A study of multilayer adsorption of asphaltenes on glass surfaces by photothermal surface deformation. Relation of this adsorption to aggregate formation in solution

Adsorption of asphaltenes on glass surface has been studied by using photothermal surface deformation spectroscopy. This technique was adapted for direct measurement of the amount of asphaltene adsorbed on glass plate surfaces. Solid-solution adsorption isotherms for asphaltenes were obtained at room temperature in toluene. Stepwise or multilayer adsorption was detected in the three cases examined (Furrial, Jobo, and Hamaca) after long contact periods. After the first layer due to glass surface saturation one (Jobo and Hamaca) or two (Furrial) further layers were measured. Analysis of results suggests that this adsorption behavior could be related to the sudden increase of aggregate concentration typical of micelle formation. For instance, below 4000 mg/L Furrial asphaltenes behave like a mixture containing two surfactants with cmc of 1500 and 3000 mg/L, respectively. A slow increase in molecular weight when measured in toluene was observed for Hamaca (from 4230 to 17420) and Cerro Negro (from 5350 to 15170) over a 3 day period, suggesting a slow formation of aggregates. Accordingly, the above stepwise adsorption takes a long time to appear (more than 3 days). Isotherms measured at shorter times only showed a continuous but not stepwise increase in adsorption with concentration.