Hi everyone!
Langmuir model is associated with monolayer adsorption, while Freundlich one is related to multilayer adsorption. I have seen in some papers that, according with the previous info, Langmuir isotherm seems to reveal chemisorption, while Freundlich equation seems inform about physisorption. But a monolayer of adsorbed molecules onto the adsorbent surface can also form by physisorption (if there is not enough space for the formation of a second layer), and multilayers can be formed by means of a combined process (the first layer through chemisorption, and the following layers through physisorption). So, I think that these models should not be used to discriminate between chemi/physisorption, only to dilucidated between mono/multilayer adsorption. I would like to know the opinion of the experts, because some papers generate confusion in this sense.
Thanks in advance :-)
When you fit a langmuir model (Qs, b and delH) if you get a heat of adsorption below 50kJ/mol, then it is physisorption, chemisorption generally has a heat of adsorption greater than 80 kJ/mol
In accordance to Krishnamurthy, as long as my observations support, Langmuir and Freundlich can distinguish mono- or multi- layer adsorption and nothing more can be inferred about chemisorption or physisorption.
I think the same. Thanks for your comment. Can you give the Krishnamurthy's reference, please?
doi: 10.1021/bk-2012-1097.ch009. This is a paper on amine grafted sorbents. You can look in similar papers or adsorption text books
I agree with Dr. Golchoobi. Langmuir and Freundlich isotherm models can not distinguish between physical and chemical adsorption. It should be used Dubinin-Raduskevich model and to calculated the mean energy of adsorption.
Good discussion and replies,I think complete and satisfactory answer has been given by esteemed members of research gate
By Langmuir and Freundlich models we can only understand whether the adsorption process is monolayer or multilayer. The heat of adsorption gives information regarding chemi- or physisorption. You have to conduct your experiment/analyses at three different temperatures (at least) to estimate the heat of adsorption.
I agree with Dr. Golchoobi. Langmuir and Freundlich isotherm models can not distinguish between physical and chemical adsorption. It should be used Dubinin-Raduskevich model and to calculated the mean energy of adsorption.
I want to ask about something here, lets assume that we have multilayers adorption which mean that the results fit well the Freundlish model and is it possible that we will have still physisorption between the molecules of the sorbates on the surface of the adsorbant? can someone clarify this to me ?
If the Freundlich isotherm were based on physicochemical foundations, it might indicate some mechanism of adsorption. But this is an empirical power function, useful for approximating part of the equilibrium curve, usually from qe=0 to qe=qm/2. As for the adsorption mechanism, it is much better to rely on physicochemical studies on the adsorbent surface, than on the description of equilibrium or kinetic data.
The major difference between the two is that the Freundlich isotherm is empirical and does not predict surface saturation i.e the adsorption increases as the quantity of adsorbing species increases. This makes Freundlich a multilayer adsorption. Langmuir and Freundlich cannot distinguish between physical and chemical adsorption.
Dubinin-Radushkevich model should be used instead. Dubinin-Radushkevich model is used for the determination of type of sorption (physisorption and chemisorption) by calculating the free energy of adsorption (E) per mole of adsorbate (KJ/mol).
From the Langmuir and Freundlich model, we can only know whether the adsorption process is monolayer or multilayer.
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Powders