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Self-diffusion in single-file zeolite membranes is Fickian at long times Peter H. Nelson
 

Summary: Self-diffusion in single-file zeolite membranes is Fickian at long times
Peter H. Nelson
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
Scott M. Auerbacha)
Departments of Chemistry and Chemical Engineering, University of Massachusetts,
Amherst, Massachusetts 01003
Received 27 January 1999; accepted 17 February 1999
We have developed a theory for self-diffusion in single-file Langmuirian zeolites of finite extent,
which has been validated by open system kinetic Monte Carlo simulations. Our theory is based on
a two-stage, Fickian diffusion mechanism, wherein a vacancy must traverse the entire file length to
produce particle displacements of one lattice spacing. For times shorter than the vacancy diffusion
time, tc , particle transport proceeds via the nonFickian, single-file diffusion mode, with
mean-square displacements increasing with the square-root of time. For times longer than tc ,
however, we find that self-diffusion in single-file systems is completely described by Fick's laws.
We find that the fraction of time in the single-file diffusion mode scales inversely with file length
for long files, suggesting that Fickian self-diffusion dominates transport in longer single-file
zeolites. Through correlations among the particle movements, the single-file self-diffusivity is
sensitive to sorption limitations for short files, and scales inversely with file length for long files.
Experimental verification of the theory by pulsed field gradient NMR and tracer zero-length column
experiments is discussed. 1999 American Institute of Physics. S0021-9606 99 51518-3

  

Source: Auerbach, Scott M. - Department of Chemistry, University of Massachusetts at Amherst
Nelson, Peter Hugo - Physical Sciences, Benedictine University

 

Collections: Chemistry