A Critique of Asphaltene Fluorescence Decay and Depolarization

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Energy & Fuels 2008, 22, 1156–1166

A Critique of Asphaltene Fluorescence Decay and Depolarization-Based Claims about Molecular Weight and Molecular Architecture Otto P. Strausz,* I. Safarik, E. M. Lown, and A. Morales-Izquierdo Department of Chemistry, UniVersity of Alberta, Edmonton, AB, Canada, T6G 2G2 ReceiVed June 2, 2007. ReVised Manuscript ReceiVed August 20, 2007

Relying on experimental and theoretical data available from the literature, it is shown that the conclusions derived from measurements of fluorescence decay and depolarization kinetic times as reported in a series of papers over the past decade (Ralston, et al. Energy Fuels 1996, 10, 623–630; Groenzin, et al. J. Phys. Chem. A 1999, 103, 11237–11245; Groenzin, et al. Energy Fuels 2000, 14, 677–684; Buenzostro-Gonzales, et al. Energy Fuels 2001, 15, 972–979; Groenzin, H., et al. Energy Fuels 2003, 17, 498–503; Badre, S., et al. Fuel 2005, 85, 1–11 and references therein) are egregiously wrong. To start with, the decay time measurements were done with inappropriate instrumentation which resulted in misleading results. Misinterpretation of the results led to the mistaken conclusion that bichromophoric type molecules are absent from petroleum asphaltene and therefore the architecture of the asphaltene molecule features a single condensed cyclic core spiked with some alkyl chains, in spite of irrefutable chemical evidence to the contrary. It was further concluded that if the asphaltene core is a single condensed ring, then the fluorescence depolarization with rotational correlation time method is applicable for the molecular weight determination of asphaltene. This is definitely not so, since, regardless of any other considerations, asphaltene is a mixture of a plethora of different, unknown components, with unknown concentrations along with innumerable different, unknown and some known chromophores portraying widely different absorption coefficients, fluorescence quantum yields, and kinetic decay times. Consequently, asphaltene fluorescence is a highly complex function of the above attributes and as such it is a totally unsuitable property for its molecular weight determination. The injection of an incorrect, single condensed ring core architecture for asphaltene has caused some confusion in asphaltene chemistry that has now hopefully been settled.

Introduction Molecular weight (MW) determination has been, and still is, a challenging problem in asphaltene chemistry. The source of complications is attributable to three basic properties of asphaltene, namely, compositional variance, size polydispersity, and, most importantly, the high propensity of the covalent asphaltene molecules to form molecular aggregates. A whole gamut of methods has been applied for the MW determination of asphaltene over the decades including chemical, physical, absolute, relative, equilibrium, and nonequilibrium methods. Over a decade ago, a novel fluorescence-based method not previously used for the MW determination of multicomponent systems was applied for asphaltene,1 and the results obtained in the intervening period have been communicated in a long series of papers.2–6 The method comprises two sequential phases: (a) measurements of the fluorescence decay kinetics of the sample asphaltenes and * To whom correspondence should be addressed. E-mail: opstrausz@ shaw.ca. (1) Ralston, C. Y.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 1996, 10, 623–630. (2) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237– 11245. (3) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (4) Buenzostro-Gonzales, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972–979. (5) Groenzin, H.; Mullins, O. C. Energy Fuels 2003, 17, 498–503. (6) Badre, S.; Goncalves, C. C.; Nonizaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2005, 85, 1–11, and references therein.

(b) measurements of their rotational correlation times (RCTs) from their rate of fluorescence depolarization (FD). The former, step a, is intended to determine whether or not the sample molecules are composed of single aromatic ring chromophores or multiple ring cluster chromophores. This information is a prerequisite for the evaluation and meaningful interpretation of the depolarization measurements which, subject to satisfying additional requirements, can be related to the MW of the sample molecules if each of the molecules features one and the same chromophore. Otherwise, the depolarization data bring to light information only about a fraction of the sample molecule and not the molecule as a whole, or information about an unresolvable composite from all of the different chromophores present. Another important prerequisite is a knowledge of the spatial geometry of the chromophore molecules. The latter, step b, yields the MW of the sample. Here, the photoexcitation is done with plane-polarized light and the angle of depolarization is measured as a function of time. The rate of depolarization can then be related through the rate of correlation time to the sample MW within the constraints of the limitations mentioned above. This has been demonstrated for solutions of pure, single-chromophore-containing molecules with known molecular geometry. However, for multicomponent systems composed of large numbers of unknown components like the asphaltene molecule, the method cannot be expected to yield meaningful results. Yet, the summary conclusions arrived at by the authors from these studies were that

10.1021/ef700320p CCC: $40.75  2008 American Chemical Society Published on Web 01/15/2008

Asphaltene Fluorescence Decay

(a) the biexponential fluorescence decay lifetime values measured in dilute solutions in the low MW end of the asphaltene spectrums1.7–3.6 and 9.4–13.0 ns for the long and slow decaysswere incompatible with intramolecular electronic energy transfer (intra-EET) processes and (b) therefore intra-EET cannot be operative in the fluorescence decay; consequently, (c) it follows that the architecture of the asphaltene molecule features a single aromatic core structure.6 Finally, (d) the MW can be derived from the measured rate of FD without any additional information about the composition or physical, chemical properties of the sample asphaltene. Now, some of the fundamental reasons as to why these conclusions are erroneous have been pointed out before,7,8 and they will be discussed in detail below involving recent, highly pertinent results on relevant hydrocarbon systems from various laboratories. Discussion Section I: The Anthryl-(CH2)n-Naphthyl and the Pyrenyl-(CH2)n-Pyrenyl Systems. The center of contention regarding the FD-RCT method suggested for the MW determination of asphaltene is the role that intra-EET processes play in the photophysics of the system. Therefore, a brief review of the theory and recent experimental results on simple, relevant hydrocarbon bichromophoric molecules containing flexible methylene bridges between the chromophore units will first be necessary. Intra-EET processes can lead to the fluorescence and phosphorescence of the acceptor chromophore, chemical alterations, excimer/exciplex formation, intramolecular electron transfer resulting in zwitter ion formation, etc. Two systems will be considered here, 9-anthryl-(CH2)n-1-naphthyl with n ) 1, 3, and 6 (AnN) and 1,3-bi(1-pyrenyl)propane (Py3Py).

