Adsorbat

Adsorbat is the substance that is adsorbed onto a surface during the process of adsorption. It can be a molecule, an ion, or an atom. The term contrasts with the adsorbent, the material that provides the surface or sites where adsorption occurs.

Adsorption takes place at interfaces such as solid–gas, solid–liquid, or liquid–liquid. Common adsorbents include activated carbon, silica, alumina, zeolites, and clays. The adsorbat may be held on the surface by physical forces (physisorption) or by chemical bonds (chemisorption). Physisorption is typically reversible and highly dependent on temperature and pressure, while chemisorption involves stronger interactions and can be slower and more selective, sometimes leading to irreversible changes.

Quantitatively, the amount of adsorbat is described by adsorption isotherms, which relate the amount adsorbed per unit mass (q) to the pressure or concentration at a given temperature. The Langmuir isotherm models monolayer adsorption on a homogeneous surface; the Freundlich isotherm describes heterogeneous surfaces; the Brunauer–Emmett–Toulan (BET) model extends to multilayer adsorption. Key parameters include qmax, the maximum adsorption capacity for a monolayer, and various equilibrium constants.

Adsorption kinetics describe how rapidly adsorption occurs, using models such as pseudo-first-order and pseudo-second-order kinetics, along with diffusion-controlled processes within particles. Experimental methods to study adsorbat behavior include volumetric gas adsorption, gravimetric techniques, and spectroscopic analyses.

Applications of adsorption and adsorbat-containing systems span air and water purification, catalysis and reactor design, chromatographic separations, gas storage, and sensor technologies.