Research On Oil-water Interface Characteristics In The Presence Of Surfactant Using Dissipative Particle Dynamics

With the development of fluid controlling technology and micro-total-analysis-system,the details and microstructure of oil-water interfacial properties regulated by surfactants have become the key point to which people pay more and more attention.By studying the macroscopic interaction mechanism of surfactant at the oil-water interface,we can guide the selections of emulsifiers,and evaluate the energy consuming in the process of oil and water separating.In this paper,we investigate the interfacial properties of oil-water system and the self-assembly behavior of surfactant on the interface,using dissipative particle dynamics(DPD).The DPD conservative parameters are calculated from all-atom molecular dynamics(MD)combined with the Flory-Huggins theory.On mesoscopic scale,the molecules are coarse-grained to simulate the corresponding real fluids.By studying the aggregation behavior of copolymers with different structures on oil-water interface,the effects of their compositions and structures on the phase behavior of oil-water system are analyzed.At the same time,the regulation of water cut,salinity,surfactant and polymer on the oil-water interface are studied.The simulation results show that the hydrophobic-hydrophilic block ratio,molecular weight and concentration of copolymer influences its aggregation behavior at the oil-water interface.Also the interfacial activity is related to its structure and molecular size.The higher the content of hydrophobic block is,the better the interfacial activity is.And the high interfacial density also leads to excellent interfacial effect.Effects of the water content on equilibrated morphology of the oil-water emulsion are studied with the help of ionic surfactant.In addition,oil-water emulsion has the patterns of the compound emulsifier at the very low and very high water content.The interfacial tension increases,reaches maximum,and then decreases with an increase of water content.Both experiments and simulations show that the inorganic salts can improve the interfacial efficiency of the ionic surfactants,and lower the interfacial tension.The addition of polymer SMAc reduces the interfacial tension,and accelerates the coalescence of oil droplets accordingly.In addition,SMAc can influence the surfactant aggregation behavior at the oil-water interface as well.Through the calculation of MD and DPD,the linear relation equation between Flory-Huggins parameters and conservative force parameters at different temperatures is fitted,and the coupling of temperature field between MD and DPD calculation is realized.The effects of temperature on the interfacial behavior of cationic(CTAB)and anionic(SDBS)surfactants at the oil-water interface are observed based on DPD.The results show that the increase of temperature can reduce the repulsive force between beads and improve the interfacial adsorption ability of surfactant,so as to achieve the purpose of reducing interfacial tension.Compared with CTAB and SDBS,SDBS has good resistance to high temperature,and can maintain a lower interfacial tension in the range of 343 to 358 K.