground-state pyrene dimerization.9 Prior to making any spectroscopic measurements, each pyrene/alkane solution was deoxygenated by vigorously bubbling Ar gas for approximately 1 h through the solution. During the deoxygenation step, the solutions were placed in an ice bath to minimize solvent evaporation. Instrumentation. The high-pressure fiber-optic-based titanium optical cell that was used for these studies has been described in detail previously.13 A high-pressure syringe pump (Isco, Model SFC-500) is filled with the pyrene/alkane solution and operated in the constant-pressure mode to continuously supply a particular deoxygenated pyrene/alkane solution through 1/16 in. stainless steel tubing to the high-pressure optical cell. The entire optical cell is located within a GC oven (Hewlett-Packard, Model 5730A) which is used for continuous temperature control. The oven temperature is monitored throughout an experiment using an insulated thermocouple (Simpson Accessories, Elgin, IL) placed within 0.5 cm of the titanium cell. A valve and flow restrictor assembly located outside the oven is adjusted during an experiment to maintain (18) Karpovich, D. S.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 3951-8. (19) CRC Handbook of Chemistry and Physics, 74th ed.; Chemical Rubber Publishing Co.: Boca Raton, FL, 1993-94; pp 6-60-6-63.
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known vibronic features are clearly observed;16,18 the I1 peak occurs at 376 nm and the I3 emission occurs at 383 nm. As the temperature is increased past the critical temperature (-‚-), we see that the pyrene emission spectrum broadens slightly. However, the intensity associated with the pyrene I1 and I3 peaks are still readily measurable under these conditions. We also note that the emission spectrum is devoid of any detectable excimer-like emission. Similar results were obtained for pyrene in each of the supercritical alkanes. The pyrene I1/I3 has been used previously to provide insight into the nature of solute-fluid interactions in supercritical water13 and CO2.9,12 We use a similar approach here to quantify the intermolecular interactions occurring in the supercritical alkane systems. It is well known that the pyrene I1/I3 can be directly related to the π* polarity scale in liquid solvents9,12,13,21
I1/I3 ) a + bπ* Figure 1. Normalized steady-state emission spectra for pyrene in n-pentane at ambient (s) and at supercritical (-‚-) conditions (T ) 201.1 °C, P ) 613.8 psia, and F ) 0.342 g/mL). a constant solution flow (∼300 µL/min) through the optical cell. The system pressure is monitored using a pressure transducer (0.03% accuracy; Omega, Stamford, CT). A He-Cd laser (Omnichrome, model 3074-20M) (325 nm) is used to excite the pyrene fluorescence, and an interference filter (10 nm fwhm, Oriel) is placed in the excitation beam path to minimize any extraneous plasma discharge from reaching the detection system. The laser beam is focused onto the proximal end of a polyimide-coated fused silica optical fiber and delivered to the high-pressure cell. The resulting pyrene fluorescence from within the high-pressure cell is collected using a second optical fiber whose output is collected and focused by a fused silica lens onto the entrance slit of an emission monochromator (bandpass ) 2 nm). After wavelength selection, the fluorescence is detected by a photomultiplier tube and sent to a personal computer for processing. The remainder of the spectrofluorometer (SLM-AMINCO 48000 MHF) is configured in the standard ratiometric mode. Measurement of the pyrene I1 and I3 band intensities is made using software provided with the fluorometer. A large range of reduced densities (Fr; where Fr ) Fexp/Fc; Fexp ) experimental density; Fc ) critical density) at a reduced temperature (Tr) of 1.01 (where Tr ) Texp/Tc; Texp ) experimental temperature; Tc ) critical temperature) was studied for each linear alkane. Special emphasis was placed on the reduced density region between 0.5 and 1.0 where a maximum in solute-fluid interactions is known to occur in other supercritical fluid systems.8 The bulk density of each alkane solution as a function of temperature and pressure was estimated by using a commercial software package (SFSolver, Isco, Inc). The solution refractive index and dielectric constant as a function of temperature and pressure were calculated using the Clausius-Massotti equation and the appropriate molar refractivity and density.20
