20 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



dipoles surrounding the particle will be correspondingly larger. This 

 is why the hydration of colloids reaches its minimal value at the 

 isoelectric point. 



The binding forces which attract the water dipoles decrease with 

 increasing distance. Thus the swarm of water molecules which are 

 hampered in their free movement becomes less dense in the outer 

 layers until in the end one reaches without noticeable transition the 

 region of the freely moving dipoles of the dispersing medium. In the 

 solvation layer the density of the water therefore decreases ex- 

 ponentially, in much the same way as the density of the atmosphere 

 with increasing distance from the earth. As thert is no sudden transition 

 from the hydration layer to the free water, such hydrophilic colloids 

 are very stable. The particles show no tendency to cluster together; 

 in a way they "have no surface at all", their surface energy is zero 

 (Fig. 2oa). 



Dehydration. If water is withdrawn from the diffuse solvation layer, 

 the difference between bound and freely moving dipoles becomes 

 noticeable. The water layer around the particle now acquires a surface 

 (Fig. 2ob) and if two such dehydrated particles meet, the surface 



Surface 



Flocculation 

 .Coacervafe droplet 



Coacervafe layer 



Fig. 20. Dehydration of colloid particles (from Bungenberg de Jong, 1932). 

 a) Diffuse hydration layer, b) definite hydration layer, r) incipient coacervation. 



energy which tends towards a minimum value will cause the surround- 

 ing water layers to unite. The colloid particles, however, cannot come 

 into direct contact with each other because of their solvation layers. 

 But they no longer possess separate layers, for these have all united 

 into a single liquid sphere. If the number of particles united in this 

 way becomes so large that they form a microscopically visible 

 conglomeration, one speaks of flakes or flocculates. These can further 

 cluster into drops (microcoacervation) and finally into a liquid layer 

 (macrocoacervation). Thus coacervates are liquids rich in colloid 

 which have been separated by means of dehydration. 



