The acid dye Alizarine Sky Blue, BS.FS, which exhibits considerable overdyeing in nylon 66 at pH 6.0, has a diffusion coefficient which is constant with conceutration. This behavior is characteristiç of a disperse dye and supports the conclusion that over dyeing is due to adsorption of an acid dye in a non-dissociated form. On the other hand, in the case of the acid dye Naphthalene Scarlet 4R, which does not show over dyeing at pH 6.0 and has a maximum dye adsorption equivalent to that necessary to saturate the amine ends, the diffusion coefficient increases considerably as the saturation of amine sites is approached.
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