(1)
where a and b are constants. In the absence of any local density effects, the π* term scales linearly with the solvent dielectric cross term, f(, n2)9,12,13
π* ) c + df(, n2)
(2)
f(, n2) ) [( - 1)/(2 + 1)][(n2 - 1)/(2n2 + 1)] (3) where c and d are constants, is the solvent bulk dielectric constant, and n is the solvent refractive index. Therefore, assuming the absence of any local density effects/fluctuations surrounding the average pyrene molecule, the pyrene I1/I3 should scale linearly with f(, n2)9,12,13
I1/I3 ) A + Bf(, n2)
(4)
An early concern centered on how thermal broadening would affect the pyrene emission spectra in these supercritical alkanes. Figure 1 presents the pyrene emission spectra in n-pentane at ambient and supercritical (T ) 201.1 °C; P ) 613.8 psia; F ) 0.342 g/mL) conditions. At ambient conditions (s), all the well-
where A is the vapor phase I1/I3 value for pyrene (0.41)16 and B, the slope of the line, is determined by the pyrene I1/I3 at high-density, liquid-like values.9,12,13 Figure 2 presents experimental pyrene I1/I3 data with associated uncertainties ((1 standard deviation for multiple measurements and experiments) as a function of reduced bulk density in supercritical n-pentane (panel A), n-hexane (panel B), n-heptane (panel C), and noctane (panel D) at Tr ) 1.01. From these data, we used eqs 1-4 to relate the pyrene I1/I3 to the local physical properties of the fluid via the dielectric cross term (f(, n2)). Figure 3 presents the pyrene I1/I3 values as a function of the dielectric cross term in supercritical n-pentane (panel A), n-hexane (panel B), n-heptane (panel C), and n-octane (panel D). The solid lines in each panel represent the theoretical I1/I3 values in the absence of any solute-fluid interactions (i.e., no local fluid density effects). Upward deviation from this theoretical line is indicative of solute-fluid local density augmentation, or increased solute-fluid clustering.9,12,13 The clear, statistically significant upward deviation of our experimental pyrene I1/I3 data for each neat supercritical alkane relative to the theoretical line (no clustering) is used to calculate the local alkane density surrounding pyrene in each of these systems.
(20) Atkins, P. W. Physical Chemistry, 4th ed.; W. H. Freeman & Co.: New York, 1990; pp 651-2.
(21) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027-38.
Results and Discussion
826 Energy & Fuels, Vol. 12, No. 4, 1998
Figure 2. Pyrene I1/I3 values as a function of reduced density in supercritical n-pentane (panel A), n-hexane (panel B), n-heptane (panel C), and n-octane (panel D) all at Tr ) 1.01.
Figure 3. Pyrene I1/I3 values as a function of the dielectric cross term, f(, n2) for supercritical n-pentane (panel A), n-hexane (panel B), n-heptane (panel C), and n-octane (panel D) all at Tr ) 1.01. The solid lines in each panel are the theoretical lines based on gas and high-density liquid alkane values.
Figure 4 presents the calculated local supercritical alkane density (Flocal) divided by the bulk fluid density (Fbulk) for the four individual alkanes investigated in this study as a function of bulk fluid density. This Flocal/Fbulk term has been used previously to estimate the extent of solute-fluid interactions occurring between pyrene and other supercritical fluid systems.9,12,13 We use Flocal/ Fbulk here to estimate the average local alkane composition surrounding the pyrene probe in each supercritical alkane. Careful inspection of Figure 4 reveals several key results. First, for each alkane system, we clearly
Niemeyer and Bright
Figure 4. Recovered local density augmentation (Flocal/Fbulk) as a function of reduced density for pyrene in supercritical n-pentane (panel A), n-hexane (panel B), n-heptane (panel C), and n-octane (panel D) all at Tr ) 1.01.
