How is the rate of adsorption related to the critical temperature of a gas? My textbook says more the critical temperature, better are they adsorbed. But I cannot really understand why. They threw in another confusing statement that says that easily liquefiable gases are readily adsorbed. I am thoroughly confused.

I did read the question How does critical temperature affect adsorption? before posting this question, and while that question was answered, it did not answer my question fully.

Edit: To clarify my area of doubt, let me say what I find confusing. Gases with lower Ct will 'prefer' the gaseous phase over the liquid phase. Which is what we want. We want gases to be adsorbed.

  • $\begingroup$ I think you have it the wrong way around in the last sentence. (roughly:) Higher Tc means stronger intermolecular interactions means more easily condensed means adsorbs more readily. $\endgroup$ – Buck Thorn Nov 24 '19 at 12:26
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    $\begingroup$ Buck, I read that article before posting this question, and while that question was answered, it wasn't to my satisfaction. $\endgroup$ – Novice chemist Nov 24 '19 at 18:45
  • $\begingroup$ As for your answer, I am still uncertain as to why we need the gas to condense. It still is in the gaseous phase when it is adsorbed. Or is it that gases with lower Tc are more difficult to tie down, so to speak. I think I am getting a hang of it. Thanks Buck. $\endgroup$ – Novice chemist Nov 24 '19 at 18:47

You can argue that the effect of the solid is to "seed" formation of the condensed phase, as a set of interactions that lower the chemical potential below that of the gas, leading to "condensation" (well, adsorption). This is all very approximate as the details matter of how (as in, through what interactions) the adsorbent interacts with the gas molecule.

In addition, the magnitude of the critical temperature may be considered as a proxy for the strength of intermolecular interactions. For instance, if you assume the gas follows the van der Waals equation of state, then the critical temperature $$T_\mathrm{c} = \frac{8a}{27bR}$$ where $a$ is a measure of the strength of attractive interactions. You can find other ad hoc justifications for associating higher $T_\mathrm{c}$ with stronger strength of adsorption. For instance, larger, more polarizable molecules will tend to have higher boiling points and critical points. Such molecules will also adsorb more strongly. However the argument that the rate of adsorption increases should be considered carefully, since steric effects may be associated with larger molecules.

Regarding the point that "we want gases to be adsorbed", well, the point is that you want to predict behavior. The book merely arms you with guidelines that allow you to make predictions. So that, for instance, if you have a number of compounds in a gaseous mixture, you can estimate which adsorb more. Or perhaps you can use that ability to select new substances with desirable properties based on the adsorption behavior of other compounds.


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