Langmuir adsorption model

This section[5] provides a kinetic derivation for a single-adsorbate case. The kinetic derivation applies to gas-phase adsorption. However, it has been mistakenly applied to solutions. The multiple-adsorbate case is covered in the competitive adsorption sub-section. The model assumes adsorption and desorption as being elementary processes, where the rate of adsorption r_ad and the rate of desorption r_d are given by

${\displaystyle r_{\text{ad}}=k_{\text{ad}}p_{A}[S],}$

${\displaystyle r_{\text{d}}=k_{d}[A_{\text{ad}}],}$

where p_A is the partial pressure of A over the surface, [S] is the concentration of free sites in number/m², [A_ad] is the surface concentration of A in molecules/m² (concentration of occupied sites), and k_ad and k_d are constants of forward adsorption reaction and backward desorption reaction in the above reactions.

At equilibrium, the rate of adsorption equals the rate of desorption. Setting r_ad = r_d and rearranging, we obtain

${\displaystyle {\frac {[A_{\text{ad}}]}{p_{A}[S]}}={\frac {k_{\text{ad}}}{k_{\text{d}}}}=K_{\text{eq}}^{A}.}$

The concentration of sites is given by dividing the total number of sites (S₀) covering the whole surface by the area of the adsorbent (a):

${\displaystyle [S_{0}]=S_{0}/a.}$

We can then calculate the concentration of all sites by summing the concentration of free sites [S] and occupied sites:

${\displaystyle [S_{0}]=[S]+[A_{\text{ad}}].}$

Combining this with the equilibrium equation, we get

${\displaystyle [S_{0}]={\frac {[A_{\text{ad}}]}{K_{\text{eq}}^{A}p_{A}}}+[A_{\text{ad}}]={\frac {1+K_{\text{eq}}^{A}p_{A}}{K_{\text{eq}}^{A}p_{A}}}[A_{\text{ad}}].}$

We define now the fraction of the surface sites covered with A as

${\displaystyle \theta _{A}={\frac {[A_{\text{ad}}]}{[S_{0}]}}.}$

This, applied to the previous equation that combined site balance and equilibrium, yields the Langmuir adsorption isotherm:

${\displaystyle \theta _{A}={\frac {K_{\text{eq}}^{A}p_{A}}{1+K_{\text{eq}}^{A}p_{A}}}.}$

In condensed phases (solutions), adsorption to a solid surface is a competitive process between the solvent (A) and the solute (B) to occupy the binding site. The thermodynamic equilibrium is described as

Solvent (bound) + Solute (free) ↔ Solvent (free) + Solute (bound).

If we designate the solvent by the subscript "1" and the solute by "2", and the bound state by the superscript "s" (surface/bound) and the free state by the "b" (bulk solution / free), then the equilibrium constant can be written as a ratio between the activities of products over reactants:

${\displaystyle K={\frac {a_{1}^{\text{b}}\times a_{2}^{\text{s}}}{a_{2}^{\text{b}}\times a_{1}^{\text{s}}}}.}$

For dilute solutions the activity of the solvent in bulk solution ${\displaystyle a_{1}^{\text{b}}\simeq 1,}$ and the activity coefficients (${\displaystyle \gamma }$) are also assumed to ideal on the surface. Thus, ${\displaystyle a_{2}^{\text{s}}=X_{2}^{\text{s}}=\theta ,}$ ${\displaystyle a_{1}^{\text{s}}=X_{1}^{\text{s}},}$, and ${\displaystyle X_{1}^{\text{s}}+X_{2}^{\text{s}}=1,}$ where ${\displaystyle X_{i}}$ are mole fractions. Re-writing the equilibrium constant and solving for ${\displaystyle \theta }$ yields

${\displaystyle \theta ={\frac {Ka_{2}^{\text{b}}}{1+Ka_{2}^{\text{b}}}}.}$

Note that the concentration of the solute adsorbate can be used instead of the activity coefficient. However, the equilibrium constant will no longer be dimensionless and will have units of reciprocal concentration instead. The difference between the kinetic and thermodynamic derivations of the Langmuir model is that the thermodynamic uses activities as a starting point while the kinetic derivation uses rates of reaction. The thermodynamic derivation allows for the activity coefficients of adsorbates in their bound and free states to be included. The thermodynamic derivation is usually referred to as the "Langmuir-like equation".[6][7]

This derivation[8][9] based on statistical mechanics was originally provided by Volmer and Mahnert[10] in 1925. The partition function of the finite number of adsorbents adsorbed on a surface, in a canonical ensemble, is given by

