Adsorbate

Adsorbate refers to a species that accumulates on a surface from a phase, such as a gas or liquid, through adsorption. The surface that provides adsorption sites is the adsorbent. Adsorption is a surface phenomenon distinct from absorption, which is uptake into the bulk.

Adsorbate can be atoms, ions, or molecules and may reside on exterior surfaces or within pores of a porous adsorbent. Common interfaces include solid–gas and solid–liquid.

Two main types of adsorption are physisorption and chemisorption. Physisorption is governed by weak van der Waals forces, generally reversible, with relatively low enthalpies (typically 10–100 kJ/mol). Chemisorption involves the formation of chemical bonds, with higher enthalpies (often over 100 kJ/mol), and is more specific and slower, sometimes irreversible.

Adsorption equilibrium can be described by isotherms. The Langmuir model assumes a single uniform layer with no interaction between adsorbed molecules; the BET model extends to multilayer adsorption and is widely used to estimate the surface area of porous materials. Temperature and pressure strongly affect adsorption behavior.

Characterization methods include adsorption isotherms, temperature-programmed desorption, infrared spectroscopy, and calorimetry. These techniques help determine surface area, binding energetics, and the identities of adsorbed species.

Applications span catalysis, environmental remediation, gas separation, sensing, and storage. Materials commonly used as adsorbents include activated carbon, zeolites, metal-organic frameworks (MOFs), silica, and various solid surfaces. Examples of adsorbed species are carbon monoxide on platinum surfaces in catalysis, water on silica, and oxygen on gold surfaces.