Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy - II. Concentration dependencies of absorptivities

The properties of molecular aggregation in toluene solutions of a crude oil and solid asphaltenes are determined almost solely by the concentration of asphaltenes, as shown by absorptivity measurements at 315-750 nm. From non-monotonic concentration dependencies of absorptivities, it is concluded that asphaltene monomers are abundant in solutions with asphaltene concentrations below 1-5 mg/l, while molecular aggregates are effectively formed above 20-25 mg/l. The most stable oligomers are a dimer and a dimer pair (Yen's "nanocrystallite" [NC]). Nanocrystallites act as building blocks for more complex aggregates at asphaltene concentrations exceeding 90-100 mg/l. These optical absorption results are supported by studies of Rayleigh scattering in asphaltene solutions. (C) 2003 Published by Elsevier Science B.V.