58 THE PROPERTIES OF SOLS AND GELS 



and even the factor of time may induce such effects in gels. It follows that 

 in experimental work with gels, if results are to be comparable, all the gels 

 used in a given experiment or set of experiments must have had identical 

 previous histories. 



Syneresis. — Syneresis may be defined as the spontaneous separation of a 

 portion of the liquid component of a gel. The liquid which separates is not 

 quite pure, however, being essentially a very dilute sol. Syneresis is a widely 

 observed phenomenon. It may be observed in both gelatin and agar gels. 

 The so-called "bleeding" of agar culture media is familiar to all who work 

 with them. The liquid which usually forms around gelatin gels which 

 have stood for some time is often a result of syneresis. It has been suggested 

 that several important biological phenomena involve the process of syneresis. 

 These include the formation of vacuoles in plant and animal cells, the separa- 

 tion of serum from a blood clot, the exudation of serum into a blister, glandu- 

 lar secretion, and the contraction of muscles. 



Thixotropy. — If a trace of sodium chloride is added to a test tube full 

 of lO per cent bentonite (a colloidal clay), and vigorously shaken, a colloidal 

 sol will result which will set to a gel after standing a few minutes. By 

 shaking, this gel can be converted to a sol which will again set upon brief 

 standing. The process may be repeated an indefinite number of times. This 

 phenomenon is called thixotropy. Protoplasm also exhibits thixotropic reac- 

 tions. Stirring of the protoplasm or subjection of a cell to pressure can be 

 shown to greatly reduce its viscosity and probably induces gel to sol changes. 

 Thixotropic phenomena may therefore play important roles in cellular physi- 

 ology. 



Discussion Questions 



1. What are the three most outstanding characteristics of colloidal systems in 



which water is the dispersion medium? 



2. The charges on colloidal micelles are assumed to be balanced by electro- 



statically equal charges in the dispersion medium. If this is true why do 

 micelles exhibit cataphoresis? 



3. How can you determine whether a given organic dye forms a solution or a 



sol when dispersed in water? 



4. Given a clear colloidal sol how would you determine whether it was hydro- 



philic or hydrophobic? Whether its micelles were positively or negatively 

 charged? 



5. List some colloidal systems found in plant cells. 



6. Why are not the micelles of hydrophobic sols flocculated by the ions of 



opposite sign in the double layer? 



7. Why are not the protoplasmic colloids of root hair cells flocculated by the 



cations and anions that enter them from the soil? 



