Discussion
Started 23 August 2024

Isoterm of Langmuir

La isoterma de Langmuir representa una adsorcion fisica o quimica?

Most recent answer

Dmitry V. Tatyanenko
St Petersburg University
I think, the Langmuir adsorption model can describe both physical and chemical adsorption.
The exact meaning of adsorption sites can differ in this cases (for chemisorption, it can be physical number of atoms/molecules of specific chemical species, whereas for physisoprtion the number of sites can just reflect the maximal number of adsorbate atoms/molecules that can "fill" the surface due to their finite lateral "size").
The binding energy is also substantially different that may give a trivial asymptotic limit in the model for chemical or physical adsorption.
Let's make some estimations. As it is often mentioned, the binding energy for physisorption is usually 10⁻²–10⁻¹ eV, whereas it is usually 10⁻¹–10⁰ eV for chemisorption. In the Langmuir adsorption model, the fractional occupancy θ depends on the temperature T, chemical potential μ and binding energy ε as
θ = [1 + exp(−(μ+ε)/kT)]⁻¹,
and 1/kT ≃ 40 eV⁻¹ for T ≃ 300 K, μ/kT ~ –16 (estimation for an ideal gas at 1 atm pressure). So, the difference in the value of ε in 1–2 decimal orders can correspond even to opposite asymptotic limits in the fractional occupancy vs the bulk concentration dependence θ(n) — a constant 'high concentration' limit θ ≃ 1 of for a high binding energy and a linear 'low concentration' asymptotic limit θn for a low binding energy.
For the estimations I presented, only moderate binding energies ε ~ 10⁻¹ eV per site can give the fractional occupancy values somewhere between the asymptotic limits of θ(n). This case may correspond to quite a weak chemical bond or a strong physical attraction.

All replies (4)

Fernando Hernández Ramírez
Instituto Tecnológico y de Estudios Superiores de Occidente
Hola Felipe:
En general, no es posible distinguir entre adsorción física o química solamente por el ajuste a un modelo. El modelo de Langmuir representa la adsorción de una monocapa solamente, pero esto puede ocurrir tanto en fisisorción como en quimisorción.
Mira esta discusión:
Ing Felipe T C
Autonomous University of San Luis Potosí
Muy amable, gracias
Abdelhak Maghchiche
University of Batna 2
La isoterma de Langmuir generalmente describe una adsorción química, donde ocurre en una sola capa con formación de enlaces entre el adsorbente y el adsorbato. Aunque está enfocada en adsorción química, también puede aplicarse a la adsorción física en ciertos casos
Dmitry V. Tatyanenko
St Petersburg University
I think, the Langmuir adsorption model can describe both physical and chemical adsorption.
The exact meaning of adsorption sites can differ in this cases (for chemisorption, it can be physical number of atoms/molecules of specific chemical species, whereas for physisoprtion the number of sites can just reflect the maximal number of adsorbate atoms/molecules that can "fill" the surface due to their finite lateral "size").
The binding energy is also substantially different that may give a trivial asymptotic limit in the model for chemical or physical adsorption.
Let's make some estimations. As it is often mentioned, the binding energy for physisorption is usually 10⁻²–10⁻¹ eV, whereas it is usually 10⁻¹–10⁰ eV for chemisorption. In the Langmuir adsorption model, the fractional occupancy θ depends on the temperature T, chemical potential μ and binding energy ε as
θ = [1 + exp(−(μ+ε)/kT)]⁻¹,
and 1/kT ≃ 40 eV⁻¹ for T ≃ 300 K, μ/kT ~ –16 (estimation for an ideal gas at 1 atm pressure). So, the difference in the value of ε in 1–2 decimal orders can correspond even to opposite asymptotic limits in the fractional occupancy vs the bulk concentration dependence θ(n) — a constant 'high concentration' limit θ ≃ 1 of for a high binding energy and a linear 'low concentration' asymptotic limit θn for a low binding energy.
For the estimations I presented, only moderate binding energies ε ~ 10⁻¹ eV per site can give the fractional occupancy values somewhere between the asymptotic limits of θ(n). This case may correspond to quite a weak chemical bond or a strong physical attraction.

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