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Asphaltene colloids are known to be sterically stabilised by
solvated resins [13]. Formation of larger asphaltene aggregates
may occur when resin molecules “desorb” from the surfaces of as-
phaltenes. The sharp dependence of the measured viscosities on
the value of TF (Figure 1) suggests that the changes in solvation is
triggered by a structural phase transition in the asphaltene/resin
molecular subsystem. In [2,3] we discussed two possible types of
these transitions at temperatures just above 30°C. One is a first-
order transition between closely packed and loosely bound struc-
tures in the adsorbed layers of resin molecules. Another possible
mechanism is the change of the surface energy of the asphaltene
aggregates due to phase transformation of their inner molecular
structure.
An important role in the observed thermal effects is evidently
played by paraffins. It has been repeatedly shown [6, 7, 12] that
the paraffins alone are unable to stabilise petroleum emulsions. In
the absence of asphaltenes, paraffin crystals remain completely oil-
wet (hydrophobic) and are dispersed in the oil phase. However,
when asphaltene aggregates adsorb on the surfaces of paraffin par-
ticles, their wettability changes. The asphaltene-solvated paraffin
particles migrate to the oil–water interface, contributing to the
formation of rigid layers around water droplet and to the emulsion
stability [6, 7].
Types of paraffin fractions may determine the role of paraffins
in the observed thermal effects in petroleum emulsions. The stud-
ied crude oils contained predominantly normal paraffins with the
melting points of 40–50°C. Bitumen are known to contain also the
microcrystalline paraffins (isoparaffin and cycloparaffin) with
higher melting points of 60–90°C.
During a sufficiently strong pre-heating (TF above both the
asphaltene/resin phase transition and the melting point of paraf-
fins) all the existing paraffin particles melt away. The new micro-
scopic crystals start appearing in the emulsion only when it is
cooled below the melting point. We presume, that large asphaltene
aggregates can not form an “adsorbed layer” around а small paraf-
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