92168.pdf

126 The solid-gas interface

Desorption

Adsorption

Figure 5.8 A hysteresis loop in physical adsorption

these circumstances, be lower than the pressure required for capillary

condensation.

Another theory of adsorption hysteresis considers that there are

two types of pores present, each having a size distribution. The first

type are V-shaped, and these fill and empty reversibly. The second

type have a narrow neck and a relatively wide interior. These 'ink-

bottle' pores are supposed to fill completely when a plpQ value

corresponding to the relatively wide pore interior is reached, but

once filled they retain their contents until p/pQ is reduced to a value

corresponding to the relatively small width of the pore neck.

In a further theory, the pores are considered to be open-ended

cylinders. Condensation will commence on the pore walls, for which

the principal radii of curvature are the pore radius and infinity, and

continue until the pore is filled with condensed liquid. Evaporation

must take place from the concave liquid surfaces at the ends of the

pore, for which (assuming zero contact angle) the principal radii of

curvature are both equal to the pore radius.

Mercury intrusion porosimetry^70

Pore size distributions are often determined by the technique of
mercury intrusion porosimetry. The volume of mercury (contact
angle c. 140° with most solids) which can be forced into the pores of
the solid is measured as a function of pressure. The pore size
distribution is calculated in accordance with the equation for the
pressure difference across a curved liquid interface,