Adsorption is the accumulation of a substance at a surface rather than in the bulk. When a gas or a dissolved solute is brought into contact with a solid, its concentration at the surface becomes higher than in the interior. The substance that is concentrated on the surface is the adsorbate; the surface that holds it is the adsorbent. Finely divided metals, charcoal, silica gel, alumina gel and clay are good adsorbents because they expose a very large surface area per gram.
Adsorption versus absorption. In adsorption the substance stays only on the surface, so the concentration changes sharply at the boundary. In absorption the substance is taken up uniformly throughout the bulk of the material, like water soaked into a sponge or ammonia dissolving through water. When both happen together the process is called sorption. Adsorption is rapid at first and then slows as the surface fills; absorption proceeds at a uniform rate.
Adsorption lowers the surface energy of the adsorbent, so it is always exothermic: $\Delta H$ is negative. During adsorption gas molecules lose freedom of motion, so the entropy change $\Delta S$ is also negative. For the process to be spontaneous, $\Delta G = \Delta H - T\Delta S$ must be negative, which requires $\Delta H$ to be sufficiently negative. As adsorption proceeds $\Delta H$ becomes less negative, and equilibrium is reached when $\Delta G = 0$.
Physisorption versus chemisorption. In physical adsorption (physisorption) the adsorbate is held by weak van der Waals forces. It has a low enthalpy of adsorption ($20$–$40\ \text{kJ mol}^{-1}$), is reversible, not specific, favoured by low temperature, and can build up multiple molecular layers. In chemical adsorption (chemisorption) the adsorbate forms chemical bonds with the surface. It has a high enthalpy of adsorption ($80$–$240\ \text{kJ mol}^{-1}$), is usually irreversible, highly specific, and forms only a single (unimolecular) layer. Physisorption can change into chemisorption as the temperature rises.
Factors affecting adsorption of gases. Adsorption increases with the surface area of the adsorbent, so porous and finely divided solids adsorb best. Easily liquefiable gases with higher critical temperatures (for example $\text{NH}_3$, $\text{HCl}$, $\text{CO}_2$, $\text{SO}_2$) are adsorbed more than permanent gases such as $\text{H}_2$, $\text{N}_2$ and $\text{O}_2$. Physisorption decreases with rising temperature (it is exothermic), while chemisorption first rises and then falls. Adsorption increases with pressure but eventually levels off when the surface is saturated.
Freundlich adsorption isotherm. At constant temperature the variation of the mass of gas adsorbed per gram of adsorbent, $\frac{x}{m}$, with pressure $p$ is given by $\frac{x}{m}=k\,p^{1/n}$, where $k$ and $n$ are constants for a given adsorbent-adsorbate pair at a given temperature, and $n \ge 1$. Taking logarithms gives the straight-line form $\log\frac{x}{m}=\log k+\frac{1}{n}\log p$. A plot of $\log\frac{x}{m}$ against $\log p$ has slope $\frac{1}{n}$ and intercept $\log k$. The equation holds over a limited range: at low pressure $\frac{x}{m}\propto p$ ($1/n \to 1$) and at high pressure $\frac{x}{m}$ becomes independent of pressure ($1/n \to 0$).
Applications. Adsorption underlies the creation of high vacuum, gas masks (activated charcoal adsorbs toxic gases), the control of humidity by silica and alumina gels, the removal of colouring matter from solutions, heterogeneous catalysis, ion-exchange and chromatographic separation, and froth flotation in metallurgy.