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fin crystal. Hence, with decreasing temperature, the newly-formed
paraffin crystals form networks, directly interacting with each
other. Direct interaction of paraffins is a weak physical one [12], so
that the resulting increase of viscosity is comparatively small and
such loosely-bound crystal networks quickly disappear when the
temperature is increased again.
During a moderate pre-heating (TF below the melting point of
paraffins, but above the asphaltene/resin phase transition) the ex-
isting paraffin particles in emulsions readily adsorb the newly-
formed asphaltene aggregates, so that their interaction properties
of these particles are notably altered. After cooling, asphaltene-
covered paraffin particles form strongly bound networks, so that
the viscosity is greatly increased and “memory” of the pre-heating
parameters is preserved at varying measurement conditions.
The relatively smaller thermal effects in the bitumen emul-
sions may be due to the weakening of asphaltene-mediated net-
works by other strong surfactants introduced into the concentrated
product.
The means by which asphaltenes, resins and paraffins inter-
act in petroleum fluids remains the subject of speculation but asso-
ciation by hydrogen bonding and by formation of charge-transfer
π–π complexes have been cited as the causative mechanisms [13].
Through these noncovalent interactions, asphaltene molecules may
influence the structure of emulsions by forming a mechanical bar-
rier around the water droplets [14].
The strength of intermolecular bonds of asphaltenes may be
evaluated from the changes in activation energies of the viscous
flow, as shown by viscosity studies in solutions, containing coal as-
phaltenes [15]. The removal of hydrogen bonds and of π interac-
tions decreased activation energies by 33 kJ/mol and by 30 kJ/mol,
respectively. These results were verified by IR-spectroscopy [16],
which showed that energies of hydrogen bonds were on average
30–50 kJ/mol (for some individual bonds up to 70 kJ/mol).
Our measurements after formation at 20°C (Figure 2) show a
step-like decrease of activation energies by 50–60 kJ/mol for flow
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