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temperatures exceeding 20°C. This decrease may be attributed to a
(reversible) thermally-induced break-up of intermolecular hydro-
gen and/or π-bonds in the asphaltene subsystems. Accordingly, an
overall decrease of activation energies after the strong pre-heating
(TF=45°C, Figure 2) should be due to an irreversible removal of
these noncovalent bonds.
Finally, formation of emulsions in the “critical” temperature
range (TF=36°C, Figure 2) induces the structural changes, charac-
terised by a large increase of activation energies of up to
300–380 kJ/mol, indicative of a possible covalent bonding. Namely,
this increase is close to the energies (≈380–420 kJ/mol) of unacti-
vated CH and CC bonds in organic compounds.
Conclusions
We found that formation temperature of emulsions has a very
strong impact on viscosity of crude oil and bitumen emulsions. At
normal technological conditions these emulsions possess a long-
term “memory” of a thermal preheating. This memory is especially
strong after formation of emulsions at “critical” temperatures of
about 36–38°C. The “critical” sharp increase of the technological
viscosity is attributed to structural phase transitions in asphaltene
molecular aggregates. In W/O emulsions and in the crude oil these
aggregates presumably stabilise the extended networks of paraffin
particles by emerging covalent bonding. The relative weakening of
the inner structures in bitumen emulsions may be due to the pres-
ence of producer-added surfactants. In all studied emulsions, the
“memory” of a strong pre-heating (formation temperatures above
the melting point of paraffins) may be explained by destruction of
noncovalent hydrogen bonds and π-interactions.
This study demonstrates the effectiveness of rheological
measurements as a means to elucidate the inner structures of pe-
troleum and bitumen emulsions over a wide range of temperatures.
With this paper we hope to stimulate further research work on
thermodynamics and mechanics of asphaltene-containing emul-
sions utilised in recovery of oil reservoirs.
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