New Answers to the Optical Interrogation of Asphaltenes: Complex States of Primary Aggregates from Steady-State Fluorescence Studies

摘要

This study examined the states of primary asphaltene aggregates in 0.2-27 mg/L toluene solutions via steadystate 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.