l8 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



(separation into two layers), classical colloid chemistry is interested in 

 the first place in phase boundaries (capillary chemistry according to 

 Freundlich, 1922). 



Thermodynamics require that all parts of a phase have exactly the 

 same energy content. This is only realized, however, when the phases 

 are so extended that the irregular distribution of energy at their 

 surface, i.e., the inhomogeneity in the immediate neighbourhood of 

 the phase boundary (Fig. 16) can be neglected (Fig. 15). Thus, the 

 classical phase theory has to forego all considerations concerning phase 

 boundaries (compare Figs. 3-11, 13, 14) because of their inhomoge- 

 neity, and its laws only apply to homogeneous regions of at least micro- 

 scopic dimensions. The properties of colloids, on the contrary, are 

 determined in the first place by the inhomogeneity of the phase boun- 

 daries, the predominant effect of which is due to the very large surface. 

 For this reason it has been suggested by Ostwald (1938) that the 

 definition "dispersed phase" should be avoided, and that we should 

 speak of the "colloid portion" of the dispersoid. 



The phase theory once seemed to hold out promise of explaining 

 the formation of new phases (separation into two strata, formation of 

 vacuoles) or the disappearance of phases (melting-in) in biological 

 systems. From the above, however, it is clear that the phase theory 

 does not hold good in colloid chemistry, since it has been developed 

 by emphasizing the homogeneity of the phase and neglecting the 

 specific properties of surfaces, while conversely, in cytological systems, 

 homogeneity usually fails and the surfaces are of quite outstanding 

 importance. Bungenberg de Jong and his fellow-workers have 

 elucidated the principles according to which visible boundary layers 

 can appear and disappear in those heterogeneous systems to which 

 the phase theory does not apply. In his theory of coacervation Bungen- 

 berg DE Jong has summarized the rules which govern these phe- 

 nomena. 



d. Coacervation 



In the separation of a sol into two non-miscible parts, the dispersing 

 medium and the dispersed portion often do not separate completely. 

 Flakes are formed which still contain a certain amount of dispersing 

 medium and therefore remain suspended. For this reason the floccu- 

 lation is usually reversible. If, however, such flakes collect into small 