Energy & Fuels, Vol. 22, No. 2, 2008 1157

Figure 1. Coupled electronic transitions of donor and acceptor leading to electronic energy transfer in thermalized conditions in solution. Also shown is the corresponding region of spectral overlap, the degree of which determines the efficiency and the mechanism of the transfer. Reprinted from ref 9.

The fundamental driving forces of the EET process are the long-range Coulombic interaction and the short-range throughspace exchange and through-bond superexchange interactions. The Coulombic interaction is treated in terms of dipole–dipole interactions and is particularly suitable for describing EET in solution involving hydrocarbon systems such as the focus of interest in the discussion here where conditions for favorable spectroscopic overlap between the emission of D* and the absorption of A are met. The exchange interactions are effective in the short range and may become dominant when the above conditions for Coulombic interactions are not fulfilled. In solution, EET is slow relative to vibrational relaxation in D* and A* and consequently the initial and final states are usually vibrationally relaxed, as shown in Figure 1. The common overlap of the D* emission and the A absorption spectra is also illustrated here, and thus the importance of the spectral overlap in driving the EET. The rate constant for the intra-EET process can be calculated from the expressions derived for long-range, through-space interaction applicable, for example, to the AnN bichromophore molecules10 Kintra-EET )

( )( ) R0

1

R

τF0

(1)

where Kintra-EET is the rate constant, τ0F is the fluorescence lifetime of D*, and In EET processes, electronic excitation energy is transferred nonradiatively from a donor molecule (D*) to an acceptor molecule (A). In the case of multichromophore groups containing molecules, the transfer may occur intramolecularly from one chromophore moiety to another. Such processes have been extensively investigated in photochemistry, photophysics, and spectroscopy. The salient features of the theoretical foundations of nonradiative energy transfer mechanisms have also been elucidated over the past six decades and validated by an extensive body of experimental results which have been reviewed up to 1996 in an excellent article.9 (7) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809– 822. (8) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Bitumen and HeaVy Oils; Alberta Energy Research Institute: Calgary, Alberta, Canada, 2003; pp 588-592 (www.aeri.ab.ca). (9) Speiser, Sh. Chem. ReV. 1996, 96, 1953–1976.

R0 )

9000K2φF ln 10 5 4

6

28π n NR

∫

fD(ν) εA(ν) ν4

dν

(2)

is the Foerster critical energy transfer distance. Here, ν is the wavenumber of fluorescence, εA(ν) the molar extinction coefficient, fD(ν) the normalized spectral distribution of the donor fluorescence, N Avogadro’s number, φF the fluorescence quantum yield of the donor, n the refractive index of the solvent, and R the center-to-center distance between A and D. The orientation factor, K, is expressed as K ) cos φDA - 3 cos φD cos φA (10) Speiser, Sh.; Hasegawa, M.; Enomoto, Sh.; Hoshi, T.; Igarashi, K.; Nishimura, Y.; Yasuda, A.; Yanazaki, T.; Yamazaki, I. J. Lumin. 2003, 102–103, 278–282.

1158 Energy & Fuels, Vol. 22, No. 2, 2008

Figure 2. Two types of stretched conformers with orthogonal (a) and parallel (b) orientations of the two end groups of N and A chromophores for A1N, A3N, and A6N. Reprinted from ref 11.

where φDA is the angle between the transition dipole moments of D and A and φD and φA are the angles between the respective transition moments and the distance vector R. Recent time-resolved, pico- and femtosecond laser excitation fluorescence studies10,11 on the A-(CH2)n-N system where n was 1, 3, and 6 in stretched polyvinyl alcohol (PVA) and nonstretched polymethylenemethacrylate (PMMA) films carried out at 10 and 296 K have yielded important evidence for the role of the various conformer modifications of the AnN molecules affecting the rate of the intra-EET process. From these studies, it has been concluded that AnN molecules incorporated into stretched films prefer a conformation in which the methylene chain is elongated and the two end groups, A and N, are separated far apart, Figure 2 and Table 1. For these conformations, the intra-EET in AnN’s on the whole can be described approximately in terms of the Foerster dipole–dipole mechanism with some contributions from through-space exchange and through-bond superexchange interactions. It is also seen from the data in Table 1 that in stretched films the fluorescence decay kinetics become more complex with increasing length of the polymethylene bridge and their description requires higher exponential decay functions. Also, their decay lifetimes increase progressively with the bridge length from 2.1 ps in A1N to 687 and 4500 ps in A6N, as predicted by the Foerster dipole–dipole interaction, eq 1, and seen from Table 2. Calculations10 on the two stretched conformers with perpendicular and parallel orientations, Figure 2, yielded values, Table 2, in reasonable agreement with the experimental results, with the experimental values lying between those for the extreme conformations. As seen from the tables, for the A3N member the intense fluorescence is emitted from the perpendicular conformer and for the A6N member both the parallel and the (11) Hashegawa, M.; Enomoto, Sh.; Hoshi, T.; Igarashi, K.; Yamazaki, T.; Nishimura, Y.; Speiser, Sh.; Yamazaki, I. J. Phys. Chem. B 2002, 106, 4925–4932.

