! CHAPTER VIII 



COLLOIDS (Cont'd) 



SUSPENSOIDS AND EMULSOIDS 



According to whether colloids form solutions that are more or less 

 viscid than the suspension medium, they are divided into emulsoids and 

 suspensoids. Examples of the former class are silicates and gelatin, and 

 of the latter, dialyzed iron and arsenious sulphide. The following char- 

 acteristics are used to distinguish between suspensoids and emulsoids: 



1. Measuring the time it takes, at a standard temperature, for a given 

 volume of the fluid to flow out of a standard pipette (10 c.c.) shows the 

 viscosity to be, roughly, inversely proportional to the time of outflow. In 

 the case of suspensoids the viscosity is no different from that of the 

 dispersion medium alone, and does not vary much when the solution is 

 cooled. The viscosity of emulsoids even in very dilute solutions is, on 

 the other hand, considerably greater than that of the dispersion medium 

 itself, and it becomes greatly increased by cooling. 



2. Suspensoids are much more readily coagulated by the addition of 

 electrolytes than emulsoids. This is particularly true when water is 

 the dispersion medium (so-called hydrosols), and when electrolytes hav- 

 ing a polyvalent ion (such as Al or Mg.) are employed. Thus, practically 

 all suspensoids are coagulated in the presence of 1 per cent of alum, 

 which has no influence on emulsoids. We shall return to this phase of 

 our subject later on. 



The division of colloids into emulsoids and suspensoids is more or less 

 arbitrary, since one class may be changed into the other, the determining 

 factor being the water content of the dispersoid. The water content of 

 suspensoids is low (lyophobe), while that of emulsoids is high. By 

 changing the relative amounts of water and solid of which a colloidal 

 solution is composed, the nature of the dispersoid may be changed. If 

 the water is diminished, the dispersoid behaves as a suspensoid and be- 

 comes readily precipitated. The practical importance of this fact is 

 that it explains the salting out of proteins a process extensively used 

 in their separation. Ordinarily these behave as emulsoids, but the addi- 

 tion of salt raises the osmotic pressure of the dispersion medium, and 

 thus attracts water from the dispersoids, with the result that they come 



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