Investigating The Structure Of Cationic/anionic Fluorinated Surfactants At The Air/Water Interface Through Molecular Dynamic Simulation

Fluorinated surfactants are most important special surfactants which play a key role in many fields. All the applications are closely related to the structure of fluorinated surfactants at the air/water interface. Therefore, it is necessary to investigate the relationship between the monomer structure and the interfacial structure of fluorinated surfactants.Molecular dynamics simulations has been widely employed to study the properties of different surfactant solutions. This thesis focus on exploring the influences of hydrophilic/hydrophobic segment structure on the interfacial structure of anionic and cationic fluorinated surfactants and their mixtures.The interaction between different surfactant chains and their influence on the interfcial structure was also explored through molecular dynamics simulations. The main conclusions are as follows:(1) The aqueous solubility of sulfonated anionic fluorinated surfactant would be enhanced with the hydrophilic chain length increasing, and the adsoebed layer of surfactants monolayer would become disordered. On the contrary, the interfacial structure would be more order when the hydrophobic chain length increase. Besides, the interfcial structure of perfluorooctane sulfonic acid(SDBS) is more orded than that for perfluorooctane sulfonate(PFOS)’s and perfluorooctanoate(PFOA)’s, It can be attributed to the interaction between the head group of perfluorooctane sulfonic acid and water molecules, it is stronger than that for PFOS and PFOA.(2) Compared with Fluorinated anionic surfactants, the adsorpted layer of cationic fluorinated surfactant(C6F13C2H4N(CH3)2R) is disorder. Their interaction between head groups would be enhanced by increasing the chain length of head group, which makes the surfactants aggregate more easily at the air/water interface and sublayer.(3) The ordered parameter of PFOS and PFOA mixture monolayer at air/water interface is between them if we mix these two anionic surfactants, the interfacial structure would be influenced by the size of the head goup. As to the mixture systems of PFOS and C6F13C2H4N(CH3)3, highly ordered monolayer would be observed at the air/water interface.The ordered parameter of the mixture is better than the two single components which may cause by the electronic interaction between head groups. The strcuture of surfactants monolayer at air/liquid interface would not influenced much by adding counterions(NaCl) to the solution.