CHAPTER VIII 



HYDROPHILIC SOLS AND GELS 



The aqueous medium of protoplasm contains salts and car- 

 bohydrates in true solution (molecular dispersion). The visible 

 structure of protoplasm is that of an emulsion. But neither of 

 these systems constitutes the ultimate substratum of life processes. 

 Those properties of protoplasm which characterize it funda- 

 mentally, such as imbibition, elasticity, and coagulation, are 

 properties of jellies. The jelly-like nature of protoplasm gives 

 to it that continuity in structure, or organization, so necessary 

 as a background for the multiphcity of reactions which, combined, 

 constitute life. It is, therefore, obvious why a knowledge of 

 gel-forming systems is so important to the study of protoplasm. 



Types of Gels. — The hydrophilic colloidal systems, of which 

 gelatin is an example, are characterized by their capacity to form 

 gels. The liquid state of these systems is known as a sol. Gra- 

 ham took the first three letters of geMin and solution to indicate 

 the firm and the fluid conditions of colloidal substances. When 

 a solution of gelatin is hot, it is a sol; on cooling (if suflaciently 

 concentrated), it sets into a gel. Some of the properties of gels 

 are retained in their sols; thus, firm gelatin jelly is elastic, and so 

 also is liquid gelatin. It is their gel qualities that so thoroughly 

 distinguish the hydrophilic colloids. 



Special terms have been devised to designate sols and gels on 

 the basis of the nature of their dispersion medium, e.g., hydrogel, 

 alcosol and xerogel. The last name was coined by Freundlich 

 to indicate a dry gel such as sheet glue. When sols become firm, 

 they are said to gelate. The reverse process is solate. Gels 

 that swell when they take up water are known as jellies; they are 

 also elastic. Sheet gelatin and glue form jellies when soaked in 

 water. Jellies, because they swell, are said to be turgescent. 

 Gels that do not swell in water are called coagula; they are not 

 usually elastic in the sense of being extensile (rubber is an excep- 

 tion). When blood becomes firm on exposure to air, when milk 

 turns into clabber or cheese, and when albumin is heated, 



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