Strausz et al.

near-perpendicular conformers are present in not very different distributions. In the case of the A1N member, to account for the smaller experimental than calculated τF values, throughbond superexchange interaction via the single methylene group has been invoked. The important conclusion from these studies with respect to fluorescence decay lifetimes reported for asphaltene and other petroleum fractions is that AnN molecules incorporated into stretched films prefer a conformation in which the methylene chains are elongated. For such a conformation, the intra-EET on the whole can be described approximately in terms of the Foerster dipole–dipole interaction mechanism, with some contributions from through-space exchange and through-bond superexchange interactions. “On the other hand, the nonstretched films exhibit an intra-EET rate which is much faster than the stretched films, indicating that A3N and A6N in films take predominantly folded conformations similar to the case of AnN’s in fluid solutions.”10 On the basis of the experimental fluorescence decay lifetimes, Table 1, it can be estimated that the fluorescence decay lifetime of a longer bridge, for example, in the A12N molecule in stretched films at 10 K, will be longer than that of the A6N molecule. Thus, assuming that the Foerster dipole–dipole mechanism applies, a crude extrapolation from the data in Tables 1 and 2 would yield the values listed in Table 3. If, however, folding is permitted like in nonstretched films or fluid solutions at 296 K, these values would be considerably reduced due to the intervention of through-space exchanges. This gives an indication of how complicated the fluorescence decay kinetics can become with the extension of the bridging chain length and how undecipherable it would get for, say, an equimolar mixture of the A(CH2)nN series of molecules where “n” would run from 1 to 23. The upper limit 23 for “n” is not an entirely arbitrary figure, since that is the upper limit derived from the R,ω-di-nalkanoic acid series from the RuVIII-catalyzed oxidation of native asphaltenes arising from the cleavage of the polymethylene bridges between aromatic core structural units in the asphaltene molecules, for example. Of course, the molecular

structure of asphaltene is far more complicated than that of the simple hydrocarbons, A(CH2)nN, investigated in the references quoted, as will be briefly discussed in the next section of this article. Nonetheless, the composite spectrum or fluorescence decay plot of even a mega mixture of a different but similar class of molecules may appear to be deceitfully simple. One of the important distinguishing features of the asphaltene molecule is the presence of various side chains, -(CH2)n-, -S-, and so forth, which hinders and effectively suppresses folding of the molecule, creating an equilibrium distribution between folded and stretched orientations of the various conformers and thereby rendering the fluorescence decay kinetics even more complicated. Returning to the A(1–12)N system, in summary, it can be concluded that, depending on the lengths of the polymethylene bridges and the experimental conditions, a theoretically possible fluorescence decay lifetime beyond the measured lifetimes reported for asphaltene, τ1 ≈ 3.6 and τ2 ≈ 13 ns, may materialize. One high-τF-value molecule extensively investigated is that of 1,3-bi(1-pyrenyl)propane (Py3Py). In a nonpolar hydrocarbon solvent, the fluorescence emission spectrum of this molecule



Table 1. Fluorescence Decay Lifetimes (ps) of Naphthalene, τF (N*), in A1N, A3N, and A6N in Stretched PVA Films and Nonstretched PMMA Films with Excitation of N at λ ) 280 nm10,11 stretched

nonstretched

10 K τF A1N A3N

296 K

A6N

τF

ampl

2.1 51 173 71 687 4500

1 0.84 0.16 0.09 0.55 (?) 0.46 (?)

10 K 1 0.65 0.35 0.18 0.52 0.30

Table 2. Computed Foerster Fluorescence Decay Lifetimes (ps)11

for Two Stretched Conformers with Perpendicular and Parallel Orientations in Comparison with Experimental Values for the A1N, A3N, and A6N Bichromophores A1N A3N A6N a

perpendicular

parallel

exptla

4.6 56 345

4.4 102 4545

2.1 51 687 4500

From Table 1.

Table 3. Fluorescence Decay Lifetime (ns) Estimates for the A8N and A12N Molecules in Stretched Films at 10 K conformers A12N A8N

perpendicular

parallel

∼16 ∼4

∼135a ∼45

a

For long bridges, the Foerster dipole–dipole mechanism would break down: the maximum value for τF possible is the value for the free donor molecule, methylnaphthalene, τF0 ≈ 70 ns.

Table 4. Average Lifetimes (ns) and Amplitudes Obtained from MEMa Analysis of Experimental Results12,13 in the λexc ) 295

nm Femtosecond Laser Flash Excitation of 1,3-Bi(1-pyrenyl)propane at λF ) 376 nm T, K

