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The effect of concentration and temperature on the microstructure of aqueous micelles of carboxylic perfluoropolyether surfactants, with two perfluoroisopropoxy units in the chain (n2) that is chlorine-terminated, are studied by SANS for the ammonium and potassium counterions. The SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multiion system reduced to an effective one component macroions system (OCM). At 28 °C, in the surfactant concentration range 0.05 to 0.12 M, the micelles display spherical shape with inner core radius of 15 Å for both counterions, and interfacial layer thickness of 4 Å. At higher concentration, both counterions provide ellipsoidal micelles, with axial ratio 2 and a limiting dimension of 13 Å. A sharp increase of temperature up to 80 °C enables the ammonium salt at 0.2 M to rearrange its ellipsoidal micelles into spherical ones, while the micelles of the potassium salt remain ellipsoidal. In all cases, the micellar size distribution is extremely narrow and the average aggregation numbers, as well as the surface charge, are found to slightly differ for the two counterions upon variation of concentration and temperature, driving ionization degrees globally spanning from 0.3 to 0.5. The interfacial hydration, the surface potential and the area per polar head at the micellar surface are discussed too.

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