${\displaystyle Z(N_{A})=\left[\zeta _{L}^{N_{A}}{\frac {N_{S}!}{(N_{S}-N_{A})!}}\right]{\frac {1}{N_{A}!}},}$

where ${\displaystyle \zeta _{L}}$ is the partition function of a single adsorbed molecule, ${\displaystyle N_{S}}$ is the number of adsorption sites (both occupied and unoccupied), and ${\displaystyle N_{A}}$ is the number of adsorbed molecules which should be less than or equal to ${\displaystyle N_{S}}$. The terms in the bracket give the total partition function of the ${\displaystyle N_{A}}$ adsorbed molecules by taking a product of the individual partition functions (refer to Partition function of subsystems). The ${\displaystyle 1/N_{A}!}$ factor accounts for the overcounting arising due to the indistinguishable nature of the adsorbates. The grand canonical partition function is given by

${\displaystyle {\mathcal {Z}}(\mu _{A})=\sum _{N_{A}=0}^{N_{S}}\exp \left({\frac {N_{A}\mu _{A}}{k_{\text{B}}T}}\right){\frac {\zeta _{L}^{N_{A}}}{N_{A}!}}\,{\frac {N_{S}!}{(N_{S}-N_{A})!}}.}$

${\displaystyle \mu _{A}}$ is the chemical potential of an adsorbed molecule. As it has the form of binomial series, the summation is reduced to

${\displaystyle {\mathcal {Z}}(\mu _{A})=(1+x)^{N_{S}},}$

where ${\displaystyle x=\zeta _{L}\exp \left({\frac {\mu _{A}}{k_{\rm {B}}T}}\right).}$

The grand canonical potential is

${\displaystyle \Omega =-k_{\rm {B}}T\ln({\mathcal {Z}})=-k_{\rm {B}}TN_{S}\ln(1+x),}$

based on which the average number of occupied sites is calculated

${\displaystyle \langle N_{A}\rangle =-\left({\frac {\partial \Omega }{\partial \mu _{A}}}\right)_{T,{\text{area}}},}$

which gives the coverage

${\displaystyle \theta _{A}={\frac {\langle N_{A}\rangle }{N_{S}}}={\frac {x}{1+x}}.}$

Now, invoking the condition that the system is in equilibrium, that is, the chemical potential of the adsorbed molecules is equal to that of the molecules in gas phase, we have

${\displaystyle \mu _{A}=\mu _{\text{g}},}$

The chemical potential of an ideal gas is

${\displaystyle \mu _{\text{g}}=\left({\frac {\partial A_{\text{g}}}{\partial N}}\right)_{T,V}}$

where ${\displaystyle A_{g}=-k_{\rm {B}}T\ln Z_{g}}$ is the Helmholtz free energy of an ideal gas with its partition function

${\displaystyle Z_{g}={\frac {q^{N}}{N!}}.}$

${\displaystyle q}$ is the partition function of a single particle in the volume of ${\displaystyle V}$ (only consider the translational freedom here).

${\displaystyle q=V\left({\frac {2\pi mk_{\rm {B}}T}{h^{2}}}\right)^{3/2}.}$

We thus have ${\displaystyle \mu _{g}=-k_{\rm {B}}T\ln(q/N)}$, where we use Stirling's approximation.

Plugging ${\displaystyle \mu _{g}}$ to the expression of ${\displaystyle x}$, we have

${\displaystyle {\frac {\theta _{A}}{1-\theta _{A}}}=x=\zeta _{L}{\frac {N}{q}}}$

which gives the coverage

${\displaystyle \theta _{A}={\frac {\zeta _{L}/(q/N)}{1+\zeta _{L}/(q/N)}}}$

By defining

${\displaystyle P_{0}={\frac {k_{\text{B}}T}{\zeta _{L}}}\left({\frac {2\pi mk_{\text{B}}T}{h^{2}}}\right)^{3/2}}$

and using the identity ${\displaystyle PV=Nk_{\rm {B}}T}$, finally, we have

${\displaystyle \theta _{A}={\frac {P}{P+P_{0}}}.}$

It is plotted in the figure alongside demonstrating that the surface coverage increases quite rapidly with the partial pressure of the adsorbants, but levels off after P reaches P₀.

The previous derivations assumed that there is only one species, A, adsorbing onto the surface. This section[11] considers the case when there are two distinct adsorbates present in the system. Consider two species A and B that compete for the same adsorption sites. The following hypotheses are made here:

1. All the sites are equivalent.
2. Each site can hold at most one molecule of A, or one molecule of B, but not both simultaneously.
3. There are no interactions between adsorbate molecules on adjacent sites.

