The Micellar Properties of Ionic Surfactants Consisting of the α,ω-Type Surfactant Ion and the Same Type Counter Ion

The micellar properties of N,N,N,N′, N′,N′-hexamethyl-1,20-icosanediyldiammonium, or the corresponding di(1-pyridinium)alkanedioates −OOC(CH2)nCOO− (n = 2—10), were studied by conductivity and pyrene fluorescence measurements. In both systems, the critical micelle concentration (cmc) varied little up to n = 4, but decreased rapidly with a further increase in n. The binding degree (β) of the alkanedioate counter ion was virtually independent of the type of surfactant ion head group and gradually increased from 0.69 at n = 2 to 0.91 at n = 10. The observed n dependences of cmc and β were similar to those of the conventional ionic surfactants with an α,ω-type counter ion, indicating the importance of the hydrophobic interaction between the hydrocarbon part of the counter ion and the micellar core. On the basis of a phase-separation model, the transfer free energy per methylene group of the counter ion from aqueous-to-micellar environments was estimated to be about −1.0 RT. The fluorescence measurements suggest that the surface layer of the micelle becomes progressively loose as n decreases. The micelle aggregation numbers of the bis (trimethylammonium) salts are of the order of 50.