272 DAVID R. BRIGGS 



ings that are ionizable when brought into contact with a liquid. 

 For example, sodium proteinate in contact with water will ionize 

 just as w^ll any electrolyte in solution but the anions, in this case, are 

 an integral part of the protein molecule (or particle) and constitute 

 point charges on the surface of or within the body of the particle. 

 The sodium ions, however, will be kinetically independent of the par- 

 ticle and, while held in the region of the particle surface by electro- 

 static forces of attraction exerted by the anions, will diffuse into the 

 body of the liquid to a distance determined by the relative magnitudes 

 of the forces of diffusion and electrostatic attraction. Other materi- 

 als, examples of which are quartz or cellulose, which do not contain 

 ionogenic groupings, will nevertheless acquire a charge when placed 

 in contact with a liquid due to a differential adsorption by the solid 

 surface of the electrolyte species present in the liquid. The hydroxy 1 

 ions present in water are more surface-active (more highly adsorbed) 

 than the hydrogen ions. Surfaces of such ionogenically inert solids, 

 then, usually bear a negative charge when in contact with water be- 

 cause of the statistically oriented state of the ions derived from water, 

 wherein the 0H~ ions are more strongly attracted toward the solid 

 surface while the H+ ions distribute themselves toward the water 

 phase in the interface region. 



As a result of this oriented distribution of the various ionic con- 

 stituents at the particle surface, an electrical potential difference will 

 exist between two layers of the fluid in the region of a particle-fluid 

 interface. One fluid layer is considered to be fixed on the surface of 

 the particle together with the fixed electrical charges of the surface, 

 while the other, containing the counter electrical charges (counter- 

 ions or gegenion), exists parallel to and a short distance normal 

 to the particle surface and is considered to be free to move mider any 

 applied shearing force with respect to the fixed layer. This potential, 

 which may exist in the region of any interface, is called the electro- 

 kinetic or zeta-potential. Its magnitude is dependent upon the net 

 charge density per unit area of the layers, upon the distance apart in 

 space at which the electrical centers of gravity of the layers exist, 

 and, as a corollary to the latter, upon the dielectric constant of the 

 medium occurring between the charged layers. The velocity of mi- 

 gration of a particle will be directly proportional to the magnitude 

 of the electrokinetic potential at its inteiface and to the magnitude 

 of the imposed field. 



Helmholtz (1879-1888), in his early treatment of the theory of the 



