Mixed micelles of homologous perfluoropolyether anionic surfactants in water

The mixtures of sodium and ammonium salts of three homologous perfluoropolyether carboxylic acids having Cl-terminated perfluoroalkyl group (Cl-PFPE) and differing in the average molecular weight (MW) were examined. The surfactants, namely n2, n3 and n4, have two, three and four PFPE units, respectively. Each surfactant was studied alone and in mixture with the other surfactants with the same counterion. NMR chemical shifts were measured for each surfactant and for the mixtures in different concentrations. For a given mixture the micelle composition, X_i, can be determined from the observation of the chemical shifts of the micellar components. It was found that Cl-PFPE surfactant mixtures form in water mixed micelles which contain the surfactants in equilibrium with monomeric species. The analysis of NMR chemical shift variations allowed evaluating the partition of the various surfactants in the mixed aggregates as a function of the total concentration. Composition of mixed micelles resembles ideal mixing predictions particularly at high surfactant concentrations.