The lifetime results are compared with fluorescence spectra and quantum yields for these solutions. The concentration effects of energy transfer and quenching ...
Fluorescence Lifetime Studies of Crude Oils X I N W A N G * and O L I V E R C. M U L L I N S t Schlurnberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877
The fluorescence lifetimes of a series of crude oils at various concentrations have been measured for UV-visible excitation and emission wavelengths. The lifetime results are compared with fluorescence spectra and quantum yields for these solutions. The concentration effects of energy transfer and quenching are large and result in a significant decrease in fluorescence lifetimes for high concentrations and for heavy crude oils. Thus, radiationless processes dominate in energy transfer. At high concentrations, energy transfer produces large red shifts in fluorescence emission spectra, while quenching produces a large reduction in quantum yields. Stern-Volmer analyses of lifetime and quenching data show a linear dependence of energy transfer and quenching rates on concentration. The rate constants are consistent with collisions which are very efficient at energy transfer and quenching, and the rates of these two processes are comparable.
Index Headings: Crude oil; Petroleum; Fluorescence; Lifetime; Energy transfer; Quenching; Spectra; Chromophore; Fluorophore.
INTRODUCTION Fluorescence lifetime data can provide insight into the dynamics of various processes after the deposition of electronic excitation energy.l In addition to contributing to fundamental understanding, fluorescence methods are useful in diverse applications, such as those involving crude oils. 2 For instance, in the petroleum industry, fluorescence is routinely employed during drilling as an indicator of the presence of crude oil. 3 For some natural materials related to crude oils, high fluorophore concentrations produced spectral red shifts and intensity reduction of fluorescence. 4 These observations are used to help gauge the evolution of these natural materials. Recently, fluorescence lifetimes of several crude oils have been measured; heavier crude oils were found to have shorter lifetimes? In general, our interest is to study the optical properties of crude oils in order to reveal systematics in crude oil spectral characteristics. This approach helps to improve understanding of the chromophore and fluorophore constituents and their dynamics in crude oils. For instance, all crude oils, from the lightest gas condensates to the heaviest tars, are found to exhibit the Urbach tail (an exponential decay) in the long wavelength electronic absorption, where all crude oil spectra are characterized by the same exponential decay factor. 6 In crude oils, the Urbach behavior, which is also found in asphaltenes, 7 is directly related to the population distribution of chromophores. 6 Attempts have been made to relate optical absorption spectra to the population of different ring sizes in asphaltenes, s One result of our optical measurements on crude oils has been the development and commercial
Received 28 October 1993; accepted 17 May 1994. * Present address: Department of Applied Science, University of California at Davis, Davis, CA 95616. ~"Author to whom correspondence should be sent.
Volume 48, Number 8, 1994
use of an in situ optical analyzer for multiphase downhole flow in oil wells. 9 Here, we study the fluorescence lifetimes, spectra, and quantum yields of a wide range of crude oils at various concentrations; the self-consistent treatment of such extensive data yields a broad understanding. A wide spectral range of both excitation and emission wavelengths is used. Lifetimes are found to decrease at high concentration and are interpreted in terms ofcollisional effects of quenching and energy transfer. Fluorescence emission spectra exhibit large red shifts at high concentration due to energy transfer, while fluorescence quantum yields drop precipitously at high concentration due to quenching. SternVolmer analyses of the lifetime and quenching data provide rates of quenching and energy transfer which are comparable. Trends towards shorter lifetimes are always found for increasingly heavy crude oils, and, with dilute solutions, for longer emission wavelength, suggesting the existence of molecular complexes in heavy crude oils. EXPERIMENTAL Lifetime data were collected with the use of beamline U9B of the National Synchrotron Light Source of Brookhaven National Laboratory. This UV-visible beamline allowed selection of desired excitation and emission wavelengths. Since our lifetimes are fairly long, data collection could proceed only when the synchrotron was operated in the single bunch mode. The short time duration (1 ns) of the synchrotron pulse obviated the need for pulse deconvolution of the data. Typical spectral widths were 6 nm for the source and 12 nm for the emission. For lifetime data collection, we also used a PTI LS-100 fluorescence spectrometer equipped with a nitrogen lamp. This line source provides output at a few discrete wavelengths; here, we used the following lines: 316, 337, and 381 nm. Again, emission spectra were collected with roughly 12-nm resolution. The time duration of the pulse as measured by the pulsed photomultiplier was about 4 ns, and deconvolving software allowed for lifetime determinations to approximately 0.5 ns. A comparison of the data collected at Brookhaven and with our spectrometer showed excellent agreement. Dilution of crude oil samples was performed with the use of benzene; and for light crude oils, n-heptane was additionally used. Both solvents were spectrophotometric grade from Aldrich Chemical Company. All samples which were used for fluorescence lifetime determinations were deaerated with nitrogen gas for approximately 12 min. A gas chromatograph septum was used as a stopper on the sample cuvette, and two GC needles were used to inject and vent the nitrogen. The effectiveness of deaeration was determined with the use of pyrene solutions; without deaeration, our measured fluorescence lifetime ofpyrene in heptane was 25 ns, and with deaeration, 495 ns.
0003.7028/94/4808-0977$2.00/0
© 1994Societyfor AppliedSpectroscopy
APPLIED SPECTROSCOPY
977
T A B L E I. The cutoff wavelength (where the electronic absorption equals three for 2-mm pathlength) and the A P I gravity of some crude oils used here.
