Aggregation of Perfluoroctanoate Salts Studied by 19F NMR and DFT Calculations: Counterion Complexation, Poly(ethylene glycol) Addition, and Conformational Effects

The aggregation of perfluoroctanoate salts in H2O is studied by F-19 NMR on solutions of LiPFO, NaPFO, and CsPFO, without and with the addition of two poly(ethylene glycol) (PEG) oligomers or molecular weight 1500 and 3400 Da, respectively, and with the addition of suitable crown ethers. The F-19 chemical shift (cs) trends fare monitored, at 25 degrees C, in a concentration range including the critical micellar concentration (cmc) or, in the presence of PEG, the critical aggregation concentration (cac). The cac values in the samples with PEG are lower than the cmc values or the corresponding samples without PEG; moreover, the F-19 cs trends above the cac and above the polymer saturation concentration reveal and help to explain some peculiarities of the aggregation process of PEG on PFO micelles, which, in the first step, seems to occur while the surfactant concentration in water is still increasing. Also in LiPFO/H2O or NaPFO/H2O solutions containing 12-crown-4 or 15-crown-5 ethers, suitable to complex Li+ or Na+ ions, respectively, the cmc decreases. On the other hand, the micellization process in the presence of crown ethers does not show other peculiarities. The prevailing conformations or the PFO chain are discussed on the basis of quantum-mechanical calculations. The theoretical chemical shifts were computed at the DFT level of theory, taking into account the effects of the environment by means of the IEF-PCM method. The helical structure is the most stable one, but anti conformations are easily accessible, in both the aqueous and fluorinated environment. The comparison between computed and experimental chemical shifts indicates that ami conformations arc more important in the micelles than in water and in CsPFO micelles than in LiPFO or NaPFO ones.