Stable water-in-crude oil (w/o) emulsions occur at many stages during the production and processing of crude oil. The breaking of these emulsions is an essential step, frequently carried out by the addition of chemical demulsifiers. The stability of any surfactant-stabilized emulsion is strongly dependent on the nature of the interfacial film, which provides a resistance to either flocculation or coalescence, towards the final equilibrium state of phase separation. Thus, effective demulsifiers must be capable of displacing preformed, rigid, incompressible interfacial films, and/or significantly reducing the rate of build-up of the surfactant layer. This work presents the results of molecular dynamics simulations of a fiat section for a w/o interface covered by a nonionic surfactant, either pure or contaminated by an anionic demulsifier. The simulation of aqueous and oleic solutions of surfactant and demulsifier were also performed by using 3D amorphous cells with 3D periodicity, while the interface was modeled by a 3D layered cell establishing 2D periodic boundary conditions. This made it possible to demonstrate changes in the amphiphile bimolecular interactions and diffusion properties at the w/o interface once the demulsifier is adsorbed, which could be related to the loss of stability of the surfactant film.