Page 136 - Introduction to Colloid and Surface Chemistry

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 plp Q value
corresponding to the relatively wide pore interior is reached, but
once filled they retain their contents until p/p Q 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.

70
Mercury intrusion porosimetry
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,