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New Answers to the Optical Interrogation of Asphaltenes. I. Monomers and Primary Aggregates from Steady State Fluorescence Studies Igor N. Evdokimov, Aleksey A. Fesan, and Aleksandr P. Losev Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00027 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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New Answers to the Optical Interrogation of Asphaltenes. I. Monomers and Primary Aggregates from Steady State Fluorescence Studies Igor N. Evdokimov,* Aleksey A. Fesan, and Aleksandr P. Losev Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, Moscow B-296, GSP-1, 119991, Russia

ABSTRACT: The size and structure of aggregating asphaltene molecules has been a controversy for several decades. In recent years, advocates of the so-called “Modified Yen Model” (MYM) describe the smallest asphaltene molecules as species with fairly large aromatic fluorophores, typically with 7–10 fused rings. This description is principally based on the experimental “optical interrogation of asphaltenes” by fluorescence techniques. We perform a series of steady state fluorescence emission (SSFE) studies of very dilute solutions with asphaltene concentrations in benzene down to 0.34 mg/L. Our results clearly show that the MYM description of the smallest asphaltenes is fundamentally wrong. First, the relevant experiments were misinterpreted because of the assumption that asphaltenes do not aggregate at concentrations of 10-25 mg/L, while new SSFE data indicate that asphaltenes form primary aggregates at concentrations as low as ca. 0.7 mg/L. Furthermore, the original MYM experiments suffered from a serious flaw in data processing, namely neglecting inner filter (self-absorption) effects which strongly distort the shapes of measured SSFE spectra. In contrast to the popular MYD description, the new SSFE experiments show that aggregating asphaltenes appear to be much smaller molecular species, typically with 1-3 ring aromatic fluorophores. By using very sensitive fluorescence techniques, such basic molecules may be identified in very dilute (≤ 0.34 mg/L) asphaltene solutions by their characteristic peaks in SSFE spectra. New SSFE peaks from primary asphaltene aggregates of 1-3 ring molecules form from hydrogen bonding at concentrations below the sensitivity limits of most other experimental techniques. On the other hand, the SSFE data show that larger (>4 ring) asphaltene molecules are apparently inactive during aggregation over the studied concentration range. According to our literature analysis, primary asphaltene aggregates may be described as multi-fluorophore supramolecular complexes with “archipelago” structures of basic asphaltene molecules.

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1. INTRODUCTION Asphaltene constituents of petroleum impact virtually all aspects of crude oil production and utilization.1,2 Undesirable asphaltene precipitation is a serious concern to the petroleum industry because asphaltenes can plug up well bores and stop oil production, in addition to blocking pipelines. These problems become more significant as the industry moves offshore into deep water, where prevention and remediation costs rise dramatically.3 However, after decades of intensive experimental and theoretical studies, the association and aggregation mechanisms of asphaltenes are still not well-characterized and are subjects of ongoing debate [cf. refs. 4-6 and references therein]. One important problem that remains is the structure of individual asphaltene molecules. Asphaltenes are known to form nano-sized colloidal particles – “nanocrystallites”,7,8 “nanocolloids”9 or “nanoaggregates” in good solvents and native crude oils at concentrations of ~100 mg/L and above.10 Less known is experimental evidence that suggests that asphaltenes can aggregate into smaller oligomeric species at much lower concentrations – down to 5 mg/L4-6 or even below 1 mg/L7,8 (for earlier experiments - cf. a review in ref. 9). This reason may explain why important conclusions on the structure of aggregating asphaltene molecules (“monomers”) in the most popular description of petroleum asphaltenes – the so-called “Modified Yen Model”10 – were improperly deduced from measurements of steady state fluorescence emission (SSFE) spectra, which were performed at asphaltene concentrations as high as 10-25 mg/L.11-21 As a typical example, Figure 1A shows a copy of an SSFE spectrum from a solution of UG8 asphaltenes (from Kuwait crude oil) for 290 nm excitation. This spectrum was first reported in a 1996 paper11 and is reproduced as a part of various figures in multiple later publications.12,19,21,10 Based on this and similar spectra, the position of the maximum at ∼450 nm was assumed to indicate a virtual absence of 1-3 ring aromatic molecules in asphaltenes and the predominance of non-aggregated molecules with 7-10 fused aromatic rings (FARs).10-21 A quantitative description of the assumed size distribution of non-aggregated asphaltene molecules (“monomers”)10 is presented in Figure 1B. As ACS Paragon Plus Environment

