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The solid-liquid interface 169

dirt redeposition. Optical brighteners are commonly incorporated

into detergents used for washing textile fabrics. These are fluorescent

dyes which absorb ultraviolet light and emit blue light which masks

any yellow tint which may develop in white fabrics.

Adsorption from solution

86

To conclude this chapter, some general comments concerning the

adsorption of material from solutions on to solid surfaces are

appropriate. Adsorption from solution is important in many practical

situations, such as those in which modification of the solid surface is

of primary concern (e.g. the use of lyophilic material to stabilise

dispersions; see page 235) and those which involve the removal of

unwanted material from the solution (e.g. the clarification of sugar

solutions with activated charcoal). The adsorption of ions from

electrolyte solutions and a special case of ion adsorption, that of ion

exchange, are discussed in Chapter 7. Adsorption processes are, of

course, most important in chromatography; however, an account of

chromatography is not included in this book (a) because other

processes, such as partition and/or molecular sieving, may also be

involved to a greater or lesser extent, depending on the type of

chromatographic separation being considered and (b) because

chromatography is far too extensive a subject to permit adequate

treatment in a relatively small space.

Solution adsorption isotherms

Experimentally, the investigation of adsorption from solution is much
simpler than that of gas adsorption. A known mass of adsorbent solid
is shaken with a known volume of solution at a given temperature
until there is no further change in the concentration of the
supernatant solution. This concentration can be determined by a
variety of methods involving chemical or radiochemical analysis,
colorimetry, refractive index, etc. The experimental data are usually
expressed in terms of an apparent adsorption isotherm in which the
amount of solute adsorbed at a given temperature per unit mass of
adsorbent - as calculated from the decrease (or increase) of solution
concentration - is plotted against the equilibrium concentration.
The theoretical treatment of adsorption from solution, however, is,