Novel molecular insight into the discrepant distributions for ionic surfactants in light oil/water and heavy oil/water systems

[Display omitted] •Discrepant distributions of SDS in two oil/water systems are elaborated by MD simulation.•Heavy oil phase provides the more suitable environment for SDS molecule than water phase.•SDS-Water-Asphaltene (S-W-A) ternary structure is fabricated via the hydrogen bonds.•S-W-A structure is responsible for the detention of SDS molecule in heavy oil phase. Surfactant exhibits great displacement efficiency for the light oil (aliphatic and aromatic hydrocarbons) reservoir, whereas the heavy components (resins and asphaltenes) in crude oil could seriously affect the interfacial activity of surfactants. Then two simulation systems (light oil/water and heavy oil/water systems) were established and the distribution behaviors of the anionic surfactant sodium dodecyl sulfate (SDS) in above two systems were investigated by molecular dynamics (MD) simulation. The simulation results revealed that some SDS molecules could stay in the heavy oil phase but not in the light oil phase. The potential of mean force (PMF) of two systems were calculated to further probe the Gibbs free energy change in the movement process of SDS molecules form water phase to oil phase. The interaction mechanisms of water/surfactant/oil in the movement process were illustrated by the corresponding system configurations and reduced density gradient (RDG) method. The water molecules in hydration shell could be pulled into heavy oil phase coupled with SDS molecules and connect with SDS and asphaltene molecules to fabricate the SDS-Water-Asphaltenes (S-W-A) ternary structure via hydrogen bonds. Furthermore, the interaction energy calculation directly demonstrated the formation of S-W-A structure should be responsible for the discrepant distributions of surfactants in light oil/water and heavy oil/water systems.