IUPAC Technical Report — Physisorption of Gases (definitions of adsorption, absorption, sorption)

Source: Thommes, Kaneko, Neimark, Olivier, Rodriguez-Reinoso, Rouquerol, Sing — Pure and Applied Chemistry 2015, 87(9-10):1051-1069, DOI 10.1515/pac-2014-1117. Credibility: High (IUPAC primary nomenclature). Confidence: high.

Verbatim definitions (the authority)

• Adsorption: “the enrichment of molecules, atoms or ions in the vicinity of an interface… adsorption takes place in the vicinity of the solid surface and outside the solid structure.”
• Absorption: “When the molecules of the adsorptive penetrate the surface layer and enter the structure of the bulk solid, the term absorption is used.”
• Sorption (umbrella): “It is sometimes difficult or impossible to distinguish between adsorption and absorption: it is then convenient to use the wider term sorption which embraces both phenomena” → sorbent, sorbate, sorptive.

Terminology

• Adsorbate = the adsorbed-state material; adsorptive = the same component still in the fluid phase; adsorbent = the solid. Quantity measured = surface excess amount (nσ).
• Adsorption modeled as essentially two-dimensional on an imaginary (Gibbs dividing) surface.

Mechanism split & energetics

• Physisorption — same forces as real-gas imperfection / vapour condensation (dispersion + short-range repulsion, plus polarisation); weak, ~20-40 kJ/mol, reversible, multilayer possible.
• Chemisorption — chemical-bond strength interactions; ~80-240 kJ/mol, often activated and monolayer.
• Prefer isosteric enthalpy of adsorption (Δ_adsʰ) over “heat of adsorption.”
• Surface area derived from monolayer capacity (BET). Pores: micropores <2 nm, mesopores 2-50 nm, macropores >50 nm.

Relevance

The citable primary backbone for a foundational adsorption-vs-absorption concept article — the surface-vs-bulk distinction and the “sorption” umbrella, in IUPAC’s own words.