Structural studies of thin AOT films by using the polarity fluorescent probe coumarin-153

The solvatochromic behavior of coumarin-153 (C153) is exploited for the characterization of thin surfactant films, which were made by dip-coating fused-silica slides in solutions of reverse micelles of bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) in cyclohexane. The original reverse micellar structure is most probably conserved but becomes very rigid when transferred into the film. Evaporation of cyclohexane trapped in the film, as well as increasing the water content in the original reverse micellar solution, does not alter either the absorption and 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 with cyclohexane molecules. Furthermore, a blue-shift, of 13 nm in absorption and of 27 nm in fluorescence indicates a transfer of C153 from a high- to a low-polarity environment. We suggest an extensive curvature change of the original structure as a possible explanation for the 'inversion' of the C-153 spectroscopic parameters, due to water adsorption.