Flocculation Due to Polymers

Bridging flocculation is the easier to visualise. This occurs when the surfaces of the droplets are attractive to the added polymer and polymer adsorption occurs at the droplet surface. When insufficient polymer is added to coat completely the droplets surfaces, encounters may occur between droplets orientated such that an area with adsorbed polymer on one drop meets an area free from polymer on a second drop. Under these circumstances the polymer molecule(s) already adsorbed to one drop may frirther adsorb to the second drop, creating a polymer bridge. The resilience of this bridge then maintains the droplets at a elose droplet-droplet separation (i.e. flocculated). 

For bridging flocculation to occur, several conditions must be met. The most obvious is that the droplet surfaces must be subject to polymer adsorption. In emulsions, the presence of surfactants at the interface, which are necessary for coalescence stability, means that only polymers with sufficient surface affinity to displace the surfactant from the siuface, or a specific affinity for the surfactant itself, may adsorb. A second condition places a lower limit on the size of the polymers in a given system. Clearly the partially coated droplets must encounter one another, coming close enough for a polymer molecule to span the gsq between the droplets. This means that the polymer molecules must be able to extend past the primary maximum in the total interaction potential. The absolute limit on polymer size will be defined by the position of the primary maximum and the radius of gyration of the polymer molecules. However, in practice, when polymer molecules become adsorbed to drop surfaces the conformation of the polymer molecule frequently alters to allow multiple attachments to the drop surface, which may reduce the distance the polymer extends into solution. 

A final condition constrains the amount of polymer that nu be added to the system and leads to an estimate of the optimum amount of polymer to maximise flocculation. If sufficient polymer is added to the system to coat the surfaces of the 

The process of adsorption of polymers from solution may take several hours to reach equilibrium conditions, during which time the tendency to flocculate may change radically. Consequently many systems undergoing bridging flocculation are often in a non-equilibrium state with respect to polymer adsorption, which makes the process difficult to model. 

If we now consider the whole emulsion (i.e. many droplets), the overall volume of solvent within the system that is depleted in polymer may be reduced if the droplets get close enough to one another i.e. flocculate) to overlap their depleted regions. By overlapping depleted regions, solvent is released to the bulk where it dilutes the polymer residing there. As this is an energetically favourable process, drops are driven in the direction of increasing flocculation.  

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Percent solid settled In tests where adsorption and flocculation were determined simultaneously, the 200 ml sample was allowed to settle for 30 seconds and then 100 ml of the supernatant was removed using a suction device and after centrifugation analyzed for residual polymer concentration. The minimum level of detection was at 0.5 to 1 ppm and the reproducibility of the adsorption measurements was 2-3%. The settled 100 ml portion was analyzed for solid content. Flocculation due to polymer addition is measured by noting % solid settled as a function of polymer concentration. 

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