Составители:
40
<47 mg/l. On the basis of this linear fit it seemed reasonable to
evaluate a modified RI
oil
from a binary mixing rule (Eq. 1). Sur-
prisingly, such evaluation resulted in a very high value of
RI
oil
= 2.0121, close to the refractive indices of amorphous carbon
and graphite (Silva, 2001).
2.5. Nature of Aggregation Effects on
Refractive Index
Literature analysis shows that, previously, aggregation effects
on RI of non-precipitating petroleum fluids have never been regis-
tered experimentally, and even the possibility of such effects has
never been discussed. Hence, a nature of the observed RI anoma-
lies requires an explanation. Studies of simple binary mixtures
have shown that RI may be affected by variations of molar vol-
umes/densities. RI of a fluid is assumed to be directly related to its
density (Gray and Gubbins, 1984). If asphaltene de-aggregation is
regarded as a dissolution, one may expect a smaller density of a
crude with de-aggregated asphaltenes and a decrease of RI in
dilute solutions, i.e. the effect qualitatively opposite to that in our
experiments. On the other hand, some non-ideal solutions exhibit
negative excess volume of mixing which may have been expected to
be related to positive excess RI. In our experiments deviations of
RI from ideal mixing rules are ∼0.1%, typical for excess RI in
binary hydrocarbon mixtures (Fermeglia and Torriano, 1999; Resa
et al., 2004). However, excess volume effects are the largest in so-
lutions with volume fractions of ∼ 0.5, while in our experiments
maximum excess RI is at volume fraction of oil close to 2⋅10
-3
(cf. Fig. 1). Moreover, a causative relationship between excess
volume and excess RI never has been proven either theoretically or
experimentally (cf. Fermeglia and Torriano, 1999 and of Resa et
al., 2004).
For the above reasons, we conclude that density mechanisms
are not responsible for the observed RI anomalies. Comparative
studies of various properties of dilute solutions indicate that non-
ideal RI behavior may be ascribed to optical dispersion effects, ac-
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