Effect of Asphaltenes on the Structure and Surface Properties of Wax Crystals in Waxy Oils

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.