Adsorption is the enrichment (positive adsorption, or briefly, adsorption) or depletion (negative adsorption) of one or more components in an interfacial layer. In certain cases a decision as to whether the actual distribution of a component between the interfacial layer and the bulk phases should be looked upon as enrichment or depletion may depend on the choice of the reference system (see §1.1.8).
The material in the adsorbed state is called the adsorbate, while that present in one or other (or both) of the bulk phases and capable of being adsorbed may be distinguished as the adsorptive. In some cases of chemisorption (see §1.1.6) adsorptive and adsorbate may be chemically different species (e.g. in dissociative adsorption).
When adsorption occurs (or may occur) at the interface between a fluid phase and a solid, the solid is usually called the adsorbent; for gas/liquid interfaces it may be in some, but not in all, cases useful to call the liquid phase the adsorbent. For liquid/liquid interfaces an arbitrary unsymmetrical nomenclature may not be appropriate.
Adsorption complex is a molecular term used to denote the entity constituted by the adsorbate and that part of the adsorbent to which it is bound. The adsorbate may or may not be in thermodynamic equilibrium with the adsorptive, though normally such an equilibrium may be reached eventually in static systems, except in some cases of activated chemisorption (see §1.1.6(f)). Quantitative definitions of adsorption are given in §1.1.8.
When two phases are put into contact, the composition of one or both bulk phases may be changed by the partition of one or more components between these phases. The transfer of a component from one phase to the other is other called absorption. In absorption the structure of the absorbent and/or the chemical nature of the adsorptive may be modified.
It is sometimes difficult or impossible to discriminate experimentally between adsorption and absorption: in such cases it is convenient to use the non-committal term: sorption (together with its derived terms sorbent, sorbate, sorptive). This term is also used as a general term to cover both adsorption and absorption when both are known to occur simultaneously.
When the adsorbate is substantially absent from the bulk of the phases forming the interface, it is said to form a spread layer (often spread monolayer when the layer is known to be only one molecule in thickness).
The term adsorption may also be used to denote the process in which molecules accumulate in the interfacial layer. When used in this sense, its counterpart, desorption, denotes the converse process, i.e. the decrease in the amount of adsorbed substance. Adsorption is also used to denote the result of the process of adsorption, i.e. the formation of adsorbate on an adsorbent. Adsorption and desorption may also be used adjectivally to indicate the direction from which experimentally derived adsorption values have been approached, e.g. adsorption curve (or point), desorption curve (or point).
Adsorption hysteresis is said to occur when the adsorption and desorption curves deviate from one another.
Adsorption from liquid mixtures. Adsorption from liquid mixtures is said to have occurred only when there is a difference between the relative composition of the liquid in the interfacial layer and that in the adjoining bulk phase(s) and observable phenomena result from this difference. A similar shift in relative composition occurs also generally (though not necessarily) in the case of adsorption from gaseous mixtures, in addition to the increase of total concentration in the interfacial layer which is the general characteristic of gas adsorption. For liquids, accumulation (positive adsorption) of one or several components is generally accompanied by depletion of the other(s) in the interfacial layer; such depletion, i.e. when the equilibrium concentration of a component in the interfacial layer is smaller than the adjoining bulk liquid, is termed negative adsorption and should not be designated as desorption. Negative adsorption may occur also in the case of adsorption from highly compressed gas mixtures.
Expulsion of a previously adsorbed component from the interfacial layer may be effected by subsequent stronger adsorption of another component; such a process is called desorption by displacement.
Adsorption isotherm is the relation between the quantity adsorbed (suitably defined, see §1.1.8) and the composition of the bulk phase (or the partial pressure in the gas phase) under equilibrium conditions at constant temperature.
Equilibrium between a bulk fluid and an interfacial layer may be established with respect to neutral species or to ionic species. If the adsorption of one or several ionic species is accompanied by the simultaneous desorption (displacement) of an equivalent amount of one or more other ionic species this process is called ion exchange.