Interaction of Methylene Blue Dye with the Surface of a Polymer Membrane during Soaking in an Aqueous Solution: Dependence on the Isotopic Composition of Water

It is known that swelling of Nafion polymer membrane in water is accompanied by unwinding of polymer fibers into the bulk of the surrounding liquid. This effect is controlled by the deuterium content in water. In this paper, we report the results of studying the adsorption dynamics of methylene blue (MB) dye on the Nafion surface for MS solutions based on natural water (deuterium content 157 ppm, unwinding effect occurs) and based on deuterium-depleted water (DDW; deuterium content 3 ppm, unwinding effect is absent). In addition, we investigated the water desorption dynamics upon drying a Nafion polymer membrane after soaking in an MB solution based on natural water and DDW. It turned out that the MB adsorption rate in MB solutions based on natural water is lower than in the DDW-based MB solutions. Finally, the water desorption upon drying was found to be accompanied by a change in the Nafion absorption spectrum. Specifically, upon water desorption, the low-frequency band of the doublet in the range from 600 to 800 nm in the absorption spectrum of Nafion with MB particles on its surface undergoes transformation with a shift to the short-wavelength region. This transition occurred earlier for Nafion soaked in a DDW-based MB solution. The found effects are related to the retardation of diffusion processes in the layer of polymer fibers unwound near the membrane surface. Thus, changing the deuterium content in the aqueous solution in which the polymer membrane is swollen by very small steps (from 3 to 157 ppm), one can control the dynamics of adsorption and desorption processes.