Page 22 - Adsorptionbypowders & poroussolids muyace

8 ADSORPTION BY POWDERS AND POROUS SOLIDS

Table 13. Definitions: porous solids
2
Term Definition
Porous solid Solid with cavities or channels which are deeper than they are wide
open PO= Cavity or channel with access to the surface
Interconnected pore Pore which communicates with da pores
Blind pore' Pore with a single connection to the surface
(Deadend pore)
Closed pore Cavity not connected to the surface
Void Space between particles
Miclopore Pore of internal width less than 2 nm
Mesopore Pore of internal width between 2 and 50 nm
Pore of internal width greater than 50 nm
~0~
Pore size Porc width (diameter of cylindrical porc or distance between opposite walls of
slit)
Pon volume Volume of pores determined by stated method
Porosity Ratio of total pore volume to apparent volume of particle or powder
Total porosity Ratio of volume of voids and pores (open and closed) to volume occupied by
solid
Open porosity Ratio of volume of voids and open pores to volume occupied by solid
Surface area Extent of total surface area as determined by given method under stated
conditions
External surface area Area of surface outside pores
Intemal surface area Area of pore walls
Tme density Density of solid, excluding pores and voids
Apparent density Density of material including closed and inaccessible pores, as determined by
stated method
In the sense of the French word 'borgru'.

consolidated agglomerates as a result of sintering or ageing. The breakdown, or
partial breakdown, of the consolidated material can be achieved by grinding. The
process of agglomeration involves the bridging or cementation of particles and
should not be confused with Osfwald ripening, which involves the growth of larger
particles at the expense of smaller ones. It is evident that an agglomerate may be
regarded as a 'secondary' particle, which always contains within it some internal
surface. In many cases the internal surface area is much larger than the external
surface area and the agglomerate then possesses a well-defined pore structure.
The classification of pores according to size has been under discussion for many
. years, but in the past the terms micropore and macropore have been applied in dif-
ferent ways by physical chernists and some other scientists. In an attempt to clarify
this situation, the limits of size of the different categories of pores included in Table
1.3 have been proposed by the International Union of Pure and Applied Chemistry
(Everett, 1972; Sing et al., 1985). As indicated, the pore size is generally specified as
the pore width, i.e. the available distance between the two opposite walls. Obviously,
pore size has a precise meaning when the geometrical shape is well defined.
Nevertheless, for most purposes the limiting size is that of the smallest dimension
and this is generally taken to represent the effective pore size. Micropores and
mesopores are especially important in the context of adsorption.
The hypothetical types of pores shown in Figure 1.1 relate to the definitions in