Microscopic Structure and Microscopic Environment of a Polyfluorinated Surfactant/Clay Hybrid Compound: Photochemical Studies of Rose Bengal

Abstract

The microenvironments of a series ofpolyfluorinated surfactants [C n F 2 n + 1 CONH(CH 2 ) 2 N + (CH3) 2 C 1 6 H 3 3 Br - ; designated as CnF-S, where n = 1-3]/clay (saponite) hybrid compounds dispersed in water-saturated benzenewere investigated based on the photochemical behavior of Rose Bengal (RB) as a probe molecule. A small-angle X-ray scattering study showed that the interlayer distance (clearance space) of the C3F-S/clay hybrid compounds was expanded up to 3.8 nm by the dispersion in water-saturated benzene from the original distance of 3.0 nm for the dried hybrid compound, indicating expansion of the hybrid compound upon swelling with the solvent benzene. The RB molecules are proposed to form ion-association complexes with the CnF-S surfactant, most likely existing in the vicinity of the ammonium group of the surfactant intercalated in the cation-exchangeable clay saponite. The absorption and fluorescence maxima of RB intercalated in the hybrid compounds were significantly red-shifted from those in aqueous solution. The hybrid compounds provide an environment with polarity between that of ethyl acetate and 1-butanol. The microscopic polarity of the hybrid compounds depends on the fluorocarbon chain length (n) of the surfactant, and the polarities increase with increasing fluorocarbon chain length. On the basis of the observed photochemical behavior of RB, including the relative decomposition rates, excited triplet lifetimes, and relative excited triplet quantum yields, the microscopic concentration of water in the hybrid layer was estimated to decrease in the order C3F-S > C2F-S > C1F-S. These differences are discussed in relation to differences in the microscopic orientational structure of the polyfluorinated surfactant within the clay layer.