48 THE PROPERTIES OF SOLS AND GELS 



originate by ionization of some of the surface molecules of the micelles. The 

 electrical charges on the micelles of protein sols arise in this way. In other 

 S3'stems the charges may originate from traces of electrolytes which are present 

 as impurities. This is probably true of agar sols. Such charges may be re- 

 garded as similar in their origin to those acquired by adsorption on the 

 micelles of hydrophobic sols, in that the charge is due to some compound 

 associated with the substance of which the micelle is composed, and not to 

 that substance itself. Most of the better known hydrophilic sols are negatively 

 charged. 



Flocculation. — Since the dispersed particles of any sol are in rapid motion 

 it would seem that repeated collisions would result in a progressive agglomera- 

 tion of the particles into larger and larger masses which eventually would 

 settle out of the system. Sols, however, are relatively stable systems, and it is 

 important to understand the mechanism by which the stability of such col- 

 loidal systems is maintained. 



The stability of hydrophobic sols is maintained almost entirely by the 

 charge which each micelle carries. Although by Brownian movement the 

 dispersed particles are repeatedly brought close together actual collision 

 seldom occurs because the shells of ions around the micelles are mutually 

 repellent. 



Reduction of the charge on the micelles of any hydrophobic sol to the 

 point where there is no difference of electrical potential across the double 

 layer results in agglomeration of the individual particles into flakes of a size 

 which rapidly settle out of the surrounding liquid. This phenomenon is 

 called flocculation, coagulation, or precipitation. The point at which there is 

 no difference of electrical potential across the double layer is known as the 

 isoelectric point of the sol. At the isoelectric point the micelles of a sol are, 

 relative to the surrounding medium, completely uncharged. As, by Brownian 

 movement, two such micelles are brought into contiguity they no longer repel 

 each other with sufficient intensity to prevent their agglomeration. By the 

 addition of other micelles such particles rapidly increase in size, soon result- 

 ing in flocculation of the sol. Merely reducing the electric charge to a value 

 approaching that of the isoelectric point is sufficient to induce instability and 

 slow flocculation in many sols. 



Flocculation is most commonly initiated by the introduction of electrolytes 

 into the system. Very small quantities of an electrolyte are often sufficient 

 to cause the flocculation of a relatively large volume of sol. The important 

 principles regarding the flocculation of hydrophobic sols by electrolytes can 

 be most easily elucidated by a discussion of some of the data in Table 1 1 . 



The flocculating effect of an electrolyte is due primarily to the added ion 



