NATO-ASI FLUID TRANSPORT IN NANOPOROUS MATERIALS COURSE .a student’s perspective and explanations from a veteran

Abstract

One of the first things mentioned in the introduction to many of the articles I’ve read about diffusion in zeolites, is that “the diffusivities reported in the literature vary by up to xxx orders of magnitude”. Although it is not always specifically stated, the implication is that these widely varying diffusivities have been measured or estimated for the same molecule/host system, and I’ve seen “xxx” as low as one and as high as four. (Interestingly enough, despite this, many authors, myself included, still report at least 2 or 3 significant figures in the diffusivities they presented in their own articles...but that is another issue.) Although most articles I’ve read have been about diffusion of relatively simple molecules in zeolites, we learned at the NATO course that large discrepancies exist between the reported diffusivities of species diffusing through nanoporous materials in general. While this can be useful to us relative newcomers to the field of diffusion, for instance when we are writing our first papers we can almost always say something like, “the diffusivities measured fell within the range reported in the literature” (if we can’t we are probably in serious trouble), but frankly it is disturbing and confusing to realize the magnitude of the discrepancies and to try to make sense of our measurements or estimates in this light. In the end, we as students are faced with the many obvious questions posed by the quote at the beginning of this paragraph: Why are there discrepancies? Why are they so large? What exactly do the different techniques for measuring diffusion actually measure and how are the diffusivities calculated from the measurements? All reported diffusivities are really estimates since no one has a digital diffusivity meter that reads out the error-free exact diffusivity of a species diffusing through a nanoporous material. Which values can we trust and to which should we compare our own estimates? What are the different diffusivities (self, transport, corrected, etc.) and how are they