Adsorbats

Adsorbats, also called adsorbates, are atoms, ions, or molecules that accumulate on the surface of a solid material (the adsorbent) during adsorption. This process concentrates the species at the interface and is distinct from absorption, which involves uptake into the bulk of a material.

Adsorption can proceed by two primary mechanisms. Physisorption involves weak van der Waals forces and is usually reversible, with low enthalpy of adsorption and sensitivity to temperature and pressure. Chemisorption involves the formation of stronger chemical bonds, can be selective for specific species, and is often irreversible or involves higher activation energies and changes to surface chemistry. Many systems exhibit both, with the dominant mechanism depending on the adsorbate and surface.

Equilibrium between the adsorbed layer and the surrounding phase is described by adsorption isotherms. The Langmuir model assumes a homogeneous surface and monolayer coverage with a finite number of identical sites, while the Freundlich model describes heterogeneous surfaces and multilayer adsorption in a somewhat empirical way. Kinetic models, such as pseudo-first-order and pseudo-second-order equations, describe how quickly adsorption reaches equilibrium.

A range of factors influence adsorption, including the surface area and porosity of the adsorbent, temperature, pressure, the nature and polarity of the adsorbate, and the presence of competing species. Regeneration or desorption can recover adsorbents for reuse, often by applying heat, solvents, or altering pressure.

Applications span environmental cleanup, gas separation and storage, catalysis, water treatment, and sensing. Common examples include activated carbon removing organics from water and CO2 capture on functionalized surfaces.