When Van Der Waals forces are involved at some physical/chemical process, also London and dipole-dipole forces should be suspected.

This term (Van der Waals forces) includes dipole-dipole, dipole-induced dipole and London (instantaneous induced dipole-induced dipole) forces, which are attractive or repulsive forces between molecular entities or between groups within the same molecular entity (Compendium of Chemical Terminology).

This is the first time I'm hearing this. I've always been told adsorption is a synonym of physisorption. Apparently, IUPAC is promoting a different concept, closer to what your text says: http://goldbook.iupac.org/A00155.html

So, adsorption encompasses both physisorption, chemisorption and "electrostatic sorption", with the latter being apart from physisorption.

In my opinion, this makes the matter more confusing, but whatever, IUPAC knows best, I suppose.

There is no sharp boundary between physisorption and chemisorption energies, but in some texts it is given around 40 kJ/mol, Conceptually, the former refers to loose binding between a sorbate-sorbent pair in which there is little perturbation in the electronic structure of both so that the PE well appears shallow. In chemisorption, a sort of chemical bond is established and there is significant perturbation of the electronic structure leading to a deep PE well and small bond length that is close in its value to a chemical bond length.

potential energy?

Yes PE stands for potential energy.

strange!

@Md Abdus Sabur: H-bonds are usually much stronger than van der Waals interactions, and they a highly directional. I prefer to regard adsorption with H-bonds as chemisorption.

The distinction between physi- and chemisorption is made according to the adsorbate bond strength, being  typically only few kcal/mole in the former case, thus requiring low temperatures to hold the molecules on the surfaace. But, common items like this can be easily found in detail in Wikipedia and other internet sites.

One of the primary distinctions between physical adsorption (physisorption) and chemisorption is the ability of adsorbed molecule to freely migrate on the adsorbing surface. This creates a mathematical shift that is similar to the distinction between the liquid state and the solid state. For physisorption the potential wells that are present are not sufficiently deep enough to localize the adsorption.

Chemisorption is characterized by localized adsorption isotherms, such as the Langmuir isotherm. Physisorption, on the other hand, is characterized by isotherms that are more characteristic of a quasi-liquid behavior such as the disjoining pressure theory (by Churaev, Starke and Adolphs) or the quantum mechanical chi theory. These theories indicate a continuity of the adsorbed phase to be continuous with the bulk phase liquid.

The popular BET theory is a localized theory and does a very poor job of fitting the isotherm, causes an anomaly for multiple adsorbates, and is inconsistent for the thermodynamics of physical adsorption. It is advisable to avoid this analysis method.

The energies of adsorption could be quite high, even in the range that most researchers consider to be characteristic of chemisorption. This is, of course, dependent upon the temperature of the observed physisorption.

Localized bonding, chemisorption, is normally attributed to covalent bonding, whereas, the delocalized attraction, physisorption, is normally attributed to the molecular attractions, London forces, dipole forces and hydrogen bonding. In other words, these are the same forces responsible for the liquid state. Since the reference here is to gas-solid systems, the ionic bonding would be very unusual and normally not relevant. However, other formulations involving ionic attractions apply for systems involving ionic solvents, for example, the Debye-Hueckel theory.

James,

I find the basic point of your answer interesting and it seems to hit the nail...

However, 'Chemisorption is characterized by localized adsorption isotherms, such as the Langmuir isotherm. Physisorption, on the other hand, is characterized by ...'

To my knowledge Langmuir isotherm is quite popular to describe monolayer adsorption, e.g. for typical physical adsorption of surfactants at fluid interfaces. Cannot see why it should not apply for monolayer adsorption without localized binding points.

Dr. Sobisch,

I respectfully disagree. For dispersed (i.e. non-localised) adsorption, the Volmer equation describes the system accurately, not the Langmuir equation. This becomes quite clear when the isotherm equations are derived from the Gibbs adsorption isotherm (or an equivalent thermodynamic approach).

Langmuir isotherm:

bC = Ø/(1-Ø)

Volmer isotherm:

bC = Ø/(1-Ø) . exp[Ø/(1-Ø)]

Kind regards,

Rui Afonso

Langmuir isotherm model is not limited to chemisorption. The equation of this model can be derived based on a kinetic approach that assumes considerable reversibility in the adsorption reaction, a characteristic that is more typical for physisorption.

Moreover, literature resources contain many cases in which this isotherm was used to describe adsorption cases in liquid-solid systems in which the sorbate-sorbent interactions are far from being of covalent nature and the process does not fit to chemisorption (e.g. aqueous cations adsorption on negatively charged surfaces).

It is true that this isotherm was originally derived for monolayer gas adsorption on solid surfaces that contain homogeneous sites, however, it has been so far applied for a wide range of cases that does not fit to its initial assumptions.

