Study of the aggregation behavior of mixed micelles of anionic surfactants and ethoxylated alcohols by time-resolved fluorescence quenching

The aggregation behavior of mixed micelles of sodium salts of anionic carboxylates, CnNa (n = 8, 10, 12), and n-butyl ethoxylated alcohols, C4EOx, (x = 0-4), were studied using the time-resolved fluorescence quenching (TRFQ) technique. Alkylpyridinium ions (CnPy+) were found to be efficient quenchers of 2,3-dimethylnaphthalene (2,3-DMN) fluorescence in anionic micelles. The quenching process is diffusion controlled and involves the migration of 2,3-DMN along the micellar surface region. The pseudo-first-order intramicellar quenching rate constant, k(Q), decreases, and the activation energy of the quenching process, E-aQ, increases with increasing micelle size. Solubilization of a small quantity of ethoxylated alcohols by micelles of C8Na or C10Na has little effect on the aggregation number of the surfactant, N-S, but results in a significant decrease in N-S in the case of micelles formed of C12Na. At a given concentration of alcohol, the average number of alcohol molecules, N-A, in the mixed micelles increases progressively with increasing number of ethylene oxide groups in the alcohol. Under these same conditions the total aggregation number, N-T = N-S + N-A, in C8Na-C4EO0-4 and C10Na-C4EO0-4 mixed micelle systems increases, while it decreases in the case of C12Na-C4EO0-4 mixed micelles.