92168.pdf

p = 2y cos 91 r

The solid-gas interface 127

(5.4)

If all the pores are equally accessible, only those for which r is greater

than y cos 0/p will be filled, i.e. each pressure increment causes a

group of smaller pores to be filled and a cumulative pore volume as a

function of pore size can be determined, as illustrated in Figure 5.9.

When the mercury pressure is reduced, hysteresis is usually

observed. This will reflect some of the mercury being permanently

trapped in 'ink-bottle' pores and, as such, the 'ink-bottle' pore

volume is given by the residual mercury entrapped when the mercury

pressure is reduced to atmospheric pressure. Hysteresis, however,

can also result from structural damage sustained due to the very high

mercury pressures involved.

A related technique for measuring the contact angle of a liquid

with a porous or finely divided solid is described on page 157.

14000

Pore diameter/nm

72 36

200 400

Pressure/atm

600

Figure 5.9 Mercury pressurisation (full line) and depressurisation (broken line) for a
sample of activated carbon