30 BACTERIOLOGICAL CHEMISTRY 



same effect is also shown when electrolytes are added 

 to colloids, the coagulation only taking place within a 

 zone of concentration, the critical value being overshot 

 by addition of an excess of the electrolyte with conse- 

 quent adsorption of the oppositely charged ion. 



Lyophilic colloids are much less sensitive to the action 

 of electrolytes than are lyophobic sols. This is due to 

 the fact that the particles of a lyophilic colloid have a 

 strong attraction for the continuous phase, usually water 

 in systems of bacteriological interest, in which they 

 are suspended. As a result of this attraction the particles 

 of the colloid become covered by a layer of water which 

 acts as a buffer between them and hinders their collision 

 and consequent aggregation even when the surface charge 

 is well below the critical value for a lyophobe system. 

 In other words, lyojDhilic colloids have a lower critical 

 surface potential than the lyophobes. It varies from ±2 

 to ±8 millivolts, according to the particular colloid 

 concerned. Accordingly, considerably more electrolyte 

 (or oppositely charged colloid) is needed to alter 

 the charge on a lyophilic sol sufficiently to cause 

 coagulation. 



Polar Groups. — The lyophilic character of proteins and 

 polysaccharides and similar substances is due to the 

 presence of polar groups. As is well laiown, the com- 

 bination of atoms to form molecules is brought about 

 through the agency of the valency electrons. In the 

 case of such ionisable compounds as sodium chloride the 

 bond is effected by the complete transfer of an electron 

 from the sodium atom, which has one more electron 

 than its stable octet, to the chlorine atom, which has 

 one electron less than its stable octet, whereby both 

 atoms attain the stable octet structure. But in the 

 process the sodium atom acquires a positive charge by 

 the loss of the negative electron, whilst the chlorine 

 atom becomes negatively charged by the gain of the 



