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.
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