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dipole interactions [15]. For the first time, the existence of such
structural units has been pointed out by T.F. Yen (cf. [3]). In crude
oils, the disperse phase of asphaltenes is stabilised by resins. In
the thermally untreated, “annealed” crudes, close to the thermody-
namical equilibrium, resins evidently form closely-packed (ener-
getically most favorable) “inert hard shells” around asphaltene
MNC’s. Association of MNC’s (e.g. via dipole-dipole interactions) is
limited by the steric action of these “shells”.
The effects, initiated at temperatures 28–30°C, may be pro-
moted by a first-order structural phase transition either in the
“shells” of resins or in the MNC’c themselves [6,15]. In both cases,
the “shells” become less compact, some resin molecules “desorb”
into the oil phase, which increases attractive interactions between
MNC’s.
As a result, colloid particles start to form more complex ag-
gregates. In asphaltene binary solutions in organic solvents, the
aggregates are evidently formed solely by asphaltene MNC’s [15],
via the diffusion-limited or the reaction-limited aggregation
(cf. [16]). In a multicomponent crude oil, the asphaltene aggregates
(in contrast to monomers or non-associated MNC’s) may act as ce-
menting species in the colloid structures formed of microparticles
of crystallising saturates (e.g. waxes, provided the oil temperature
is below the wax appearance temperature, WAT) [17,18].
By cooling a crude oil below 18–20°C, some newly-acquired
properties of the asphaltene/resin subphase may be temporarily
conserved in this temperature range. In conditions of a decreased
thermal movement, asphaltene/wax aggregates evidently form ex-
tended networks, which lead to the dramatic increase of dynamic
viscosity, as seen in Fig. 1. At low ambient temperatures such “su-
percooled” state of the crude oil is certainly unstable but rather
long-lived, the ordered structures are only partially broken in a
shear flow. However, this non-equilibrium oil phase is easily de-
stroyed by re-heating the sample to temperatures above the as-
phaltene/resin phase transition (>28–30°C) or, better, above the
WAT (39–40°C in the studied crude oil).
44
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