Abstract:

This study used the dissipative molecular dynamics simulation method (DPD) to study the self-assembly behaviors of the amino-modified polymer surfactant polyimethylsiloxane and the carboxy-modified silica nanoparticle on a silicone oil-water interface, and explored 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) were investigated by examining the interface properties (interface thickness and interface tension) and structural properties (rotation radius). The results showed that nanoparticles and surfactants spontaneously assembled into nanoparticle surfactants at the oil-water interface. When ω_S=0.005—0.2, it can effectively reduce the interfacial tension. While ω_S=0.2—0.5, this assembly behavior caused the interfacial tension to increase with increasing ω_S. For ω_S of 0.2, the radius of gyration of surfactant was the largest and the fluctuation with time was the smallest, and the interfacial tension of the system was the lowest, indicating that the stretching effect of surfactant molecules was more obvious. The interface tension increased with increasing surfactant chain, showing that the proportion of end amino groups in surfactants played a dominant action on interface behaviors. For the surfactant chain of 8, the feature of the lowest interfacial tension and the largest thickness was highlighted. The interfacial tension decreased as the nanoparticle diameter increased from 2.44Å to 12.21Å, and increased as ω_N raised from 0.0165625 to 0.1325. When the interaction parameter between the surfactant and nanoparticle was raised from 9 to 21, the variation in interaction parameter had no influence on the interface properties.

Key words: self-assembly, molecular simulation, interfacial tension, surfactants, nanoparticles, polymers

摘要：

采用耗散分子动力学模拟方法，研究了氨基修饰的表面活性剂聚二甲基硅氧烷与羧基修饰的二氧化硅纳米颗粒在硅油-水界面上的自组装行为，通过分析油-水界面性质（界面厚度和界面张力）和活性剂的结构性质（回转半径），探讨了活性剂质量分数ω_S和纳米颗粒质量分数ω_N等对界面性质的影响，阐明了自组装过程中界面性质的影响因素。结果表明，纳米颗粒和表面活性剂在油-水界面上自发组装成纳米颗粒表面活性剂，当ω_S=0.005~0.2时，纳米颗粒表面活性剂可有效降低界面张力；而在ω_S=0.2~0.5时，该组装行为使界面张力随ω_S而提高。当ω_S=0.2时，活性剂的回转半径最大、随时间波动最小，表明活性剂分子被拉伸效果更明显，此时体系的界面张力最低。活性剂结构对界面张力影响较大，活性剂链长在8~17时，端氨基占比较大，即链长最短时，体系的界面张力最低，界面厚度最大。当纳米颗粒直径在2.44~12.21Å(1Å=0.1nm)时，界面张力随颗粒直径增大而降低；当ω_N=0.0165625~0.1325时，界面张力随ω_N而提高。当活性剂与颗粒的作用参数在9~21时，作用参数对界面性质无影响。

关键词: 自组装, 分子模拟, 界面张力, 表面活性剂, 纳米粒子, 聚合物