PROPERTIES OF MATTER 45 



Adsorption 



Many systems show a relatively high concentration ol the solutes 

 present at the various surfaces between phases. This selection phe- 

 nomenon is called adsorption and in its general form may be illus- 

 trated by the increased concentration of certain surface-active mate- 

 rials in the film at the solution-air interface. Frequently adsorption, 

 sometimes called sorption, involves neither particles nor surfaces of 

 colloidal dimensions, although the adsorbed layers are usually thin, 

 from one to a few molecules in thickness. However, adsorption is 

 commonly discussed in connection with colloidal systems, partly 

 because colloid chemists did much of the early work on adsorption 

 and partly because the colloidal systems present relatively great surface 

 areas on which adsorption can occur. 



With a given weight of material total area increases with increasing 

 subdivision. Thus a colloid presents a vastly greater area for concen- 

 tration of material than does an equal weight of a coarse dispersion. 

 Natiually dispersion of the adsorbing material into the individual 

 molecules increases the total area still more. However, it seems that 

 small molecules often possess secondary force fields somewhat different 

 from those of aggregates of these same molectdes, and adsor})tion 

 diminishes if the particles get too small. 



Classical valence bonds do not seem to participate in adsorption. 

 Instead secondary forces like those between the molecules in a li(]uid 

 or the interactions between the dipolar molecules of water lead to 

 adsorption. An accurate general theory of adsorption is still not 

 available, but a number of empirical expressions have been proposed. 

 Usually called adsorption isotherms, the equations can be applied to 

 certain aspects of adsorption in certain cases only. 



The extent of adsorption is known to depend upon the natures of 

 the solvent, adsorption surface, and the material adsorbed. It usually 

 decreases with increasing temperature and may be affected by violent 

 stirring. Additional solutes and changes in pH, dielectric constant, 

 and concentration modify adsorption, often drastically. 



The phenomenon is used to concentrate all manner of materials, 

 gases, liquids, or dissolved solids. These concentrations are used in 

 isolation, purification, separation, deodorizing, detoxifying, decolor- 

 izing, and many other laboratory and practical processes. Adsorption 

 may be so effective as to concentrate the minute traces of nutritional 

 materials in distilled water enough to permit the growth of micro- 

 organisms at the surface of the suspended adsorbing material. 



