588 APPENDIX 



Me]\ibrane Potential. — It will be observed that the outside layer 

 has the same total electric charge as the inside layer although op- 

 posite in kind. There is then a difference in potential between 

 those two layers. The stability of colloids runs parallel with the 

 drop in the potential in the double layer. This drop in potential 

 is due to changes in the outside layer. It is now known that this 

 layer is more complex than has been indicated here and that one 

 portion only of the total potential difference is important in colloid 

 stability. 



Effect of Cations on Colloids. — It has long been known that free 

 hydrogen ions in the dispersion medium profoundly affect the 

 stabilit.y of colloids. When their concentration is sufficient to bring 

 al)out comi)lete neutralization of charge, tlie colloid becomes very 

 unstable and is readily precipitated. This is called the isoelectric 

 point. This is in accordance with the observation tliat colloidal 

 particles liaving like charges repel each other. 



Critical Potential. — The point in the drop in potential where 

 the colloidal particles barely repel each other is called the critical 

 potential, a fall below which indicates the loss of the property of 

 repulsion. This discussion applies more nearly to suspensoids than 

 to emulsoids. In the latter group of colloids the dispersion medium 

 enters into intimate relationship with the dispersed material. 



Factors Governing' Stability of Emulsoids.— It is thought that 

 there are at least two factors involved in maintaining the dispersed 

 state of emulsoids. One factor is the charge and potential and the 

 other is the so-called film of water around each particle. Removal 

 or reduction in amount of this water fihn favors precipitation. If 

 an adequate water film is maintained, a drop in potential may not 

 lead to precipitation. This will be more fully discussed under 

 Cohesion and Precipitation. 



Migration of Charged Particles. — Cataphoresis. — As is evident 

 from the preceding discussion, one of the important properties of 

 colloids is that the dispersed particles are electrically charged with 

 respect to their surroundings. When an electric current is passed 

 through a colloid, any negatively charged particles will travel to- 

 ward the positive pole, or anode, while positively charged particles 

 will travel toward the cathode or negative pole. This migration 

 phenomenon is called cataphoresis. If the dispersed particles have 



