Tritium exchange experiments on thin microporous silica layers

Abstract

The tritium-hydrogen exchange and the tritium distribution in tritium-labelled thin microporous silica layers was studied with a washing and etching technique. The tritium distribution was found to be homogeneous along the whole thickness of the layer. The exchange process can be resolved in three diffusion processes which can be related, in order of decreasing rate, to tritium: (i) in the physisorbed water molecules in the micropores; (ii) in the silanol groups in the micropores; and (iii) in bulk intra-skeletal silanol groups, respectively, in agreement with the description of the microporous silica layer given by Grigorovich et al. The diffusion rate of the water molecules resembles the diffusion rate of water molecules in ice I. The diffusion coefficient of the hydrogen in the micropore silanol groups (5 × 10 −17 m 2 s −1) is much higher than the self-diffusion coefficient D H ≈ 10 −21 m 2 s −1, calculated from the previously studied sodium-hydrogen exchange processes in the silica film using the interdiffusion coefficient according to Doremus. However, the former diffusion coefficient is compatible with the diffusion coefficient of the “free” hydrogen ions, calculated from the sodium-hydrogen exchange process using an interdiffusion coefficient according to Helfferich by which the dissociation of the silanol groups in the micropores is taken into account, if the dissociation constant is about 10 −9.