52 



THE PROPERTIES OF SOLS AND GELS 



sol until the charges on the micelles are neutralized. Although now at its 

 isoelectric point, unlike hydrophobic systems, the sol does not flocculate. If, 

 however, alcohol now be added to the system, immediate flocculation occurs, 

 because of a dehydration of the micelles, resulting in an elimination of the 

 only remaining stability factor in the system. 



Briefly then, in order to flocculate a hydrophilic sol, its micelles must be 

 both dehydrated and electrically discharged, except in the occasional systems 

 in which micelles are uncharged, in which dehydration alone will suffice. 

 Dehydration alone of a hydrophilic sol with charged micelles results in its 

 conversion into a lyophobic sol. 



The inter-relationships among the factors involved in the stability and 

 flocculation of hydrophilic sols are shown in Fig. 7 which is self-explanatory. 



HYDROPHILIC SOL, (cHARGED) 



HYDROPHILIC SOL. (uNCHARGEd) 



DISCHARGED 



BY electrolyte:^ 



DEHYDRATION 

 BY ALCOHOL 



REHYDRATION 

 BY WATER 



DEHYDRATION 

 BY ALCOHOL 



DISCHARGED 

 BY ELECTROLYTE 



FLOCCULATING MICELLE 

 LYOPHOBIC SOL. 



Fig. 7. Diagrammatic representation of the flocculation of a micelle of a 



hydrophilic sol. 



The addition of relatively large quantities of certain electrolytes to hydro- 

 philic sols will result in their flocculation without any previous removal of 

 the water of hydration of the micelles by means of alcohol or any other 

 dehydrating agent. Only very soluble salts are effective in this way. It is 

 evident that this phenomenon, usually called "salting out," involves a more 

 complicated reaction than that taking place in simple flocculation and must 

 be distinguished from the latter phenomenon. Three salts which are especially 

 suitable for salting out hydrophilic sols are (NH4)oS04, MgS04, and 

 NaoS04. In these three salts the ions, especially the anions, acquire relatively 

 large quantities of water of hydration. The result of the addition of a very 



