Isotope Exchange Studies of Self-Diffusion in Organic Plastic Crystals

A technique that has been fairly extensively used in the study of self-diffusion in ionic 1 and rare gas crystals 2,3 is the isotopic vapour exchange technique. In these experiments the kinetics by which isotopically labelled molecules exchange between the bulk solid and vapour phases is monitored. The rate determining step in this process is the diffusion rate in the solid since the exchange of molecules at the interface and diffusion in the vapour are both relatively fast. Provided that the surface area involved in exchange, A, can be determined then self-diffusion coefficients in the solid, D, can be evaluated. Compared to the conventional serial sectioning technique of determining diffusion coefficients this technique requires considerably less "handling" of the specimen and is ideally suited for the study of solids with low melting points and volatile solids. Organic plastic crystals have relatively high vapour pressures and in some cases present handling problems. There is considerable interest in diffusion in these crystals 4 and we have developed apparatus for their study by the isotopic vapour exchange technique.