Interfacial Behaviour And Structure-Function Relationship Of Surfactants

In this thesis molecular simulation techniques, such as Dissipative Particle Dynamics simulation (DPD) and Molecular Dynamics simulation (MD), were used to investigate the molecular behavior of mixed surfactants at oil/water, gas/water interfaces and in the bulk phase.And interfacial tension and foam stability of the mixed surfactant systems were measured through macroscopical experiment methods. By combining molecular simulation and experiment results, the synergistic effect of the mixed surfactant systems and the influence of inorganic ions on the molecular behavior, inetrfacial activity and foam stability were discussed.The interfacial array behaviors of synergistic surfactants on molecular level were obtained preferably and the mechnisms revealed provided theory foundation for the design and application of the mixed surfactant syatem.According to the above, this thesis is divided into three parts:Firstly, the synergistic effect between ionic surfactant sodium dodecyl sulfate and zwitteroinic surfactant lauramidopropyl betaine were investigated through measuring interfacial tension and foam stability combined with computer simulation The effects of temperature, inorganic salts and the proportion of components in the mixed binary systems on the synergistic property were discussed. The results indicated that they showed good synergism. Interfacial activity of mixed system was significantly better than unitary system and was enhanced when there was Ca2+or Mg2+existed. They would be a good candidate system for foam system used under high salinity condition. The molecular behavior of the surfactants at oil/water interface and the foam films were similar. When the proportion of components of SDS and Betaine was4:6, the oil/water interfacial activity and foam stability of the binary system both got better effects with Ca2+or Mg2+existing. The molecular array behaviors of SDS and Betaine at air/water and oil/water interface were described under high salinity condition using Molecular Dynamics simulation, it was found there was very strong interaction between the Ca2+or Mg2+and the head groups of SDS and Betaine, which adjusted the array states of the surfactant molecules in the interface layer, thereby influenced the property. Ca2+and Mg2+Were coexisted and the influence of Mg2+dominated with sea water as medium.Secondly, synergistic effects between nonionic-ionic bi-head surfactant fatty alcohol polyoxyethylene ether sodium sulfate AES and zwitterionic carboxyl betaine or sulphobetaine were investigated, oil/water interfacial activity and foam stability of fatty alcohol polyoxyethylene ether sodium sulfate AES and carboxyl betaine or sulphobetaine were discussed. The effects of inorganic salts and the proportion of components in the mixed binary systems on the interfacial activity were investigated. It was found that there were synergistic effects between AES and betaine. The interfacial activity of mixed systems was better than unitary system. The foam stability of sea water was bad compared with inorganic salt system separately. Interfacial activity was better when mass ratio of AES and carboxylbetaine was7:3-1:9, while AES and DSB was8:2-3:7. In the mixed system of AES and carboxylbetaine, the stronger interaction of COO" and Ca2+resulted in no Ca2+distribution in the distance of5A around SO4-; while in the mixed system of AES and DSB, the stronger interaction of SOs-and Mg2+resulted in no Mg2+distribution around SO4-.Thirdly, dissipative particle dynamics and molecular dynamics simulations were used to investigate the interface behaviors and bulk phase behaviors of SDS, betaine and the mixed system with different flavor being used as oil phase, t was found that adding betaine made dispersion effect better and the surfactant molecules array more ordered in the mixing system, and te flavors were dispersedbetter..The innovations of the thesis are as follows:1. By combined computer simulations with experiments the oil/water interfacial activity and foam stability of synergistic surfactantswas investigate on molecular scale;2. The influence of the multivalent inorganic cations on the interfacial adsorption behaviors of the surfactants and the synergistic effect was discussed, optimum system has high interfacial acivity and foam stability using sea water as medium was 3. Molecular simulation methods were used to describe the behavior of surfatants in bulk phase and interface with different flavors used as oil phase, and the effect of synegistic effect of surfactants on the oil phase dispersion capability was found.