4 STUDIES IN GELS III 



phenomenon, too, can be followed by means of X-rays, since the 

 new chain lattice shows new interferences, while the orieinal ones 

 disappear. These conversions are termed permufoid or topochemical 

 reactions, because the reacting groups undergo chemical changes 

 within the crystal lattice itself without dissolution of the molecules. 

 The characteristic feature of these reactions lies, therefore, in the fact 

 that chemical changes take place in the solid state, in contrast to the 

 classical formula: corpora non agiint nisi fluida. 



Intramicellar swelling clearly demonstrates the great similarit}- 

 between swelling and dissolution. As has been shown by Katz (1924), 

 in both cases the same physico-chemical phenomena take place (heat 

 of swelUng, volume contraction and swelling pressure as a result of 

 solvation), the only difference being that swelling occurs very slowly 

 because of the slow Brownian movement of the macromolecules. And 

 if in some way or other these form a network, only limited swelling 

 takes place and the state of a sol is not reached. 



Shrinkage. Most gels encountered in nature are liable to swell to 

 a certain extent. On drying, the behaviour depends on the properties 

 of their gel frame. If this possesses meshes with fixed contours, such 

 as, for instance, silica gels, the decrease in volume does not correspond 

 tothelossof water. The dry system is a porous body, i.e., it has changed 

 into an air-containing aerogel. 



If the gel framework is flexible, however, the meshes will graduallv 

 close on continued shrinkage till finally the micellar strands touch on 

 all sides. The result is a horny, brittle xerogel without perceptible 

 porosity. The drying process of these xerogels is very problematic. If 

 we assume the gel to be isotropic, it must possess a gel frame of random 

 arrangement. Were we to apply this principle of randomness also to 

 amicroscopic regions (Fig. 53a, p. 69), the framework obtained when 

 the molecular or micellar strands approach each other would be a 

 loose structure with numerous interstitial or intermicellar spaces. In 

 that case the xerogel would possess a lower density than the crystalline 

 substance and it would have a white and untransparent appearance 

 as a result of the light diffraction caused by the air-containing spaces. 

 This only applies, however, to aerogels, whereas xerogels solidify to 

 completely transparent glassy substances. If the density of the crys- 

 talline micellar strands is determined by mean? of X-rays and compared 

 with the density of xerogels, the discrepancy found is only about 10% 



