Syneresis

SYNERESIS, contraction of a gel and exudation of liquid on its surface. Already Graham noted that many gels (e.g., the gel of silicic acid) after some time following solidification exude droplets of liquid on their surface, the number of which gradually increases, forming a continuous liquid layer. At the same time, the volume of the gel decreases, it contracts and separates from the walls of the vessel containing it. The magnitude of syneresis shows very different dependence on the concentration of the colloidal substance in different gels. Thus, silicic acid gives the greatest syneresis in more concentrated gels, while gelatin and agar primarily in more dilute ones. Similarly, other colloids can be divided into two groups, with syneresis increasing with increasing concentration of the colloidal substance in some, and decreasing in others. At suitable concentrations, syneresis is observed in all elastic gels. As well-known examples of syneresis, one can name the contraction of clotted milk or of a blood clot, from which serum is pressed out in the process. As the given examples show, during syneresis the gel exudes not pure water, but a diluted colloidal solution of the same substances that served for the formation of the gel. The theory of syneresis remains to this day controversial. On one hand, during syneresis, forces of attraction between colloidal micelles may play a role, gradually leading to their approach, and thereby to the compaction and contraction of the network structures underlying the gel. On the other hand, the possibility of gradual dehydration, partial liberation of hydration water of the micelles themselves, cannot be excluded.