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.

This study provides the first nanoscale characterization of the thin films responsible for oil/water emulsion stability using Transmission Electron Microscopy (TEM). High-resolution images revealed that they are on average 2.5 μm large and 24 nm thick in toluene and can easily fold and wrinkle. The films consisted of vertical stacks of wormlike asphaltene clusters with a thickness of 2.5–9 nm and a length-to-thickness aspect ratio of about 5. These clusters resulted from the aggregation of the most interfacially active asphaltenes, whose structure is slightly different from that of bulk asphaltenes, leading to interconnected viscous hydrogels in solution and significant interfacial tension reduction at the interface. The films provided a protective layer around water droplets on which bulk asphaltenes adsorbed. Thus, one side of the films became rough and hydrophobic while the other side facing water remained smooth and hydrophilic. This amphiphilic character was later verified through contact angle measurements on film-coated quartz chips. TEM micrographs indicated the presence of small clusters that adsorbed in multilayered patches and medium-to-large clusters that adhered randomly. As these aggregates built up at the interface, they perforated the film and compromised its integrity. Therefore, the conditions that promote the adsorption of bulk asphaltenes on surfaces (e.g., a low bulk concentration or a high heptane content) will reduce emulsion stability.