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the “Modified Yen Model” was further developed, the above assumptions from SSFE studies were employed as a well-proven foundation to interpret data from other experimental techniques.22-31

Figure 1. Typical experimental SSFE spectrum of asphaltenes (A) that was employed in the “Modified Yen Model” to deduce the size distribution of non-aggregated asphaltene molecules (B) (for experimental details and references cf. text). In this paper, we report new results on SSFE spectra in very dilute solutions of asphaltenes. We show that non-aggregated asphaltenes may be observed only at sufficiently low concentrations that do not exceed 0.6 mg/L, where the predominant fluorescent species are small 1-3 ring aromatic fluorophores that emit at 280-330 nm. We also show that the 450-500 nm peaks at higher concentrations are related to emissions from primary aggregates of these small fluorophores but not from individual asphaltene “monomers”, as erroneously assumed in the “Modified Yen Model”. Hence, the basic assumption of this model should be discarded and the legitimacy of the model’s predictions should be reconsidered.

2. EXPERIMENTAL 2.1. Materials. Samples of heavy crude oil (10.9 wt. % asphaltenes) were collected from the well head at an oilfield in northern Russia. n-C7 (heptane insoluble) solid asphaltenes were separated from the crude oil according to the standard ASTM D 6560 method.32 A stock solution of solid asphaltenes in benzene (99.8 % “chemically pure” grade) were prepared with an asphaltene concentration of 3110 mg/L to avoid undesirable oxidation during our lengthy experiments. Before ACS Paragon Plus Environment

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use, this solution was stored for two weeks in a hermetically sealed 125 ml glass vessel in the dark and at ambient pressure and temperature (23.1-25.2 oC). 2.2. Preparation of Dilute Asphaltene Solutions. The concentration of asphaltenes in each dilute solution was verified by weighing ca. 50 ml of pure benzene before and after mixing with a pre-determined volume of a stock solution (weighting accuracy of 0.1 mg). Each prepared solution was intermixed with a glass rod and poured into a dark glass vessel for equilibration. For decades, the importance of the equilibration time (kinetic effects) in optical studies of asphaltene solutions has not been recognized. Changes in the aggregation state of asphaltenes have been implicitly assumed to be virtually instantaneous. For instance, in refractive index studies,33 asphaltene solutions of varying compositions were prepared and measurements were performed a few minutes after mixing. In the aforementioned steady state fluorescence studies,11-21 the period between the preparation of solutions and collection of data did not exceed 12 min.12 However, asphaltene aggregation/dissolution have recently been proven to be kinetic phenomena and can take several hours,34 days or weeks in some cases.35 Consequently, optical studies were performed at various times after sample preparation, namely, after 1, 3, and 6 hours or 1, 3, 5, and 7 days, to ensure that the asphaltene aggregates in the studied dilute solutions were in their representative equilibrium states. During the first day after mixing, strong kinetic effects were observed in all the studied solutions. The results virtually stabilized after a period of two or three days; hence, asphaltenes in solutions that aged over a standard period of 3 days were regarded to be in their equilibrium states. 2.3 Measurements of Optical Absorption. Optical absorbance spectra A(λ), which have been employed to correct SSFE data for inner-filter effects (cf. below), were measured at the 2701100 nm range under an SF-56 Spectrophotometer (ZAO “OKB SPECTR”, Saint-Petersburg,


