Characterization of the Interfacial Material in Asphaltenes Responsible for Oil/Water Emulsion Stability
Youssra Rahham, Kaustubh Rane, Lamia Goual
Abstract
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 π–π 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.