Self-diffusion and conduction in hydrogen bonded solids. Part 2.—Self-diffusion of C6H514COOH and C6H5COO3H in single crystals of benzoic acid

Abstract

A study has been made of the self-diffusion of the labelled species C₆H₅¹⁴COOH and C₆H₅COO³H in single and polycrystalline specimens of benzoic acid. The two tracers diffuse in a similar manner in the latter where grain-boundary diffusion may predominate. In the pure single crystals the tritium tracer diffuses more rapidly than the 14-carbon tracer and by a different mechanism. The 14-carbon labelled tracer diffuses with the characteristics of molecular vacancy self-diffusion. From a consideration of the effects of ambient conditions and the residual concentration of water in the samples, it is concluded that the rapid tritium diffusion process is extrinsic in nature and that it is a consequence of the exchange of the labile proton with included water coupled with the interstitial diffusion of the water molecule in the lattice.