Anomalous permeation of the [P5W30O110]15− polyoxoanion in polyelectrolyte multilayer films

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The integration of polyoxoanions into functional thin films is of major interest to increase their use as catalysts or to provide new properties to the films. The incorporation of polyoxoanions in polyelectrolyte multilayer (PEM) films has been widely used for such aims. In most investigations, the polyoxoanions were used as a component of the layering process and there is only limited work investigating the interactions of polyoxoanions with preformed PEM films. Herein, we investigated the incorporation of the 30-tungsto-5-phosphate [P5W30O110]15− polyoxoanion into PEM films made from hyaluronic acid (HA) and poly(allylamine hydrochloride) (PAH) as a function of the polyoxoanion's concentration. Surprisingly, we found an increased amount of incorporated [P5W30O110]15− upon a decrease in its bulk concentration. Confocal Raman microscopy allowed us to investigate the concentration profile of the polyoxoanion across films of about 3 μm in thickness. Homogeneous distribution was only found for films fed with polyoxoanions at the lowest bulk concentration, namely 1.2 × 10−6 M, whereas the films fed with solutions at higher concentrations (1.2 × 10−4 M) showed local enrichment in POM at the film–solution interface. We explain the lower amount of incorporated polyoxoanion at high bulk concentration by the formation of a polyoxoanion-rich barrier reducing further diffusion of the anions in the deeper part of the films.