The Interfacial Activities Of Surfactants At Oil/Water Interface:Molecular Dynamics Simulations

Surfactants are widely used in daily life and industrial production.Studying their interface properties by molecular simulation can obtain information of the microscopic behavior of surfactant molecules,which helps to understand the interface adsorption mechanism and gains insight into the relationship of structure and properties of molecules.By means of atomistic molecular dynamics(MD)simulations,we explore the adsorption and interfacial activities of three typical surfactants at three different oil/water interface and simulate the displacement of heavy oil aggregates adsorbed on the surface of rock,including the nonionic type surfactant octylphenol polyoxyethylene(10)ether(OP-10),the cationic type surfactant dodecyl trimethyl ammonium bromide(DTAB),and the anionic type surfactant sodium dodecyl benzene sulfonate(SDBS).In the n-dodecane oil-layer/water system,the interfacial adsorption structures and the capabilities of reducing the oil/water interfacial tension of three surfactants with different surface concentration are explored.The molecular orientation of surfactants,the interface thickness,the radial distribution function(RDF)of water molecules around the polar head group,and the mean squared displacement(MSD)of water in the first hydration layer are investigated by using the simulation trajectory.Simulation results exhibit that all three kinds of surfactants can form stable adsorption layer at the oil/water interface,however,the capabilities of reducing the oil-water interface tensions are quite different.The OP-10 molecule can not reduce the interfacial tension of n-dodecane oil-layer/water greatly in comparison to two ionic surfactants.The n-dodecane oil-layer/water interface can reach a lower interfacial tension value with the concentration increasing of two ionic surfactants.In the oil-droplet/water system,the microscopic adsorption process of surfactant molecules randomly distributed in the aqueous phase onto the oil-droplet surface is simulated.It is found that under the influence of the OP-10 molecules,the free spherical nanosized oil-droplet undergoes a structural transition from a spherical aggregate state to a rod-like aggregate state.In the polar oil molecule oil-layer/water system,it is found that the polar oil molecules randomly distributed in the oil phase migrate to the oil/water interface to form an adsorption film and reduce the interfacial tension.By adding or removing benzene ring between the headgroup and tail of these three surfactant molecules,the effect of the benzene ring of surfactant molecule on interfacial tension is explored,it is found that the surfactant without benzene ring can reduce the n-dodecane oil-layer/water interfacial tension better.At the same time,under the non-equilibrium conditions,the displacement behavior of the heavy crude oil aggregates adsorbed in the pores of nano-scale rock in the three different surfactant solutions are simulated,the results show that the ionic surfactants have better oil displacement effect.