Counter Anion Effect on the Self-Aggregation of Dimethyl-di-N-octylammonium Cation: A Dual Behavior between Hydrotropes and Surfactants

Self-aggregation of eight dimethyl-di-N-octylammonium salts ([DiC(8)]) has been investigated as a function of the nature of the counteranion. Tensiometry, conductimetry, and [DiC(8)]-selective electrode measurements highlighted three different behaviors and led to a rationalization of the aggregation process depending on the counteranion: hydrophilic anions (MoO42-, WO42-, SO42-, F-) give only unimers and micelles, whereas less hydrated anions form unimers, dimers, and either one micelle-like structure (NO3-, Br-) or two micelle-like structures (CH3SO3-, Cl-). Small-angle neutron and dynamic light scattering confirms the unusual behavior of [DiC(8)][Cl], which forms two types of aggregates: (i) disk or vesicles between 10 and 30 mM and (ii) ellipsoidal micelles above 30 mM. For [DiC8][MoO42-], the formation of ellipsoidal micelles is supported between 10 and 300 mM. Finally, shapes and sizes of the aggregates are confirmed by molecular dynamic experiments.