It is noted in several research articles that activated carbon has a better affinity for non-polar molecules . Why does such a phenomenon occur. Is it due to its morphological nature or does it have to do with the fact that the pore size of the activated carbon are small for polar molecules. I wanted to know the exact chemistry mechanism behind the process
What kind of compounds are you speaking about ?
Adsorption isotherms of water vapor on activated carbons are Type V isotherms (doi:10.1016/0008-6223(91)90139-A ) because the surface of the material is hydrophobic. However, there are oxygen-, sulfur- and nitrogen-containing groups on the surface of the activated carbons (doi: 10.1021/la3048938). Therefore while the adsorbed values are low at low pressure, after reaching a certain value of p/p0, the adsorbed amounts will sharply increase due to the pore filling.
But if you compare the adsorption of CO (polar molecule) and N2 (non polar molecule) on activated carbons, you will see higher values for CO than for N2 (doi:10.1080/01496390902729130)
Dear Amit,
According to most of the sources I found, activated carbon binds to most substances through London dispersion forces. This should mean that it adsorbs larger molecules and non-polar molecules preferentially since they would have larger dispersion forces.
With my best regards
Prof. Bachir ACHOUR
Thank you everyone for your inputs.
The compounds I am dealing here are organic pesticides, salts of potassium , sodium, calcium and mangenese and certain co-ordination complexes.
Nitrogen is not totally nonpolar, it is quadrupole-ar. This makes it weaker adsorbed than e fully polar molecule as water or co.
It is due to its hydrophobicity and hydrophilicity. Pure carbon is hydrophobic in nature but as the amount of oxygen associated with the carbon surface increases, it becomes more hydrophilic. So this influences its adsorption of polar and non-polar molecules.
Dear Amit,
Activated carbon is usually made of neutral carbon atoms with no electrical gradients between molecules. That's why carbon does not attract polar molecules in preference to nonpolar molecules.
http://www.epa.state.oh.us/portals/27/engineer/eguides/carbon.pdf.
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