30 INTERFACIAL PHENOMENA 



Certain examples of adsorption have also been recognized which are desig- 

 nated by the name of chemical adsorption. Chemical reactions are involved 

 in adsorption phenomena of this type. The familiar reaction of I2KI with 

 starch in which the starch stains purple is often considered to be an example 

 of chemical adsorption. 



Actually there exists no clean cut distinction between mechanical, electri- 

 cal, and chemical adsorption. The several phenomena intergrade, and ele- 

 ments of more than one of these mechanisms are usually operating whenever 

 adsorption occurs. Since the surface or interface of even pure water bears 

 an electric charge, at least minor electrical effects are involved in even the 

 simplest adsorption phenomena. 



Adsorption of Water. — Water itself is strongly adsorbed at certain types 

 of interfaces. The adsorption of water is a phenomenon which can be re- 

 garded as more nearly similar to electrical adsorption than to mechanical 

 adsorption. Water molecules, while not charged in the sense that ions are, 

 are strongly "polar" (Chap. X) and behave somewhat as charged bodies. 

 Water-vapor molecules become adsorbed on many different kinds of surfaces. 

 The water-vapor adsorbed upon glass weighing bottles often becomes an ap- 

 preciable source of error in accurate quantitative work. The particles in 

 certain colloidal systems have the property of adsorbing large quantities of 

 water, as shown in the next two chapters. A large portion of the water which 

 moves into various types of substances in the process of imbibition (Chap. IX) 

 becomes adsorbed upon the internal surfaces of the imbibing substance. 



The Biological Significance of Adsorption. — Adsorption phenomena are 

 known to play a manifold role in living organisms, probably being involved 

 in practically all cell activities. Protoplasm and many other constituents of 

 plant cells are essentially colloidal, and adsorption phenomena are of general 

 occurrence in colloidal systems. Within plant cells many interfaces occur, 

 as at the boundaries between the protoplasm and vacuole, protoplasm and cell 

 wall, and nucleus and cytoplasm, at all of which interfacial concentration of 

 solutes undoubtedly occurs. The adsorption of certain compounds at the cyto- 

 plasmic interfaces is generally believed to exert a marked influence on the 

 permeability of the cytoplasm. It is quite possible that interfacial tensions play 

 an important role in the process of cell division. Imbibitional phenomena, 

 of basic importance in the water relations of plant cells, involve the adsorp- 

 tion of water. The action of enzymes as well as of other catalysts is gen^ 

 erally believed to involve adsorption phenomena. Much information regard- 

 ing the structure of cells has been gained by the use of dyes which are differ' 

 entially adsorbed by various constituents of cells. Chromosomes, for example, 

 are so named because they strongly adsorb certain stains. 



