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sistent with the size of asphaltene aggregates. Accordingly, when
the asphaltene particles/aggregates are dissolved to substantially
smaller units, they no longer produce stabilising effects.
Based on the above arguments, it can be concluded that the
asphaltene fractions, when present, are primarily responsible for
the stability of the emulsion’s inner structure. The degree of as-
phaltene aggregation determines the strength of this structure
(emulsion “tightness”). Hence the observed sharp variations of vis-
cosity may be attributed to some specific aggregation processes in-
volving asphaltene molecules.
4.2. The “trigger” mechanism of asphaltene
aggregation after thermal preload
Asphaltene colloids are known to be sterically stabilised by
solvated resins (12). Formation of larger asphaltene aggregates
may occur when resin molecules “desorb” from asphaltenes. The
sharp dependence of the measured viscosities on the value of TF
(Fig.2) suggests that the changes in solvation is triggered by a
structural phase transition in the asphaltene/resin subsystem. In
(1, 2) we discussed two possible types of these transitions at tem-
peratures just above 30°C. One is a first-order transition between
closely packed and loosely bound structures in the adsorbed layers
of resin molecules. Another possible “trigger mechanism” is the
change of the surface energy of the π-stacked asphaltene aggre-
gates due to phase transformation of their inner molecular struc-
ture.
4.3. The role of waxes
It has been repeatedly shown (5, 6, 11) that the wax fractions
(aliphatic hydrocarbons) alone are unable to stabilise petroleum
emulsions. In the absence of asphaltenes, wax crystals remain
completely oil-wet (hydrophobic) and are dispersed in the oil phase.
However, when asphaltene aggregates adsorb on the surfaces of
wax particles, their wettability changes from completely oil wet-
ting to a mixed (intermediate) wetting. The alteration of wettabil-
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