τ1

τ2

334 293 273 253 230 77

2.6 6.4 8.9 14.6 37.7 209

40.4 47.5 67.0 66.8

τ3 189 142 221 211 259

a1

a2

a3

0.898 0.939 0.958 0.963 0.985 1.000

0.078 0.041 0.032 0.024

0.024 0.020 0.010 0.013 0.015

pyrene 77

521

τF

ampl

2.9 18 153 56 380 4090

1.000

Maximum entropy method. Concentrations: 9.3 × 10-6 mol/L in the liquid phase, 2.43 × 10-7 mol/L at 77 K. a

consists of a strong, structureless excimer band (λmax ∼500 nm) and a much weaker monomer band with distinct vibrational structure (370 e λF e 400 nm). Fluorescence decays of Py3Py at λF ) 376 nm where only the monomer emits and in the temperature range 77–334 K have been investigated12,13 employing different multiexponential functions for the evaluation of fluorescence decay lifetimes. The results are collected in Table 4. As seen from Table 4, in liquid n-heptane solutions, the values of τ1, τ2, and τ3 tend to increase from a value of 2.26 ns at 334 K with decreasing temperature, reaching their maximum of 37.7 ns at the lowest temperature employed, 230 K. Upon changing the matrix to a 1:1 methylcyclohexane-methylcyclopropane frozen glass, τ1 increases further to a value of 209 ns at 77 K. (12) Siermiarczuk, A.; Ware, W. R. J. Phys. Chem. 1989, 93, 7609– 7618; Chem. Phys. Lett. 1987, 140, 227–280. (13) Siemiarczuk, A.; Wagner, B. D.; Ware, W. R. J. Phys. Chem. 1990, 94, 1661–1666.

3.5 19 197 37 188 3800

296 K ampl 1 0.58 0.42 0.50 0.41 0.09

τF 2.7 14 140 46 138 3100

ampl 1 0.74 0.26 0.63 0.32 0.05

Under the same condition, the free pyrene molecule has a τ1 value of 521 ns. It should be noted that the high Py3Py τF values compared to the A3N τF values, Table 1, are in a large extent due to the extremely long τF0 value of the free Py molecule compared to that of the free methylnaphthalene molecule, 521 versus 70 ns. Section II: Some Relevant Structural Features of the Asphaltene Molecule. As has been extensively discussed in the literature,7,8,14–30 the asphaltene molecules possess loose, flexible architectures in which aromatic and naphthenic ring structural units are attached to the asphaltene core by -(CH2)n-, -S-, and possibly alkyl ester -C(O)-O- and -O-C(O)and alkyl ether -O- bridges. Along with bridges, these structural elements are also present as side chains.31 (14) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171–1187. (15) Yen, T. F. In Chemistry of Asphaltenes; Bunger, J. W., Li, L. C., Eds.;Am. Chem. Soc. Adv. Chem. Series; American Chemical Society: Washington, DC, 1981; Vol. 195, pp 39–51. (16) Ignasiak, T. M.; Kemp-Jones, A. V.; Strausz, O. P. J. Org. Chem. 1977, 42, 312–320. (17) Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1985, 2, 131–137. Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 2, 177–184. Mojelsky, T. W.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1986, 3, 43–51. Strausz, O. P. Am. Chem. Soc. Prepr. DiV. Geochem. 1988, 33, 264–268. In AOSTRA Technical Handbook; Hepler, L. G., Shi, C., Eds.; 1989; Vol. 3, pp 34– 73. In Fourth Intl. Conf. UNITAR/UNDP; Meyer, R. F., Wiggins, E. J., Eds.; 1988; Vol. 2, pp 607–628. In AIChE Symp. Ser.; Heck, R. H., Degnan, Th. F., Eds.; 1989; Vol. 85, pp 1–6. Am. Chem. Soc. Prepr. DiV. Petrol. Chem. 1987, 32, 396–397. Am. Chem. Soc. Prepr. DiV. Petrol. Chem. 1989, 34, 395-400. (18) Sinninghe Damste, J. S.; Kock-van Dalen, A. C.; Leew, J. W.; Schenck, P. A J. Chromatogr. 1988, 435, 435–452. (19) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1988, 4, 117–131. (20) Payzant, J. D.; Lown, E. M.; Strausz, O. P. Energy Fuels 1991, 5, 445–453. (21) Mojelsky, T. W.; Ignasiak, T. M.; Frakman, Z.; McIntyre, D. D.; Lown, E. M.; Montgomery, D. S.; Strausz, O. P. Energy Fuels 1992, 6, 83–96. (22) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355– 1362. (23) Strausz, O. P.; Lown, E. M.; Mojelsky, T. W.; Peng, P. Prepr. ACS DiV. Fuel Chem. 1998, 43, 917–923. (24) Strausz, O. P.; Mojelsky., T. W.; Faraji, F.; Lown, E. M.; Peng, P. Energy Fuels 1999, 13, 207–227. (25) Peng, P.; Strausz, O. P. Energy Fuels 1999, 13, 248–265. (26) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M.; Kowalewski, I.; Behar, F. Energy Fuels 1999, 13, 228–247. (27) Peng, P.; Fu, J.; Sheng, G.; Morales-Izquierdo, A.; Lown, E. M.; Strausz, O. P. Energy Fuels 1999, 13, 266–277. (28) Wang, Z.; Liang, W.; Que, G.; Qian, J.; Yang, G. Pet. Sci. Technol. 1997, 15, 559–577. (29) Su, Y.; Artok, L.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 1265–1271. (30) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287–296. (31) Behar, F.; Pelet, R.; Roucache, J. Org. Geochem. 1984, 6, 587– 595. Rubinstein, J.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1887–1893. Ritchie, R. O. S.; Roche, R. S.; Steedman, W. Fuel 1979, 58, 523–530.


