Design, synthesis, and structure-activity relationships of oil-soluble fluorinated surfactants with fluorocarbon/hydrocarbon hybrid chain

This study presents an exploration into the relationship between the molecular structure of oil-soluble fluorinated surfactants and their surface properties across a variety of organic solvents. A series of oil-soluble surfactants with fluorocarbon/hydrocarbon hybrid chains, N-methyl-N-alkyl-4-perfluoroalkylsulfoxybenzylamines (FmHn), were designed and synthesized, and their surface activity was evaluated in 12 organic solvents with different polarities. The surface parameters, including CMC, gamma CMC\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\gamma }_{CMC}$$\end{document}, Gamma max\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Gamma }_{max}$$\end{document} and Amin\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${A}_{min}$$\end{document}, were measured in n-hexadecane, m-xylene, and DMSO, allowing for an in-depth analysis of the influence of molecular structure on these surface properties. Results indicate that an increase in the length of the fluorocarbon chain generally enhances surface activity, leading to a reduction in the CMC value and an increase in the effectiveness of surface tension reduction. However, the impact of the hydrocarbon chain length on surface activity is more complex and dependent on the polarity of the organic solvents. In low-polarity solvents, surface activity is improved with a longer hydrocarbon chain, whereas in high-polarity solvents, a shorter hydrocarbon chain is more beneficial. Consequently, a "polarity-directionality" strategy was proposed to tailor the molecular structure of surfactants to optimize performance in solvents with varying polarities, resulting in a significant reduction in surface tension. Specifically, F8H12 was identified as particularly effective in low-polarity n-alkanes and cycloalkanes, F6H8 was most effective in medium polarity aromatics, and F8H4 or F6H4 was ideal for larger polar solvents. These findings enrich the understanding of the structure-activity relationship in oil-soluble fluorinated surfactants and offer new perspectives for the development of high-performance surfactants with reduced environmental impact.