44 



THE PROPERTIES OF SOLS AND GELS 



of colloidal systems depends upon the concentration of the particles of the 

 disperse phase. In most sols the total mass of the disperse phase is not only 

 relatively small ; but it is present in the form of particles which are much 

 larger than molecules or ions. The negligible osmotic pressures of sols are 

 a necessary corollary of the relatively small concentration of dispersed particles. 

 Viscosity. — The viscosity of a fluid is its resistance to flow. The more 

 viscous a liquid the less readily it will flow. Glycerine, for example, is much 

 more viscous than water. The viscosity of hydrophobic sols never varies ap- 

 preciably from that of the dispersion medium — water. Unlike hydrophobic 

 sols the viscosity of hydrophilic sols is usually greater than that of the dis- 

 persion medium. The viscosity of hydrophilic sols 

 increases appreciably with increase in the concentra- 

 tion of the sol, but the relation is not a linear one 

 (Fig. 5). The rapid increase in the viscosity of 

 hydrophilic sols with increasing concentration is 

 ascribed to the hydration of the micelles. Increas- 

 ing the concentration of the disperse phase results 

 in a decrease in the relative amount of free water 

 present due to the association of a larger propor- 

 tion of the water with the micelles. This reduces 

 the "fluidity" of the sol, hence raises its 

 viscosity. 



The viscosity of all liquids, including sols, is 

 influenced by temperature. In general, increase in 

 temperature decreases viscosity. In hydrophilic 

 systems this reduction in viscosity with increase in temperature is probably due 

 to two factors : the decrease in viscosity of the medium itself, and the decrease 

 in the hj'dration of the micelles. 



Electrical Properties. — The dispersed particles of all hydrophobic sols 

 carry electrical charges. A colloidal system, however, is electrically neutral, 

 because for every charge carried on a micelle an equal charge of opposite 

 value is carried by ions in the dispersion medium. The situation is similar to 

 that in a solution of an electrolyte. Although the individual ions are charged, 

 for every negative charge carried by an anion an equal positive charge is car- 

 ried by a cation. In some colloidal systems the dispersed particles are nega- 

 tively charged, in others positively charged, but normally all the dispersed 

 particles in any one sj'stem bear a charge of the same sign. 



Whatever the origin of the micellar charges they invariably are produced 

 in such a way as to involve the release of ions into the dispersion medium 

 which may therefore be regarded as also being charged. When the micelles 



CONCENTRATION OF DISPERSE PHASE 



Fig. 5. Relation of 

 relative viscosity of hydro- 

 phobic and hydrophilic sols 

 to the concentration of the 

 disperse phase. 



