Competitive adsorption of sodium naphthenates and naturally occurring species at water-in-crude oil emulsion droplet surfaces

The stability of water-in-crude oil emulsions is partly due to a complex, rigid layer formed at the water droplet surface that is likely made of large molecules such as asphaltenes. In highly diluted bitumen, where water droplet surfaces are relatively free of low molecular weight surfactant species, surface rigidity is prevalent. The properties of this layer change dramatically upon addition of a low molecular weight surfactant, sodium naphthenates (NaN), to the water phase. A rigid droplet surface becomes flexible as the concentration of NaN is increased. Interfacial tension measurements, made on individual micron-sized droplets, indicate that the water–oil interface is increasingly dominated by the added surfactant. Without the surfactant, over days, the interfacial tension of such a system decreases indicating long term, irreversible adsorption, similar to the behaviour observed for proteins, or a slow reorganization of the adsorbed macromolecules into lower energy configurations. It appears that the adsorbed NaN prevents adsorption of less surface-active, higher molecular weight material. Although water droplets remain stable despite surfactant addition, the nature of the stability changes. In the presence of added surfactant the droplets flocculate—a phenomenon not observed in its absence.