No 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 multifluorophore supramolecular complexes with “archipelago” structures of basic asphaltene molecules.

This study examined the states of vanadyl porphyrins in toluene solutions of n-heptane solid asphaltenes via measurements of near-UV−visible absorption spectra. Low intensity of the characteristic Soret absorption peak of porphyrins in most samples with various asphaltene concentrations C A indicated that porphyrins are bonded to individual basic (one to three ring) molecules of asphaltenes (at C A ≤ 0.5 mg/L), within porous supramolecular structures of most primary asphaltene aggregates (at C A = 0.6−30 mg/L) as well as in all colloidal-size complexes at higher asphaltene concentrations, up to C A = 1880 mg/L. However, porphyrin−asphaltene bonds appear to be weak and may be disrupted merely by dilution to some proper final asphaltene concentrations. Namely, for some specific values of C A (close to 4 and 12 mg/L) we observed sharp increases of porphyrin Soret absorption peaks attributed to the appearance of free, nonbonded, porphyrin molecules in these samples. We suggest that these specific dilutions of solid asphaltenes result in such equilibrium molecular systems where the active centers of asphaltenes are effectively engaged in internal bonds of primary asphaltene aggregates and, hence, are not available for aggregation with any foreign molecules.

This study examined the states of primary asphaltene aggregates in 0.2–27 mg/L toluene solutions via steady-state fluorescence emission (SSFE) techniques. The experimental results do not support the conventional models of “consecutive aggregation” with interdependent states of aggregates and with monotonic increase of the complexity of aggregates with increasing concentration. The observed concentration dependencies of SSFE spectra were strongly nonmonotonic, with several intervals of “apparent re-entrance” when the measured properties in more-concentrated solutions returned to those in less-concentrated ones. Literature analysis revealed qualitatively similar nonmonotonic/“re-entrant” effects of asphaltene concentration in experimental results of other research groups. We suggest that the complex effects of concentration are consistent with autonomous kinetic routes to the independent states of molecular aggregates observed in most experiments with asphaltenes in good solvents.

This study examined the dominant mechanisms of near-ultraviolet−visible−near-infrared (NUV−vis−NIR) light attenuation in toluene solutions with asphaltene concentrations (C A ) in the wide range of 0.20−1880 mg/L. The analysis of experimental results indicates that in the near-ultraviolet range (270−430 nm) the main mechanism at all C A is electronic absorption by 1−4 ring aromatic chromophores and by vanadyl porphyrins. In the visible range (430−730 nm), no distinct peaks of electronic absorption by larger aromatic chromophores were found and all light attenuation spectra followed a λ −4 dependence, consistent with the dominance of Rayleigh scattering mechanisms. In the near-infrared range (730−1100 nm), a significant mechanism of light attenuation by more dilute solutions (C A < 30−40 mg/L) is photon absorption via excitation of C−H stretching vibrations in aromatic chromophores. However, for more concentrated solutions (C A > 120−130 mg/L), the most important light attenuation mechanism in the NIR range also becomes Rayleigh scattering.