Colloidal dispersions kinetics, flocculation

Stability" is a ubiqitous word as applied to colloidal dispersions it may apply to more than one physical process (e.g. flocculation, settling, Ostwald ripening) and may be used in either the kinetic or thermodynamic senses. We restrict discussion here to flocculation and consider first the thermodynamic aspects. 

There are two general theories of the stability of lyophobic colloids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these colloids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of colloids. 

Stability in colloidal dispersions is defined as resistance to molecular or chemical disturbance, and the distance the system is removed from a reference condition may be used as a measure of stability. The stability can be analyzed from both energetic and kinetic standpoints. The kinetic approach uses the stability ratio, as a measure of the stability. W is defined as fhe ratio of the rate of flocculation in the absence of any energy barrier to that when there is an energy barrier due to adsorbed surfactant or polymer. These processes are referred to as rapid and slow flocculation with rate constants kj and kg, respectively, such that W = kjlk. The stability of colloidal suspensions can be evaluated using various techniques. In practice, two methods are mainly used sedimentation and rheology measurements. 

Regulation of the physico-chemical properties of colloid dispersions by polyelectrolytes is of great interest from the theoretical and practical points of view [89]. A consideration of the peculiarities of absorption of polyelectrolytes on disperse particles can be of help to clarify some aspects of kinetics and mechanism of flocculation, elemental acts of flocculation, as well as to provide the se-... 

The predictions of different quantitative criteria for stability-instability transitions were investigated [461], having in mind that the oscillatory forces exhibit both maxima, which play the role of barriers to coagulation, and minima that could produce flocculation or coalescence in colloidal dispersions (emulsions, foams, suspensions). The interplay of the oscillatory force with the van der Waals surface force was taken into account. Two different kinetic criteria were considered, which give similar and physically reasonable results about the stability-instability transitions. Diagrams were constructed, which show the values of the micelle volume fraction, for which the oscillatory barriers can prevent the particles from coming into close contact, or for which a strong flocculation in the depletion minimum or a weak flocculation in the first oscillatory minimum could be observed [461]. 

To prepare stable PVC latexes by emulsion polymerization of VC, considerable attention must be paid to the choice of a proper emulsifier. The emulsifier greatly affects the reaction kinetics and the physico-chemical and colloidal properties of the final polymer product and/or the dispersion [98]. The choice of the proper emulsifier is a complex problem because of its manifold functions, e.g. the surf ace tension of the aqueous solution, emulsification or solubilization of oil-soluble monomers or additives, and the protection of latex particles against flocculation [99]. 

If the maximum value of the potential energy is much higher than the natural kinetic energy of about kgT, about (15—25) 57 , then we may expect that our colloid is stable (metastable, to be precise). A secondary minimum can occur in some cases which correspond to a reversible flocculation. If the potential energy is lower than these values, and certainly when it is smaller than kgT, then the dispersion is unstable. 

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