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practically no time effects at slower rates, hence such data sets
were considered to be representative of the “equilibrium” states of
solutions. In particular, the RI results reported in this paper were
obtained with a rate of concentration increase equal to 0.14 vol. %
of oil per hour.
2.4. Break-up of Ideal Mixing Rules in
Dilute Solutions
It is clear from Figure 1 that in dilute solutions the data show
“anomalous” deviations of RI from the simple ideal mixing rule
(solid line in Fig. 1). Two most obvious “anomalies” are: 1) the
“concentrated” approximation does not terminate at (n
2
–1)/(n
2
+2)
for pure solvent; 2) the measured concentration dependencies are
non-monotonous, with an extremum at oil content of about
0.2 vol. % and a sharp drop of RI at lower contents. Such “anoma-
lous” behavior of RI may seem highly unusual. However, a litera-
ture survey reveals a number of publications, containing experi-
mental results which, subjected to a proper analysis, show specific
features which may be attributed to the above mentioned “anoma-
lies”. Apparently these features have not been noticed by the au-
thors. E.g., Goual and Firoozabadi, 2002 have measured RI vs. wt.
% in toluene of a Hamaca heavy crude. By fitting a straight line to
the respective graph, they found extrapolated RI of “pure crude”.
However, in the graph they do not plot data for pure toluene, while
extrapolations to zero concentrations result in inconsistency of ex-
trapolated RI of toluene (1.4987). If the reference value of RI for
toluene (1.4969) is included in the data set, the results of Goual
and Firoozabadi, 2002 show an obvious “anomalous” drop of RI at
oil concentrations below 1 wt. %, qualitatively similar to that ob-
served in our experiments. Taking into account earlier evidence of
peculiar behavior of other properties in dilute oil solutions (cf. In-
troduction), we ascribe the observed “anomalies” of RI to phenom-
ena of de-aggregation of oil asphaltenes. Hence, in the following we
refer to the respective asphaltene concentrations in oil-toluene so-
lutions.
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