You are not giving much background, but as far I as I can tell, you need to enter the pN2 and pCO2 (that is, gas phase) and the solution of the method (IAST, I guess?) should return the qCO2 and qN2.

or in other words...

pN2 and pCO2 are the partial gas pressures at the conditions that you want to know (you enter this to the solver), and qCO2 and qN2 are the adsorbed amounts at the conditions that you want (the solver should return this to you).

Then you can calculate selectivity by the equation that you wrote.

Thanks Fernando 🙏I already made these calculations, I was only confused in case of partial pressure for mixture of 15%/85% CO2/N2, so according to my results using the selectivity equation,

S=q1/q2 x p2/p1,

I just did this way,

qCO2 (0.85)/qN2 (0.15)

First, if you want to use IAST, I would recommend to not use Langmuir-Freundlich for selectivity calculations. This model is thermodynamically not consistent when p --> 0 (there is no Henry region in this model) and should not be used with IAST. There are several pure component isotherm models that can be used in IAST: i.e. Langmuir, Multi-sites Langmuir, Toth, Jensen-Seaton...

Secondly, if you want to use the pure component model to calculate the selectivity of the system CO2/N2 at 1 bar with the gas phase molar ratio equal to, respectively, 0.15 and 0.85, then the formula is: S = (qCO2/qN2)/(0.15/0.85).

For example: if qCO2 = 5 mol/kg and qN2 = 0.2 mol/kg, the Selectivity will be: (5/0.2)/(0.15/0.85) = 141.7#

Regards

yep, thanks, I did the same, and I used dual site Langmuir freundlich not Langmuir freundlich model

Thanks, Fernando !! already made these calculations, I was only confused in case of partial pressure for a mixture of 15%/85% CO2/N2, so according to my results using the selectivity equation, S=q1/q2 x p2/p1, I just did this way, qCO2 (0.85)/qN2 (0.15) I tried this but can you please help me to get a complete graph from zero pressure to 1 bar because 0.85 and 0.15 method gives only one point, not a full graph of selectivity as reported in literature.