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The presence of alkyl side chains and bridges attached to aromatic structural units has been proven by ruthenium-ioncatalyzed oxidation (RICO),14,23–30 that neatly converts them to their corresponding n-alkanoic acids and R,ω-di-n-alkanoic acids, respectively:

These structural elements can also be liberated31as mixtures of n-alkanes and 1-alkenes by thermolysis. Every single one of the three dozen or so different asphaltenes investigated afforded these products with carbon numbers of up to 23–29 and beyond in varying yields. These concordant results established clearly and unambiguously the presence of alkyl bridges connecting aromatic ring structures and aromatic attached alkyl side chains as universal structural features of the asphaltene molecules. Similarly, the presence of aromatic-naphthenic and naphthenicnaphthenic bridges has also been established. Asphaltenes in general are rich in sulfur, and their average sulfur content is about 6%. The sulfur is present mostly in the form of condensed thiophenes > acyclic sulfides > cyclic sulfides. The C-S bond in sulfides can be broken efficiently and selectively by nickel boride reduction to H2S. If the sulfides are present as acyclic sulfides, then the removal of the sulfur atom results in the breakup of the molecule and a drop in its MW. When Athabasca asphaltene was subjected to nickel boride reduction, the MW decreased about 4-fold,8,14 from ∼4800 to ∼1200 g/mol corresponding to the excision of three sulfur atoms representing three sulfide bridges,

Additional structural features identified in asphaltene molecules relevant to the present case are as follows: • n-alkyl side chains from RuVIII oxidation as methyl esters, up to C30, Figure 3 • R-C1–C4 branched n-alkyl side chains from RuVIII oxidation as methyl esters up to C31, Figure 4 • R,ω-di-n-alkyl bridges from RuVIII as dimethyl esters, Figure 5 • n-alkyl- and monomethyl-n-alkylbenzene up to C28, Figure 6 • aromatic condensed naphthenic rings, usually with alkyl side chains, Figure 7 • n-alkylthiophenes up to C29, Figure 8 • n-alkylthiolanes and thianes up to C29, Figure 9 • n-alkylbenzothiophenes up to C24, Figure 10 • n-alkyldibenzothiophenes up to C25, Figure 11 • some other condensed n-alkylthiophenes up to C26, Figure 12 • 9-n-alkylfluorenes, various biological markers attached directly to aromatic carbon in the asphaltene core or via one or two sulfur atoms, etc. Regarding high-sulfur asphaltenes, the most abundant aromatic structural units of the covalent asphaltene moleculessas determined from the analysis of their thermolysis productssare the condensed thiophenes, n-alkyl-substituted benzo- and dibenzothiophenes, Figures 10 and 11 and their higher condensed derivatives. Among the smaller sulfur aromatics identified were various alkyl-substituted one-to-five-ring species. Figure 12 shows the concentration distribution of some of the prominent

Figure 3. GC-MS total ion current chromatograms of the alkanoic acid methyl esters from Boscan and Duri asphaltenes (reprinted from ref 26), an unknown asphaltene (Kowaleski, I.; Behar, F.; Strausz, O. P. Unpublished results), and Saline Lake asphaltene (reprinted from ref 27). The numbers refer to the chain length of the esters, and the asterisks denote isoprenoid acid esters.

series as obtained from field ionization mass spectrometric studies, and of fluorenes, Figure 13, after oxidation to more readily analyzable species. The above structural units and structural elements have been estimated to account in an aggregate amount of slightly over 50 wt % of Athabasca asphaltene. While the quantitative aspects of these results show considerable variations from asphaltene to asphaltene, the qualitative aspects exhibit a remarkable invariance, testifying to the general validity of the results discussed.



was applied for multicomponent systems only in the work reported in refs 1-6 and therein. In multicomponent systems, however, it is not solely the MW of the components that will determine the rate of FD kinetics, but the concentration and absorption coefficient as well as the quantum yield and lifetime of fluorescence of each component will together cooperatively govern the composite FD kinetics. Moreover, when it is taken into consideration that, • in light of the huge number of different chromophores present in asphaltene, section II, some molecules will exhibit bi- or higher exponential decay kinetics, and • for the extraction of the MW values from the experimental data a knowledge of the spatial geometry of the excited state of each molecule is necessary, it will become obvious that the smooth appearance of the composite biexponential FD plots shown in Figure 1, Figures 8 and 9 of ref 6, and in other figures of refs 1-6 is highly misleading. These attributes of the FD-RCT method alone would be sufficient to disqualify it for the MW determination of asphaltene or any multicomponent systems comprised of mixtures of unknown molecules. However, in reality, the system is even more complicated and there are additional reasons of which either one by itself is again sufficient for disqualifying FD-RCT as a meaningful method for the MW determination of asphaltene. As has been briefly outlined in section II, the homologous series of alkylaromatic compounds, alkylbenzenes, benzothiophenes, dibenzothiophenes, fluorenes, etc., identified in the thermolysis products are attached to the asphaltene core by -CH2, -S-, and possibly some -O- bridges, revealing the bi- and multichromophoric architectural pattern of the asphaltene molecule. It is reasonable to assume that the monoalkylaromatic molecules, e.g., the 1-n-alkyldibenzothiophene series, Figure 11, occupy predominantly terminal positions in the alkyl bridge and in the asphaltene molecule. Higher alkyl-substituted aromatic

structural units may occupy terminal but probably mostly internal positions. Figure 4. GC-MS cross-scan (m/z ) 88, 102, and 116) chromatograms showing the presence of R-methyl, R-ethyl, and n-propyl branched alkanoic acids in the Saline Lake asphaltene. The numbers refer to the chain lengths of their methyl esters. Reprinted from ref 27.

