HEAT OF IMBIBITION 99 



does, it forms such small crystals of ice that the colloid structuie is 

 not destroyed. Although it is so tlillicult to dri\e oil, that a silica 

 gel, for instance, can only partially be dried in a vacuum at 300° C, 

 about 5 per cent, of the water remaining after 6 hours' heating 

 (Neuhausen and Patrick), and an alumina gel cannot be dried by 

 heating for 2 or 3 days at 500° C, yet some relationship does 

 exist between the " free " and the " bormd " water. Under 

 certain conditions, as yet undefined, boimd water may become 

 free again, and the reverse. Many physiological processes may 

 depend on an equilibrium between free and bound water. For 

 example, certain enzymes proceed towards synthesis under one 

 set of conditions and towards hydrolytic splitting under the 

 opposite set. The former conditions are generally admitted to be 

 when the reacting substances are concentrated, ie., when by 

 imbibition colloids have removed water from the sphere of activity, 

 and the latter when dilution takes place. 



Heat of Imbibition (p. 55). During the process of compression 

 a considerable amount of heat is set free. If the swelling takes 

 place relatively slowly, as with laminaria, it is not easy to demon- 

 strate the development of heat, but, in the case of colloids which 

 rapidly imbibe water, even such a value obtained by merely stirring 

 the collcid with a thermometer during the process of imbibition is 

 appreciable (Part II.). The amount of heat developed depends on 

 various factors. At the isoelectric point the main factor is the 

 amount of compression of water produced. 



TABLE XVIII 

 Pressure and Heat of Imbibition of Hydrogels 



Gel. ('oiiii)ressioii (atmos.). tJram calories per gram of gel. 



Dry Gelatin . . over 300 5-7 



Dry soluble Starch . over 2,500 (Rodewald) 6-6 



Dry Gum Tragacanth . over 400 10-3 



It is obvious from Table XVIII. that other factors besides 

 compression play a part. Starch always exerts a large osmotic 

 pull because it is never free from ions. This may, in part, explain 

 the large compression without a correspondingly large evolution 

 of heat. 



The amount of water imbibed and the rate of imbibition are 

 lowest at the isoelectric point (Fig. 21;. The addition of either 

 acid or alkali greatly increases both rate and quantity. This effect 

 is due to the formation of salts of the colloid, and the colloidal ions 

 so produced exert a pure osmotic ])ull on the water — the gel 

 itself acting as a semipermeable membrane to its own ions. The 



7—2 



