monolayer has been obtained (roughly speaking), no further accumulation
takes place. Solutes showing this kind of behavior are calledsurfactants.
The accumulation of a compound at a surface or an interface is called
adsorption. Adsorption is a very common phenomenon and can occur at all
solids or liquids in contact with a gaseous or a liquid phase. The compound
adsorbing is called theadsorbate, the material onto which it adsorbs is the
adsorbent. The following notation will be used, either as such or in
subscripts: S¼solid, A¼air (or a gaseous phase), W¼water (or an
aqueous solution), and O¼oil.
The adsorbent can be a solid or a liquid. The adsorbate is dissolved in
a liquid or is (present in) a gas. Adsorption on an A–S interface concerns,
for instance, adsorption of water from moist air on a solid; see Section 8.2.
Also other volatiles can adsorb from air, e.g., flavor compounds. At the
liquid–solid interface, the solid generally is in contact with solvent as well as
adsorbate, i.e., solute molecules. For a liquid–liquid interface (generally O–
W) the adsorbate may be soluble in both liquids. The molecules adsorbed
FIGURE10.4 Effect of the concentrationcof various solutes on the surface tension
gof water.