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Introduction
A typical petroleum fluid contains millions of different molecules
and is a hybrid of a solution and a colloidal dispersion (of crystal-
lizing waxes, self-associating asphaltenes, etc.). Hence, under-
standing its phase behaviour should be expected to be a challenge.
The surprise is that relatively simple (although, possibly, “uncon-
ventional”) experiments can shed light on details of phase trans-
formations in such complex fluids.
E.g., for long time a commonly accepted view in petroleum sci-
ence has been that asphaltenes molecules start to form colloidal
particles (“micelles”, “aggregates”, “agglomerates”) only at concen-
trations above a “CMC” of about several grams per litre (Andersen
and Christensen, 2000; Sheu, 1996). However, our recent meas-
urements of optical absorptivities in “unconventional” highly di-
luted toluene solutions of oils and of solid asphaltenes (Evdokimov
et al., 2003a,b) have revealed that association of asphaltene
monomers apparently begins at concentrations not exceeding
1–2 mg/l, while massive aggregation of asphaltenes occurs at
60–100 mg/l (cf. Fig.1).
Fig. 1. Optical absorptivity ε of asphaltenes at
540 nm as a function of asphaltene
concentration C in crude oil/toluene solutions
(adapted from Evdokimov et al., 2003c).
These results have been further supported by our measure-
ments of refraction indexes and of dielectric constants in crudes of
different origin (submitted for publication). An independent con-
formation of asphaltene aggregation at 50–75 mg/l has been re-
cently obtained in absorbance/fluorescence experiments (Goncalves
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