Nanoscale Characterization of Thin Films at Oil/Water Interfaces and Implications to Emulsion Stability

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 mu m 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.