Micellar formation of cationic surfactants

The micellar structure of six alkyl trimethylammonium halides was studied via conductivity. It was found that the aggregation number increased with the decreasing carbon chain length. Furthermore, Br(−) significantly enhanced the micellar formation over Cl(−). However, the aggregation number and ionization degree remain similar for both anions. The modelling results validate that the counter-anions affect micellar formation via equilibrium constants, instead of their hydration size. In particular, the association constants between surfactant (both monomer and micelle) and Br(−) are significantly higher than Cl(−). This is consistent with the qualitative description of hydrated Br(−) in the literature. The experimental and modelling results confirm that micelles are formed via “ion-paring/hydration” structure, instead of the conventional “packing” concept.