STRUCTURAL STUDIES OF THIN AOT FILMS BY USING THE POLARITY FLUORESCENT-PROBE COUMARIN-153

The solvatochromic behavior of coumarin-153 (C153) is exploited for th e characterization of thin surfactant films, which were made by dip-co ating fused-silica slides in solutions of reverse micelles of bis(2-et hylhexyl) sulfosuccinate sodium salt (AOT) in cyclohexane. The origina l reverse micellar structure is most probably conserved but becomes ve ry rigid when transferred into the film. Evaporation of cyclohexane tr apped in the film, as well as increasing the water content in the orig inal reverse micellar solution, does not alter either the absorption a nd fluorescence maxima or the red-edge excitation shift (REES) of C153 . When the film adsorbs a certain amount of water from the atmosphere, the C153 environment becomes more fluid and seems to get in contact w ith cyclohexane molecules. Furthermore, a blue-shift, of 13 nm in abso rption and of 27 nm in fluorescence indicates a transfer of C153 from a high- to a low-polarity environment. We suggest an extensive curvatu re change of the original structure as a possible explanation for the 'inversion' of the C-153 spectroscopic parameters, due to water adsorp tion.