Heterocoagulation process

The description of dynamic adsorption layers under the condition of bubble/bubble or bubble/particle interaction is much more complex than the consideration of dynamic adsorption layers of individual bubbles discussed in the present chapter. It is still more difficult to control dynamic adsorption layers experimentally under conditions of the above-mentioned interactions. Because of these experimental difficulties, the role of mathematical modelling is extremely important in studying coagulation and heterocoagulation processes in foams and emulsions. 

Beyond electrostatic and hydrophobic forces, the heterocoagulation process could be controlled by secondary molecular interactions. We will briefly highlight with some examples the hydrogen bonding, ji-ti interactions, and speciflc molecular interactions obtained from complementary DNA strands, and biotin-avidin complexation. 

In the light of shell formation on colloidal particles our consideration would not be complete if we do not mention about surface controlled precipitation. The described in [45] results demonstrate that slow heterocoagulation process of polymer at the suspension of colloidal particles is a way to controlled coating the particles by polymeric films (Fig. 2.6). Precipitation could be caused either by non-soluble complex formation between polyelectrolyte and multivalent ions (Fig. 2.6A) or by mixing the polymer solution with non-solvent (Fig. 2.6A). 

Fig. 2.6. Schematic representation of heterocoagulation process of polymers in suspension of colloidal particles. Polymers precipitation is caused either by complexation with multivalent ions (A) or by mixing with nonsolvent (B)...

One mechanism that is consistent with the observed properties of the particles in these suspensions involves the dissolution of amorphous Si02 and adsorption of soluble silicate on the Fe(OH)3 surface. This process could occur in parallel with the heterocoagulation mentioned earlier. Soluble silicate species might then compete with Se03 or PO4 for surface sites as suggested by Goldberg (8) for the P04/silicate/goethite system. Sorption of silicate species onto Fe(OH)3 need not affect cationic adsorbates. Benjamin and Bloom (10) demonstrated that adsorption of cations is often minimally affected by anion adsorption even under conditions where anion-anion competition is severe (11). 

Unlike the original flotation whose elementary act is complicated by an inertia impact and the accompanying deformation of bubble surface, microflotation is completely a colloid chemical process and it can be described in terms of modem colloid chemistry as orthokinetic heterocoagulation (Deijaguin Dukhin, 1960). 

The interpretation of the attachment process during flotation of small particles in terms of the heterocoagulation theory, as suggested by Derjaguin Dukhin (1960), has been considered in a number of review articles on the theory of flotation (Joy Robinson 1964, Usui 1972, Rao 1974) and was further confirmed in many studies (Derjaguin Shukakidse 1961, Jaycock Ottewil 1963, Rubin Lackay 1968, Devivo Karger 1970, Collins Jameson 1977). 

Since heterocoagulation is a stochastic process, great care needs to be taken not to end up with large fractal clusters or flocks of the two colloidal components. Driving forces to promote adhesion of inorganic nanoparticles onto the surface of polymer latex particles, or vice versa, can be based on a variety of forces, such as electrostatic attraction, hydrophobic interactions, and secondary molecular interactions such as (multiple) hydrogen bond interactions and specific molecular recognition (e.g. complementary proteins like avidin-biotin). 

In the previous section, we have seen that hard inorganic nanoparticles can adhere onto the surface of polymer latex particles via a stochastic process of collisions, which was referred to as heterocoagulation. Once deposited onto the surface of the latex particles, the strength of adhesion governed by attractive forces such as electrostatic attraction, the hydrophobic effect, and hydrogen bond interactions needs to outbalance repulsive forces and the entropy gain achieved when nanoparticles detach. This potential detachment of nanoparticles from the surface of the polymer latex particle is typically induced by the thermal energy of the system, k T (where is the Boltzmann constant and T is temperature). 

The methods of destroying aerosols, whether liquid or sohd, are numerous. Some of the more common are illustrated in Figure 13.7. One of the most important from a practical standpoint is the use of a spray (usually water) to wash the aerosol from the gas phase. As already mentioned, for aerosols, almost every collision between aerosol particles, collisions with container walls, or collision with a water droplet will be sticky. For two aerosol particles the result is homocoagulation for the other cases the process is heterocoagulation. In each case the result will be an increase in the size of... 

Figure 2 TEM image of deposition of Ti02 nanoparticles on single CNTs during heterocoagulation by colloidal processing [51]...

The combined Ni nanoparticles deposition and PTFE suspension from the sulfamic-acid electrolyte during nickel-coating at room temperature leads to polymer inclusion into the Ni deposit. The polymer may contribute up to 20 wt%. The process is controlled by varying the cathode current density and the concentration of the suspension introduced. The relationship between the homocoagulation and heterocoagulation interactions of metal particles and polymer is determined by the rates of electrochemical deposition and the trapping of Ni in the PTFE systems. 

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