Isomerizable N-Alkylmerocyanine Dyes as Probes of Micellar Solubilization Sites

Article abstract of DOI:10.1021/j100133a032

The thermal cis to trans isomerization of N-alkylmerocyanine dyes is catalyzed by both cationic and anionic micelles.The catalysis by cationic micelles of hexadecyltrimethylammonium bromide (CTABr) increases markedly with substrate N-alkyl group length and is exceptionally pronounced for N-alkyl groups larger than pentyl.Rate data fit the pseudophase model of micellar catalysis for CTABr concentrations between 0.005 and 0.1 M and binding constants (K_S) and micellar rate constants (k_m) were evaluated.Both K_S and k_mincreased dramatically as the N-alkyl group was varied from N-methyl to N-pentyl so that the increase in catalysis with substrate hydrophobicity is due not only to increased substrate binding to the micelle but also to enhanced reactivity within the micelle.At CTABr concentrations greater than 0.1 M, k_m is not constant but increases, reflecting changes in micelle structure at bigh surfactant concentration.Eyring activation parameters were determined for the overall rate constant.The increase in reactivity as a function of N-alkyl group hydrophobicity was shown to be due primarily to an increase in ΔS^<*>.