Importance of asphaltene aggregation in solution in determining the adsorption of this sample on mineral surfaces

Adsorption rate constants and adsorption isotherms on silica have been obta ined for toluene solutions of Furrial asphaltenes. For initial solution con centrations of 5, 20 and 50 mg l(-1) an adsorption rate proportional to sol ute concentration was observed with an average first order rate constant of (1.17+/-0.3) 10(-3) min(-1). For more concentrated solutions (200 and 400 mg l(-1)), we found the same initial rate constant. However. at long times significant lower rates were apparent, We found that desorption of asphalte nes from the surface was very slow and could be neglected. Also, we found t hat the adsorption isotherms. measured for solution concentrations (c(s)) b elow 500 mg l(-1), changed with time over a period of days where adsorption on the surface (v) grows slowly with time. A slow tendency from L-type to H-type was observed for the adsorption isotherms measured at 18, 48 and 96 h. Under the conditions of concentration and time examined, these results a re consistent with the formation of asphaltene multilayer at the silica sur face and with the adsorption of aggregates (dimers, trimers, etc.) as well as single asphaltene molecules. In view of these results we suggest that fo r solutions concentration below 50 mg l(-1), small aggregates are adsorbed on the surface at a rate similar to the single molecules until the silica s urface is covered or saturated. After this 'saturation' adsorption continue s on the asphaltene layer at a much smaller rate. (C) 2000 Elsevier Science B.V. All rights reserved.