Adsorption

Adsorption is the process by which atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to a solid surface, forming a molecular film at the interface. Adsorption differs from absorption, in which species diffuse into the bulk of a phase rather than remaining at the surface. Adsorption occurs due to attractive forces between the adsorbate and the surface, and is commonly categorized as physical adsorption (physisorption) or chemical adsorption (chemisorption).

Physisorption involves weak van der Waals interactions and is usually reversible; chemisorption involves the formation of chemical bonds and is often stronger and more specific. Temperature, pressure, contact time, adsorbate concentration, and the surface area and porosity of the adsorbent strongly influence adsorption.

Equilibrium adsorption is described by isotherms, with Langmuir isotherm assuming a homogeneous surface and monolayer adsorption, and Freundlich isotherm describing heterogeneous surfaces and multilayer adsorption. Kinetic models describe rate, including pseudo-first-order and pseudo-second-order.

Adsorption has wide applications: purification and separation of gases and liquids, water treatment, catalyst support, and analytical techniques such as chromatography. Regeneration or desorption allows reuse of adsorbents; techniques include heating, solvent washing, or chemical treatment.

Common adsorbents include activated carbon, silica gel, alumina, zeolites, clays, and metal-organic frameworks. Characterization often uses BET surface area to quantify available surface, and pore size distribution.