Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy: I. Structure of the absorbance spectrum

The aim of this paper is to provide new data on the properties of molecular aggregation in toluene solutions of crude oils and of solid asphaltenes. The shape of the optical absorption spectrum was found to be sensitive to the details of asphaltene aggregation processes. In dilute solutions, these processes are apparently determined by the net concentrations of asphaltenes; other oil constituents play a secondary role. Our experimental data indicate that molecular solutions of asphaltenes are possible only for concentrations below 1 mg/l. With increasing asphaltene content, more and more complex molecular aggregates are formed. In particular, asphaltene dimers evidently are the predominant species in the range of 5–15 mg/l, while stable “nanocrystallites” (dimer pairs) are predominant at concentrations ≈90 mg/l. Aggregates at higher concentrations may be viewed as assemblies of such “crystallites”. The observed gradual aggregation process is distinct from conventional micellisation phenomena with step-like changes at critical micelle concentrations (CMCs).