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"Consumption" of matter from a gas or from an opening of the interface between them (or from the surface of a solid body). In other words, adsorption is the "consumption" by an adsorbate from a volume of gas on the surface of the adsorbent. Adsorption is a particular case of sorption.

The molecules of the adsorbate falling on the surface of the adsorbent are retained by the surface force for a period of time, which depends on the natures of the adsorbent and adsorbate, on the temperature $T$ and on the pressure $p$, after which they leave the surface (are desorbed). Under conditions of thermodynamic and molecular equilibrium, the rates of adsorption and desorption are equal. The relation between the relative pressure $\phi=p/p_s$ of the adsorbent and the relative concentration $\theta=c/c_s$, where the index $s$ stands for the limit value at a constant temperature, is known as the adsorption isotherm.

Langmuir's equation of mono-molecular adsorption has the form

$$\phi=\frac{\theta}{k(1-\theta)},$$

where $k$ is the equilibrium constant which roughly describes the interaction between the adsorbent and the adsorbate.

Brunauer's equation  is commonly used in the case of a homogeneous surface of the adsorbent and poly-molecular adsorption.

Posnov's formula , which is empirical, is widely employed for capillary bodies:

$$\frac1\theta=A\ln\phi+1,$$

where $A$ is a coefficient which varies with the temperature and with the structure of the adsorbent.

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