Theory question - modeling Langmuir behavior

[dkreller]

Hi,

I know this is mainly a forum that is devoted to practical applications of chromatography - but I know that there are some very deeply knowledgable folks on this forum and I am convinced that some of you people know a lot about various aspects of chromatography and chromatography-related theory.

For purposes of modeling things that are happening on my (in-house packed) columns I am interested in estimating the rates of adsorption and desorption of an adsorbing solute to my stationary phase. I was never the big theory guy in school but I am familiar with equilibrium thermodynamics and basic langmuir adsorption theory.

Of course, I understand that if equilibrium is reached in an adsorbing system, the rates of adsorption and desorption are equal. Does anyone have brief suggestions on how one should proceed to try to calculate those rates?

Thanks for your consideration,

David

[danko]

Hi David,

From what I understand, you need to calculate "K" for a specific compound and under specific conditions (i.e. mobile phase, separation temperature etc.) K=(Tr – t0)/T0
If you need some kind of universal adsorption/desorption constant, I’m not sure you’ll ever be able to calculate it. Ideally, every single compound will be adsorbed and de- sorbed at a specific for it (and the conditions) rate. That’s what makes a given separation possible.
Best Regards

[Uwe Neue]

In most chromatographic systems, the adsorption and desorption kinetics are much faster than the diffusion in and out of the particles. So unless you have system that is MUCH worse than a comparable system with equal particle size, I would not worry about kinetics.

In order to give you better advice, I would need to know a lot more about what you are doing. You can contact me directly, if you want.

[dkreller]

Dear Uwe,

Thanks for your post and offer to discuss this with me. I will take you up on your offer of contacting you. I still have your email address in my records. Indeed the interactions are not like those in normal chromatographic separations - as a continuation of the research in my laboratory - that you are aware of - I have begun looking at the interaction of a pure compound (phthalic acid) on an aluminum oxide coated quartz sand by my LC technique. There are strong adsorption interactions b/w the adsorbate and adsorbent that lead to adsorption that is irreversible at least on the timescale of the column adsorption experiment. Even outside of flowing systems - I want to improve my understanding of adsorption and desorption kinetics in Langmuirian systems.

Have a nice weeken, and I'll be in touch soon,

Best Regards,

David

[HW Mueller]

How are you going to understand non-equilibria with equilibrium theories?