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phase of the crude oil. More detailed information on the magnitude
of these bonds may be obtained by the analysis of the “current” ac-
tivation energies of the viscous flow U, determined in the narrow
(2°C) intervals of the measurement (flow) temperature T
m
.
The dependencies U(T
m
) are shown in Fig. 3 for three oil sam-
ples, subjected to different thermal pretreatment: T
pt
=19, 36 and
45°C. In the studies of petroleum systems, an Arrhenius activation
energy is frequently assumed to be equal to an energy of associa-
tion of asphaltenes [3]. A more correct procedure appears to be an
evaluation of the intermolecular bonds from the changes in U, as
shown by viscosity studies in solutions of coal macromolecules, in-
cluding asphaltenes [19]. In [19], the removal (by chemical means)
of hydrogen bonds and of π interactions decreased activation ener-
gies by the values, close to the bond strengths directly determined
by IR-spectroscopy [20]. The IR average strengths of hydrogen
bonds were 30–50 kJ/mol, while some individual bonds had the
strengths of up to 70 kJ/mol. Our results of Fig. 3 show that, de-
pending on the pre-treatment temperature, different types of bonds
may be formed in the crude oil, as indicated by different values of
activation energies in samples, cooled to 10–13°C. These bonds are
evidently removed by re-heating the oil to the temperatures of the
assumed phase transition: at T
m
≈28°C the values of U in all sam-
ples decrease to a new level of ≈8.5–10.5 kcal/mol (≈36–44 kJ/mol),
almost independent on the pre-treatment history of the crude oil.
The corresponding changes of U may be interpreted as the average
strengths of the respective bonds. In particular, in samples stored
for a long time at room temperature (T
pt
=19°C), the strength of the
bonds is 15–20 kcal/mol (63–84 kJ/mol), typical for noncovalent
hydrogen bonding in asphaltenecontaining systems [20]. The re-
sults of Fig. 3 show, that the above discussed phase transition in
the sample, pre-treated at T
pt
=36°C, triggers the formation of
bonds with the strengths up to 85–95 kcal/mol (360–400 kJ/mol),
indicative of a possible covalent bonding. Namely, this is close to
the energies ≈380–420 kJ/mol, frequently cited for unactivated CH
and CC bonds in organic compounds. Finally, in oil samples, pre-
46
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