Understanding mechanisms of asphaltene adsorption from organic solvent on mica

The adsorption process of asphaltene onto molecularly smooth mica surfaces from toluene solutions of various concentrations (0.01-1 wt %) was studied using a surface forces apparatus (SFA). Adsorption of asphaltenes onto mica was found to be highly dependent on adsorption time and asphaltene concentration of the solution. The adsorption of asphaltenes led to an attractive bridging force between the mica surfaces in asphaltene solution. The adsorption process was identified as being controlled by the diffusion of asphaltenes from the bulk solution to the mica surface with a diffusion coefficient on the order of 10(-10) m(2)/s at room temperature, depending on the asphaltene bulk concentration. This diffusion coefficient corresponds to a hydrodynamic molecular radius of approximately 0.5 nm, indicating that asphaltene diffuses to mica surfaces as individual molecules at very low concentration (e.g., 0.01 wt %). Atomic force microscopy images of the adsorbed asphaltenes on mica support the results of the SFA force measurements. The results from the SFA force measurements provide valuable insights into the molecular interactions (e.g., steric repulsion and bridging attraction as a function of distance) of asphaltenes in organic media and hence their roles in crude oil and bitumen production.