Measurement of coagulation forces by ultracentrifugation

Abstract

A technique, using an ultracentrifuge, has been developed to measure forces of coagulation of colloidal sols. Such measurements are appropriate for the study of sols stabilized by adsorbed long chain molecules where classical methods of electrolytic coagulation suffer the disadvantage of perturbing the conformation of these molecules both in solution and in the adsorbed state. The validity of the method has been tested by studying classical AgI sols, the doublelayer parameters of which were varied by addition of KNO₃. The extent of coagulation, which is dependent on centrifuge speed, does not change with time after 30 min. We propose that coagulation begins in the lowermost layers of the sediment and proceeds upwards when the centrifuge speed is increased. Accordingly, the critical forces for coagulation can be evaluated. The critical force is found to be almost constant over the concentration range 0.010 to 0.050 mol dm^–3 KNO₃, in excellent agreement with the theoretical treatment.

Measured critical forces are rather lower than those calculated from an analysis which considers only nearest neighbour interactions in the sediment. Interactions between more distant particles would lower the force barrier, though this effect would be expected to be small at the values of xa used here.