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Russia). Ten-millimeter sample and reference (benzene) quartz cuvettes were used, and the measurements were performed at 24±0.5 oC. 2.4 Measurements of Steady State Fluorescence Emission (SSFE) Spectra. SSFE measurements were conducted by using a Cary Eclipse fluorescence spectrophotometer with classic 90o geometry and 10-mm quartz sample cuvettes. The excitation wavelengths varied from 265 nm to 420 nm. For all samples the same procedure of recording the SSFE spectra was used. Namely, for all spectra we employed excitation and emission slits with 5-nm bandwidths; the spectra were recorded with 1-nm intervals in the total 265-900 nm wavelength range available in the particular instrument. The wavelength scan rate was set to 120 nm/min. Note that spectrophotometers with diffraction gratings, in addition to the principal (“first order”) emission spectrum of the studied substance, may exhibit several other (“second order”, etc.) spectra of progressively smaller intensities, which may also be registered by the detector in the employed wide range of emission wavelengths36 - cf. Figure 2 below. As other modern spectrophotometers, our instrument had the option of suppressing higher order spectra via absorption of some wavelengths ranges by introducing specially designed optical filters. We did not employ this option, because absorption spectra of these filters were not specified in the instrument’s manual; hence, some artifacts of unknown origin could appear at the tails of the measured/analyzed first order SSFE spectra. 2.4.1 Data Corrections for “Inner Filter Effects”. Accurate fluorescence measurements crucially depend on accounting for “inner filter effects” (IFE).36 IFEs can be segregated into “primary IFEs” from the attenuation of the excitation light with λex and “secondary IFEs” that attenuate the fluorescence emission with λem. In this study, we employed the absorbance-based approach for IFE correction,36 where the observed fluorescence intensity ( I λobs ex , λem ) is converted into the corrected fluorescence intensity ( I λcorr ex , λem ) for each pair of excitation and emission wavelengths by the following transformation:


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obs {0.5×[ A ( λex ) + A ( λem )]} I λcorr ex ,λem = I λex ,λem × 10

The factor 0.5 in the power exponent appears in the 90o geometry because absorbance (excitation) and fluorescence (emission) are assumed to occur at the midpoint of the cuvette. 2.4.2 Data Corrections for the Solvent Background and Instrumental Effects. Figure 2 shows SSFE data that were obtained with pure solvent (benzene) in the sample cell. In this particular spectrophotometer’s optical scheme, both first-order and second-order emission spectra were observed at all the studied excitation wavelengths. For the 265-nm excitation (Figure 2A), only solvent fluorescence of very low intensity was registered. For the excitation wavelengths of 300-420 nm, broad solvent fluorescence spectra were accompanied by much stronger (greater than two orders of magnitude) narrow peaks of stray excitation light (cf. Figure 2B). On the one hand, these excitation peaks interfered with the SSFE spectra of asphaltenes and had to be removed before data analysis; on the other hand, these peaks appeared to be useful for calibrating the intensity of excitation light beams at various λex values (cf. Figures 4 and 5).

Figure 2. Fluorescence emission spectra from a cuvette with pure solvent, obtained with 265-nm (A) and with 300-nm (B) excitation. The vertical straight lines denote the excitation wavelengths. Accordingly, after the IFE corrections, the measured SSFE data for the asphaltene solutions were further processed by:


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1) Removing all the data points for the second-order (so) spectra at λem > λso≈0.5λex; the exact value of λso was identified by the sharp rise of intensity at the base of second order peak of excitation light. 2) Removing the data points for the first order excitation peak at λem ≤ λex + 8 nm (for 300420-nm excitation). The intensity of excitation peak was registered and stored for further normalization of some experimental data (cf. Figures 4 and 5). 3) Subtracting the background solvent fluorescence from the remaining range of emission wavelengths.

