44 GENERAL BIOCHEMISTRY 



forces holding the giant molecules together are ordinary valence 

 forces. 



The other colloids consist of aggregates of smaller molecules, not 

 necessarily all alike, held together by attractive forces often called 

 the secondary forces. These interactions are strong enough to yield 

 rather large particles, although there occasionally is equilibrium 

 between particles of different sizes. Sometimes the colloidal system is 

 unstable and the individual particles coalesce, forming a coarse disper- 

 sion and perhaps precipitating altogether. In other cases they may 

 separate into the component molecules, yielding an ordinary solution. 



Clearly either of the foregoing changes coidd be reversed to form 

 dispersions of the colloidal type. Thus grinding with machines called 

 colloid mills can break large particles into those of colloidal size. 

 Moreover, certain added materials induce a spontaneous breakdown 

 called peptization with formation of a colloidal system. Conversely, a 

 reduction in temperature, prolonged standing in solution, a suitable 

 chemical reaction or agent, or the addition of another solvent may 

 cause molecides to associate. Thus there are two general types of 

 methods, those causing dispersion and those causing aggregation, and 

 there are many variations for either method. 



Stabilization and Precipitation 



These two types of treatment of colloidal systems are comple- 

 mentary. When stability is desired, all factors provoking precipitation 

 are avoided, and conversely. In general, stabilization is aided by (1) 

 an optimum choice of temperature, (2) addition of stabilizing or pep- 

 tizing agents, (3) keeping the system dilute, and (4) using a good 

 solvent. In the first instance, many colloidal solutions are destabilized 

 by freezing and thawing or by heating, especially boiling. In the 

 second, peptizing agents prevent further aggregation by coating the 

 surfaces of the particles and making them repel each other or at least 

 preventing adhesion when they do collide. In the third, in a dilute 

 system collisions are less frequent, and when an equilibrium exists, 

 dilution favors the smaller aggregates below the range of precipitation. 

 Finally, solvents corresponding somewhat in structure and polarity 

 to the dispersed phase help reduce the forces of aggregation. 



Precipitation is promoted by reversing the above procedures, as by 

 freezing or boiling, omitting peptizers, concentrating, and adding poor 

 solvents. In addition, precipitation results when suitable chemical 

 agents are added, heavy metals, acids or bases in some cases, or other 

 substances changing the particle surface to allow adhesion on contact. 