"It is true that this isotherm was originally derived for monolayer gas adsorption on solid surfaces that contain homogeneous sites, however, it has been so far applied for a wide range of cases that does not fit to its initial assumptions."

In such cases, it should be regarded as no more than a mathematical correlation with no physical meaning. One should be particularly careful regarding the estimate of the maximum adsorption capacity, which diverges widely depending on the equation used to fit the data.

Dear colleague R. Alfonso,

I agree that in such cases the interpretation of the model constants must be made carefully, as experimental conditions can be very influential on the results. In some cases even the changes in studied concentration range can cause the isotherm fit to switch from Langmuir model to Freundlich model. However, I tend to disagree with considering such fits as meaningless mathematical correlation. May be the way the isotherm constants are interpreted need to be revised/modified, and may be it need to be broken into more case-specific parameters.

Interesting!

Yes, Langmuir isotherm model is not limited to chemisorption. I quite agree with Prof. Talal Shahwan. There are adsorption reactions which involve the formation of monolayer (Langmuir isotherm), but then, the bond(s) formed as a result of sorbate-sorbent interaction(s) are characterised by weak intemolecular forces like Van der waal forces, etc.

An error made by many researchers is that they restrict Langmuir isotherm model to chemisorption. This is not true!

I think it's interesting that Dr. Shanwan mentioned the shift from the Langmuir model to the Freundlich isotherm.  The Freundlich isotherm is a special case of the two models that I mentioned.  It make a great deal of difference what the energy of adsorption is and the range picked for temperatures, pressures and concentration as to what the particular approximation to the overall model is a good fit.  For example, the BET does a reasonable job between 0.05 and 0.30 (interpreted as) monolayers, but for many cases is very poor outside these limits.

With respect to the Langmuir isotherm, simultaneous measurements of the isotherm and integral heats of adsorption by Berg demonstrated that even for adsorptions of 12 to 18 kJ/mol the Langmuir isotherm does not hold.  (He and others at the Morrison lab did not do extensive work in this area - considering the difficulty of doing the simultaneous measurements, that is not surprising.)  Instead, the multilayer adsorption isotherms were observed.  It may be that to get local adsorption the local potential wells need to be significantly deeper than the overall potential on the surface.  Thus, there would be a trade-off between the high entropy of a mobile layer on the surface versus the energy gained by the molecules being "stuck" in the local potential wells.

Most of my experience has been with gas adsorption on solids, so my comments should be taken lightly for any other system.  I have done some work with electrolytes in aqueous solutions in electrolytic cells.  For these systems I didn't find large deviations from the Debye-Hueckel theory.

Dear Dr. Condon

As you noted in the last paragraph what applies for gas-solid adsorption is not necessarily valid for liquid-solid adsorption.

Regarding the observation of multilayer gas adsorption, I think it can readily occur in many cases. As is known, the high entropy effects of a gas necessitate significant perturbation of the electronic structure of the gas molecule for chemiorption to take place, which results in a deep PE well. The distortion in the electronic distribution of the first adsorbed layer of gas molecules can readily induce multilayer coverage on the surface specially if the polarizability of the gas molecules is high. Of course this should not be generalized as the nature of surface adsorption being physical or chemical (molecular, dissociative) depends on many factors like electronic structure of gas atoms and surface atoms, surface structure, surface temperature, gas pressure...etc.

Dear Dr. Shahwan,

I think you are right, in some cases, the Langmuir isotherm is still valid even if some of the original assumptions behind its 1918 kinetic derivation are not valid. I probably was still thinking about my previous answer, and interpreted your comment in its context, which is obviously incorrect.

I'm sorry for the mistake. Thanks for correcting me.

Rui Afonso (WITH NO "L")

With pleasure and best wishes, Dr. Afonso.

Thank you for all the answers!

Would like to ask one more thing - is ion exchange considered as part of the chemisorption or it is a "separate" process? Thanks!

And about Langmuir model - I have seen that a lot of researchers (in the field of biosorption) are using it in their studies! In other words - it is widely used, even the assumptions are no valid (mono layer sorption, uniform sorption sites, etc.)

 dear Dr. Omorogie, 

thanks for your valuable guidance. I want to know that do you have any example of adsorption process where both Freundlich and langmuir models are satisfied but thermodynamic data shows physisorption. If yes then please send me the link. 

Hello, I have a question associated with this energy of bonding, I have run adsorption essays at different temperatures and determined enthalpy, Gibbs free energy and entropy changes by Van't Hoff equation, what parameter can be used to determine physi or chemisorption? I mean what parameter can be used to determine this range

Physisorption. Physical adsorption is the simplest immobilization method, in which the biomolecules are attached to the surface through weak bonds such as van der Waals forces, hydrogen bonding, or hydrophobic interactions.