In regard to the origin of the cyclic structural units, simple, well-founded chemical logic dictates that they were formed by the cleavage of one or perhaps two bonds in the aliphatic bridge connecting them to the asphaltene core and not by some excision operation from a large, single ring constituting the core of the asphaltene molecule, as suggested in refs 1-6:

Section III: The FD-RTC Method for MW Determination. The method was originally developed for the MW determination of a solute in a single component solution and

The bulk of the sulfur in crude oil is present as alkylsubstituted aromatic and naphthenic condensed thiophenes and the n-alkylbenzo- and n-alkyl-dibenzothiophenes, Figures 10-12, are major structural units of the high-sulfur asphaltene molecule. They are fluorescent when irradiated in their first absorption band around 283–295 nm with fairly short lifetimes. The experimental value of the florescence decay lifetime in deaerated dilute cyclohexane solution for dibenzothiophene is reported32 to be 3.0 ns and without deaeration, 2.7 ns. The shortness of the lifetime and the low value of the fluorescence quantum yield, (32) Nijegorodov, N.; Luhanga, P. V. C.; Nkoma, J. S.; Winkoun, D. P. Spectrochim. Acta, Part A 2006, 64, 1–5.


Strausz et al.

Figure 5. Gas chromatograms of the R-ω-di-n-carboxylic acid methyl esters from the organic phases of the RICO of Boscan, Duri, and X asphaltenes and the combined phase of the RICO of the Saline Lake asphaltene, along with plots of the number of bridges as a function of the length for the Gudao and Arabian asphaltenes. The numbers refer to the chain lengths of the esters. The Boscan and Duri plots are reprinted from ref 26. The X plot is from Kowaleski, I.; Behar, F.; Strausz, O. P. Unpublished results. The Saline Lake plot is reprinted from ref 27. The Gudao plot is reprinted with permission from ref 28. Copyright 1997. Taylor & Francis. The Arabian plot is reprinted from ref 29.

φF ) 0.08, are due to the internal heavy atom effect of sulfur promoting efficient intersystem crossing, S1–T1, to the lowest triplet state.32 In the following, the discussion will be focused on the shortest wavelength spectral group with λexe ∼280–290 nm and λF ∼330 nm dealt with in refs 1-6 which, however, would not limit the generality of the conclusions.

If, in the asphaltene molecule, the n-alkyldibenzothiophene (or any other suitable intra-EET donor) is attached to the asphaltene core via an aromatic moiety capable of acting as an electronic excitation acceptor, many series of bichromophoric substrates would be generated with the dibenzothiophene as the donor moiety. The efficiency within a series would vary with the length of the methylene bridge and would be governed by the same



Figure 6. GC-FID trace of the alkylbenzenes in Athabasca asphaltene pyrolysis oil. The numbers above the clusters of peaks indicate the total number of carbon atoms in the corresponding alkylbenzenes. The inset is an expansion of the area about C20. Peak m, meta-monomethyl n-alkylbenzene; peak n, n-alkylbenzene; peak o, ortho-monomethyl n-alkylbenzene; peak p, para-monomethyl n-alkylbenzene. Reprinted from ref 20.

Figure 8. Cross-scan chromatograms for m/z ) 97, 111, and 125 showing the thiophenes in Athabasca asphaltene pyrolysis oil. The numbers above the peaks correspond to the total number of carbon atoms in the thiophene. The peaks in the bottom panel marked by an asterisk correspond to the 2-n-alkyl 5-ethylthiophenes; the remaining peaks are unidentified. Reprinted from ref 19.

Figure 7. Distribution of the aromatic hydrocarbons in the pyrolysis oil of Athabasca asphaltene as determined by field ionization mass spectrometry. Reprinted with permission from Payzant, J. D.; Rubinstein, I.; Hogg, A. M.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1187. Copyright 1979. Elsevier.

interactions as in the case of the AnN bichromophores. Since the fluorescence lifetime of dibenzothiophene is lower than that of methylnaphthalene, 2.7 versus 70 ns, the fluorescence lifetime of the series may be expected to be shorter as well with a limiting upper value of about 2.7 ns, the lifetime of the free dibenzothiophene molecule. A similar consideration applies to the series of alkylated phenylthiophenes, diphenylthiophenes, and benzothiophenes. Most of these compounds absorb at 290 nm and fluoresce at 330 nm, and any of them could act as donor molecules to appropriate aromatic and heteroaromatic acceptors. The fluorescence lifetimes of all of these molecules are even shorter than that of dibenzothiophene, 32–383 ps, and the phenylthiophenes exhibit biexponential decay kinetics.33

Figure 9. Cross-scan chromatograms for m/z ) 87, 101, and 115 showing the n-alkyl sulfides in Athabasca asphaltene pyrolysis oil. The numbers above the peaks represent the number of carbon atoms in the molecule. Reprinted from ref 19 and Payzant, J. D.; McIntyre, D. D.; Mojelsky, T. W.; Torres, M.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1989, 14, 461.

(33) We are grateful to Professor W. R. Ware and Dr. D. R. James formerly at the University of Western Ontario for this information.

Among the homoaromatics that have been detected in asphaltene and may act as donors in bichromophoric systems


Figure 10. Cross-scan chromatograms for m/z ) 147 and 161 showing the n-alkylbenzo[b]thiophenes in the pyrolysis oil of Athabasca asphaltene. The numbers above the peaks correspond to the total number of carbon atoms in the benzo[b]thiophenes. Reprinted from ref 19 and Payzant, J. D.; McIntyre, D. D.; Mojelsky, T. W.; Torres, M.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1989, 14, 461.

Strausz et al.

Figure 12. Distribution of the aromatic sulfur compounds in the pyrolysis oil of Athabasca asphaltene as determined by field ionization mass spectroscopy. Reprinted with permission from Payzant, J. D.; Rubinstein, I.; Hogg, A. M.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1187. Copyright 1979. Elsevier.

