Molecular Simulations on the Coalescence of Water-in-Oil Emulsion Droplets with Non-ionic Surfactant and Model Asphaltene

Abstract

Water droplets in crude oil can be stabilized by the adsorption of interfacially active components, such as asphaltenes. Demulsifiers like non-ionic surfactants are commonly used to destabilize the water-in-oil emulsions. In this work, molecular dynamics simulations and free energy calculations were performed to study the coalescence of water droplets coated with both model asphaltene and non-ionic surfactants [PEO-PPO-PEO copolymer (SurP) or Brij surfactant (SurB)]. For the first time, we quantitatively studied the interaction force between water droplets in the presence of both asphaltenes and demulsifiers and addressed the effect of solvent property on the coalescence process. At the droplet surface, demulsifiers adsorbed closer to the water phase and formed more hydrogen bonds with water molecules compared to asphaltenes, indicating the capability of demulsifiers to break the asphaltene film. Comparing the two non-ionic surfactants, VO-79/SurP complexes formed a single-layer film on the droplet surface, while a two-layer structure was formed by VO-79/SurB complexes. This led to a higher repulsive force during droplet coalescence when SurB was present, regardless of the type of solvent. Comparing the two different solvents (toluene vs heptane), for the same adsorbates, the interfacial film was more compact in heptane and there were fewer dispersed VO-79. For VO-79/SurB adsorbates, the bridging of VO-79 led to a smaller repulsion during droplet coalescence when the solvent was heptane, while the difference is insignificant for VO-79/SurP adsorbates. This work suggests that the energy barrier and interaction force for droplet coalescence is highly dependent on the structure of interfacial films, thus providing atomic-level insights into the demulsification mechanisms of water-in-oil emulsions in the presence of surface-active asphaltenes.