adsorption

When you use activated charcoal to soak up a poison, the toxin does not soak into the charcoal like water into a sponge; instead it sticks to the enormous surface of the charcoal. That sticking of molecules onto a surface, in greater concentration than in the bulk, is adsorption. Note the spelling: adsorption (sticking to a surface) is different from absorption (soaking into a volume).

Molecules accumulate at a surface or interface because doing so lowers the system's free energy, for instance by satisfying unbalanced surface forces. Adsorption can be physical (weak, reversible van der Waals attraction, called physisorption) or chemical (strong covalent-like bonding, called chemisorption). The amount adsorbed at a given temperature, plotted against the concentration or pressure of the adsorbate, gives an adsorption isotherm such as the Langmuir or Freundlich curve.

Adsorption is woven through pharmaceutics. Surfactants adsorb at interfaces to lower tension and stabilise emulsions; drugs can adsorb onto excipients, filters, or container surfaces and be lost from solution; activated charcoal and clays adsorb toxins or drugs in the gut, which is exploited in poisoning treatment but can also cause unwanted drug interactions. A useful caveat: adsorption of a drug onto packaging or a syringe is a real loss mechanism that must be checked during formulation, especially for low-dose proteins.