PREPRINT of the paper, published in: Journal of Petroleum Science and Engineering. 2003. V.37. Iss.3-4. P.145-152. Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy II. Concentration dependencies of absorptivities I.N. Evdokimov*, N.Yu. Eliseev, B.R. Akhmetov Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, Moscow B-296, GSP-1, 119991, Russia Abstract 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. Keywords: Petroleum fluids; Asphaltenes; Molecular aggregates; Colloidal suspensions 1. Introduction Asphaltenes constitute a very important fraction of crude oils, petroleum residues, bitumens, as well as of oil- or bitumen-based emulsions. The tendency of asphaltenes to aggregate distinguishes them from other oil constituents. Asphaltene aggregation may be the cause of complex non-linear effects in such phenomena as adsorption at solid surfaces, precipitation/deposition, fluid’s rheology, emulsion stability, etc. Until now, the structure of asphaltene molecules and the precise nature of the asphaltene–asphaltene interactions are not well known, which precludes reliable theoretical modelling of the properties of asphaltene aggregates. Hence, of primary importance are experimental studies, which may reveal new details of the aggregation process, especially at its initial stages of formation of the simplest aggregates. In our previous publications (Akhmetov et al., 2002; Evdokimov et al., 2003), we have presented experimental evidence for the influence of asphaltene aggregation in dilute solutions on positions of absorption bands in the near-ultraviolet/visible spectroscopy (near-UV/Vis) range. On the basis of these results, we concluded that single asphaltene molecules are predominant only in solutions with asphaltene concentrations below 1–5 mg/l, while asphaltene dimers are most abundant species at concentrations close to 10–15 mg/l. The most stable aggregates are apparently 2 nm quasispherical dimer pairs or ‘‘nanosized crystallites’’ (Yen et al., 1961; Dickie and Yen, 1967), predominant at concentrations 90 mg/l. The observed multiple-stage aggregation process was regarded as quite distinct from conventional micellisation phenomena with a single critical micelle concentration. In the present paper, we report new near-UV/Vis absorption results, which reveal non- monotonic concentration dependencies of asphaltene absorptivities. These results are supplemented by Rayleigh scattering measurements. The experimental data confirm our previous aggregation model, which is further substantiated by an analysis of asphaltene molecular weight (MW) distributions measured by other authors. 2. Experimental 2.1. Samples The solid asphaltenes have been precipitated with n-heptane (40/1 dilution at room temperature) from a mixture of Tatarstan crude oils. The crude was collected directly from a