STUDIES ON THE GELATION OF SILICIC ACID SOLS. PART IV. ON THE DEVELOPMENT OF YIELD VALUE .AND STRUCTURE OF SILICIC ACID SOLS

The rate of change of turbidity with time of silicic acid sols increases, becomes maximum approxi­mately at or near the time of gelation and then gradually decreases. The time of gelation can be deter­mined graphically from turbidity measurements. The viscosity of freshly prepared silicic acid sols is very low. As ageing continues it increases at first very slowly, then with appreciable speed and finally, a little before gelation, it shoots up to a very high value. A gradual increase in the extent of aggregation of silicic acid in the sol state is noticeable from the viscosity data of different systems by the application of Stammlinger's viscosity relation Yield value develops roughly at about 75%, 80% and 86% of the time of gelation in the cases of sole prepared 'by using HCI, H2S04 and H3PO4, respectively at room temperatures. Shaking prevents the formation of 'macro-structures' but does not hinder the formation of 'micro-structures' in silicic acid sols. Ultrafiltration of silicic acid sols in HCl system indicates that appreciable aggregation of silicic acid sols does not take place before 82% of the time of gelation at room temperatures