Synthesis and Characterization of a Series of Biofriendly Fluoroether Betaines

Fluoroether approach has been extensively employed as an alternative to conventional fluorinated surfactants; however, it faces challenges such as inadequate performance of short fluoroether chains and environmental contamination from long fluoroether chains. This study aims to address the conflict between environmental compatibility and the performance of fluoroether surfactants. The design of the surfactants is centered around the OC chain (CF₃(OCF₂)_<i>n</i>-), utilizing reduced fluorine content to achieve superior surface activity. A series of surfactant molecules featuring elongated hydrophobic chains and dual-chain structures have been synthesized and characterized. All of the molecules have good surface activity at low concentrations. Among them, CF₃(OCF₂)₄CH₂O(CH₂)₃SO₂NH(CH₂)₃N⁺(CH₃)₂CH₂COO^- (OC4F-B), which employs a strategy of extending the hydrophobic chain by insertion of -CH₂O(CH₂)₃-, shows the best performance. This surfactant can achieve a surface tension of 20 mN/m at a concentration of 0.005 wt %, which is 20 times lower than that required for CF₃(OCF₂)₄CONH(CH₂)₃N⁺(CH₃)₂CH₂COO^- (OC4-betaine) to attain the same surface tension. In contrast, (CF₃(OCF₂)₃CH₂O(CH₂)₃SO₂NHCH₂CH₂)₂N⁺(CH₃)CH₂COO^- (DOC3F-B), utilizing both dual-chain and extended carbon chain strategies, exhibits a critical micelle concentration that is diminished by 2 orders of magnitude relative to OC3-betaine, reaching to 0.047 g/L. Merely adjusting the length of the fluorocarbon chain is insufficient to reconcile the conflict between the performance and environmental impact of fluorinated surfactants. This study introduces an innovative approach by incorporating hydrocarbon hydrophobic chains and adopting a dual-chain design, thereby addressing this challenge effectively.