3. RESULTS AND DISCUSSION 3.1. Most Dilute Asphaltene Solutions. Predominance of 1-4 Ring Aromatic Fluorophores. In this section, we present SSFE data that were obtained for a benzene solution with an asphaltene concentration of 0.34 mg/L. According to these results and a comparison with data for higher concentrations (cf. the following sections), we conclude that asphaltenes exist in nonaggregated states at 0.34 mg/L. The data points that merge into thick lines in Figure 3 are the corrected SSFE spectra of asphaltenes for excitation wavelengths of 265, 280 and 300 nm (normalized to the maximum intensities). For comparison, the thin solid lines show the normalized standard SSFE spectra of some individual hydrocarbon molecules with one to four fused aromatic rings, which are labeled by corresponding numbers in the upper part of Figure 3. If not indicated otherwise, these SSFE spectra are reproduced from two open access databases.37,38 The position of any SSFE peak crucially depends on the number of aromatic rings in a particular molecule, while the effects of various substituents are comparatively small.37,38


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The data analysis shows that the structure of the asphaltene SSFE spectra in Figure 3 indicates the existence of underlying characteristic peaks (with positions that are defined by the nature of the emitting fluorophores and not dependent on λex) in the following wavelength ranges: 275-310 nm, 315-335 nm and 360-380 nm. A comparison with the standard spectra shows that the emission in the first range may be attributed to fluorophores with a single aromatic ring – like in substituted benzenes (cf. spectra “1” and “1A” for 1,4-dicyanobenzene39 and styrene). The wavelength range of 315-335 nm may be attributed to emission from fluorophores in non-substituted or substituted molecules with two fused aromatic rings (cf. spectra “2” and “2A” for naphthalene and 1-methylnaphthalene). Finally, a structure at 360-380 nm may be attributed to emission from fluorophores in individual molecules (substituted or not) with three fused aromatic rings (cf. spectra “3” and “3A” for phenanthrene and anthracene). We also observed a possible small contribution from compact 4-ring fluorophores, which emit in the same wavelength range (cf. spectrum “4” for pyrene, which practically coincides with the spectrum for anthracene).

Figure 3. Data points - corrected spectra of fluorescence emission from 0.34 mg/L asphaltene solution, which were obtained from short-wavelength excitation (265-300 nm). Thin solid lines – reference emission spectra for some 1-4 ring aromatic fluorophores. ACS Paragon Plus Environment

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For the 265-nm excitation (upper part of Figure 3), the predominant species that contribute to the SSFE spectrum of asphaltenes evidently contain one and two ring aromatic fluorophores. Only a low-intensity characteristic feature at the tail of the spectrum may be attributed to emission from fluorophores with three fused aromatic rings (and, possibly, from compact 4-ring fluorophores). For the 280-nm excitation (middle part of Figure 3), emission from single ring fluorophores (and, to some extent, from fluorophores with two fused rings) is strongly suppressed, as expected from the wavelength dependencies of the respective absorbancies.36,38 While the predominant characteristic emission peak remains that from 2-ring fluorophores, the broad characteristic peak from 3-ring (and from some 4-ring) fluorophores has comparable intensity. The measured relative intensities of characteristic fluorescence peaks are not directly proportional to the relative abundances of the molecules with respective fluorophores in the studied samples because of differences in individual extinction coefficients, quantum yields, etc.36 Hence, without additional information from other experimental techniques, the size (or MW) distribution of asphaltenes cannot be derived solely from fluorescence measurements, as assumed, e.g., in the “Modified Yen Model”10 (cf. Figure 1B). The erroneous nature of these assumptions has been pointed out before40,41 with the conclusion that such an interpretation may yield numerical information with no physical meaning.41 Additionally, as noted above, the positions of characteristic peaks indicate only a number of aromatic rings in individual asphaltene fluorophores, while any conclusions regarding the nature of particular substituents would be merely speculative. Moreover, our experiments have shown that characteristic emission peaks from specific fluorophores in the SSFE spectra of most dilute asphaltene solutions are no longer observed at excitation wavelengths above 300 nm. In the particular case of λex=300 nm (bottom part of Figure 3), one-ring fluorophores are no longer excited; the characteristic two-ring peak is still resolved at ∼327 nm, but no features are present along the smooth continuous tail of the spectrum that indicate


