Study of Asphaltene Adsorption on Kaolinite by X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectroscopy

The interaction between asphaltenes and clay is crucial in understanding wettability changes in petroleum reservoirs and in oilsands production. In this study, we report the changes in surface properties and composition of kaolin clay as a result of exposure to solutions of asphaltenes. Adsorption experiments were conducted at 25 degrees C with solutions of asphaltenes in toluene at concentrations ranging from 0.05 to 5 mg/mL. The wettability of the modified kaolinite surface was characterized by contact angle measurement. Chemical composition changes of the surface because of asphaltene adsorption were assessed using time-of-flight secondary ion mass spectroscopy (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and elemental analysis. The contact angle data indicated that, upon asphaltene adsorption, the clay particles changed from water-wet to bi-wet. Both ToF-SIMS and XPS measurements indicated that the kaolinite surface was never completely covered by asphaltenes based on the concentration of Al and Si on asphaltene-treated kaolinite surfaces. ToF-SIMS analysis indicated that, with more asphaltenes covered on the kaolinite surface, the relative intensities of C3H5+/Al+ and C3H5+/Si+ increased and an inverse linear correlation between the contact angle and surface Si+ concentration was observed. The maximum thickness of adsorbed asphaltenes, assuming complete surface coverage, was estimated to be 11 nm based on XPS depth profile results on a model silica surface, with a mean value of 3 nm. The XPS depth profile also indicated no preferential adsorption of nitrogen- or sulfur-rich species at the interface between asphaltenes and kaolinite.