Effect of Dissolved Polymer on Colloid Adhesion

Dissolved polymer molecules behave as particles (Fig. 10.18(e)) which exhibit internal Brownian movement The difference between a polymer particle and a clay particle is that the polymer can change its size because it is swollen by 

In the weakly bound state, shown in Fig. 10.19(a), most of the polymer molecules coil up in solution to form particles which are considerably smaller than the extended chain length, A fully extended chain made up of iV segments each of length I would be Nl long, whereas a polymer chain is known to coil up in 

However, some of the molecules are adsorbed onto the particle surface, and these are in a dynamic equilibrium, constantly jumping on and off the particle. 

The force acting between silica surfaces in polyacrylic acid solution has been measured by atomic force microscopy, giving the interaction curve shown in Fig. 10.19(b). At large separations, the particles follow the van der Waals attraction curve as the silica surfaces come closer together. But then the polymer molecules cause a repulsion as the polymer molecules resist desorption from the surfaces. This repulsion increases to a certain point but then the particles jump into a secondary minimum. As the particles are pushed further together, the polymer is squeezed out of the gap and eventually the silica particles make intimate contact in the primary minimum. 

Such complex restructuring of the interfaces, with polymer molecules changing positions substantially as the particles approach, is quite different from the situation when polymer is strongly bonded to the particles. 

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