34 



COLLOIDAL SYSTEMS 



attributes of colloidal systems. In general, if the dispersed particles fall within 

 the range of o.ooi — o.i /x in diameter the system is considered a colloidal 

 system, if larger than this a suspension or an emulsion, and if smaller a solu- 

 tion. The individual molecules of some substances (certain dyes, some pro- 

 teins) are so large as to bring them within the colloidal range of dimensions. 

 Hence molecular dispersions of such substances are simultaneously both solu- 

 tions and colloidal systems. The limits generally accepted for the range of 

 sizes of colloidal particles have been somewhat arbitrarily set and actually 

 there is no sharp boundary bet^veen colloidal systems and suspensions on the 

 one hand, nor between colloidal systems and solutions on the other. There 

 is a perfect gradation in properties from one type of system to the next. The 

 properties of suspensions or emulsions in which the suspended particles are of 

 small dimensions approach those of colloidal systems, while the smaller the 

 dispersed particles in a colloidal system the more closely it approaches a solu- 

 tion in its properties. The important points discussed in this section are sum- 

 marized in Table 8. 



TABLE 8 CLASSIFICATION OF DISPERSE SYSTEMS ACCORDING TO PARTICLE SIZE AND 



VISIBILITY 



Types of Colloidal Systems. — Colloidal systems are frequently classi- 

 fied on the basis of the original physical state of the two components which 

 have been combined in forming the system (Table 9). Gas-in-gas systems 

 do not exist, since gases do not form molecular aggregates. Of the eight 

 types listed, the solid-in-liquid systems and liquid-in-liquid systems are of by 

 far the greatest importance from the standpoint of living organisms. 



The Nomenclature of Colloidal Systems. — The word colloid is derived 

 from the Greek roots KOLLA (glue) and EIDOS (appearance). Thomas 

 Graham, a prominent early investigator of colloidal phenomena, who did his 

 most important work just after the middle of the nineteenth century, used this 



