Asphaltenes, as the
most polar component of crude oils, may interact
with wax molecules during the wax crystallization, thus changing wax
precipitation characteristics and wax crystal structure and affecting
the gelation behavior of crude oils. In the present work, waxy model
oils of variable asphaltene contents (0–0.20 wt %) were prepared
in order to systematically examine the effect of asphaltenes on the
wax precipitation characteristics and the gelation behavior of waxy
oils and to look into the structure and the surface electrical properties
of wax crystals formed in oils of different asphaltene contents. By
means of differential scanning calorimetry and multiple rheometries,
it was found that asphaltenes play a significant role in inhibiting
the precipitation of wax crystals and delaying the gelation, as well
as in weakening the strength of the gel structure. It was observed
by polarized optical microscopy that as the asphaltene content increases,
the size of wax crystals gradually decreases, and the shape of wax
crystals transforms from rodlike into elliptical. Furthermore, both
the average aspect ratio and the average perimeter of wax crystals
decrease, while the boundary box fractal dimension increases, demonstrating
that the morphology of wax crystals becomes more complex. Based on
the X-ray diffraction (XRD) pattern, the grain size obtained with
the Scherrer equation presents a descending trend with the increase
of asphaltenes. It could be inferred that the reduction of grain size
is one of the reasons leading to the smaller sizes of wax crystals.
The XRD result also showed that with higher asphaltene content, the
lattice parameter c of wax crystals obviously increases,
indicating that the conformation of wax crystals gets more disordered,
which results in the weakening of gel strength, corresponding to the
decline of the measured yield stress of the waxy oil gel. With higher
asphaltene content, the zeta potential of wax crystals goes up, which
indicates that with more negative charges adsorbed on the wax crystal
surfaces, stronger electrostatic double layers have formed. The greater
electrostatic repulsion among wax crystals would interfere with the
growth and aggregation of wax crystals, thus holding up the gelation
of waxy oils.