Figure 11. Cross-scan chromatogram for m/z ) 197 showing the n-alkyldibenzothiophenes in the pyrolysis oil of Athabasca asphaltene. The numbers above the peaks indicate the chain lengths of the 1-nalkyl groups and A, B isomeric methyl groups. Reprinted from ref 20.

in the right spectral range, 290 and 330 nm for excitation and fluorescence, are series of alkylated naphthalenes, acenaphthenes, biphenyls, fluorenes, etc. (Figure 13). All of these and the sulfur compounds discussed so far contribute to the fluorescence decay plots shown in Figure 3a and Figure 9 in refs 1 and 6, respectively. Now, comparing the fluorescence decay time values for the A(CH2)nN system10,11 (where n ) 1, 3, 6) measured at 10 K in stretched films, which nearly coincide with the theoretically computed values, with those measured in nonstretched films at 296 K which is similar to the case of A(CH2)nN in fluid solution, Table 1, one can see that • there is in both cases only one slow-decaying conformer with τF values of 3.1–4.5 ns; • in the nonstretched film the role of the slow-decaying conformer is quite minor, whereas in the stretched film this conformer plays a major role; • the faster fluorescence decays (smaller τF values) in A3N and A6N in nonstretched films are attributed to the phenomenon of folding, shortening the through-space distance between the A and N chromophores in the molecule; • this in turn brings into play the through-space exchange interaction which accelerates the rate of intra-EET;

Figure 13. GC-MS of the fluorenols from Athabasca asphaltene pyrolysis oil. Upper trace, total ion current; middle trace, m/z ) 181, the base peak of the mass spectra of 9-n-alkylfluoren-9-ols; bottom trace, m/z ) 195, the base peak of the mass spectra of monomethyl9-n-alkylfluoren-9-ols. The number above the peaks corresponds to the total number of carbon atoms in the fluorenols. Reprinted from ref 20.

• it can be estimated that over 90% of fluorescence from the AnN (n ) 1–6) system in fluid solution has a τF value less than 150 ps at 296 K.


The structure of the A(CH2)nN molecules differs from that of the bichromophore molecules in the asphaltene in that the latter contains an abundance of side chains, n-alkyl, somewhat branched n-alkyl, n-alkylester, n-alkylether, n-alkylthiolane, n-alkylthiane, n-alkylthiophene, n-alkylbenzene, and various biomarker molecules which, in the case of Athabasca asphaltene, amount to about four side chains plus one sulfide bridge per covalent molecule of MW ∼1200 g/mol. The steric interference exerted by these side chains with aromatic stacking is an important factor in the stability of asphaltene solutions which is manifested by the significant drop in stability upon partial dealkylation by thermal treatment. Therefore, it can be predicted that in dilute asphaltene solutions the folding phenomenon would be markedly reduced relative to the polymethylene-bridged bichromophore hydrocarbon molecules discussed in section I, and thus, dilute asphaltene solutions could be described by the dipole–dipole interaction and consequently the fluorescence decay lifetime values for -(CH2)1–6- bridges vary in a similar fashion to those for stretched A-(CH2)1,3,6-N molecules, Table 1. For extended length methylene bridges, lower limits for τF values of the highest amplitude conformers could be very roughly estimated as τF ∼4 and ∼6 ns for n ) 8 and 12, respectively. For chain lengths longer than n ) 12, the value of τF would keep on increasing, gradually approaching the lifetime of the free donor, methylnaphthalene, ∼70 ns. The experimentally measured values reported in refs 1-6, 3.5 and 13 ns, could be reached around (CH2)13-(CH2)14, that is, at chain lengths well within the experimentally measured chain lengths (CH2)2-(CH2)25 occurring in asphaltene, Figure 5. Now, regarding the claim put forth in refs 1-6 that the measured lifetime values for the fluorescence decay from dilute asphaltene solutions, ∼3.5 and 13 ns, are too long for intraEET processes and therefore such processes do not occur in the photophysics of asphaltene, we can conclude that such claims are ill-founded, since values up to τF1, τF2, τF3 ) 6.4, 47.5, and 142 ns for Py3Py and 0.071 (0.09), 0.687 (0.55), and 4.5 (0.46) ns for A6N have been measured experimentally, Tables 1 and 2. For longer polymethylene bridges, through-space dipole–dipole based estimates predict even larger values, but for very long chains, the dipole–dipole interaction breaks down and the expected upper limits would approach the τF0 values of the free D* molecules, 521 and 70 ns, respectively. Quite aside from the foregoing arguments, the work reported in refs 1-6 suffers from fatal experimental inadequacies. It is stated in refs 1 and 6 that the spectra and fluorescence decay measurements were done “at beamline U9B at the National Synchrotron Light Source at Brookhaven National Labs” using 2 ns pulses. However, with 2 ns exciting light pulses, it would be just about impossible to detect fluorescence with τF e 300 ps or to measure lifetimes much less than about 500–600 ps in such feeble light beams as emerging from highly dilute asphaltene solutions. Thus, the only conformers in Tables 1 and 2 for which τF values could be obtained would be the slowestdecaying ones in the triexponential decay kinetics of A6N, 3.1 and 4.5 ns. This is the reason that in frontline studies of intraEET fluorescence the excitation light source employed was picosecond or, even better, femtosecond laser pulses.10–12


