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.
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