The equation
log(1/1-Q) = logKAE + blogC
is called Adejo-Ekwenchi Adsorption Isotherm, where Q and C are surface coverage and initial conc, of the adsorbate, respectively.
All systems we have worked seem to, reasonably, follow the model. Can anyone prove otherwise?
Dr. Adejo
Dear Gert Van der Zwan,
If you understand the question well, we are trying to see if the equation can be applied to solute systems.
However, to your postulate, you know that there is no surface coverage at C=0, and therefore the equation can only hold for a system for which concentration is greater than zero. You also know that surface Q cannot never be zero, if it is then there is no adsorption. At any adsorption Q MUST be greater than zero, please.
The fundamental ideal here is the relationship between the difference between total surface coverage and the fraction covered and the concentration of inhibitor. This is an inverse relationship.
1/1-Q = KCb
What will be the equation if Q = 0, and Q can only be zero when C = 0, and no adsorption. And the equation as said is applicable to adsorption.
The physical meaning of b is simple. It be be used to distinguish between physical and chemical adsorption. The read the attached, please.
Langmuir equation does not hold for ALL systems, as you said (MANY).
Please applied this equation to surface coverage system and see. The fundamental underlining principle of this isotherm is the thermodynamics. And as you know isotherm are formulated mainly on three principles: thermodynamics, kinetics and surface potential.
I wait to hear from you, please.
Thanks.
Dr. Adejo
I think we agree that when C = 0, Q = 0, no adsorption. If no adsorption where then can you applied the equation. Q is surface coverage not surface area please. When do you have Q, I guess only when adsorption occurs.
If C= 0, KCb will be what, I guess zero also, then anything equal to zero will be what sir?
You can think of applying the equation ONLY when there is adsorption.
Yes the equation many not hold for ALL systems which is the crux of my question. You out there should get to work and let us know if there is any?
My Prof. that equation is very SOUND if you try it, please.
Dr. Adejo
Dear Gert Van der Zwan,
Pls can you recognise this equation
Q/1-Q = K'Cy
I know you know it is well established surface coverage isotherm equation, in short the El-Awady isotherm. What will be the unit of both side? Do they work out?
I really appreciate your commends, but try the isotherm and let us have you the feedback.
Dr. Adejo
As indicated by Gert van der Zwan, if C-> 0 we must have Q -> 0 and the equation 1/1-Q = K C ^ b is certainly not verified with low concentrations. In addition, the Langmuir isotherm has a theoretical thermodynamical and kinetics justification, what this equation has not. The problem is that the Langmuir equation considers an uniform surface with a well-defined value of K. In practice, the heat of adsorption, and thus K, often vary with Q either because the adsorption sites are different or because the adsorbed molecules interact with each other. To obtain a general equation of absorption, we should integrate the equation of Langmuir from Q = 0 to Q = 1. Such an integrated equation does not exist. An approximate solution was proposed by Temkin but his approximations imply that the resulting equation, Q = K C ^ b, is not valid for very large or very small concentrations.
Guy sir,
Remember that many known isotherms either don;t not hold at low while some don't at high concentration. Does not holding at low low concentration make it completely useless? One thing I keep on saying is for the equation to be applicable there must be adsorption. Can we get surface coverage Q without adsorption and does surface coverage has any unit?
One thing is clear there is no formula for the derivation of an isotherm and for some isotherm there are limiting conditions, in fact Freundlich isotherm is a clear example. Not reducing the isotherm to Henry's law at low concentration does not completely invalid Freundlich isotherm does it?
There continue the discussion, the world is a learning field.
Thank to you all
Dr. Adejo
Im working all the issues you have addressed. Good been out there with people like you. But in science issue is usually thoroughly assessed before comments. It will be good to study all known isotherm. The applicability of the Adejo-Ekwenchi is our next head and its being tackled and be presented soon.
Thank you guys
Dr. Adejo
I fully relate to what Gert Van der Zwan wrote.
Any thermodynamically sound isotherm equation for c->0 should give coverage ->0 (lim Q(c->0) =0) (even you consider several machanisms, e.g physi- and e.g. chemisorption - with much higher K - if not adsorbate is present, adsorbed amount will be 0).
Unless it is purely empirical equation to be used in a narrow concentration range (here C.max/C.min=5 or less), e.g. as a smoothing tool. As it was indicated earlier, similar behavior near Q -> 1 will be shown by e.g. Tóth isotherm (or Langmuir-Freundlich, Generalized Freundlich (Sips), etc.).
Here we have a problem that it is proposed as just "isotherm" and not "isotherm approximationg behavior in some data range". BTW, even well known isotherms which reduce to (Kc)^n near c=0 have a problem, because this form implies infinite max. adsorption energy, but at least they do have a much wider compatibility range (several orders of magnitudes of C)