40 THE PROPERTIES OF SOLS AND GELS 



closelj^, just as more bricks can be stacked in a given space if arranged regu- 

 larly, than if tossed in indiscriminately. As a result of this packing the water 

 in these oriented shells has a greater density than that in the bulk of the liquid ; 

 in other words there is an actual contraction in the volume of the liquid as- 

 sociated with the micelles. 



The fact that micelles may, under certain conditions, lose their water of 

 hydration very rapidly would seem to favor the latter theory. It is possible, 

 of course, that in some hydrophilic systems the water is actually dissolved in 

 the micelles, that in others it is present only as a shell of oriented molecules, 

 while in still others both of these two suggested modes of hydration may exist. 



The important properties of sols will now be summarized, with special 

 attention to differences between the properties of sols of the hydrophilic type 

 and sols of the hydrophobic type. 



Dilution. — Most sols are extremely dilute. In other words the actual 

 mass of substance dispersed throughout the dispersion medium is extremely 

 small in proportion to the total volume of the system. The well-known 

 colloidal gold sols, for example, rarely contain more than i g. of gold dis- 

 persed per liter of sol. Other hydrophobic sols are correspondingly dilute. 

 Most hydrophilic sols are also extremely dilute, although there are some 

 exceptions to this statement, as very viscous sols of such substances as starch 

 and gum acacia can be prepared which contain lO parts or more of dispersed 

 material per lOO parts of sol. 



Slow Rate of Diffusion of the Dispersed Particles. — In general the 

 particles which make up the disperse phase of a sol diffuse much more slowly 

 than substances in true solution, i.e. in the molecular or ionic state. The slow 

 rate of diffusion of micelles as compared with most ions and molecules is 

 clearly correlated with the relatively large size of the colloidal particles. 

 There is, however, no sharp line of demarcation in terms of diffusion rates 

 between colloidal micelles and solutes. The diffusion rates of some solutes 

 with large molecules are not perceptibly faster than those of the dispersed 

 particles of colloidal systems in which the micelles are relatively small in size. 



Filterability. — Sols are usually filterable; that is they pass through or- 

 dinary filter papers without any appreciable separation of the disperse phase 

 from the dispersion medium by the filter. Usually there is some loss of the 

 disperse phase due to an initial adsorption when the sol first comes in contact 

 with the filter. Sometimes this may be very considerable. Since the pores in 

 ordinary filter papers are about 2-5 /a in diameter, and even porcelain filters, 

 such as those widely used in bacteriological work, have pores 0.2-0.6 fi in 

 diameter, it is easy to understand why micelles with diameters in the size 

 range o.ooi-o.i fi pass through. 



