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the manner of introducing one component into the other (e.g. from
a vial or from a pipette), the manner of stirring the mixture (by
hand, in a mechanical stirrer), experimental time intervals (be-
tween mixing, stirring and measurements), thermal history of a
sample during these intervals, etc. The most pronounced “history
effects” were in solution with oil contents of ≈0.2–0.5 vol. %. The
respective asphaltene concentrations of about 60–150 mg/l corre-
spond to conditions of the expected phenomena of de-aggregation of
asphaltenes (cf. Introduction). Hence, the “history effects” may be
attributed to the complex kinetics of asphaltene dissolution which
is known to vary significantly depending on the physical state of
the system (Hammami et al., 2000; Cosultchi et al., 2003). We
agree with Cosultchi et al., 2003 that the differences between the
reported data for crude oil and asphaltene solutions may be often
related to non-standard times and procedures of dissolution. In
most of the experimental procedures, the time to reach the dissolu-
tion of the solid matrix of aggregated asphaltenes and the equilib-
rium of the solution is mentioned only as a minor factor.
To obtain consistent concentration dependencies of RI and to
reduce the scatter of results, we have employed a procedure of
sample preparation used in our previous studies (Evdokimov et al.,
2003a,b,c). Operationally, this technique is “titration of toluene
with oil”. Crude oil is introduced into a large volume of toluene by
minute quantities, to facilitate quick redistribution of maltenes
over the sample so that asphaltene aggregates are almost immedi-
ately subjected to the environment of solvent molecules. In each
experiment with dilute solutions a concentration of a single sample
is increased step-wise, at equal time intervals (0.5–10 min.), by
means of consecutive additions to toluene (100 ml) of calibrated oil
drops (m
drop
=10.3±0.3 mg). Hence, the concentration increments in
solutions are 0.012 vol. % for the crude oil (3.7 mg/l for oil’s asphal-
tenes). At the beginning of each time interval a solution is gently
stirred with a glass rod, at the end – RI measurements are per-
formed. The results revealed consistent dependence on the rate of
concentration increase above ≈0.2 vol. % of oil per hour. There were
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