4. Adsorption

Surface pressure

Surface pressure

It shows how much does the surface tension decrease 

γ = surface tension [mN/m or mJ/m²]

γ₀ = initial surface tension [mN/m or mJ/m²]

Π=surface pressure [mN/m or mJ/m²]

Adsorption

Molecules adhere to a surface. Adsorption is the formation of surface layer. 

• Molecules with affinity for a surface

– Hydrophobicity

– Opposite charge

Simple adsorption

v=Volume of sample (m³)

c₀=inital concentration of particle (kg/m³)

c_eq= equilibrium concentration of particle (kg/m³)

A=Total surface area of particles (m²)

Adsorption isotherm

Relationship between adsorbed amount and equilibrium concentration of absorbate in surrounding fluid at constant temperature

A completely covered surface by a monolayer of surfactant

Typically ≈ 1-2 mg/m²

A completely covered surface by a monolayer of protein or polymer

Typically ≈ 1-5 mg/m²

A completely covered surface by a monolayer of macromolecules

Adsorbed amounts can be higher than proteins/polymer

How large should adsorbed amounts be to indicate multilayer adsorption?

Higher adsorbed amounts (>5mg/m² ) can indicate multilayer adsorption

What does Gibbs’ adsorption isotherm assume?

Equilibrium

CMC in surface tension vs log(concentraion of surfactant

Gibbs’ adsorption isotherm

c_i= concentration [kg/m³]

R=Gas constant=8.3145 J/K⋅mol

T=temperature [K]

γ = surface tension [mN/m or mJ/m²]

CMC in adsorption isotherm diagram

The critical micellar concentration (cmc) can be determined from a logarithmic adsorption isotherm diagram

Assumptions for Langmuir isotherm

– Reversible adsorption

– Equilibrium i.e. rate of adsorption=rate of desorption

– Homogenous surface

– Monolayer formation

Tails, trains and loops on adsorbed macromolecules

Protein adsorption

• Practically all proteins are surface active.

– Hydrophobic/Hydrophilic amino acids.

• Adsorption can cause unfolding and denaturation at surfaces.

• Protein adsorption at hydrophobic surfaces is practically irreversible, although displacement can occur.

Competetive adsorption

One surface active species is displaced by another

Associative adsorption

Both species adsorb and form a mixed layer at the interface

Multi-layer adsorption

One species form a second layer on top of the first layer. Commonly seen with opposite charges.

Adsorption of large molecules

– High adsorption energies.

– Relatively low gain in entropy upon desorption.

– Can be irreversible.

Is adsorption reversible?

Depends on:

– Adsorption energy.

– Entropy of mixing.