FLOCCULATION 51 



shortly hydrophilic sols are much less easily flocculated by electrol3'tes than 

 hydrophobic sols and this is undoubtedly the basis for protective action. 



We turn now to the question of the mechanism of the flocculation of 

 hydrophilic sols. The micelles of such sols may or may not carry an electrical 

 charge, but whether charged or not such colloidal systems are stable. Hydro- 

 phobic sols, as already shown, are stable only when the micelles bear an 

 electrical charge. One of the most important differences between hydrophobic 

 and hydrophilic sols is the possession by the latter of a second stability factor, 

 which in itself is effective in keeping such sols stable for long periods of time. 

 In our previous discussion we have seen that uncharged micelles of hydro- 

 phobic sols soon agglomerate and settle out of the dispersion medium. Why 

 do not the uncharged micelles of a hydrophilic sol behave in the same way? 

 This is probably due to the effect of hydration upon the properties of the 

 dispersed particles. The micelles of all hydrophilic sols, it will be remem- 

 bered, are highly hydrated. The first layer of molecules of water of hydra- 

 tion is probably so firmly bound to the particle as to virtually constitute an 

 integral part of the micelle itself. Surrounding this are other layers of water 

 molecules more or less completely oriented depending on their distance from 

 the surface of the particles. Each such particle is "cushioned" against impacts 

 with other particles by its enveloping shell of oriented water molecules. 

 Agglomeration of the micelles is thus prevented, and hence even uncharged 

 hydrophilic sols are stable. 



The possession of two stability factors by h^'drophilic sols complicates the 

 mechanism of flocculation in such systems. The manner in which floccula- 

 tion of hydrophilic sols may occur can be illustrated by reviewing a simple 

 experiment. The experimental material is a dilute (about O.i per cent) sol 

 of agar-agar. Such a sol is perhaps the most typical example of a simple 

 hydrophilic system. If a relatively large volume of alcohol be added to a 

 small portion of such a sol, the micelles lose their water of hydration, and 

 the sol acquires the cloudy, bluish, opalescent appearance tj'pical of many 

 lyophobic sols. In fact it now is a lyophobic sol, and shows all the typical 

 properties of such systems. The alcohol, which is a powerful dehydrating 

 agent, has robbed the micelles of their shells of oriented water molecules. 

 Nevertheless, the sol is still stable, since the micelles retain their original 

 negative charges. Finally, let a drop of a solution of an electrolyte such as 

 AICI3 be added to the sol. The sol now flocculates almost immediately since 

 the only remaining stability factor — the electrical charge — has been destroyed 

 by the addition of the electrolyte, and the opalescent cast of the sol disappears. 



The stability factors of a hydrophilic sol can also be dissipated in the 

 opposite order. Suppose that the AICI3 solution first be added to the agar 



