CHARACTERIZATION BY ATOMIC-FORCE MICROSCOPY OF ADSORBED ASPHALTENES

Asphaltenes extracted from crude oils of various origins have been irreversibly adsorbed from filtered very dilute solutions in toluene onto freshly cleaved mica. After drying, the analysis under air by atomic force microscopy (AFM) of the mica surfaces reveals, in some instances, the presence of discoids of dimensions approximately 2 nm x 30 nm. These dimensions are compatible with literature data resulting from small-angle neutron scattering experiments for asphaltenes in a good solvent. We observe in the present case an increase in the discoid sizes with the sulfur content of the asphaltenes, but there seems to be no correlation with their molecular weight measured by tonometry. The adsorption kinetics are accompanied by well-correlated changes of the surface topography, of its wettability as' measured by the contact angle of a sessile drop, and the adhesion force of the local probe. The two last properties oscillate in phase during the first hour of adsorption, a phenomenon which we attribute to the amphiphilic character of asphaltenes. Upon adsorption from unfiltered solutions, objects identifiable with two-dimensional diffusion-limited aggregates showed up on the mica surfaces. These fractal-like objects were easily characterized by AFM, with ''finger'' lengths typically a few microns, width 1 mu m and thickness 10-20 nm. We attribute these objects to flocs formed in solution or at the mica/solution interface, as a result of the destabilization of some of the asphaltenes.