observe local density augmentation surrounding the pyrene solute. Second, the maximum degree of local density augmentation occurs, over the range investigated, at approximately one-half the critical density in each alkane, and the degree of augmentation decreases, approaching unity, at the highest liquid-like bulk fluid densities. This result is fully consistent with those observed in other supercritical fluid systems9,12,13 and corresponds to a decrease in local density augmentation at more liquid-like fluid densities where the local and bulk density approach one another. Third, as the alkane chain length increases, the maximum degree of local density augmentation generally increases. For example, between the reduced density region from 0.5 to 1.0, the maximum density augmentation surrounding pyrene increases from about three in supercritical n-pentane to nearly six in supercritical n-octane. It is well known that the primary intermolecular forces acting in the alkanes are dispersion forces.22 Dispersion forces occur between nonpolar molecules when a temporary dipole is induced in the electron cloud due to the proximity of another molecule. The electronic polarizability (R) is associated with the extent to which this dipole may be induced, where a larger R is indicative of a larger induced dipole, leading to increased dispersion forces. As the alkane chain length increases, the corresponding electronic polarizabilities subsequently increase from a value of 10.05 × 10-24 cm3 for n-pentane to 15.56 × 10-24 cm3 for n-octane (see Table 1).23 The electronic polarizability of pyrene can be estimated as 20.33 × 10-24 cm3 by simply adding the characteristic polarizabilities of its bonds.24 Because of the higher electronic polarizability for pyrene, we (22) Atkins, P. W. General Chemistry, 2nd ed.; W. H. Freeman & Co.: New York, 1990; pp 866-8. (23) Huyskens, P. L.; Luck, W. A. P. Intermolecular Forces, An Introduction to Modern Methods and Results; Zeegers-Huyskins, T., Ed.; Springer-Verlag: Berlin, 1991; Chapter 1.
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to determine if Flocal/Fbulk vs carbon number will continue to track the critical density data/predictions. Conclusions and Implications
Figure 5. Effects of alkane chain length on the degree of density augmentation surrounding pyrene. The maximum density augmentation between Fr ) 0.5 and 1.0 (b) is represented as a function of carbon number. Shown also is the bulk fluid critical density for linear alkanes from experiment (2) or from computer simulations (9) as a function of carbon chain number.
This work provides a viable methodology that appears useful for studying and quantifying local solvation phenomena in complex fuels under supercritical conditions. Using the approach presented in this paper, local density augmentation is observed surrounding pyrene dissolved in supercritical n-pentane, n-hexane, n-heptane, and n-octane at Tr ) 1.01. For all alkanes studied, the maximum augmentation is observed at approximately one-half the critical density. Our results also show that the degree of local density augmentation increases as the alkane chain length increases and parallels the fluid dispersion forces. However, a correlation with the bulk fluid density suggests that the extent of local density augmentation may actually drop for longer linear alkanes beyond n-octane, but further experimental work is needed to confirm this hypothesis. Although this study provides a basis for probing solvation phenomena in fuels, more work is clearly needed to understand real aviation fuels. The phase equilibria for binary straight-chain alkane mixtures26 as well as straight and branched binary alkane mixtures27 are known. Therefore, simple binary alkane mixtures are an obvious jump point to begin to further the understanding of some more complicated phenomena (e.g., preferential solvation) occurring in supercritical aviation fuels. Finally, one might question if density augmentation occurs only near Tc. The similarity between the behavior of the alkanes and other fluids studied to date5-9,12-15 argue that the largest degree of density augmentation is observed near Tc, but this phenomena occurs, albeit to a lesser degree, reasonably far away from Tc also.
propose that as the alkane chain length and R increases, this leads to greater dispersion forces and a greater degree of solute-fluid clustering. In support of this, we observe that the maximum local density augmentation surrounding pyrene in n-octane (between Fr ) 0.5 and 1.0) is approximately 2-fold greater than that in npentane correlating with the electronic polarizability for n-octane which is approximately 1.5 times greater than that for n-pentane. Figure 5 presents the maximum Flocal/Fbulk value for each of the respective alkanes as a function of the alkane carbon number (b) along with critical densities recovered experimentally (2) or from computer simulations (9) for linear alkanes up to C24.25 We observe an initial increase in the maximum degree of local density augmentation with increasing alkane chain length from supercritical n-pentane to n-heptane with a “leveling off” observed after n-heptane. For the C5-C8 alkanes, we see a direct correlation between the maximum Flocal/Fbulk and the critical density vs carbon number. Further studies of longer chain alkanes (>C8) are clearly needed
EF980039D
(24) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992; Chapter 5. (25) Smit, B.; Karaborni, S.; Siepmann, J. I. J. Chem. Phys. 1995, 102, 2126-40.
(26) Martin, M. G.; Siepmann, J. E. J. Am. Chem. Soc. 1997, 119, 8921-4. (27) Berro, C.; Laı¨choubl, F.; Rauzy, E. J. Chem. Eng. Data 1991, 36, 474-8.
Acknowledgment. Financial support for this research was provided by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, United States Department of Energy (DEFGO290ER14143), the Air Force Office of Scientific Research, Summer Research Extension Program (F49620-93-C-0063), and the American Chemical Society Analytical Division Fellowship to E.D.N. sponsored by DuPont Company.
