화학공학소재연구정보센터
Energy & Fuels, Vol.30, No.6, 4494-4503, 2016
New Answers to the Optical Interrogation of Asphaltenes: Monomers and Primary Aggregates from Steady-State Fluorescence Studies
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."