56 THE PROPERTIES OF SOLS AND GELS 



example, a 4 per cent gelatin sol gelates at about 28° C, but the resulting gel 

 must be raised to about 31° C. before solation will occur. For agar gels the 

 temperature spread between gelation and solation is much greater. The 

 former process occurs at approximately 40° C. ; the latter at about 85° C. 

 The viscosity of a heat reversible sol increases steadily with a decrease in 

 temperature and shows no sudden change as the sol passes into the gel 

 condition. 



The Structure of Gels. — Numerous theories purporting to explain the 

 structure of gels have been advanced, and there is no doubt that there are 

 elements of truth in most of them. Since it seems improbable that the mole- 

 cular architecture of all gels is the same, it is unlikely that any one of the 

 proposed theories will apply equally well to all gels. The structure of gels 

 is too fine to be resolved by the microscope, and only rarely has the ultra- 

 microscope revealed anything when it has been used as an instrument for study- 

 ing gel structure. Necessarily, therefore, most evidence of the structure of 

 gels is indirect. 



There seems to be little doubt that in non-elastic gels of the silicic acid 

 type that the solid phase is crystalline, and forms a sort of a rigid frame- 

 work. The liquid phase is held in the interstices of this solid framework. 

 In some respects this may be regarded as the simplest type of gel structure. 



One of the older theories of the structure of elastic gels is the so-called 

 "honeycomb theory." According to this theory the fluid phase of the gel is 

 discontinuous, being separated into minute chambers or compartments, 

 bounded on all sides by films of the solid, or at least of a more solid phase. 

 The analog^^ between such a cellular structure and a honeycomb is obvious. 

 The "solid" phase is not necessarily supposed to be composed of the chemically 

 pure, originally dispersed material; it may represent simply a phase which is 

 relatively rich in the dispersed substance as compared with the fluid phase. 

 In a gelatin gel, for instance, it might be supposed that the solid phase is 

 relatively rich in gelatin but poor in water while the converse is true for the 

 fluid phase. 



At the present time an hypothesis more generally favored is that both 

 the solid and liquid phases of a gel are continuous. The solid phase is usually 

 visualized as a meshwork of long, tangled fibrillae of ultramicroscopic, or 

 sub-ultramicroscopic dimensions ; the spaces within this interwoven mesh being 

 occupied by the fluid phase. This theory is often called the "brushpile" theory 

 in allusion to the supposed jumble of intermeshing threads of the solid phase. 

 The remarks made in the preceding paragraph concerning the constitution of 

 the two phases are also valid for this theory. 



This last theory appears to be most acceptable in the light of the known 