As was mentioned before, asphaltenes, with minor exceptions, are rich in sulfur and over 60% of the sulfur is present in the form of alkylated thiophenes, benzo-, dibenzo-, and higher benzo-, naphtheno-, and arylthiophenes. Athabasca asphaltene with a sulfur content of 8% would, with an average MW for the thiophenes of about 220–250 g/mol, contain about 35% thiophenes. Therefore, the aggregate fluorescence landscape of the countless number of bichromophore (multichromophore) molecules in Athabasca asphaltene can be expected to be dominated by the thiophene chromophores as the donor moiety. Since the fluorescence decay lifetimes of the free thiophenes are much shorter than those of the homoaromatic chromophores discussed above, their average fluorescence decay lifetimes as donor chromophores in bichromophore molecules will also be much shorter and consequently a larger proportion of them will avoid detection due to instrumental limitations. Summary and Conclusions Measurements of fluorescence decay lifetimes, as reported in refs 1-6 and references cited therein, emerging from dilute asphaltene solutions led the authors to the conclusion that the values obtained, 3.5 and 13 ns, were incompatible with bichromophoric emissions. This, in turn, induced them to conclude that the asphaltene fluorescence originates from molecules having a single condensed aromatic ring structure. If that is so, then fluorescence depolarization-rotational correlation time measurement would lead to the determination of an average molecular weight of the asphaltene solute. However, this stream of logic and measurements is erroneous at every level as summarized below: • Fluorescence decay time measurements on highly complex mixtures of constituent molecules with unknown molecular weights, molecular shapes, concentrations, extinction coefficients, fluorescence decay times, kinetics, quantum yields, etc., may yield numerical information with no physical meaning (e.g., one major component with low extinction coefficient and fluorescence quantum yield may be obscured by a minor component with high values for the above properties). • Bichromophoric molecules of the types expected in asphaltene molecules may assume more than one conformeric structure with different fluorescence properties. The different conformers with different foldings and relative orientations of the chromophore moieties may follow different intra-EET paths, involving long-range throughspace Coulombic, Foerster type dipole–dipole interaction, short-range through-space electron exchange, and throughbond super electron exchange interaction, resulting in differing rates and bi- and higher exponential decay kinetics. The reaction rates may vary among the various conformers by as much as 2 orders of magnitude. This can render the decay kinetics even for a single component system complicated and for a multicomponent system even less or not at all interpretable. • For bichromophoric systems, fluorescence decay times for the slowest-decaying conformers as large as 3.1–4.5 ns and for a triexponential decay kinetics as large as 6.4, 47.5, and 142 ns for the three conformers have been reported in the literature. This fact invalidates the argument that the magnitude of τF measured by the authors of refs 1-6 exclude bichromophoric molecules as the source of the fluorescence from asphaltene solutions. • Chemical studies have brought to light irrefutable evidence for the existence of polymethylene and sulfide-bridged


•

•

•

•

•

bichromophoric molecules as principal components of asphaltene. Another important structural feature of the asphaltene is the presence of alkyl, ether, ester, and other side chains of length up to C30 attached to the ring structures. The side chains inhibit intramolecular stacking of the aromatic ring moieties and diminish the role of throughspace exchange interaction, thereby restricting the intraEET process to the Foerster type dipole–dipole interaction. The effect of this on the fluorescence is an increase in the value of τF relative to molecules without side chains. That suggests substantially larger values for τF for polymethylene bridges in the asphaltene, -(CH2)e23-, than in the model molecules, -(CH2)e6-, studied, and consequently a large proportion of fluorescence with decay lifetimes exceeding the values reported in refs 1-6 or ref 10 or 11. Chemical studies revealed the presence of acyclic sulfides in sulfur-containing asphaltenes forming bridges between molecular core segments. Intra-EET may occur through such bridges as well. Aside from the argument centered on the magnitude measured for τF, overwhelmingly convincing evidence against a single condensed aromatic sheet forming the core of the asphaltene molecule comes from all of the chemical structural studies reported on asphaltene. Mild thermal decomposition of asphaltenes yields a plethora of small aromatic and naphthenic molecules providing irrefutable evidence against a single core molecular framework and support for a network of small ring molecules connected by methylene, sulfide, and perhaps ether and ester bridges. Finally, the fluorescence decay lifetime measurements reported in refs 1-6 suffer from a fatal instrumental

Strausz et al.

limitation. The exciting flash time duration was 2 ns which would not permit the detection of τF of less than about 300 ps or the measurement of τF less than 500–600 ps, especially not in a decay kinetic curve of such enormously complex mixture like asphaltene dispersed in a highly dilute solution. • Also, the rate of fluorescence depolarization measurements for molecular weight determination and their interpretation suffer from shortcomings similar to those of the fluorescence decay times discussed above. Additionally, as was shown34 over 20 years ago, the yield of fluorescence per unit weight strongly depends on the (VPO) molecular weight of the asphaltene; it monotonically falls off with increasing molecular weight of the separated fractions of asphaltene and the highest molecular weight fractions (>17 000 g/mol) barely emit. Regrettably, the injection of the fluorescence depolarizationrotational correlation time molecular weight determination method with its seriously mistaken corollaries into asphaltene chemistry has caused a measure of confusion that hopefully now has been resolved and this episode in asphaltene chemistry has been closed for good. In the final epilogue, we wish to state that the literaturereported molecular weight values and inferences postulated with respect to the molecular architecture of asphaltene based on fluorescence decay and depolarization kinetic and rotational correlation time studies of asphaltene should in their entirety be disregarded. EF700320P (34) Yokota, Y.; Scriven, F.; Montgomery, D. S.; Strausz, O. P. Fuel 1986, 65, 1142–1149.

A Critique of Asphaltene Fluorescence Decay and Depolarization

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