NMR investigation on micellization of ammonium/tetraalkylammonium perfluorooctanoates

The micellization of ammonium/tetraalkylammonium perfluorooctanoates (C7F15COON(CnH2n+1)4, n = 0–4) was investigated by 1H and 19F NMR. Tetraalkylammonium chlorides were also studied by 1H NMR to simulate the layer of bound counterions on micellar surface. The critical micelle concentration (cmc) determined by NMR was found to decrease with the size of counterions. It was directly proved from the chemical shift change that the longer the alkyl moieties of counterions, the tighter is the contact between perfluorooctanoate headgroups and counterions. Possible conformations of counterions bound to micellar surface were proposed from the chemical shift analysis. The hydrophobicity and structures of counterions with long alkyl chains such as N(C3H7)4+ and N(C4H9)4+ seemed to result in folded, shrink or penetration conformation, rather than ideal models with symmetrically stretched alkyl chains.