Colloid destabilization

The collision efficiency factors, describing the extent of the colloid destabilization, within certain limits are equal under perikinetic and orthokinetic conditions (3). 

The colloidal dispersion resulting from thermal/chemical oxidative conversion can rapidly retrograde, which makes immediate neutralization of the acid product to a pH 8.0 to 8.5 mandatory. Discoloration of the paste is lessened when sodium bisulfite is added to the starch slurry and/or sodium sulfite added to the paste. Multivalent ions in the paste can induce colloidal destabilization, and may require the addition of a sequestrant."... 

A typical aqueous suspension polymerization process consists of dispersed monomer droplets and an oil-soluble initiator (50-500 pm). In order to prevent colloidal destabilization, a suspending agent is added to the water in addition to continuous stirring of the monomer dispersion. 

Key words Colloid destabilization with metal ions — colloid collision — shear flow — liquid-solid-separation — homogeneity and heterogeneity of coagulating systems... 

When the colloid destabilization occurs, the kinetics of collapse is often fast. Let us consider that we have a colloid system containing originally n particles per volume. Once destabilized, the particles aggregate by colliding with each other and thus the concentration of particles drops with time. Typieally this phenomenon is represented by the seeond-order equation, often called Smoluchowski model ... 

Relaxation measurements on Agl colloids destabilized by.coulostatic impulses (using Ag-AgI electrodes) have shown durations of the order of 1 ms [10]. This would seem to indicate that relaxation in the Stern layer is slower than Brownian motion. The issue has not been resolved for oxides. One may expect shorter relaxation times because of the fast diffusion of the proton and the OH ion in the strongly structured surface hydration layer. 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

In the simplest emulsions just described, the final separation is into two Hquid phases upon destabilization. The majority of emulsions are of this kind, but in some cases the emulsion is divided into more than two phases. One obvious reason for such a behavior is the presence of a material that does not dissolve in the oil or the water. One such case is the presence of soHd particles, which is common in emulsions for food, pharmaceuticals, and cosmetics. Another less trivial reason is that the surfactant associates with the water and/or the oil to form a colloidal stmcture that spontaneously separates from the two hquid phases. This colloidal stmcture may be an isotropic Hquid or may be a semisoHd phase, a Hquid crystal, with long-range order. 

Our model predicts destabilization of colloidal dispersions at low polymer concentration and restabilisation in (very) concentrated polymer solutions. This restabilisation is not a kinetic effect, but is governed by equilibrium thermodynamics, the dispersed phase being the situation of lowest free energy at high polymer concentration. Restabilisation is a consequence of the fact that the depletion thickness is, in concentrated polymer solutions, (much) lower than the radius of gyration, leading to a weaker attraction. 

Colloid stability conferred by random copolymers decreased as solvent quality worsened and became increasingly solvent dependent around theta-conditions. However, dispersions maintain some stability at the theta-point but destabilize close to the appropriate phase separation condition. 

Simple electrolyte ions like Cl, Na+, SO , Mg2+ and Ca2+ destabilize the iron(Hl) oxide colloids by compressing the electric double layer, i.e., by balancing the surface charge of the hematite with "counter ions" in the diffuse part of the double... 

Specifically sorbable species that coagulate colloids at low concentrations may restabilize these dispersions at higher concentrations. When the destabilization agent and the colloid are of opposite charge, this restabilization is accompanied by a reversal of the charge of the colloidal particles. Purely coulombic attraction would not permit an attraction of counter ions in excess of the original surface charge of the colloid. 

Relationship between MnC>2 colloid surface area concentration and ccc of Ca2+ a stoichiometric relationship exists between ccc and the surface area concentration in case of Na+, however, this interaction is weaker, so that primarily compaction of the diffuse part of the double layer causes destabilization. 

In steric stabilization the colloids are covered with a polymer sheath stabilizing the sol against coagulation by electrolytes. In sensitization or adsorption flocculation, the addition of very small concentrations of polymers or polyelectrolytes leads to destabilization (Lyklema, 1985). 

As shown in Fig. 7.7d polymers can destabilize colloids even if they are of equal charge as the colloids. In polymer adsorption (cf. Fig. 4.16) chemical adsorption interaction may outweigh electrostatic repulsion. Coagulation is then achieved by bridging of the polymers attached to the particles. LaMer and coworkers have developed a chemical bridging theory which proposes that the extended segments attached to one of the particles can interact with vacant sites on another colloidal particle. 

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