Syntheses and aggregation behavior of pyridine-based CH–CF hybrid surfactants

Synthesis and application of non-bioaccumulable fluorinated surfactants: a review

Abstract

Due to negative effects of conventional fluorinated surfactants with long perfluorocarbon chain (CxF2x+ 1, x≥7) like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), these conventional long perfluorocarbon chain surfactants have been restricted in many industrial applications. Nowadays, their potential non-bioaccumulable alternatives have been developed to meet the requirements of environmental sustainable development. In this paper, the recent advances of potential non-bioaccumulable fluorinated surfactants with different fluorocarbon chain structures, including the short perfluorocarbon chain, the branched fluorocarbon chain, and the fluorocarbon chain with weak points, are reviewed from the aspects of synthesis processes, properties, and structure-activity relationships. And their applications in emulsion polymerization of fluorinated olefins, handling membrane proteins, and leather manufacture also are summarized. Furthermore, the challenges embedded in the current non-bioaccumulable fluorinated surfactants are also highlighted and discussed with the hope to provide a valuable reference for the prosperous development of fluorinated surfactants.

Graphical abstract

Synthesis and Characterization of a Novel Class of Zwitterionic Fluorocarbon Surfactants Based on Perfluorobutyl

Perfluorooctane sulfonate (PFOS) and its derivatives had been banned due to their potential environmental hazards, although they possessed excellent surface activity. An effective method to solve this problem was to shorten the fluorocarbon chain of these surfactants from C°H17 to C4F9. As previous studies had shown, zwitterionic surfactants possess higher surface activity but have lower toxicity compared to other types of surfactants. In view of this, a class of novel zwitterionic fluorocarbon surfactants (n-CFNA-Br) with perfluorobutyl moiety was synthesized in this work. Their structures were characterized by FTIR, 1H NMR, 13C NMR, 19F NMR and MS. The results showed that all synthesized n-CFNA-Br had almost the same minimum surface tension, but their critical micelle concentration (CMC) decreased with increasing length of hydrophobic carbon chain. In pure water, the surface tension at the CMC (γCMC) of the four n-CFNA-Br were about 20 mN/m, and the CMC values were 7.73 mmol/L for 1-CFNA-Br, 4.70 mmol/L for 2-CFNA-Br, 4.13 mmol/L for 3-CFNA-Br, and 3.36 mmol/L for 4-CFNA-Br, indicating high efficiency and effectiveness. In 0.1 mol/L NaCl, the CMC values reduced to less than half of the CMC values measured in the pure aqueous surfactant solution, while the surface tensions γCMC remained almost unchanged, indicating good salinity tolerance of the synthesized surfactants. The acidic surfactant solutions exhibited similar CMC values to the saline solutions, but the surface tension γCMC increased slightly to 25 mN/m. However, further investigation showed that the n-CFNA-Br surfactants exhibited poor surface activity in alkaline solution (0.1 mol/L NaOH). In the pH range of 6.6 to 10.4, white precipitates appeared in the surfactant solutions after some time, indicating that the n-CFNA-Br are not suitable for use in alkaline systems with pH greater than 6.6.

Synthesis and Monolayer Behaviors of Novel Hybrid Corynomycolic Acids Containing Semifluoroalkyl Groups

In this work, novel hybrid-type corynomycolic acids [hybrid-OH and hybrid-COOH, with semifluoroalkyl groups (Rf-(CH2)n-: Rf = C4F9, n = 6 and Rf = C6F13, n = 3) located on the carbon atoms attached to the hydroxyl and carboxylic acid groups (C-OH and C-COOH), respectively] were successfully synthesized. The behaviors and formation of hybrid corynomycolic acid monolayers at the air-water interface were investigated by surface tension and surface pressure-area (π-A) measurements to clarify the effects of the Rf chain length, position of the semifluoroalkyl group, and surfactant molecule stereochemistry. Compared to dialkyl corynomycolic acid, both the critical micelle concentration (CMC) and the surface tension at the CMC (γCMC) of hybrid corynomycolic acids were reduced by the presence of the Rf group. With respect to the surface tension versus log concentration (γ vs. log C) isotherms, all syn-isomers of the hybrid-OH and hybrid-COOH acids showed two break points, while the anti-isomers showed only one break point. These different isotherms can be explained in terms of the steric repulsion between the two hydrophilic groups (OH and COO(-)), which depend on the stereochemistry of the surfactant. No effect of the location of the semifluoroalkyl group was observed. With respect to the formation of a monolayer film, four parameters-the lift-off area (AL), zero-pressure molecular area (A0), maximum of the Gibbs elastic modulus [EG (max)], and monolayer collapse pressure (πc)-were measured. Both AL and A0 of all hybrid corynomycolic acids were larger than the corresponding dialkyl acids due to the bulky and rigid Rf groups. Interestingly, syn- and anti-hybrids had almost identical isotherms on compression, although the values of πc of anti-hybrids were higher than those of syn-isomers. In addition, the values of EG (max) of hybrid-COOHs were slightly larger than those of the corresponding hybrid-OHs. Using the nascent soap method (agent-in-oil method), we found that anti-F4C6-OH (a hybrid corynomycolic acid) is a promising emulsifier for a ternary system comprising octane, water, and perfluoropolyether oil.

Self-Assembly and Rheological Properties of a Pseudogemini Surfactant Formed in a Salt-Free Catanionic Surfactant Mixture in Water

The surface and bulk properties of bola-type dicarboxylic acid (sebacic acid, SA) and zwitterionic surfactant tetradecyldimethylamine oxide (C14DMAO) mixtures in aqueous solutions were studied. Surface tension measurements indicate a pronounced synergistic effect between SA and C14DMAO. In bulk aqueous solutions, rich phase behavior was observed with a varied SA-to-C14DMAO ratio (ρ) and a total surfactant concentration. Typically at ρ = 0.5, a novel pseudogemini surfactant (C14-S-C14) forms, driven by electrostatic interaction and hydrogen bonding. The C14-S-C14/H2O system exhibits rich phase behavior induced by the transition of aggregates. With increasing concentration of C14-S-C14, one can observe a viscous L1 phase, an L1/Lα two-phase region where a birefringent Lα phase is on the top of an L1 phase, a single Lα phase, and finally a mixture of an Lα phase and a precipitate. Microstructures formed in the Lα phases were determined by freeze-fracture transmission electron microscopy (FF-TEM) and cryogenic-transmission electron microscopy (cryo-TEM) observations. Polymorphic aggregation behavior was observed with the formation of a variety of bilayer structures including unilamellar vesicles, onions, and open and hyperbranched bilayers. Rheological measurements showed that the Lα phases are viscoelastic and sensitive to temperature where a quick loss of viscoelasticity was observed at elevated temperature.