Molecular Dynamics Simulation Of The Aggregation Behavior Of Surfactants With Different Hydrophobic Tail Chain Structures At The Interface

In recent years,surfactants have been widely used in petroleum exploitation,daily chemical industry,medicine and biotechnology due to their unique properties.Molecular simulation method can reveal the properties of surfactants from the microscopic point of view.The research results have important theoretical significance and practical value for guiding the molecular design of surfactants.In this paper,molecular dynamics simulation method was used to study three octadecyl sulfate surfactants with different degrees of tail chain branching,which are expressed as C-18 S,C18S(FO-180N),C18S(FO-180).The aggregation behavior and the effect on the interfacial properties in the air/water,oil/water interface system and the n-octane/water,n-decane/water,and n-dodecane/water interface systems,respectively.The details are as follows:(1)Molecular dynamics simulation was used to simulate the aggregation behavior of octadecyl sulfate surfactants with different hydrophobic tail chains at the air/water interface.The interface formation energy,density distribution,interfacial tension,number of hydrogen bonds,diffusion coefficient,radial distribution function and spatial distribution function were used as evaluation indexes to investigate the interfacial properties of octadecyl sulfate with different branching degrees of hydrophobic tail chain at air/water interface,and the mechanism of their differentiation was discussed.The results show that the three surfactants have different ability to reduce interfacial tension.The surfactant C18S(FO-180)which has the highest degree of hydrophobic tail chain branching can cause the lowest interfacial tension,and the aggregation structure formed at the interface is more stable than other systems.The results show that the polar head group of C18S(FO-180)system has a stronger intermolecular interaction with the surrounding water molecules.By changing the structure of hydrophobic alkyl tail chain of surfactant,the aggregation behavior at the interface and the interfacial properties can be changed.(2)The effect of branching degree of hydrophobic tail chain on the aggregation behavior of surfactant at oil/water interface was investigated at the molecular level by analyzing the interface aggregation morphology,density distribution,single molecule occupied area,radial distribution function and coordination number of interfacial aggregation.The results show that the higher degree of branching of the hydrophobic alkyl tail chain,the closer the arrangement of surfactant molecules are at the interface,and the stronger the interaction between hydrophilic head group and surrounding water molecules.The results of interfacial tension and interface formation energy show that with the increase of branched degree of hydrophobic alkyl chain,the ability of surfactant molecules to reduce the interfacial tension of oil-water gradually increases,and the system becomes more stable.(3)The effects of different oil phases(n-octane,n-decane and n-dodecane)on the interfacial properties of three surfactants with different degree of hydrophobic tail chain branching were studied by molecular dynamics simulation.The density distribution,interfacial tension,order parameter,radial distribution function and coordination number were used as evaluation indexes.The results show that when the oil phase is n-dodecane,the structure of the monolayer is the most ordered,and the polar head group has the strongest binding with water.By analyzing the ability of surfactant to reduce the interfacial tension of the system at the different interfaces of different oil phase,the results show that the change of molecular structure of oil phase will affect the properties of surfactant at the interface.