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the existence of the characteristic 3-ring peak. Furthermore, as augmented below, the intensity maximum at ca. 350 nm (which is essential, 50 nm above λex) is not a characteristic emission peak from some specific fluorophores but belongs to a previously unreported continuous fluorescence spectrum that is collectively shaped by a multitude of various fluorophores with a number of fused aromatic rings ≥4 and, possibly, by some aggregates of molecules with these fluorophores. The differences in the position and intensity of this maximum are determined by some universal mechanism of the “cutoff” of a common continuous emission spectrum by the particular λex. In support of these conclusions, Figure 4 shows SSFE spectra of asphaltenes for excitation wavelengths of 320-420 nm. All the spectra are normalized to equal excitation light beam intensity via dividing by the registered intensities of first order excitation peaks (cf. section 2.4.2).

Figure 4. Corrected spectra of fluorescence emission from 0.34 mg/L of asphaltene solution, which were obtained with long-wavelength excitation (320-420 nm). The spectra are normalized to the intensity of the excitation light.

No characteristic peaks (independent of λex) are visible in Figure 4, while the positions of all the “cutoff” emission maxima are almost exactly 50 nm above the respective excitation wavelengths. Above the “cutoff” peaks, all the spectra exhibit common “collective” wavelength dependence, which is fairly linear up to ∼550 nm. On the log-linear scale in Figure 5A, this dependence is linear at longer wavelengths, indicating exponential behavior. Further analysis has shown that an apparent common underlying mechanism of “collective” fluorescence is also present in wavelengths ranges below the “cutoff” peaks. Namely, all the spectra in these ranges (normalized to the peak intensity ACS Paragon Plus Environment

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values) may be reduced to a single master curve merely via shifting each spectrum along the abscissa axis by subtracting the respective value of λex from the registered values of λem. For short, the difference λem-λex may be called “reduced wavelength”. The resulting plot of the discussed master curve is presented in Figure 5B.

Figure 5. A – The common behavior of the falling parts in all the spectra from Figure 4 is emphasized by a log intensity scale. B – The common “cutoff” nature of all the rising parts in these spectra is revealed by plotting the fluorescence intensity versus “reduced wavelength” (λem-λex).

3.2. Less Dilute Asphaltene Solutions. Predominant Primary Aggregates of 1-4 Ring Aromatic Fluorophores. In this section, we compare the “most dilute” results (for 0.34 mg/L) with SSFE data that were obtained for a benzene solution with double the concentration (i.e., 0.68 mg/L). Additionally, we report the SSFE spectra that were measured at typical concentrations for basic experiments of the “Modified Yen Model” (cf. Introduction). In the figures below, the respective data sets are numbered as follows: “1” - 0.34 mg/L; “2” - 0.68 mg/L; “3” – 15 mg/L; and “4” – 27 mg/L. The fluorescence intensity naturally increases as the concentrations of the studied molecules in the solutions increases. Assuming no structural changes in the emitting fluorophores (caused, e.g., by aggregation), the intensity increases concomitantly at all wavelengths and the shape of the emission spectra remains unaltered. Then, all the spectra for various concentrations would collapse


