Why adsorption and desorption branches of N2 sorption isotherm of polymaric compounds are not close at relative low pressure. The reason that I found in literature is the ability of polymers to swell in non- solvent but how it affects the isotherm?
When the loop produced by adsorptton and desorption branches of N2 isotherms don't close, this can be due to a leak in the experiment (99% of the cases) or to dimensional changes of the material, indeed. I personally never met this situation with polymers as the solubility of N2 inside is about zero, especially at 77K and at so low pressure. Therefore, I think that it is most probably due to a leak or to uncertainties of the measurement. I indeed expect that most polymers have very low surface areas, from where a high uncertainty and the related possibility of a desorption branch not identical to the adsorption at low relative pressure.
Alain
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