adsorbaat

An adsorbaat, or adsorbate, is a substance that adheres to a solid surface, the adsorbent, by adsorption. The interaction can be physical (physisorption) or chemical (chemisorption). The distinction is based on the strength and nature of the interaction: physisorption involves relatively weak van der Waals forces and is usually reversible, while chemisorption involves the formation of chemical bonds with surface atoms and is typically stronger and more site-specific.

In physisorption, binding is weak and often temperature-dependent, with energies on the order of tens of kilojoules per mole. Chemisorption features higher binding energies, can be highly selective for particular surface sites, and may proceed with changes in the electronic structure of the adsorbate and substrate. Adsorption also depends on environmental conditions such as temperature and pressure or concentration, and on surface characteristics such as roughness and chemical composition at active sites.

Adsorption is often described using surface coverage, theta, representing the fraction of available sites occupied by adsorbates. Classical models include Langmuir theory, which assumes a uniform, monolayer adsorption, and Freundlich theory, which accounts for heterogeneous surfaces. The Brunauer–Emmett–Teller (BET) model extends to multilayer adsorption and is widely used to estimate surface area from gas adsorption data.

Characterization methods include adsorption isotherms, temperature-programmed desorption, calorimetry, and spectroscopy, which together provide information on binding energies, coverage, and identity of the adsorbed species.

Applications of adsorbates span catalysis, environmental remediation (for example, activated carbon removing pollutants), gas separation, and sensing technologies. Examples of common adsorbates include carbon monoxide on metal surfaces, water on oxides, and various organic molecules on porous materials.