ORGANIZATION OF SOLS 



19 



drops or into a coherent liquid layer, we have to do with a phenomenon, 

 for which Bungenberg de Jong (1932) introduced the term 

 coacervation (Fig. 20); in English: piling up (acervus — pile). 



Hydration. The colloid particles in a sol are solvated, which means 

 that molecules of the dispersing medium adhere to the particle. In the 

 special case of water, this solvation is designated as hydration, since in 

 that case water molecules are bound by the colloid particle. The 





W H 



\ 



/- / 1 \ x" ^^ 



Fig. 17 



Fig. 18 



Fig. 19 



Fig. 17. Model of a water molecule and scheme of dipoles - Fig. i8. Hy- 

 dration of an isoelectric colloid particle - Fig. 19. Hydration of a charged 

 colloid particle (from Pallmann, 193 i). 



attraction is brought about by electrostatic forces, for, in a water 

 molecule the electric charges are not distributed uniformly, because 

 the two positive hydrogen atoms are separated in space from the 

 doubly charged negative oxygen. For that reason a water molecule in 

 an electric field behaves like a molecular rod with two different 

 electric poles and is therefore designated as a dipole (Fig. 17). Similarly, 

 in a colloid particle the electric charges are usually not distributed uni- 

 formly, not even if the particles are isoelectric, i.e., if their positive 

 and negative charges cancel each other so that outwardly they appear 

 neutral. In Fig. 18 a particle has been sketched, the negative charges 

 of which are situated towards its surface. This has a polarizing effect 

 on the water molecules in the immediate neighbourhood of the 

 particle. These water molecules follow the particle in its Brownian 

 movement as the so-called solvation or hydration layer.' If the colloid 

 particle is not neutral but carries an excess negative or positive charge 

 as a result of dissociation of H- or OH -ions, the swarm of oriented 