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into a single wavelength dependence when normalized to the total intensity (equal area of the measured spectra). If aggregation is present, the equal area spectra would not be identical and would exhibit zones of enhanced intensity because of emission from appearing aggregates and zones of diminished intensity from the respective aggregating fluorophores.36 Figure 6A shows SSFE spectra (normalized to equal areas of I(λ) graphs) for the 265-nm excitation of four samples with the above-listed asphaltene concentrations. Deviations from the spectrum of non-aggregated fluorophores (curve “1”) are observed at concentrations as low as 0.68 mg/L (curve “2”), which indicates the appearance of primary aggregates (PA) – possible types of these aggregates are discussed in Section 3.3. At higher asphaltene contents, PAs clearly become the predominant fluorescent species and the intensity maximum shifts from 295-310 nm to 440-465 nm. Apparently, aggregation processes are most intense below 15 mg/L because only marginal changes in the equal area spectra occur after further increasing the asphaltene concentration to 27 mg/L (compare curves “3” and “4”). We subtracted the spectrum (“1”) for non-aggregated fluorophores from spectra of more concentrated samples (“2”,”3”,”4”) to emphasize the features that reflected fluorescence emission from newly-formed PA species and to reveal the individual fluorophores that were predominantly involved in aggregation. The resulting increments of fluorescence intensity are presented in Figure 6B. Positive intensity increments (net SSFE spectra of PA) are almost identical for the 15 mg/L and 27 mg/L solutions. The substructure in these PA spectra may be attributed to the aggregation of individual fluorophores with different numbers of fused aromatic rings (for details – cf. Figure 9 and the respective discussion). Figure 6B shows that PA spectrum for the 0.68 mg/L solution has a low intensity (the respective data “2” are multiplied by a factor of 10). Moreover, the short-wavelength part of this spectrum is underdeveloped compared to the spectra for more concentrated solutions. The reason may be explained by considering negative intensity increments. In the 15 mg/L and 27 mg/L solutions, the relative contents of all the excited 1-3 ring fluorophores almost equally ACS Paragon Plus Environment

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decreased because of their aggregation. However, the 0.68 mg/L solution exhibits practically no changes in the wavelength ranges up to 310 nm, which indicates that the respective 1-ring fluorophore (cf. Figure 3) are still present in non-aggregated states.

Figure 6. Above – equal-area spectra of fluorescence emission from solutions 1-4 with increasing asphaltene concentrations (for mg/L values – cf. text). Below – increments of fluorescence intensity in solutions 2-4 with respect to the most dilute solution 1. Excitation wavelength – 265 nm.

At longer excitation wavelengths (280-340 nm), the positions of the intensity maxima in the SSFE spectra at 420-460 nm are also consistent with the predominance of PA emission. Moreover, both the positive and negative intensity increments for the 0.68 mg/L solution become comparable to those for the more concentrated 15 mg/L and 27 mg/L solutions – cf. the data in Figure 7, which were obtained with 300-nm excitation. At still longer excitation wavelengths, the PA features in the SSFE spectra for asphaltene concentrations of 0.68, 15 and 27 mg/L are virtually identical. The corresponding local intensity maxima become less pronounced and the main intensity maximum is observed in the same wavelength range as in the most dilute solution of asphaltenes (0.34 mg/L) – cf. the data in Figure 8A, which were obtained with 360-nm excitation. Finally, at λ≥400 nm (cf. Figure 8B), no new features that can be attributed to emission from emerging primary aggregates are registered in the data for “less dilute” asphaltene solutions. All the ACS Paragon Plus Environment

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SSFE spectra exhibit common “collective” wavelength dependence, similar to what is observed in the “most dilute” (0.34 mg/L) samples (cf. Figures 4 and 5).

Figure 7. Same as in Figure 6 but for an excitation wavelength of 300 nm.

Figure 8. Equal-area spectra of fluorescence emission from solutions 1-4 for long excitation wavelengths – 360 and 420 nm. A more detailed analysis of fluorescence emission spectra from primary asphaltene aggregates (i.e., positive intensity increments) for 265-320-nm excitation reveals the presence of several “characteristic” sub-peaks (with only relative intensities, but not the positions, which depend


