Characterization of the Interfacial Material in Asphaltenes Responsible for Oil/Water Emulsion Stability

This study aimed at a better understanding of the structure and properties of the most interfacially active asphaltenes responsible for the stability of oil/water emulsions. Interfacial material (IM) was extracted from three different crude oils using a modified solvent washing procedure. Structural parameters of IM were analyzed and compared to those of a parent asphaltene. High-resolution microscopy imaging was performed to correlate IM microstructure to their interfacial properties. Although IM and asphaltene fractions had comparable molecular weights, IM species had a smaller aromatic core than asphaltenes with at least one linear fatty acid chain containing a sulfinic or carboxylic group. This subtle difference conferred them with an amphiphilic character that promoted their self-assembly into 2-6 nm thick worm-like aggregates in solution instead of spherical clusters. IM molecules interacted with water through their fatty acid chains while simultaneously pi-pi stacking with adjacent molecules. These two types of interactions favored their multilayer growth and the formation of stable interfacial films around water droplets that significantly lowered the oil/water interfacial tension. High-resolution microscopy imaging of the interfacial films revealed the presence of flexible nanosheets that are 10-50 nm thick. The nanosheets provided a large surface area on which asphaltene clusters (smaller than 20 nm) and fine clay particles (100-200 nm) adsorbed. Even though multiple washings were performed to extract IM, there were still traces of asphaltenes present in this fraction as well as 10-30 wt % of clays, depending on the type of crude oil. The latter induced some artifacts when analyzing the properties of the most interfacially active asphaltenes.