Electrokinetic properties of asphaltene colloidal particles: Determining the electric charge using micro electrophoresis technique

In this work, the electrokinetic properties of asphaltene particles have been investigated. Micro-electrophoresis method by applying DC electric field, was utilized to different mixtures containing asphaltene to determine its electric charge. It was observed that in the case of using n-heptane and its mixture with toluene (heptol), the asphaltene particles were showed to be positively charged however for toluene itself, they expressed no tendency toward the electrodes. While it is expected that larger asphaltene aggregates carry higher electric charge, the results contradictorily showed that they are mainly governed by gravity rather than electro-static force and that "aggregation" reduces the charge density of asphaltene particles. Using a digital microscope equipped with image processing software, the average net electric charge of each asphaltene molecule was calculated to be 5. 62 x 10(-28) C/molecule or x 3. 51 x 10(-9) (e) over bar /molecule. As this value is much less than 1 (e) over bar, it can be concluded that asphaltene molecules do not possess intrinsic electric charge and the apparent charge is mostly induced through the medium, generated by rearrangement of asphaltene molecular structure. Thereafter, based on calculated charge and considering governing forces (gravity, friction and electric force), the electrophoretic mobility of asphaltene particles in dynamic mode was mathematically modeled and verified under different electric field intensities. The results illustrated satisfactory agreement with the obtained experimental data. Knowing the asphaltene electric charge, the magnitude of streaming potential can be calculated, subsequently the model will be able to predict the velocity ranges at which colloidal asphaltene flocculation is possibly avoided. Finally the effect of flow rate, electric potential and solvent composition was investigated on electrophoretic mobility. It was found that, the higher the fluid flow rate, the greater is the mobility. However, increasing the electric potential resulted in electrophoretic mobility reduction of asphaltene particles. Moreover, critical increase of electrophoretic mobility was observed for n-heptane contents of more than 50 wt.% in heptol, which refers to the onset point of asphaltene nano-aggregates.