100 DISPERSE SYSTEMS 



effect of a very slight increase in pH on swelling is much more 

 pronounced than a larger increase — e.g. 1 gram of gelatin at pH 4.-7 

 imbibes 7 c.c. of water, while at ^^H 4-4, 21 c.c, water will be taken 

 up. Increasing the hydrogen ion concentration still further 

 produces a fairly steady increase in swelling power till at jjH of 3-4 

 about 35 c.c. of water have been absorbed. The extra amount 

 absorbed by increasing the hydrogen ion concentration still further 

 is inappreciable. When acetic acid or similar weak acid is used 

 to acidify the gel atypically large results are produced, due, according 

 to Loeb, to a diminution in the cohesion of the gel brought about 

 by the high concentration of those acids necessary to give a pH 

 of 3-2. 



It is obvious from our discussion of the isoelectric point that the 

 addition of salts to a gel will tend to depress its power of imbibition 

 to a value approximating that found at the isoelectric point due to 

 their inhibitory action on the io7iisation of the gel. Salts cannot 

 exert an osmotic effect of their own if the gel is freely permeable 

 to them (see Diffusion). 



Explanations may now be offered as to why a limiting value 

 is placed on the amoimt of water imbibed by a gel. The force 

 causing swelling, whether due. as at the isoelectric point to pure 

 capillarity, or to the ionised gel exerting an osmotic effect, is 

 opposed by the elastic forces of the gel. The gel molecules or gel 

 ions exert a cohesive force which has to be overcome. In the case 

 of substances like laminar la, the cohesion is very great and does 

 not permit the molecidcs to be forced very far apart. On the other 

 hand, gelatinous substances have their particles separated suffi- 

 ciently to make the gel soft, and, finally, if water were freely 

 admitted, the gel would become more and more like a sol. 



Syneresis. Graham found that if gels were left undisturbed for 

 some time they underwent contraction and expressed a quantity 

 of their dispersion medium containing some of all the colloidal and 

 crystalloidal matter present. This process he called syneresis 

 (coalescence), and it is common to all gels, but in different degree. 

 Tvpical examples are separation of serum from blood-clot, whey 

 from curded milk, and weeping of agar slants. 



The behaviour of most organic gels is complicated not only by 

 the presence of electrolytes, and by the fact that the content of 

 the intermicellar fluid in electrolytes may be rapidly altered, but 

 also by the fact that the dispersed substance is a mixture of 

 closelv related substances. Thus agar-agar, a carbohydrate 

 superficially similar to the protein-hydrate gelatin, consists of at 

 least two substances a and ^ agar-agar which are mutually con- 

 vertible under certain conditions. Purified, a agar-agar is prac- 



