ELECTRICAL PROPERTIES 45 



are negatively charged, the dispersion medium is positively charged and vice 

 versa. Electrostatic attraction therefore exists between the surface charges 

 of a colloidal particle and the ions of opposite charge in the dispersion medium. 

 The result is that surrounding each colloidal particle with its charged surface 

 is a "shell" of ions of opposite charge. 



This arrangement of charges at the surface of a micelle is called an elec- 

 tric double layer. Similar electric double layers exist also at boundaries be- 

 tween solid surfaces and liquids, as for example along the walls of a capillary 

 tube. Figure 6 represents the distribution of the electrical charges around two 

 micelles, one positively charged, one negatively charged. The innermost layer 

 of ions in the dispersion medium is probably compactly oriented around the 

 oppositely charged micelle, which is in turn surrounded by progressively more 

 diffuse layers. That is, while most of the ions are close to the charged surface 

 of the micelle, some are farther away, although with increasing distance from 



1- 



B 



Fig. 6. Diagrammatic representation of the electric charges around micelles: {A) 

 positively charged, (B) negatively charged. 



the surface of a micelle, the number of ions associated with that micelle de- 

 creases rapidly. 



The ions of the double layer are in dynamic equilibrium with undissociated 

 molecules at the periphery of the micelle proper. Anions and cations of the 

 double layer are continually uniting and forming uncharged molecules which 

 become part of the micelle. Contrariwise molecules at the surface of the 

 micelle are continually dissociating into cations and anions. If the micelle is 

 negatively charged the anions adhere to its surface; if positively charged, the 

 cations. The ions of the opposite charge become part of the outer shell around 

 the micelle. 



The presence of this electrical double layer results in a difference of elec- 



