Dynamic Asphaltene−Resin Exchange at the Oil/Water Interface: Time-Dependent W/O Emulsion Stability for Asphaltene/Resin Model Oils

Abstract

The critical electric field (CEF) technique was used to determine the time-dependent stability of water-in-oil emulsions in which asphaltenes stabilize the film. Stabilizing films comprising purely asphaltenes were observed to increase monotonically in stability with time. However, in the presence of resins, particularly in mass ratios of resins to asphaltenes of 0.5−1.0, the stability of the emulsions as probed by CEF were observed to exhibit a very sharp local maximum. Similar behavior was observed in dilatational interfacial rheology experiments using an oscillating drop tensiometer. The dilatational modulus (ε) for the stabilizing film, as obtained from the variation of interfacial tension with interfacial area, of an aging asphaltene/resin model oil droplet in water exhibited a time-dependent local maximum. Values of ε were nominally lower for asphaltene/resin model oil systems than asphaltene model oil systems, qualitatively similar to CEF trends. These observed phenomena are similar to the “Vroman effect”, observed in competitive protein adsorption. One plausible explanation is that resin-solvated asphaltenic aggregates are able to diffuse and adsorb to the interface more quickly than larger pure asphaltenic aggregates, but then a molecular rearrangement occurs in which resins become the primary adsorbent in the monolayer by reptation through the consolidated asphaltene network, displacing the asphaltenes and reducing the stability and the dilatational elasticity.