ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes

Fluoroalkyl esters of 3-carboxy pyrroline nitroxide, FPn   (n=8n=8 and 12), containing (n−2n−2) CF2 groups in the side-chain, were used as novel ESR spin probes of fluorinated micellar systems. The method was applied to study aqueous solutions of perfluoropolyether surfactants of the general formula Cl(C3F6O)2CF2COOX, consisting of two perfluoroisopropoxy units and the counterion X = Na+ or NH+4. By measuring the change of 14N hyperfine splitting with surfactant concentration the critical micellar concentration of the ammonium salt was determined at temperatures of 297, 313 and 333 K. The ESR line shape was also examined as a function of surfactant concentration and of temperature in the range 120–360 K. The results are discussed in terms of solubilization and local environment of the probes in micelles of different size and shape, depending on the surfactant concentration and the kind of the counterion.

Graphical abstractPyrroline nitroxide derivatives with a fluorinated side-chain are suitable spin probes for CMC determination of fluorinated surfactants, and their ESR spectra are sensitive to changes of the micellar structure.Figure optionsDownload full-size imageDownload as PowerPoint slide