Molecular dynamics study on emulsified oil droplets with nonionic surfactants

Abstract

The stability of heavy crude oil is closely related to the netlike structure formed by the aggregation of asphaltene and resin molecules inside the oil droplets, and the aggregation is too complex to accurately obtain the microstructure of these molecules experimentally. In this paper, molecular dynamics (MD) simulation was performed to investigate the structure of emulsified oil droplets on the molecular level. The simulation results showed: (i) The stacking structure of asphaltene and resin molecules form the netlike structure of the oil droplet, and it is also considered as the main reason of high stability and viscosity of heavy crude oil. (ii) When nonionic surfactants were introduced, some of them can adsorb to the surface of oil droplet and change its hydrophilicity; others can enter the interior of the oil droplets and destroy the stacking structure of asphaltene or resins inside the oil droplet. The decreased viscosity of emulsified heavy oil in the present of surfactants attributed to the increase of hydrophilicity on the oil drop surface and the damage of stacking structure inside the oil drops. (iii) the steered molecular dynamics (SMD) simulation proved that the mechanical properties of emulsified heavy oil droplet become easier to be destroyed with the increase of surfactant concentration, and it indicates that nonionic surfactant is beneficial to promote demulsification and viscosity reduction of crude oil.