Flow Cytometry Study of the Non-Fickean Diffusion of Small-Charged Molecules in Poly(diallyl dimethyl ammonium chloride)/Poly(styrene sodium sulfonate) Multilayers: Impact of the Layer Number, Top Layer, and Concentration of Diffusing Molecules

Abstract

The diffusion of sodium dithionite (S₂O₄^2-) through polyelectrolyte multilayers of poly(diallyl dimethyl ammonium chloride) (PDADMAC)/poly(styrene sodium sulfonate) (PSS) assembled on colloidal particles with the layer-by-layer technique is studied by means of flow cytometry and quenching assay. Fluorescence is provided by a layer of (7-nitrobenz-2-oxa-1,3-diazol-4yl) amino) hexanoate (NBD)-labeled poly(allyl amine hydrochloride) assembled below the PDADMAC/PSS multilayer. NBD is quenched by a redox reaction with S₂O₄^2-. NBD quenching is fast at short times but strongly retarded at longer times. Quenching is faster for PDADMAC as the top layer and for increasing concentrations of S₂O₄^2-. The quenching kinetics of NBD is described with a model assuming a non-Fickean diffusion of S₂O₄^2-, with diffusion coefficients that depend on time with an inverse power law. Diffusion coefficients show little dependence on the number of layers but are highly dependent on the concentration of S₂O₄^2-. Increasing the concentration of S₂O₄^2- over 10 mol/m³ results in a decrease of the diffusion coefficient, more evident at longer times. The non-Fickean behavior for S₂O₄^2- diffusion is explained on the basis of the trapping of dithionites in the multilayers.