50 THE PROPERTIES OF SOLS AND GELS 



micelles of a hydrophobic sol is a differential effect between the anions and 

 cations but the influence of the ion of opposite charge predominates. 



The mechanism of the flocculation of a hydrophobic sol is too complex 

 a process to be considered in more than a general way in an introductory 

 discussion. In general flocculation results from destruction of the double 

 layers around the micelles. There is a close analog}' between the flocculation 

 of a hydrophobic sol by an electroh'te and a precipitation reaction between one 

 electrolyte and another. The micelles of hydrophobic sols may be regarded 

 as giant ions bearing numerous charges. 



When CaClo is added to a solution of Na2S04 slightly dissociated CaS04 

 is precipitated, leaving paired Na+ and Cl~ ions in the solution. An an- 

 alogous phenomenon results when CaCl2 is added to a negatively charged 

 AS2S3 sol in which the outer shells of the double layers are composed of 

 hydrogen ions derived from the dissociation of adsorbed HoS. The resulting 

 flocculant consists of uncharged particles of AS2S3 + Ca, while the H+ and 

 Cl~ ions left behind in the solution pair off in the usual manner, forming 

 hydrochloric acid. In other words Ca++ ions have replaced the H+ ions 

 in the outer shells of the micelles. The result, however, is an unstable 

 particle, since Ca++ ions do not remain in the dissociated state when as- 

 sociated with AS2S3 micelles which owe their negative charge to the dissocia- 

 tion of adsorbed H2S molecules. They combine with such micelles, thus 

 dissipating their electrical charges. Flocculation of the uncharged micelles 

 then ensues. 



Flocculation may also be initiated by introducing into a hydrophobic sol 

 another hj^drophobic sol with micelles bearing a charge of opposite sign. If 

 Fe(OH)3 sol be slowly added to an AS2S3 sol a point will be reached at 

 which complete flocculation will occur. The particles of the two sols will 

 settle out as an intimate mixture. The same phenomenon occurs when any 

 negatively charged hydrophobic sol is added to any positively charged hydro- 

 phobic sol in suflicient quantity, or vice versa. This process is called mutual 

 flocculatioji. When this phenomenon occurs the ions of the outer zone of 

 one kind of particle pair off with the oppositely charged ions of the outer shell 

 of the other kind. 



The addition of a small amount of a hydrophilic sol, such as a gelatin or 

 gum arabic sol, to a hydrophobic sol makes flocculation of the latter by elec- 

 trolytes or micelles of opposite charge difficult or impossible. This effect of 

 a hydrophilic on a hydrophobic sol is termed protective action. Protective 

 action is apparently due to the adsorption of the micelles of the hydrophilic 

 sol around the micelles of the hydrophobic sol. The properties of the sol, 

 therefore, become essentially those of the hydrophilic system. As will be seen 



