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.
Keywords: Chemical engineering; Conductivity; Micelles; Physical chemistry; Surfactants.