Nanosecond pulse radiolysis study of metal aggregation in polymeric membranes

After an electron pulse, the transient optical absorption spectrum and the decay rate in a perfluorosulfonated ion-exchange membrane such as Nafion swollen by a 2-propanol/water mixture are found to be similar to those of the solvated electron in the same free solvent. The radiolytic yield is G(es-) = 2. In membranes swollen by a solution of silver ions, chosen as a model system for the study of metal aggregation in a microheterogeneous environment, the end-of-pulse spectrum is that of Ag2+ (G = 3.7). The following step is a fast dimerization of Ag2+ giving Ag42+, occurring inside one single hydrophilic cavity, with a rate slightly higher than in the free solvent. On the contrary, the further coalescence rate drops suddenly to a value 104 times lower than in a free solution, due to the slower diffusion of the transient species of silver aggregation through the hydrophilic channels. This process competes on the 10-1-s range with a corrosion of the smallest aggregate by H3O+ contained in the same cavity. Thus the polymeric medium allowed the authors to observe directly the corrosion of a noble metal aggregate by the acidic cations. In a repetitive pulse regime, it can be observed that the aggregates generated in the primary pulses act as growth centers and induce the coalescence of the new atoms further produced, thus protecting them from corrosion. 36 refs., 10 figs., 1 tab