Molecular-diffusion and Fluorescence Energy-transfer Studies in Thin Surfactant Films

Thin surfactant films have been formed by dip-coating glass slides in a solution of reversed micelles containing titanium isopropoxide. The alkoxide is slowly hydrolyzed in the presence of reversed micelles since hydrolysis competes with hydration of surfactant polar groups. Adhesion of the surfactant on the glass slide is assisted by incompletely hydrolyzed alkoxide through the following possible mechanism: the alkoxide adheres by-Si-O-Ti-bonds and the surfactant follows by hydrophobic attraction to the isopropyl groups. The films are transparent and visually uniform. Three surfactants, forming reversed micelles, have been investigated: one nonionic, Triton X-100; one anionic, AOT; and one cationic, hexadecyldimethylbenzylammonium chloride. The environment provided by the surfactant film has been studied by fluorescence probing. In particular, we have analyzed pyrene excimer formation as well as energy transfer between pyrene, acting as donor, and coumarin-153 or N-n-heptyl-4-(((dimethylamino)phenyl)ethyl)-pyridinium bromide, acting as acceptor. In Triton and hexadecyldimethylbenzylammonium chloride films, pyrene excimer formation is diffusion-controlled while in AOT films excimer largely comes from pyrene aggregation. Pyrene excimer formation capacity decreases in the presence of cosulubilized poly(vinyl methyl ether) chains. Energy transfer data indicate that coumarin-153 is randomly distributed in the films, but N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide is not randomly distributed but shows a tendency to aggregate. Generally speaking, the benzyl-group-bearing surfactants form fluid structures while AOT may provide a restricted low dimensional environment