Chemistry · Ch 10 — Surface Chemistry
Adsorption and Absorption
Adsorption and Absorption
A solid's surface behaves quite differently from its bulk interior. Atoms deep inside a crystal or a lump of metal have all their bonding capacity satisfied by neighbouring atoms on every side, but atoms sitting exactly at the surface are missing neighbours on the outward-facing side, leaving them with unsatisfied ('residual') valency or force of attraction. This residual force is what lets a solid surface reach out and hold onto whatever gas, liquid or dissolved species happens to touch it — the process called adsorption. Charcoal adsorbing ammonia gas, silica gel adsorbing water vapour, and charcoal removing coloured impurities from a sugar solution are three everyday illustrations, and in every case the attracted species accumulates specifically AT the surface rather than spreading through the interior — which is exactly what marks adsorption out as a surface phenomenon.
This is sharply different from absorption, a bulk phenomenon in which the absorbed substance distributes itself uniformly all through the volume of the absorbing material (a sponge soaking up water is a familiar example of absorption, not adsorption). The vocabulary used throughout this chapter follows directly from this distinction: the solid (or, less commonly, liquid) material that does the attracting is called the adsorbent; the substance that gets attracted and held is the adsorbate; and the surface of separation between the two phases, where the adsorbate's concentration builds up, is called the interface. When the concentration of a substance actually builds up at the interface, this is called positive adsorption; on the rare occasions where the concentration at the interface is instead LOWER than in the bulk, it is called negative adsorption. The reverse of adsorption — physically removing the adsorbed substance from the surface it is stuck to — is called desorption.
A wide range of gaseous adsorbates is used to illustrate adsorption in the laboratory: He, Ne, , , and , along with dissolved species such as NaCl or KCl solutions, can all be adsorbed by a suitable adsorbent. On the adsorbent side, silica gel and metals such as Ni, Cu, Pt, Ag and Pd, together with certain colloids, are commonly used because their surfaces provide abundant, accessible active sites. …
Worked out. In practice, adsorption and absorption often occur together and are hard to separate experimentally, so M.C. Bain introduced the umbrella term 'sorption' to describe the simultaneous occurrence of both processes at once, without committing to which one dominates. Separately, T. Graham used the term 'occlusion' specifically for the sorption of gases on metal surfaces, such as hydrogen being taken up by finely divided palladium — a usage that survives in the phrase 'occl …