Features of the adsorption of methylene blue and ascorbic acids on a polymer membrane “Nafion”

Transmittance coefficient of polymer membrane “Nafion” during adsorption of methylene blue and ascorbic acid from aqueous solutions onto the membrane was investigated by infrared (IR) Fourier spectrometry. A change in the membrane color and transmittance in the IR range during swelling of the polymer in the studied solutions was found, which is associated with chemical reactions occurring in the “membrane-solution” system. It was found experimentally that the adsorption and desorption rates of distilled water, oxidized Methylene blue (Mb$^+$) and reduced colorless leuco-form of methylene blue (MbH$^0$) on the polymer membrane are close and lie within the following limits: adsorption rate 0.029–0.031 min$^{-1}$, desorption rate 0.010–0.011 min$^{-1}$. The adsorption and desorption rates of ascorbic acid from aqueous solution were 0.021 and 0.08 min$^{-1}$, respectively. During the sorption of Methylene blue leuco-form, we detected a change in membrane color. Thus, a redox reaction occurred in the membrane. Upon prolonged drying of the membrane, due to the proton-exchange properties of the membrane, the leuco-form of methylene blue oxidized to the initial state of Mb$^+$. This fact may indicate the manifestation of metachromatic properties of the system “Nafion – Methylene blue”. The system consisting of Methylene blue adsorbed on sulfonated perfluorocarbon can be considered as a model of dehydrogenase, a biocatalyst capable of hydrogen transfer. The “Nafion” membrane, due to its high cationic conductivity, promotes proton transfer and thus affects the rate of redox reactions involving Methylene blue.