46 



THE PROPERTIES OF SOLS AND GELS 



trical potential between the micelle and the intermicellar liquid. The mag- 

 nitude of this difference of potential varies with different sols and with the 

 same sol under different conditions. 



The usual charges borne by the micelles of some of the better known hy- 

 drophobic sols are shown in Table lO. 



TABLE lO SIGN OF THE ELECTRICAL CHARGE ON THE PARTICLES OF 



CERTAIN HYDROPHOBIC SOLS 



The usual method of determining the sign of the charge upon the dis- 

 persed particles of sols is to observe the direction of their migration in an 

 electrical field. If two electrodes are so arranged as to dip into a hydrophobic 

 sol contained in a suitable vessel, and the electrodes connected with a direct 

 current of proper potential, it will be found that the dispersed particles will 

 move toward one of the poles. By suitable arrangements, using a microscope 

 or ultramicroscope (depending upon the size of the particles), the migration 

 of the particles may be actually watched. In a positively charged ferric hydro- 

 xide sol, for example, the micelles will move towards the negatively charged 

 electrode, while in a negatively charged arsenious sulfide sol they will move 

 towards the positively charged pole. This phenomenon is known as cata- 

 phoresis or electrophoresis. 



As a micelle moves under the influence of an electric current only the 

 innermost layer of the electric double layer — the one which determines its 

 electrical charge — clings to the micelle and moves with it. This inner layer 

 slides past the oppositely charged ions of the outer shell of one micelle after 

 another as it moves towards the anode if negatively charged, or towards the 

 cathode if positively charged. Simultaneously ions of the outer layer move 

 towards the opposite pole from the one towards which the micelles migrate. 

 There is a close analogy between this phenomenon and the behavior of the 

 ions of an electrolyte during electrolysis. 