As derived using kinetic considerations, the equilibrium constants for both A and B are given by

${\displaystyle {\frac {[A_{\text{ad}}]}{p_{A}\,[S]}}=K_{\text{eq}}^{A}}$

and

${\displaystyle {\frac {[B_{\text{ad}}]}{p_{B}\,[S]}}=K_{\text{eq}}^{B}.}$

The site balance states that the concentration of total sites [S₀] is equal to the sum of free sites, sites occupied by A and sites occupied by B:

${\displaystyle [S_{0}]=[S]+[A_{\text{ad}}]+[B_{\text{ad}}].}$

Inserting the equilibrium equations and rearranging in the same way we did for the single-species adsorption, we get similar expressions for both θ_A and θ_B:

${\displaystyle \theta _{A}={\frac {K_{\text{eq}}^{A}\,p_{A}}{1+K_{\text{eq}}^{A}\,p_{A}+K_{\text{eq}}^{B}\,p_{B}}},}$

${\displaystyle \theta _{B}={\frac {K_{\text{eq}}^{B}\,p_{B}}{1+K_{\text{eq}}^{A}\,p_{A}+K_{\text{eq}}^{B}\,p_{B}}}.}$

The other case of special importance is when a molecule D₂ dissociates into two atoms upon adsorption.[11] Here, the following assumptions would be held to be valid:

1. D₂ completely dissociates to two molecules of D upon adsorption.
2. The D atoms adsorb onto distinct sites on the surface of the solid and then move around and equilibrate.
3. All sites are equivalent.
4. Each site can hold at most one atom of D.
5. There are no interactions between adsorbate molecules on adjacent sites.

Using similar kinetic considerations, we get

${\displaystyle {\frac {[D_{\text{ad}}]}{p_{D_{2}}^{1/2}[S]}}=K_{\text{eq}}^{D}.}$

The 1/2 exponent on p_D2 arises because one gas phase molecule produces two adsorbed species. Applying the site balance as done above,

${\displaystyle \theta _{D}={\frac {(K_{\text{eq}}^{D}\,p_{D_{2}})^{1/2}}{1+(K_{\text{eq}}^{D}\,p_{D_{2}})^{1/2}}}.}$

Also known as the two-site Langmuir equation. This equation describes the adsorption of one adsorbate to two or more distinct types of adsorption sites. Each binding site can be described with its own Langmuir expression, as long as the adsorption at each binding site type is independent from the rest.

${\displaystyle q_{\text{total}}={\frac {q_{1}^{\text{max}}K_{1}a_{2}^{\text{b}}}{1+K_{1}a_{2}^{\text{b}}}}+{\frac {q_{2}^{\text{max}}K_{2}a_{2}^{\text{b}}}{1+K_{2}a_{2}^{\text{b}}}}+\dots ,}$

where

${\displaystyle q_{\text{total}}}$ – total amount adsorbed at a given adsorbate concentration,

${\displaystyle q_{1}^{\text{max}}}$ – maximum capacity of site type 1,

${\displaystyle q_{2}^{\text{max}}}$ – maximum capacity of site type 2,

${\displaystyle K_{1}}$ – equilibrium (affinity) constant of site type 1,

${\displaystyle K_{2}}$ – equilibrium (affinity) constant of site type 2,

${\displaystyle a_{2}^{\text{b}}}$ – adsorbate activity in solution at equilibrium

This equation works well for adsorption of some drug molecules to activated carbon in which some adsorbate molecules interact with hydrogen bonding while others interact with a different part of the surface by hydrophobic interactions (hydrophobic effect). The equation was modified to account for the hydrophobic effect (also known as entropy-driven adsorption):[13]

${\displaystyle q_{\text{total}}={\frac {q_{1}^{\text{max}}K_{1}a_{2}^{\text{b}}}{1+K_{1}a_{2}^{\text{b}}}}+q_{\text{HB}}.}$

The hydrophobic effect is independent of concentration, since ${\displaystyle K_{2}a_{2}^{\text{b}}\gg 1.}$ Therefore, the capacity of the adsorbent for hydrophobic interactions ${\displaystyle q_{\text{HB}}}$ can obtained from fitting to experimental data. The entropy-driven adsorption originates from the restriction of translational motion of bulk water molecules by the adsorbate, which is alleviated upon adsorption.

The Freundlich isotherm is the most important multi-site adsorption isotherm for rough surfaces.

