Page 24 - Adsorption by Powders and Porous Solids

CHAPTER 1. INTRODUCTION

1.1. Cross-section of a hypothetical porous grain showing various types of pores: closed (C),
blind (B), through 0, interconnected (I), together with some roughness (R) (Rouquerol, 1990).

Table 1.3. In addition to closed pores and open pores, we may distinguish between
blind pores (or dead-end pores) and interconnected pores. Pores which are open at
both sides of a membrane or porous plug are termed through pores.
Porosity is usually &fmed as the ratio of the volume of pores and voids to the
volume occupied by the solid. However, it should be kept in mind that the recorded
value of porosity is not always a simple characteristic property of the material, since
it is likely to depend also on the methods used to assess both the pore volume and the
volume of the solid. The pore volume is usually regarded as the volume of open
pores, but it may include the volume of closed pores. Moreover, the recorded value
may depend on the nature of the probe molecule or the experimental conditions.
It is not always easy to distinguish between roughness and porosity or between
pores and voids. In principle, a convenient and simple convention is to refer to a solid
as porous only if the surface irregularities are deeper than they are wide. Furthermore,
the area of a rough surface is regarded as an external surface area, whereas the area of
the pore walls is the internal surface area. We prefer to regard the porosity as an intrin-
sic property of the material and to designate void as the space between particles, which
is dependent on the conditions of packing (and the particle coordination number).
It is evident that the description of many real porous materials is complicated by a
wide distribution of pore size and shape and the complexity of the pore network. To
facilitate the application of certain theoretical principles the shape is often assumed
to be cylindrical, but this is rarely an accurate portrayal of the real system. With some
materials, it is more realistic to picture the pores as slits or interstices between
spheroidal particles. Computer simulation and the application of percolation theory
have made it possible to study the effects of connectivity and tortuosity.
Pore structures can be created in a number of different ways. intracrystalline pores
are an inherent part of certain crystalline structures, e.g. of zeolites and certain clays.
These pores are generally of molecular dimensions and are arranged as highly regular
networks. A second type of porous material is composed of an assemblage of small
particles (as mentioned earlier). The pore structure of the consolidated system (e.g. a
xerogel) is mainly dependent on the size and packing density of the primary particles:
the process is therefore constitutive. A third route is subtractive since inherent
Parts of the original structure are removed to create the pores, e.g. the thermal