CHAPTER V 



THE PROPERTIES OF SOLS AND GELS 



The discussion in this chapter will deal only with sols in which water 

 is the dispersion medium. Such systems are classified into the two groups 

 of hydrophilic sols and hydrophobic sols. The dispersed particles in all such 

 colloidal systems fall within the size range of o.ooi to o.i fx, but their size 

 may vary greatly with different sols. Even in a given sol the particles may 

 exhibit a wide range of dimensions, although in some they are quite homogene- 

 ous in size. 



Most of the important differences between hydrophilic and hydrophobic 

 sols are due to the hydration of the dispersed particles in systems of the 

 former type. There is no general agree- 

 ment regarding the exact physico-chemi- 

 cal relationship between the micelle and 

 its water of hydration ; but only two con- 

 ceptions have any wide currency at the 

 present time. One of these relates this 

 hydration to an actual solution of some 

 of the water in the substance of the 

 micelles (Fig. 4, //). 



A more probable theory, however, 

 holds that no actual solution of the 

 medium in the micelles occurs, but that 

 water molecules are oriented around each 

 dispersed particle forming a "shell" many 

 layers of molecules in thickness (Fig. 4, 

 B). It is presumed that the first layer of 



oriented water molecules is so firmly attached to the micelle that it is essentially 

 an integral part of it. The successively enveloping layers of water molecules 

 are also oriented, but with increasing distance from the periphery of the micelle 

 the forces of attraction and orientation progressively decrease. In this zone 

 there is a gradual transition from water molecules wTiich are practically all 

 oriented to those which are completely unoriented. Oriented molecules fit to- 

 gether more closely than unoriented molecules. They are "packed" more 



39 



Fig. 4. Diagrammatic representa- 

 tion of possible relationships between a 

 micelle and its water of hydration: 

 (A) solution of water in the micelle. 

 The proportion of dissolved water pres- 

 ent is represented as decreasing toward 

 the center of the micelle. (B) orien- 

 tation of the water molecules as a 

 "shell" around the micelle. 



