Precipitation of Asphaltenes and Resins at the Toluene–Silica Interface: An Example of Precipitation Promoted by Local Electrical Fields Present at the Silica–Toluene Interface

Abstract

Adsorption and desorption of asphaltenes and resins occurring at the silica–toluene interface were used as a tool to investigate phase separation or precipitation promoted by the surface. Adsorption isotherms for this system were obtained as usual, employing commercial silica plates immersed in toluene solutions. After the adsorption of the sample (asphaltenes, asphaltene subfraction A2, resins I, and resins II), the plate was removed, contacted with and retracted from toluene to remove trapped solution, and then immersed again in fresh toluene to measure the desorption. In all cases, the amount desorbed was either insignificant or a small fraction of the amount expected. This could be analyzed in terms of precipitation at the interface promoted by local electrical fields present at the silica–toluene interface; these fields promote molecular polarization, dispersion interactions, and piling up, leading to precipitation. As a result, when the plate with the adsorbed precipitate (material of very low solubility in toluene) is contacted with fresh toluene, desorption is either insignificant or very low. The combine effects of the local electrical fields, molecular polarization, and dispersion interactions are so effective that resin precipitation at the above interface was also obtained. These ideas were found coherent with preliminary atomic force microscopy (AFM) measurements performed on asphaltenes deposited on a glass surface.