54 



THE PROPERTIES OF SOLS AND GELS 



result not in flocculation, but in the conversion of the system into a lyophobic 

 sol. Addition of a suitable electrolyte to this sol will result in a discharge of 

 the particles and their consequent flocculation. These relations are shown 

 diagrammatically for a protein sol in Fig. 8. 



HYOROPHILIC SOL 

 tCHARGED) 



HYOROPHILIC SOL 

 (.UNCHARGED) 



HVDROPHILIC SOL 

 (CHARGED) 



DEHYDRATION 

 BY ALCOHOL 



DEHYDRATION 

 BY ALCOHOL 



* + -< 

 X \ I / y 



^V /^ DISCHAR 



•* y\^^_^\^ SY ELECT 



HARGED 



ROLYTE 



FLOCCULATING MICELLE 



Fig. 8. 



LYOPHOBIC SOL LYOPHOBIC SOL 



Diagrammatic representation of the flocculation of a micelle of a protein sol. 



Properties of Gels. — Under certain conditions most hydrophilic sols 

 change into gels. Any gelatin sol which is not too dilute, for example, will 

 "set" upon standing and form a gel. Everyone is familiar with such gelatin 

 gels, often as they appear on the table under the guise of desserts. Other 

 familiar gels are the agar gels widely used as a medium for the culturing of 

 bacteria, fungi, algae, etc., ordinary household "jellies" which are basically 

 pectin gels, and starch gels. The latter also sometimes come before our eyes 

 by the dessert route, under the name of corn starch puddings. Some hydro- 

 philic sols, however, do not ordinarily form gels. This is true of sols of gum 

 acacia and of some protein sols. 



The gel-forming capacity of some substances is very remarkable. A gelatin 

 sol containing as low a proportion as one part of gelatin to one hundred of 

 water will usually gelate. Agar gels containing only 0.15 per cent of agar 

 can be prepared. Li such a gel one part of agar has the property of removing 

 the liquidity from nearly 700 parts of water. 



Since gels form from two-phase colloidal systems, it is generally assumed 

 that they also are two-phase systems. In all of the gels which we shall have 

 occasion to consider, the liquid phase is water, although gels in which one 

 component is some other liquid are well known. Gels are usually rigid 



