Adsorption (Latin: adsorbere = to suck in) means the accumulation of particles (atoms, molecules, ions, etc.) of one or more types of particles from a liquid or
gaseous phase on the surface of a solid or a liquid.

Since the liquid or gaseous substance accumulates on the surface, there is always a change in concentration at the phase interface during this process. Adsorption is usually spoken of when there is a gas/solid interface.

A substance that accumulates (adsorbs) particles from a neighboring gaseous or liquid phase at its interface is called an adsorbent or substrate.

The particles that are in the adjacent gaseous or liquid phase and accumulate on the adsorbent are referred to as adsorptive.

## Degree of coverage

The surface of an adsorbent contains a certain number of adsorption sites. The quotient of the number of occupied adsorption sites divided by the number of available adsorption sites is described by the degree of coverage Θ.

Θ = Number of occupied adsorption sites / Number of existing adsorption sites

The degree of coverage Θ is therefore a measure of the adsorption on a surface.

Θ=0: all places are free

Θ=1: all places are occupied

The degree of coverage can also be specified as a percentage.

The rate at which a surface is covered by the adsorbent depends on how fast the adsorbent can absorb the energy of the impacting particle. If this happens too slowly, the particle will be pushed back into the gas phase. So not all the particles hitting the surface of the adsorbent are adsorbed.

The proportion of collisions that result in adsorption is called the adsorption probability.

adsorption probability = frequency of adsorption of particles /frequency of impact of particles

The probability of adsorption decreases with increasing degree of surface coverage.

## Physisorption

Physisorption is based on the effect of van der Waals forces between adsorbate and adsorbent. Although these interactions are weak, they extend over large distances, since adsorption on the existing adsorbate layer is also possible due to van der Waals forces, and several adsorption layers can form in this way.

When a particle approaches a surface, two opposing forces act on this particle: a repulsive force and an attractive force. The magnitude of these forces depends on the distance r between the particle and the surface.

The repulsive force is proportional to r−12 while the attractive force is proportional to r−6. There is a superimposition of the forces of attraction and repulsion acting on the particle, the sum of which leads to the following expression for the potential energy of the particle:

E(r) = −Br6 + Ar12

The function obtained is called the Lennard-Jones (6,12) potential.