Flocculated phase

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

In contrast to this, based on experimental results, a new theory has recently been developed by us [37], which is applicable to all heterogeneous polymer systems [88,89], Our main principle is to define the non-equilibrium character of colloidal dispersions in polymeric matrices and to interpret the experimental findings (phase separation, dispersion-flocculation phase transition, [84]) as dissipative structures . This term has been first introduced by Prigogine [77a,b] for... 

Therefore, the following hypothetical mechanism has been developed in accordance with the dispersion/ flocculation phase transition model for conductive blends and in accordance with results published about impact modification. 

Once an associative thickener is completely desorbed from the latex, it behaves like a non-adsorbing conventional thickener and can flocculate the latex by the depletion mechanism (see Section 13.3.1.1). Flocculation phase diagrams can be constructed showing the state of flocculation or deflocculation of the latex at a specified solids content, over a range of thickener and surfactant or cosolvent concentrations [97]. Because of their lower molar mass and less volumefilling backbones, the threshold concentration for desorbed HEUR thickeners to cause depletion flocculation is much higher than that for high molar mass cellulose ethers. 

These results give rise to a picture of the drying process like that drown in Figure 14.6 [12c]. In this picture one sees an ordered dispersion in the bulk with a particle separation of 13 run. and a flocculated phase at the air-water intoface. According to van Tent, as water is lost, fire particle density in the bulk remains constant, but the floe layer at the surface becomes tbidmr until it reaches the substrate. 

This image has similarities to that presented CroU in his modd of the drying process. CroU envisions the top l er to be transparent and essentially dry, but with a percolation network of tir pores. This sits atop the flocculated phase, which in turn rests upon tire aqueous dispeisian. Tent s results require the flocculated phase to be wet, witii die particles separated by a water-swollen hydrophilic membrane. If, howevo-, there were a diin transparmt layer at the top which grew steadUy in thickness, the changing interference between reflections at the upiper and lower interfaces would almost certainly be doected in die UV-visible transmission measurements. 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

If there is particle—particle interaction, as is the case for flocculated systems, the viscosity is higher than in the absence of flocculation. Furthermore, a flocculated dispersion is shear thinning and possibly thixotropic because the floccules break down to the individual particles when shear stress is appHed. Considered in terms of the Mooney equation, at low shear rates in a flocculated system some continuous phase is trapped between the particles in the floccules. This effectively increases the internal phase volume and hence the viscosity of the system. Under sufficiently high stress, the floccules break up, reducing the effective internal phase volume and the viscosity. If, as is commonly the case, the extent of floccule separation increases with shearing time, the system is thixotropic as well as shear thinning. 

The sequence, flocculation — coalescence — separation, is compHcated by the fact that creaming or sedimentation occurs and that this process is determined by the droplet size. The sedimentation velocity is monitored by the oppositely directed forces which form the buoyancy and the viscous drag of the continuous phase on the droplet ... 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

Solutions of polychloroprene adhesives containing metal oxides and r-butyl phenolic resin may show phasing (e.g. clear upper layer and flocculated lower layer of metal oxides) on standing for days or months. To recover the full utility... 

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