Asphaltenes and Other Natural Surfactants from Cerro Negro Crude Oil. Stepwise Adsorption at the Water/ Toluene Interface: Film Formation and Hydrophobic Effects

Abstract

The adsorption of asphaltenes and other natural surfactants at the water/toluene interface has been studied here, using both the ring and pendant drop method to measure the interfacial tension (ç). The dependence of ç on the logarithm of the sample concentration (ln C) (represented as the ç(ln C) isotherm) leads to a stepwise isotherm similar to that corresponding to the adsorption of asphaltenes at the toluene/silica interface. As is the case for the toluene/silica interface, the stepwise ç(ln C) isotherm seems to be the result of adsorption at the toluene/water interface of molecules, as well as aggregates formed in solution. Once adsorbed, these aggregates flocculate at the interface, leading to the well-known three-dimensional asphaltene films. In agreement with previous evidence from thermal lens experiments, the onset of asphaltene aggregation was observed near a sample concentration of C ) 60 mg/L. Although the presence of aliphatic (acid fraction soluble in heptane, AFSH) and other types of acidic components in the asphaltene mixture increases the interfacial activity of asphaltenes, the removal of these components does not prevent the adsorption and leads to a film with an interfacial tension of ç ) 26 mJ/m2. This shows that such asphaltene films are less hydrophobic than what might be expected. As suggested by the detailed description of the results given in this paper, the asphaltene interfacial film is a mixed film formed by both AFSH and asphaltenes, where the role of AFSH is to reduce the value of ç and the role of asphaltenes is to reduce hydrophobic effects by forming a film that effectively separates oil from water.