3 STRUCTURE OF GELS 73 



however, the reticular structure of gels can be photographed directly 

 in the electron microscope (Fig. 58). As will be explained in the next 

 paragraph, the submicroscopic strands or strings which form the gel 

 frame will be designated as micellar ttrands. Thus the submicroscopic gel 

 structure is a micellar framework. 



The transition into the amicroscopic domain is of particular im- 

 portance. Whereas corpuscular disperse systems in this case become 

 real solutions and are no longer accessible to colloid chemical- 

 methods of research, reticular systems remain colloids even if the 

 thickness of the strands of their framework is reduced to amicroscopic 

 dimensions, i.e., to the cross-section of a single molecule. Thus in net- 

 work systems there is no lower Umit to the colloid domain ; they re- 

 main gels irrespective of whether their network is submicroscopic or 

 amicroscopic. Examples are the polystyrene gels mentioned on p. 67, 

 (Fig. 52, p. 68), or the methyl cellulose gels prepared by Tavel (1939) 

 with the aid of oxalyl chloride, or pectin gels prepared by Deuel 

 (1947a) with ethylene oxide. In these cases the strings of the network 

 are chain molecules and the gel structure is a fine molecular frameivork. 



Comparison of corpuscular and reticular systems. The properties of net- 

 work gels differ in principle from those of sols with their corpuscular 

 dispersed particles. This is clearly demonstrated by Table X. 



Whereas a liquid capable of solvating a substance will disperse 

 corpuscular colloids, reticular colloids remain a coherent mass into 

 which the solvating medium can penetrate to a certain extent only 

 (limited swelling). In this case the dispersing medium would be 

 better characterized as an imbibition medium (see p. 81 and 84), since 

 the colloid substance is not dispersed into separate particles. In the 

 coacervation of sols an equilibrium liquid poor in colloid and the 

 coacervate layer rich in colloid are formed (Fig. 21, p. 22). In reticular 

 coacervates, however, the equilibrium liquid contains no colloid, be- 

 cause the latter is insoluble in the reticular state. For example, after 

 gelation of a gelatin solution, no gelatin is found in the supernatant 

 liquid (cf. p. 21). 



In reticular colloids the mutual position of their submicroscopic 

 elements is fixed, so that a structure results. It follows that gels possess 

 a certain elasticity, although often only slight, indicating that the 

 forces acting in the junction bonds are weak. Typically intermediate 

 between gels and sols are gel solutions, whose particles impede each 



