Growth of Cetyltrimethylammonium Chloride and Acetate Micelles with Counterion Concentration

Time resolved fluorescence quenching measurements with pyrene as probe are employed to determine aggregation numbers for hexadecyltrimethylammonium acetate (CTAOAc) and chloride (CTACl) micelles as a function of the concentration of detergent and added common-counterion salt. The aggregation numbers, NA, of CTACl are roughly twice those of CTAOAc micelles at equivalent concentrations of detergent and salt, consistent with the known relative counterion binding affinity (chloride ⪢ acetate). For both detergents, the increase of NA with the net concentration of counterions in the intermicellar aqueous phase ([Yaq]) follows the relationship found previously for anionic micelles: log NA = log No + γ log[Yaq], where γ and No are constants. However, compared to anionic micelles, for which γ = 0.2-0.25, spherical micelles of both CTAOAc and CTACl exhibit less pronounced growth, with γ = 0.1.