Toward mechanistic understanding of asphaltene aggregation behavior in toluene: The roles of asphaltene structure, aging time, temperature, and ultrasonic radiation

Abstract

Many investigations have been carried out to provide a good understanding of the colloidal state of asphaltenes in appropriate solvents such as toluene or natural medium. Despite these attempts, the mechanism of asphaltene aggregation and its dependency on the structure of asphaltene molecules has not been fully understood yet. In this study, three different types of asphaltenes, namely asphaltene A, asphaltene B, and asphaltene C, were characterized using elemental analysis and energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and X-ray diffraction (XRD). Then, dynamic light scattering (DLS) was used to measure the size of asphaltene aggregates in toluene. The effects of asphaltene structure, composition, concentration, temperature, ultrasonic radiation and aging time on the aggregation behavior of the asphaltenes were investigated. The results show that as asphaltenes concentration in toluene increases, a slight increase in asphaltene aggregates size is observed. In addition, the size of asphaltene aggregates strongly depends on asphaltenes structure and composition. Asphaltene C, which has the lowest aromaticity, polarity (heteroatoms), and number of fused rings in each asphaltene sheet shows the lowest aggregates size. Both polarity and aromaticity play key roles in asphaltene instability or aggregates size. Asphaltene A with higher polarity and more functional groups but lower aromaticity displays a higher aggregates size compared with asphaltene B. Furthermore, aging time can significantly increase the size of asphaltene aggregates from few nanometers after 15 min aging time to few microns after 1 month. Moreover, ultrasonic radiation can decrease the size of asphaltene aggregates down to an optimum value. Furthermore, with increasing temperature, asphaltenes aggregate size decreases due to increasing solubility.