92168.pdf

170 The solid-liquid interface

in general, more complicated than that of gas adsorption, since

adsorption from solution always involves competition between

solute(s) and solvent or between the components of a liquid mixture

for the adsorption sites. Consider, for example, a binary liquid

mixture in contact with a solid. Zero adsorption refers to uniform

mixture composition right up to the solid surface, even though

(unlike zero gas adsorption) both components are, in fact, present at

the solid surface. If the proportion of one of the components at the

surface is greater than its proportion in bulk, then that component is

positively adsorbed and, consequently, the other component is

negatively adsorbed. Apparent, rather than true, adsorption isotherms

are, therefore, calculated from changes in solution concentration.

Examples of apparent adsorption isotherms for binary liquid mixtures

are given in Figure 6.11. Within the context of certain assumptions,

foj-57

C CD

•2-0

0-E

Q--5 c

+ 1

-1 Z

0 0.2 0.4 0.6 0.8 1.0

Mole fraction of benzene

(M 7 at equilibrium

""ft O>

2-oO £ o

OJ*>

-8 ^ -0.5

ll

0 2ao

-1.0

__ — |

^svs

X.

Xx

^NS.

*^«— •

I

/

/

*/y

0.2 0.4 0.6 0.8

Mole fraction of chloroform

at equilibrium

1.0

Figure 6.11 Composite (surface excess) isotherms for the adsorption of (a) benzene
from solution in methanol on to charcoal^177 and (b) chloroform from solution in carbon
tetrachloride on to charcoal^178 (By courtesy of (a) American Chemical Society, (b)
Journal of the Chemical Society)