Novel Molecular Insights into the Effects of Ethoxy Group Number on Emulsification and Viscosity Reduction of Anionic–Nonionic Surfactants

Abstract

The high density and viscosity of heavy crude oil seriously affect the production and transportation processes, and surfactant emulsification and viscosity reduction technology are regarded as the ideal methods to enhance oil recovery of heavy oil reservoirs. In the present paper, sodium dodecyl sulfate and sodium dodecyl ether sulfate are used to study the effects of the number of introduced ethoxy (EO) groups on the emulsification and viscosity reduction of surfactants through the experimental method, molecular dynamics simulation, and quantum chemical calculations. The results of optical microscopy and viscosity measurement show that the surfactants can change the emulsion type of heavy oil to cause a remarkable reduction of emulsion viscosity. Then, the molecular density, interfacial thickness, and hydrogen bond relaxation time are computed to study the change of the properties of an interfacial film with additional surfactants. The interaction among surface-active substances is evaluated by radial direction functions and the independent gradient model based on the Hirshfeld partition method, which is confirmed through the steered molecular dynamics simulation method. The simulation results indicate that the surfactants containing more EO groups show better adsorption behavior to enhance the interfacial hydrophilicity and destroy stacking structures among asphaltene and resin molecules, which is conducive to the emulsification and viscosity reduction process. This work presents a nanometer view of the effects of the number of introduced EO groups on the emulsification and viscosity reduction mechanisms of anionic–nonionic surfactants.