Occlusion of Foreign Molecules in Primary Asphaltene Aggregates from Near-UV-Visible Absorption Studies

摘要

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.