Dye–surfactant interaction: chain folding during solubilization of styryl pyridinium dyes in sodium dodecyl sulfate aggregates

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Cationic dyes are known to interact with membranes and, therefore, are used as membrane probes. The alkyl chains of the dyes play a role in modifying the environment of the chromophore and affect the nature of interaction with the micelle. A number of cationic dyes with varying alkyl chains (C 1 to C 18) have been synthesized and their interaction with sodium dodecyl sulfate (SDS) is reported. From the absorption, fluorescence spectra and binding constant values it has been proposed that: (a) C 1–C 8 dyes solubilize in the micelle and the chromophore occupies a methanolic/ethanolic environment, (b) C 10–C 14 dyes exist both as an extended monomer and as folded species in presence of SDS, which is reflected in two Gaussian peaks, (c) the experimentally obtained absorption spectra for C 16 and C 18 dyes can be explained by three Gaussian peaks due to monomer ⇋ dimer in the absence of surfactant and two such peaks due to monomer and folded monomer in the presence of surfactants. The binding constant and fluorescence intensity values support this proposition.