Investigating The Structure Of Fluorinated Surfactants At Air/Liquid Interface Through Molecular Simulations

Fluorinated surfactants are widely employed in the application fields ofemulsifiers of the emulsion polymerization of fluorinated olefin, extinguishingadditives, ink wetting additives, which can be attributed to the advantages of highthermal and chemical stability and the hydrophobic and oleophobic properties offluorocarbon segment. All the applications are closely related with the structure offluorinated surfactants adsorbed at air/liquid interface, it is necessary to investigatethe relationship between the monomer structure and the structure of fluorinatedsurfactants at air/liquid interface, which provides guidance and theoretical basis forthe design and synthesis of surfactants.Recently, molecular dynamics simulations were widely employed to illustrate thestructure of surfactants at air/liquid interface from molecular level, which presents asupplement to traditional experimental methods. However, the relationship betweenthe monomer structure and the interfacial structure of fluorinated surfactants is stillunclear unit now. This thesis focus on exploring the effects ofhydrophilic/hydrophobic segment structure of polyoxyethylene nonionic fluorinatedsurfactants on their interfacial structure and explaining the effects of intermediateconnection segment structure of sulfonated anionic fluorinated surfactants on thestructure at air/liquid interface through molecular dynamics (MD) simulations.Besides, different head group and a counter ion were used to reveal their influences ontheir interfacial structure. The main conclusions were as follows:(1) For the polyoxyethylene nonionic fluorinated surfactants, when thehydrophobic chain segments are the same length, the thickness of interfacial waterlayer increases with the hydrophilic chain length increases, while the arrangement ofhydrophobic segments become on the air/liquid interface d disordered On the contrary,when the hydrophilic chain segments shows the same structure and length, thesystems with a shorter hydrophobic segments will dissolved in the aqueous phase, thearrangement of hydrophobic segments at the air/liquid interface become more andmore ordered with the hydrophobic chain length increasing. (2) Perfluorocarbon surfactants reduce the surface tension of water solutionbetter than that for hydrocarbon ones. An ordered anisotropic orientation was detectedat air/liquid interface of perfluorocarbon surfactants while the hydrocarbon8H2EOchains present more disordered. The arrangement of surfactant chains adsorbed at theair/liquid interface is dominated by the VDW parameter of hydrophobic groups. Withreplacement of one CF2group in8F2EO with a CH2group, the interfacial structurewas not significantly influenced, when more CF2groups were replaced with CH2groups, the structure of the surfactant at air/liquid interface becomes disordered asthat for hydrocarbon surfactants.(3) The hydrophilic of sulfonic acid groups would be enhanced when theintermediate segment chain segments was introduced to connect the hydrophilicsegments with the sulfonated anionic fluorinated surfactant. Different functionalgroups could be employed as connected groups, while the segment consist of athioether bond (sulfur) or hydrocarbon ones would influence the hydrophilic of headgroups little. While the connected segment contains the functional groups carbonyland imino would enhanced the hydrophilic of head group effectively.(4) When the hydrophilic groups of ionic surfactant is the same type, thestructure of water layer around head group would be influenced little. The counter ionalso influences the structure of water layer surrounding the perfluorooctane sulfonatedhead groups, then the solubility in the aqueous phase of surfactants would beinfluenced. The arrangement of the fluorocarbon segment at the air/liquid interfacechange little(5) Both the solubility of surfactants and their arrangement are influenced by thetype of hydrophilic head groups. The solubility of cationic surfactant is best and thehydrophobic segments pack tightly but disordered; anionic surfactant is the secondone, as to the the hydrophobic segments, they arrange tightly and ordered; and theonionic one is the worst, the hydrophobic segments arranged loosely and disordered.