ELECTRICAL PROPERTIES 47 



The sign of the charge on the dispersed particles of any sol can be de^ 

 termined by cataphoresis. Particles considerably above the colloidal range 

 of dimensions frequently can be shown to exhibit cataphoretic migration. The 

 phenomenon can be demonstrated with bacteria, unicellular algae, spores, etc. 

 Practically all such small living organisms are negatively charged. 



The dispersed particles of hj^drophobic sols apparently acquire their charges 

 either by electrolytic dissociation or by adsorption. In some such systems the 

 charges seem to arise as a result of ionization of some of the molecules com- 

 posing the micelle. The ions released into the dispersion medium become the 

 outer envelope of the double layer, leaving the micelle with a residual and 

 compensating charge of ions of opposite sign. Individual molecules of some 

 substances are large enough to fall within the colloidal range of sizes. The 

 dye Congo Red, which is the sodium salt of a complex organic acid, is an 

 example of such a substance. Dispersed in water this compound produces 

 sodium ions and a colloidal anion, the latter, of course, being negatively 

 charged. 



In other systems the electrical charge on the dispersed particles is ap- 

 parently acquired by adsorption of either the positive or negative ions of 

 an electrolyte, the ion of opposite charge becoming the outer shell of the 

 double layer. Ferric hydroxide sols ^ are normally positively charged. The 

 charge on the micelles of these sols is often ascribed to the adsorption of 

 Fe+ + + ions of the FeCls from which ferric hydroxide sols are usually 

 prepared, the Cl~ ions forming the outer layer around the micelles. Similarly 

 the negative charge of the micelles of arsenious sulfide sols is often ascribed 

 to the adsorption of HoS which is used in the preparation of this sol. Dis- 

 sociation of HoS results in the release of H+ ions into the dispersion medium 

 leaving the dispersed particles with a residual and compensating negative 

 charge. 



Similarly it is believed that some substances acquire an electrical charge 

 by adsorbing hydrogen or hydroxyl ions — more frequently the latter — from 

 their water dispersion medium. Certain inert substances such as cellulose, 

 carbon, quartz, collodion, etc., are believed to become charged in this way. 

 All of these substances acquire a negative charge when in contact with water, 

 indicating that the hydroxjd ions are adsorbed, the hydrogen ions becoming 

 the outer shell of the double layer. 



The micelles of some hycJrophilic sols are charged ; those of others are not. 

 As in hydrophobic systems the dispersed particles of different hydrophilic sols 

 acquire their charges in different ways. In some the charges on the particles 



2 Most investigators believe that the so-called ferric hydroxide sol is actually a 

 sol of hydrated ferric oxide (Fe203( H20)x). 



