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 obtained for toluene solutions of Furrial asphaltenes. For initial solution concentrations of 5, 20 and 50 mg l(-1) an adsorption rate proportional to solute 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 asphaltenes from the surface was very slow and could be neglected. Also, we found that the adsorption isotherms. measured for solution concentrations (c(s)) below 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 are consistent with the formation of asphaltene multilayer at the silica surface and with the adsorption of aggregates (dimers, trimers, etc.) as well as single asphaltene molecules. In view of these results we suggest that for 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 surface is covered or saturated. After this 'saturation' adsorption continues on the asphaltene layer at a much smaller rate. (C) 2000 Elsevier Science B.V. All rights reserved.