Self-diffusion and ion-exchange kinetics have been studied in hydrated sodium and potassium chabazites by a new 'flow method' and the values for the self-diffusion coefficients obtained from these area-dependent experiments rationalized by comparison with results from an area-independent experiment on hydrated calcium chabazite. In each case the self-diffusion coefficients follow the Arrhenius equation and the activation energy was found to be about 7-8 kcal.mole-1. The results have been discussed in terms of the crystalline structure and values for the entropies of activation indicate that the self-diffusion process may involve the co-participation of water molecules and cations. A numerical analysis procedure has been evolved and a programme developed for a digital electronic computer, to solve the diffusion equation with spherical symmetry and a concentration-dependent interdiffusion coefficient. This coefficient includes the self-diffusion coefficients of the exchanging ions and an 'activity correction' term derived by irreversible thermodynamics, which is itself concentration-dependent. The computer programme has been described. Computed results have been obtained for a number of limiting functions of the correction term and compared with the results of the Heifferich and Plesset treatment for an ideal system (57). The new treatment was found, in particular, to yield strikingly improved results for the 'test' system Ca++/Sr++-chabazite. The implications and limitations of the new treatment have been discussed. The experimental ion-exchange kinetics for the Na+/K+-chabazite system were compared with the predictions of the new treatment, using data obtained from self-diffusion and exchange equilibrium studies (158). Although the agreement was poor, the kinetics may be linked with the preferential selectivity of the exchanger for one of the exchanging species which the extended treatment seeks to encompass.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:622999 |
| Date | January 1967 |
| Creators | Brooke, Naurice Michael |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/17702 |
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