Gelation in ReversiUe Colloidal Systems. 



99 



When once formed the phases have considerable stability. If the 

 droplets are composed of a solid solution one may, by the addition of 

 water, cause them to increase to relatively vast dimensions without 

 their being destroyed, as they increase in size their refractive index 

 approximates more and more to that of the external phase until 

 finally they are lost sight of. The addition of alcohol, however, once 

 more brings them into view and causes them to shrink. Owing to 

 this stability once a configuration is established one has to far overstep 

 the conditions of its formation in order to destroy it. This would 

 account for the remarkable hysteresis observed in reversible gels. 

 Thus a 10 per cent, solution of gelatine in water sets at 21° and melts 

 again at 29*6°, and solutions of agar in water set at temperatures about 

 35° and melt at temperatures about 90°. Similarly with the ternary 

 mixtures. In one holding about 35 per cent, gelatine, the internal 

 and external phases separate at 20°, but they mix again only at 65°. 

 When water is added to a ternary mixture so as considerably to swell 

 the droplets the system is unstable, and the two phases mix at once 

 when it is mechanically agitated. 



The properties of the ternary system : alcohol, gelatine, and water 

 are the following : — 



i. Below a certain temperature it exists in two phases separated by 



a well-defined surface. The temperature at which the separa- 

 tion occurs depends upon the relative proportion of the com- 

 ponents in the mixture. Increasing the proportion of gelatine 

 raises it ; as does also an increase in the proportion of alcohol. 

 An increase in the proportion of the common solvent, water, 

 however, lowers the temperature at which the biphasic cha- 

 racter develops. 



ii. Both phases are at first fluid ; with further fall in temperature 



one becomes solid. 



iii. The surface of separation is curved and discontinuous. In some 



cases, strictly as a secondary change, the discontinuous masses 

 of the internal phase become continuous with one another. 



iv. The more slowly the two phases are established the less is the 



surface which separates them both in extent and in curva- 

 ture. 



V. The solid solution phase is formed sometimes on the concave, 

 sometimes on the convex side of the surface of separation. 

 The former happens when the proportion of gelatine is small, 

 the latter when it is large. 



It follows from the last (v) of these properties that a hydrogel may 

 be built on two very diff'erent plans. It may consist of a solid mass 

 containing spherical fluid droplets, or of solid droplets which, by 

 hanging one to the other, form a framework in the spaces of which 



