ELECTROKINETICS 



369 



Freundlich and Rona suggested that only a part of the outer 

 diffuse layer of ions is actually mobile ; that is to say, the colloidal 

 particle carries with it not only the innermost single and closely 

 adhering (Helmholtz) layer but also some of the outer ions. 

 This being true, the particle, regarded as an electrode, becomes 

 somewhat larger than its actual boundary — its surface of metal. 

 The adhering ions are probably attached to the particle by 

 secondary forces (secondary valence, adsorption, van der Waal 

 forces). Electric charges (in this case, ions) are subject to heat 

 motion. Near the surface of the particle the electrostatic forces 



^H- - I - + 



+ 





l^'\ - 



f^-^ 

 h 



+ 



t&\ -1 



I- 



w 



+ - 



■|- 

 S 



+ 



+ 



w 



& = ^zO. 



A B 



Fig. 161. — Potential distribution at the surface of a particle. W is the wall 

 of the particle; S the plane of slippage; C the outer free cloud of ions; ^ the 

 electrokinetic potential; and e the thermodynamic potential. {After H. Midler.) 



are so large that the motion due to temperature is neghgible. 

 This area extends out mto the fluid for a short space until at a 

 sufficient distance the ions overcome the electrostatic forces and 

 move freely. The interface, where the inner adhering ions move 

 or slip with the particle against the outer free ions, is known as 

 the -plane of slippage (S, Fig. 161A). 



If our hypothetical particle were at absolute zero, it would be 

 found to possess a rigid Helmholtz double layer of positive and 

 negative ions equal in number and placed within ionic diameters 

 of each other. But owing to the heat motion of the ions, at 

 ordinary temperatures, the ions outside the plane of sUppage 

 distribute themselves in a diffuse double layer (corresponding to 

 the density of the atmosphere about the earth). Far off in the 

 body of the solution (C, Fig. 16L4) the number of positive and 



