It is often useful to consider the adsorbent/fluid interface as comprising two regions. The region of the fluid phase (i.e. liquid or gas) forming part of the adsorbent/fluid interface may be called the adsorption space while the portion of the adsorbent included in the interface is called the surface layer of the adsorbent (see Figure 2 p. ).
With respect to porous solids45, the surface associated with pores communicating with the outside space may be called the internal surface. Because the accessibility of pores may depend on the size of the fluid molecules, the extent of the internal surface may depend on the size of the molecules comprising the fluid, and may be different for the various components of a fluid mixture (molecular sieve effect).
When a porous solid consists of discrete particles it is convenient to describe the outer boundary of the particles as external surface.
It is expedient to classify pores according to their sizes:
The terms intermediate or transitional pores, which have been used in the past are not recommended.
In the case of micropores, the whole of their accessible volume may be regarded as adsorption space.
The above limits are to some extent arbitrary. In some circumstances it may prove convenient to choose somewhat different values6.
The area of the surface of the non-porous solid as defined in §1.1.2 is usually greater than that calculated from the macroscopic dimensions of the surface by a factor called the roughness factor.