22 FUNDAMENTALS OF SUBMIC ROSC OPI C MORPHOLOGY 



charges tend to unite, but because of their solvation layers can only- 

 approach each other to a certain extent. The attraction is counteracted 

 by the hydration as by a spring (Fig. 22) and thus no coagulation 

 takes place which would annul the charges, but again a coacervate 

 occurs which now contains particles of opposite charge. So, in addi- 



Equilibrium liquid 



Coacervate 



Fig. 21. Coacervation of gelatin at 41"^ C. 

 Isoelectric gelatin sol + alcohol as sensi- 

 tizer. Equilibrium liquid = solution of 

 water and alcohol. Coacervate = gelatin + 

 small amounts of water + alcohol. 



v^AA\AAA 



Fig. 22. State of stability of colloid particles 

 (from BuNGENBERG DE JoNG and Bonner, 

 1935). Attraction by opposite electrical 

 charges (arrows). Repulsion by solvation 

 layer (spring). 



tion to sensitizers, electric charges are apt to cause coacervation. For this 

 the sols must have opposite charges; e.g., gelatin (positive) and gum 

 arable (negative) or lecithin (positive) and nucleic acid (negative). 

 In this case the aggregation is designated as complex coacervation, since 

 two oppositely charged kinds of particles take part in the flocculation. 

 In many cases colloid particles can be made to reverse their charge by 

 adding neutral salts, when the familiar valency rules apply, viz., on 

 the addition of polyvalent cations, negative particles change their sign 

 more easily according as the valency of the cation is higher, while 

 positive particles behave in a similar way with respect to polyvalent 

 anions. Negatively charged phosphatides, for example, reverse their 

 charge on the addition of CaClg. In the sol, the phosphatide particles 

 which have already become positive and those which have so far re- 

 mained negative attract each other, and in this way a separation occurs 

 which has been called autocomplex coacervation, because in this instance 

 similar but oppositely charged particles attract each other. 



Morphologically the coacervation shows many features which have 

 their counterpart in the phenomena occurring in cells. In the first place 

 the vacuolization calls for mention. If, in a system consisting of equili- 



