STABILITY OF RADICAL INTERMEDIATES IN MICROSCOPICALLY HETEROGENEOUS MEDIA - PHOTOLYSIS OF WATER ADSORBED ON SILICA-GEL STUDIED BY ESR AND DSC

Unstable radical intermediates obtained by 77 K UV irradiation of silica gel swollen by water containing a small amount of hydrogen peroxide have been studied by electron spin resonance (ESR). The identity and reactivity of the radicals have been compared with results obtained by UV irradiation of bulk H2O/H2O2 solutions. In the solutions, OH and HO2 radicals were detected, with an initial intensity ratio that depended on the concentration of H2O2. These radicals decay at 110 and 150 K, respectively; the decay can be correlated with the corresponding relaxation regions detected in pure ice. In the UV-irradiated gels, signals from two additional radicals (A and B) have been detected. Radical A, with g-tensor components 2.0015, 2.0079, and 2.0571 in the gels swollen by H2O, appears after annealing of the swollen gel to 150 K and disappears at about 200 K. The decay of the A radicals is associated with the appearance of type B radicals, which are stable up to 230 K and have g-tensor components 2.0021, 2.0092, and 2.0273 in the gels swollen by H2O. We propose that radicals A and B represent O2-radical ions in two different orientations with respect to the silica network. The appearance of the radicals at different annealing temperatures suggests a transition from an end-on to a side-on trapping site, as the temperature is raised. The properties of the water adsorbed on the gel are profoundly modified by the silica network, as found in DSC experiments: the melting process starts at about 220 K and covers a range of 40 K.