Adsorption (Wikipedia) — surface vs bulk, physisorption/chemisorption, isotherms

Source: https://en.wikipedia.org/wiki/Adsorption (cites IUPAC, Langmuir 1918, Brunauer-Emmett-Teller 1938). Credibility: Medium-high (well-sourced tertiary). Confidence: high (corroborated by IUPAC).

Key points

• The distinction stated plainly: adsorption = “adhesion of atoms, ions, or molecules… to a surface,” forming a film on the exterior; “adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface.” Absorption = a substance “dissolved by or permeates a liquid or solid” (bulk). Sorption encompasses both; desorption is the reverse.
• Physisorption vs chemisorption: physisorption = weak van der Waals, reversible, low activation energy; chemisorption = covalent/chemical bonding, stronger, often irreversible, significant activation energy.
• Thermodynamics: adsorption is exothermic (ΔH_ads < 0); equilibrium constant vs temperature follows van ‘t Hoff. Surface atoms are under-coordinated, so they attract adsorbates — more pore area = more capacity.
• Isotherms: Langmuir (1918) θ = KP/(1+KP) assumes equivalent sites, one molecule per site, monolayer; BET (1938) extends to multilayer physisorption. A finite monolayer/site saturation is an adsorption fingerprint.

Relevance

Accessible scaffolding for the adsorption-vs-absorption concept article — the surface-vs-bulk test plus the isotherm/thermodynamic vocabulary.