Crude oil Gas condensate (GC) Hunt oil Escravos North Sea Sales Texl41
Cutoff k (nm) 308 406 618 771 999 1453
All fluorescence spectra were collected with the PTI LS-100 fluorescence spectrometer fitted with a quantum counter and an extended red PMT. The emission spectra were corrected with the use of the PMT efficiency curve. Absorption spectra were collected with a Cary 5 UVvisible/NIR absorption spectrometer. Fluorescence lifetime and spectral measurements were performed both in the front-surface mode for neat crude oils and for highly concentrated solutions of crude oils and in the transmission mode for dilute solutions; for the dilute case the absorption for a 10-mm pathlength was kept below 0.3 optical density (OD) for the excitation wavelength. The optical absorption of crude oils monotonically increases sharply at shorter wavelength, 6 so that the OD of the solutions at the emission wavelengths is much lower than 0.3; we have not observed significant self-absorption effects. Fronhsurface and transmission measurements performed on the same solution for both lifetime and spectral measurements showed no significant differences. Quantum yield data were collected with a 10-W tungsten-halogen lamp with a 380-nm short-pass filter from Omega Optical (OD = 6 out-of~band rejection). The source light was collected by a bifurcated fiber optic with the common end placed in a small test tube filled with index matching fluid. The test tube was then placed in the various crude oil solutions exciting fluorescence. The fluorescence was collected by the common end of the fiber optic, passed through a 415-nm long-pass filter (Omega), and detected by a Newport Model 835 optical power meter. By performing quantum yield measurements with different distances between the end of the fiber optic and the end of the test tube, we were able to determine when light penetration into the more dilute solutions changed the effective collection geometry, thus invalidating the quantum yield measurement for those solutions of low concentration. A series of crude oils, from light to heavy, was used in this work. Table I lists several of the crude oils, the cutoff wavelength, defined as the wavelength at which their electronic absorption strength is OD = 3 for a 2-ram pathlength, and their API gravity. Gas condensate is nearly colorless, while Tex 141 has the appearance of asphalt. Data Fitting. Figure 1 shows a typical fluorescence decay curve which was obtained for the crude oils. Because the decay curve is not linear in this plot, the data fitting requires more than a single exponential decay. Only a few of our decay curves could be accurately fit with the use of a single decay curve. In all cases, a fit using two exponential decays could reproduce the data, thereby providing a long- and a short-decay component. The x-square values for the two component fits were approximately 2. 978
Volume 48, Number 8, 1994
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Figure 1 shows such a fit. O f course, crude oils are complex mixtures, and the corresponding fluorescence decay curves may, in fact, consist of three or more decay components. However, the data are not sufficient to obtain more than two components from a fit; that is, increasing the number of decay components did not significantly decrease the x-square values. Simple computational modeling shows that a two-component analysis of a multicomponent decay does reflect the constituent lifetimes and populations of the original curve, but, of course, some information is lost. All of our analysis will be based on a two (or one)-component fit. Generally, the population of the short-lifetime component is much greater than that of the long-lifetime component for all crude oils. For the lighter crude oils, the longer-lifetime component does increase in population up to ~ 50%. Table II lists the populations and lifetimes for different concentrations and emission wavelengths for Sales crude oil with 316-nm excitation. For the Stern-Volmer plots, we have constrained the least-squares fits to pass through the origin. We are interested in the comparison of slopes of different plots and found better consistency with this constraint. The minim u m number of points (not including the origin) for any of our Stern-Volmer plots is four. R E S U L T S AND D I S C U S S I O N The dependence of the fluorescence emission spectra on concentration is shown in Fig. 2 for two crude oils; each of the curves is arbitrarily normalized to have a maximum value of 1. For each of the neat crude oils, the fluorescence emission spectrum is significantly red-shifted from the source (316 nm), and is quite broad. With increasing dilution, the peak in the emission curve monotonically shifts towards the blue until, for the very dilute solution, the maximum of the emission curve shows only a small Stokes shift of 45 nm from the source. Furthermore, with increasing dilution, the emission peak also narrows in width, with the largest change occurring with the initial dilution. The heavier crude oil, Sales, exhibits a more pronounced red shift for the neat solution than
T A B L E II.
Populations and lifetimes (in nanoseconds) for Sales crude oil at different dilutions and emission wavelengths for 316-nm excitation.
370 n m Dilution
440 n m
560 n m
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0.71 0.29 0.16 0.84 0.09 0.91 0.15 0.85
1.80 1.15 7.89 2.03 11.84 2.94 17.5 3.53 14.3 2.9
0.67 0.33 0.09 0.91 0.16 0.84 0.11 0.89 0.14 0.86
2.38 0.79 10.02 2.74 14.77 3.08 16.82 3.5 13.48 2.9
0.5 0.5 0.10 0.90 0.13 0.87 0.11 0.89 0.14 0.86
3.03 1.03 11.40 2.89 14.67 2.71 16.72 3.11
0.56 0.44 0.11 0.89 0.15 0.85 0.14 0.86
1:25 1:125 1:25,000
the lighter crude oil, North Sea. The spectrum for a given dilution of North Sea oil is quite similar to the Sales solution with an additional dilution of a factor of five. The cause for these substantial spectral changes with mere changes in concentration is electronic energy transfer. Largely, dilution does not alter the identity of chromophores in the crude oil; we have determined that only small changes in the absorption spectra of crude oils are 1
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