Molecular Simulation Study On Adsorption Characteristics Of Anionic Surfactant At Heterogeneous Interfaces

As an indispensable chemical additive,surfactants have been widely used to reduce the hydrophilicity of coal,inhibit methane hydrate,lubricate automobile engines,enhance crude oil desorption,and improve oil displacement efficiency.Surfactants are widely used in the petroleum industry,especially in enhanced oil recovery(EOR)processes.Since surfactant adsorption on reservoir rock means the loss of a valuable component,it makes surfactant flooding economically less feasible.In this work,the molecular dynamics(MD)simulation method is used to study the adsorption behavior of anionic surfactants on the calcite surface under different charge states for the first time.The adsorption process was systematically analyzed using adsorption energy,mean square displacement(MSD),relative concentration distribution(RCD)and radial distribution function(RDF),revealing the adsorption mechanism.The results show that the adsorption of sodium dodecyl sulfate(SDS)on the surface of calcite is the strongest.When different charges were added to the calcite surface,sodium dodecylbenzenesulfonate(SDBS)was the most stable on the surface with a positive charge state of 10 e,with the largest adsorption energy and the smallest diffusion coefficient.The adsorption energy of SDBS becomes weaker as the length of the surfactant tail carbon chain increases,which is mainly attributed to the van der Waals interaction(i.e.,long-range dispersion interaction)between surfactants.Monolayer or bilayer surfactants can be deduced from RCD.The RDF curves indicate that hydrogen bonding plays an important role in the interaction of the calcite surface,water molecules and surfactant head groups.In order to further discuss the effect of temperature on the adsorption of surfactants on the rock surface,the MD method is used to study the adsorption of SDBS on the calcite surface at temperatures of 313 K,333K,353 K,373K and 393 K.By analyzing the equilibrium state,MSD,RDF and RCD,the results show that the energy of the system tends to be stable at about 75 ps,and the energy after stabilization is-5428.48kcal/mol,indicating that the thermodynamic equilibrium has been reached;through the MSD image of SDBS,it can be concluded that the temperature rise high will make the surfactant unstable;it can be seen from the RDF image that the change of temperature will affect the formation of hydrogen bonds between atoms;it can be concluded from the RCD image that 313 K shows that the adsorption of the active agent is the best.The adsorption of SDBS became more and more unstable with increasing temperature,and two hydration layers were formed.In this chapter,the adsorption of surfactants at different temperatures is studied by molecular dynamics simulation,and its adsorption configuration and microscopic mechanism are analyzed,which provides reference value and theoretical guidance for the experimental exploration of further enhanced oil recovery.In order to explore the effect of surface activity on the oil-water interface,the molecular dynamics simulation method(MD)is used in this chapter to obtain the equilibrium configuration of the system and the adsorption morphology of surfactants,and explore the alkyl groups with different carbon chain lengths.The adsorption of sodium sulfonate on the surface of octane-calcite was compared with the oil-water adsorption state when no surfactant was added,and four parameters such as interface formation energy(IFE),MSD,RDF and RCD were further analyzed,which explained the oil from the rock surface from the microscopic level.The results show that when no surfactant is added to the system,the IFE is the smallest,the diffusion of water molecules is the most intense,and the oil molecules are still stably adsorbed on the calcite surface.With the increase of the number,the IFE of the system gradually increases,the system gradually tends to be stable,the diffusion of water molecules tends to be gentle,and the octane molecules begin to move away from the rock surface;the RDF curve shows that the head group atoms of the surfactant are through hydrogen bonds.connected.The analysis shows that the more carbon atoms,the more stable the oil-water-rock system.The results provide important molecular-scale insights into the adsorption of charge-modulating anionic surfactants on calcite surfaces.By studying the effect of surfactants at the oil-water interface,it is concluded that surfactants can accelerate the release of octane molecules from the calcite surface.separation,provides guidance for finding the optimal flooding conditions to reduce surfactant loss,and provides a feasible route on how to accelerate oil separation.