${\displaystyle \theta _{A}=\alpha _{F}p^{C_{\text{F}}},}$

where α_F and C_F are fitting parameters.[14] This equation implies that if one makes a log–log plot of adsorption data, the data will fit a straight line. The Freundlich isotherm has two parameters, while Langmuir's equations has only one: as a result, it often fits the data on rough surfaces better than the Langmuir's equations. However, the Freundlich equation is unique; consequently, if the data fit the equation, it is only likely, but not proved, that the surface is heterogeneous. The heterogeneity of the surface can be confirmed with calorimetry. Homogeneous surfaces (or heterogeneous surfaces that exhibit homogeneous adsorption (single-site)) have a constant ${\displaystyle \Delta H}$ of adsorption as a function of the occupied-sites fraction. On the other hand, heterogeneous adsorption (multi-site) have a variable ${\displaystyle \Delta H}$ of adsorption depending on the sites occupation. When the adsorbate pressure (or concentration) are low, high-energy sites are occupied, and as the pressure (or concentration) increases, the lesser-energy sites become occupied, resulting in a lower ${\displaystyle \Delta H}$ of adsorption.[15]

A related equation is the Toth equation. Rearranging the Langmuir equation, one can obtain

${\displaystyle \theta _{A}={\frac {p_{A}}{{\frac {1}{K_{\text{eq}}^{A}}}+p_{A}}}.}$

Toth[16] modified this equation by adding two parameters α_T0 and C_T0 to formulate the Toth equation:

${\displaystyle \theta ^{C_{T_{0}}}={\frac {\alpha _{T_{0}}p_{A}^{C_{T_{0}}}}{{\frac {1}{K_{\text{eq}}^{A}}}+p_{A}^{C_{T_{0}}}}}.}$

This isotherm takes into account indirect adsorbate–adsorbate interactions on adsorption isotherms. Temkin[17] noted experimentally that heats of adsorption would more often decrease than increase with increasing coverage.

The heat of adsorption ΔH_ad is defined as

${\displaystyle {\frac {[A_{\text{ad}}]}{p_{A}[S]}}=K_{\text{eq}}^{A}\propto \mathrm {e} ^{-\Delta G_{\text{ad}}/RT}=\mathrm {e} ^{\Delta S_{\text{ad}}/R}\,\mathrm {e} ^{-\Delta H_{\text{ad}}/RT}.}$

He derived a model assuming that as the surface is loaded up with adsorbate, the heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbate–adsorbate interactions:

${\displaystyle \Delta H_{\text{ad}}=\Delta H_{\text{ad}}^{0}(1-\alpha _{T}\theta ),}$

where α_T is a fitting parameter. Assuming the Langmuir adsorption isotherm still applied to the adsorbed layer, ${\displaystyle K_{\text{eq}}^{A}}$ is expected to vary with coverage as follows:

${\displaystyle K_{\text{eq}}^{A}=K_{\text{eq}}^{A,0}\mathrm {e} ^{\Delta H_{\text{ad}}^{0}(1-\alpha _{T}\theta )/kT}.}$

Langmuir's isotherm can be rearranged to

${\displaystyle K_{\text{eq}}^{A}p_{A}={\frac {\theta }{1-\theta }}.}$

Substituting the expression of the equilibrium constant and taking the natural logarithm:

${\displaystyle \ln {\big (}K_{\text{eq}}^{A,0}p_{A}{\big )}={\frac {-\Delta H_{\text{ad}}^{0}\alpha _{T}\theta }{kT}}+\ln {\frac {\theta }{1-\theta }}.}$

Brunauer, Emmett and Teller (BET)[18] derived the first isotherm for multilayer adsorption. It assumes a random distribution of sites that are empty or that are covered with by one monolayer, two layers and so on, as illustrated alongside. The main equation of this model is

${\displaystyle {\frac {[A]}{S_{0}}}={\frac {c_{B}x_{B}}{(1-x_{B})[1+(c_{B}-1)x_{B}]}},}$

where

${\displaystyle x_{B}=p_{A}K_{m},\quad c_{B}={\frac {K_{1}}{K_{m}}},}$

and [A] is the total concentration of molecules on the surface, given by

${\displaystyle [A]=\sum _{i=1}^{\infty }i[A]_{i}=\sum _{i=1}^{\infty }iK_{1}K_{m}^{i-1}p_{A}^{i}[A]_{0},}$

where

${\displaystyle K_{i}={\frac {[A]_{i}}{p_{A}[A]_{i-1}}},}$

in which [A]₀ is the number of bare sites, and [A]_i is the number of surface sites covered by i molecules.