Studies on Coagulation. I. On the Velocity of Gelation and Hydrolysis of Gelatin Sol.

GELATION of hydrophilic colloids has frequently been studied in regard to its 'gelation time' (usually called the coagulation time) i.e. the time necessary for the gelatinising of a sol. The definition of 'gelation time,' however, is somewhat arbitrary and differs with the observer. Some observers consider a sol is gelatinised, when the vessel containing it can be inverted without spilling. Others consider it gelatinised, when it adheres as an amorphous mass to a rod inserted in it. Still others consider it gelatinised, when a small gLass bead thrown into it stops half-way down. Gelation, however, is not a sudden change of state, but a kinetic process with respect to time. Therefore it is more important and fundamental to study the course of a whole process of gelation than to study simply the gelation time. It is not yet known exactly, what change of state the colloidal constituents undergo in the process of gelation. The process is called gelation only when a hydrophilic sol, separating into two phases, gradually increases its viscosity until it becomes an amorphous solid mass throughout. Thus the degree of gelation can be estimated by measuring the increment of viscosity, and the rate at which the viscosity increases is called the 'gelation velocity.' Von Schroder [1903] studied this problem as follows. He heated a gelatin sol at 1000 for a certain time, and then cooled it suddenly to 250. He now kept the sol at this latter temperature, and measured its viscosity twice, first, after a lapse of five minutes, and again, after sixty minutes. The second value was of course larger than the first one. He called the difference of these two viscosity values Aq, and the time interval between the measurements (55 minutes) At, and assumed that a represented the coagulability of the At sol. He found that the longer the sol was kept at 1000, the smaller were both the viscosity at the first measurement and