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ity allows the wax crystals to have affinity to the aqueous phase.
The asphaltene-solvated wax particles migrate to the oil–water in-
terface, contributing to the formation of rigid layers around water
droplet and to the emulsion tightness (5, 6).
Types of wax crystals and their thermal properties may de-
termine, how wax could interfere with the thermally-induced ef-
fects in petroleum emulsions. The studied crude oils contained
predominantly normal paraffin waxes with the melting points in
the range of 40–50°C. Bitumen are known to contain also the
microcrystalline (isoparaffin and cycloparaffin) waxes with higher
melting points, in the range of 60–90°C.
During a sufficiently strong thermal preload (TF above both
the asphaltene/resin phase transition and the wax melting point)
all the existing wax particles melt away. The new microscopic wax
crystals start appearing in the emulsions only when it is cooled be-
low the melting point. We presume, that large asphaltene aggre-
gates can not form an “adsorbed layer” around small wax crystals.
Besides, above the phase transition point asphaltenes may even
become wax inhibitors, because larger aggregates no longer serve
as crystallisation centres. With decreasing temperature, the newly-
formed wax crystals form networks, directly interacting with each
other. Direct interaction of the wax crystals is a weak physical one
(11), 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 thermal preload (TF below the wax melt-
ing point, but above the asphaltene/resin phase transition) the ex-
isting wax 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 wax
particles form strongly bound networks, so that the viscosity is
greatly increased and “memory” of the thermal preload is pre-
served at varying measurement conditions.
The relatively smaller thermal effects in the bitumen emul-
sions may be due to the weakening of asphaltene-mediated net-
35
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