Sorption mode vs diffusion mode  

Posting by Jasper Michels on 24 Nov 2008 at 16:53:54.


Again I have a very basic and possibly silly question: If a sorption isotherm for a certain material is Henrian (linear relationship between partial vapour pressure and vapour concentration), does this automatically mean that the diffusion coefficient of the vapour inside this material is not a function of concentration?

Thanks again!


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    On 27 Nov 2008 at 17:50:44 Composite Analytica posts:

    This is not a silly question at all. To formulate the answer in the appropriate chemical - physical terminology takes some time and paper, hence I will try to keep it practical:

    Henry behaviour occurs in dilute solutions.

    If the solubility of the chemical stays below 0.1 vol%, the variation of the water vapour diffusion coefficient in a polymer is so small, that in most applications one may assume that the diffusivity is a constant (however for advanced electronics and medical controlled release applications care must be taken).

    Although the isotherm for several species in polymers looks like Henrian in case of an uptake between 0.1 and 1 vol%, the diffusion coefficient is often not constant at all. This depends on the polymer and solvent.

    Above 1 vol%, diffusivities in polymers should never be treated as a constant.

    Good Luck,
    Composite Analytica

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