92168.pdf

The solid-gas interface 133

already partially covered with a monomolecular layer, lateral

interaction with the adsorbed gas molecules will be involved in

addition to interaction with the solid. In this respect, the heat of

adsorption might be expected to become more exothermic with

increasing surface coverage, as, for example, shown in Figure 5.12c.

The shape of a multilayer physical adsorption isotherm depends on

the tendency of each adsorbed monomolecular layer (particularly the

first layer) to be completed before any adsorption into further layers

takes place. This situation is favoured if the adsorption energy for the

layer being completed is significantly more exothermic than that for

commencing further adsorbed layers. As a rather extreme example,

Figure 5.12c shows this kind of variation of adsorption energy with

surface coverage for the physical adsorption of a gas on a fairly

homogeneous solid surface. The corresponding adsorption isotherm

_ 30

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ll-«= O

I "I-

MOT3 u*

J2 O

9- 10

(a)

\

X•*^^

^x

"V,

T

0 o

a E

"•2 .2

<5 15-

j= 8

o"o

0.5 1.0 1.5

200

100

_T 20

0~o

o ^

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<5 o

0^ 0

(c)

t*^ \

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1.0 2.0 3.0

Figure 5.12 Adsorption energy and surface coverage, (a) Physical adsorption of
nitrogen on rutile at 85 K^71. (b) Chemisorption of hydrogen on tungsten^169 , (c)
Physical adsorption of krypton on graphitised carbon black^166. (See Figure 5.6) (By
courtesy of (a) Science Progress, (b) Discussions of the Faraday Society and (c) The
Canadian Journal of Chemistry)