Dissipative Particle Dynamics Simulation Study On Emulsifcation Properties Of Gemini Surfactants At Oil-water Interface

Gemini surfactants possess higher interfacial activity compared with conventional surfactants and are expected to be used in tertiary oil recovery.The structure-effective relationship between Gemini surfactants with different structures and their interfacial emulsification properties is not clear,which affecting the development of high-efficiency oil-displacing agents.The molecular formula of the Gemini surfactants and their derivatives studied in this thesis is:([(C₁₂H₂₅)_n（C₆H₃SO₃^-）O（CH₂）_mO（C₆H₃SO₃^-）(C₁₂H₂₅)_n]2Na⁺).The dissipative particle dynamics simulation at the mesoscopic scale was used to study the emulsification performance of Gemini surfactants at the oil-water interface.By changing the spacer group and the length of the hydrophobic chain in the Gemini surfactant,the influence of its molecular structure on the emulsification performance was revealed.This offers a theoretical basis for the development of high-performance Gemini surfactants for oil displacement.Specific research contents include:（1）The aggregation behavior of Gemini surfactants with different hydrophobic chain lengths at the water/n-dodecane interface was simulated.The interfacial tension,density distribution,interfacial thickness,and radial distribution functions under different concentrations were calculated.The spatial distribution of each component in the system was analyzed.It was concluded that the increase of hydrophobic chain length of Gemini surfactants leads to the decrease of critical micelle concentration,and the self-assembled morphological phase diagram of Gemini surfactants at different concentrations and hydrophobic chain lengths was drawn to elucidate the emulsification mechanism of Gemini surfactants at the oil-water interface.The concept of micro-mixing between the hydrophobic chain ends and the oil phase was proposed to reveal the reason why Gemini surfactants can form microemulsions without co-surfactants.（2）The accumulation pattern of Gemini surfactants with different spacer group lengths at the water/n-dodecane interface was simulated.The interfacial tension and density distribution at different spacer group lengths were systematically analyzed and the aggregation behavior of Gemini surfactants at the interface was observed.It was discovered that the interfacial activity of the Gemini surfactant would be strengthened with the increase of the spacer group length,and the water/n-dodecane interfacial tension was calculated for surfactants with different sapcer group lengths at different interfacial coverage.The density distribution in the direction of the oil-water interface suggests that the spacer group will occupy the position of the hydrophilic group at the interface.The mechanism of the microscopic influence of the length of the spacer group on the interfacial activity as well as the aggregation morphology of the Gemini surfactant was illustrated.（3）The emulsification behavior of straight-chain surfactants at the water/benzene interface and the water/octane interface were simulated.The aggregation morphology of surfactants in benzene and n-octane were discovered.The two-dimensional density map of surfactants at the oil-water interface profiles was obtained,and the spatial distribution structure of surfactants at different oil-water interfaces was clarified.The"similar solubility principle"between surfactants and oil phases was proposed,illustrating the effect of the interaction between hydrophobic chains of surfactants and oil phases on the emulsification performance of the system.In summary,through the simulation study of the oil/water/surfactant system,the emulsification mechanism of Gemini surfactants at the oil-water interface and straight-chain surfactants in different oil phases was analyzed,in order to achieve the purpose of efficient screening and even design of surfactant molecules,provide theoretical guidance for experiments,and provide kinetic criteria for the stability of（micro）emulsions.