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on λex). For comparison, all the PA spectra for the 27 mg/L asphaltene solution (data points, which merge into thick lines) in Figure 9 are normalized to the intensities of the main sub-peak at 460 nm. The thin lines show the normalized SSFE spectra for some simplest aggregates of molecules with 14 ring aromatic fluorophores.39,42-45 The above discussion of negative intensity increments indicates that short-wavelength PA sub-peaks, which are centered at 370-380 nm, may be attributed to aggregates of some single-ring aromatic fluorophores. In contrast to individual fluorophores, the positions of the SSFE peaks of these aggregates appear to depend on the nature of particular substituents. Literature analysis shows that the characteristic SSFE peaks for molecular complexes of purely hydrocarbon benzene derivatives are observed at much shorter wavelengths, typically at 320-340 nm – as an example, cf. spectrum “1A” in Figure 9 for styrene aggregates.42 These peaks shift to the wavelength range that is observed in our study only when benzene ring substituents in the aggregated molecules contain heteroatoms, including nitrogen – cf. spectrum “1” for aggregates of 1,4-dicyanobenzene.39 The presence of nitrogen-containing functional groups in asphaltenes is known to be the cause of strong hydrogen-bonding intermolecular interactions, which are much more effective than the π–π stacking mechanisms of aromatic rings.46,47 Another pair of asphaltene PA sub-peaks in Figure 9, which were suggested by local maxima/shoulders at ∼420 nm and ∼430 nm by literature analysis, may be attributed to aggregates of two-ring aromatic fluorophores – as an example, cf. spectrum “2” for a naphthalene dimer.44 Finally, the most intense sub-peak at 460 nm and a shoulder at ∼475 nm may be assigned to complexes of 3ring fluorophores (cf. solid line spectrum “3A” for anthracene aggregates43) with a possible contribution from associates of most compact 4-ring fluorophores (cf. dash-dot line spectrum “4” for a pyrene aggregate45).


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Figure 9. Data points – spectra of characteristic fluorescence emission from primary asphaltene aggregates in solution 4 (with 27 mg/L asphaltenes) for 265-320 nm excitation. Thin lines reference emission spectra for the simplest aggregates of some molecules with 1-4 ring aromatic fluorophores.

With increasing λex, the short-wavelength parts of asphaltene’s characteristic PA spectra in Figure 9 progressively disappear (“cutoff”), while the coincident long-wavelength tails of these spectra remain unaltered in the discussed interval of λex=265-320 nm. As illustrated in Figure 10, the characteristic PA peaks (with fixed positions) are no longer visible at λex≥340 nm and the “cutoff” processes start to affect the long-wavelength tail of the spectra. Thus, the most prominent feature of a PA spectrum becomes a non-characteristic “cutoff” peak, which is positioned ca. 130 nm above the particular excitation wavelength. The intensity of this “cutoff” peak gradually decreases with increasing λex and specific features, which may be attributed to primary asphaltene aggregates are no longer registered at λex≥400 nm (cf. the data in Figure 8B).

Figure 10. Residual “cutoff” fluorescence emission spectra from primary asphaltene aggregates in 27 mg/L asphaltene solution, observed with 320-380 nm excitation.


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One of the important conclusions from the above experimental data is that the intensity maxima in SSFE spectra that are measured at typical asphaltene concentrations and excitation wavelengths for basic “Modified Yen Model” experiments10 are shaped by the emission processes in primary asphaltene aggregates and not those in non-aggregated asphaltene molecules (“monomers”10). Moreover, apart from an incorrect interpretation of the data, fundamental flaws are present in the measurement procedures of these experiments. In particular, one necessary requirement, which is discussed in all textbooks on fluorescence measurements,36 is correcting data for “inner filter”/self-absorption effects (cf. Section 2.4.1). The necessity and importance of “inner filter” corrections in asphaltene studies was emphasized in multiple publications by various research groups.48-51 However, such corrections have never been performed in “Modified Yen Model” SSFE experiments11-21 because of the assumption that no “complications from self-absorption” occur at asphaltene concentrations of 10-25 mg/L because “optical densities OD of all solutions were kept below 0.2”. This assumption appears to be unfounded and wrong. First, no direct comparisons of the measured “raw” SSFE spectra with those that are corrected for “inner filter”/self-absorption effects are present in the discussed publications. Second, the statement of OD4 ring) aromatic fluorophores are apparently inactive during aggregation over the studied concentration range (≤27 mg/L).


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ACKNOWLEGEMENTS The authors acknowledge support from the Nedra-Test Research and Testing Establishment, LLC, Moscow, Russia in purchasing the experimental equipment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] , URL: http://eee.gubkin.ru Notes The authors declare no competing financial interest.

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