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where F(RI) = (n
2
– 1)/(n
2
+ 2) and φ
oil
is a volume fraction of a
crude oil.
Obviously, the assumption of “an ideal mixture” is valid only
provided there are no compositional changes in the complex petro-
leum “component”. Indeed, noticeable deviations from simple mix-
ing rules have been observed when asphaltenes flocculate and
commence to precipitate out of a crude oil solution (Taylor et al.,
2001; Buckley, 1999; Buckley et al., 1998). However, presently
there is no experimental evidence whether the RI may be sensitive
to specific aggregation phenomena of asphaltenes which remain
suspended in solution.
Conventionally, “critical micellisation concentrations”
(“CMCs”) of asphaltenes in solutions were revealed by various ex-
perimental techniques (Sheu, 1996). The “CMC” values for asphal-
tenes in toluene range from ≈2 g/l (Castillo et al., 1998) to ≈5 g/l
(Andersen and Christensen, 2000). Over the past years it became
clear that conventional “CMCs” are not signatures of primary
monomer-micelle transformations, but reflect some secondary
processes of aggregation into complex colloid structures. Indeed, it
has been experimentally proven that distinct aggregation stages of
asphaltenes are observed at concentrations well below the conven-
tionally reported “CMCs”. E.g., in our previous publications
(Evdokimov et al., 2003a,b,c) we have presented experimental data
which indicate that aggregation of asphaltene monomers com-
mences at concentrations as low as 2–5 mg/l, while massive forma-
tion of asphaltene “molecular nanoclusters” is observed at concen-
trations ≈90–150 mg/l. Aggregation processes were no longer de-
tected at asphaltene concentrations above 500–700 mg/l, indicative
of some “steady” state of asphaltene aggregates in more concen-
trated solutions (up to 1.6 g/l).
In this paper, we report RI measurements in toluene solutions
of a Tatarstan crude oil. Results for concentrated solutions (with
asphaltenes in a “steady” aggregated state) are in agreement with
ideal mixing rules. An “anomalous” behavior of RI in dilute solu-
tions is attributed to de-aggregation of asphaltenes. The RI data
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