Molecular Dynamics Study On The Influence Of Self-assembly Behaviors Of Nanoparticles And Surfactants On The Properties Of Silicone Oil/Water Interface

When surfactants and nanoparticles have complementary functional groups（e.g.,ion-pairing）,nanoparticle surfactants are generated on interfaces,thereby significantly affecting the interface tension,which has wide applications in chemical industries.The nature of the surfactant and the nanoparticles have a vital impact on self-assembly.This study uses the dissipative molecular dynamics simulation method（DPD）to study the self-assembly behaviors of the amino-modified polymer surfactant polydimethylsiloxane and the carboxy-modified silica nanoparticle on a silicone oil-water interface,and explores the internal mechanisms in self-assembly behaviors for a wide range of surfactant mass fraction.The effects of surfactant and nanoparticle mass fractions（ω_S,ω_N）are investigated by examining the interface properties（interface thickness and interface tension）and structural properties（rotation radius）.The results show that nanoparticles and surfactants spontaneously assemble into nanoparticle surfactants at the oil-water interface.Whenω_S=0.005-0.2,it can effectively reduce the interfacial tension.Whileω_S=0.2-0.5,this assembly behavior causes the interfacial tension to increase with increasingω_S.Forω_S of 0.2,the radius of gyration of surfactant is the largest and the fluctuation with time is the smallest,and the interfacial tension of the system is the lowest,indicating that the stretching effect of surfactant molecules is more obvious.The interface tension increases with increasing surfactant chain,indicating that the proportion of end amino groups in surfactants plays a dominant action on interface behaviors.For the surfactant chain of 8,the feature of the lowest interfacial tension and the largest thickness is highlighted.When the number of end amino groups in a surfactant is 1-6,the more end amino groups,the greater the radius of gyration of the surfactant,the greater the interface thickness of the system,and the lower the interfacial tension.When a surfactant contains 2,4,or 6 end amino groups,the self-assembly effect when the end amino groups are located at both ends of the surfactant is higher than that when the end amino groups are all located at one end.At this time,the interfacial tension of the system is lower,the interface thickness is larger and the radius of gyration of surfactant is larger.The results show that the properties of the nanoparticles also affect the interfacial properties.The interfacial tension decreases as the nanoparticle diameter increases from 2.44（?）to 12.21（?）,and increases asω_N raises from 0.0165625 to 0.1325.When the interaction parameter between the surfactant and nanoparticle is raised from 9 to 21,the variation in interaction parameter has no influence on the interface properties.When the oil phase volume of the total volume（φ）in the system accounts for 0.36364-0.71429,with the increase of the oil phase volume,the interface thickness and the radius of gyration of surfactant in the system increase while the interfacial tension decreases.The hydrophobicity of the nanoparticles has a greater impact on the self-assembly.When the repulsion parameter between the nanoparticles and water(a_s-w)is 28.7-38.3,as the a_s-wincreases,the nanoparticles are more hydrophobic,the interface thickness of the system and the radius of gyration of surfactant decreases while the interfacial tension of the system increases.While a_s-w=38.3-48.7,with the increase of a_s-w,the interface thickness of the system and the radius of gyration of surfactant increases while the interfacial tension of the system is reduced.When a_s-w=38.3,the nanoparticle is neither lipophilic nor hydrophilic,and the center of the nanoparticle is roughly located at the center of the oil-water interface.At this time,the interfacial thickness of the system is the smallest and the interfacial tension is the highest due to the presence of surfactants.