Steric stabilization DLVO theory

Colloid stability depends significantly on the (often) attractive van der Waals forces and the (often) repulsive electric and steric forces. DLVO theory accounts for the van der Waals and electric forces and these are the ones which are discussed in this chapter. The main... 

DLVO Theory. The overall stabiUty of a particle dispersion depends on the sum of the attractive and repulsive forces as a function of the distance separating the particles. DLVO theory, named for Derjaguin and Landau (11) and Verwey and Overbeek (12), encompasses van der Waals attraction and electrostatic repulsion between particles, but does not consider steric stabilization. The net energy, AGp between two particles at a given distance is the sum of the repulsive and attractive forces ... 

The stabilization mechanisms of colloidal materials have been described in Derjaguin-Landau-Verway-Overbeek (DLVO) theory [8, 9]. Colloids stabilization is usually discussed in terms of two main categories, namely charge stabilization and steric stabilization. 

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of coagulation, polymer-induced forces, and steric stabilization (Chapter 13)... 

A number of chapters have been overhauled so thoroughly that they bear only minor resemblance to their counterparts in the first edition. The thermodynamics of polymer solutions is introduced in connection with osmometry and the drainage and spatial extension of polymer coils is discussed in connection with viscosity. The treatment of contact angle is expanded so that it is presented on a more equal footing with surface tension in the presentation of liquid surfaces. Steric stabilization as a protective mechanism against flocculation is discussed along with the classical DLVO theory. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like surface hydration and steric effects, have received considerable attention over the past several decades [194,278],... 

The principles of colloid stability, including DLVO theory, disjoining pressure, the Marangoni effect, surface viscosity, and steric stabilization, can be usefully applied to many food systems [291,293], Walstra [291] provides some examples of DLVO calculations, steric stabilization and bridging flocculation for food colloid systems. 

According to Deijaguin-Landau-Verwey-Overbeek (DLVO) theory, a cornerstone of modem colloid science, two types of forces exist between colloidal particles suspended in a dielectric medium electrostatic forces, which result from an unscreened surface charge on the particle, and London-van der Waals attractive forces, which are universal in nature. The colloidal stability and rheology of oxide suspensions, in the absence of steric additives, can be largely understood by combining these two forces (assumption of additivity). 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

The DLVO theory, which has been used with success to explain stabilization mainly in aqueous systems, states that the overall interaction energy is the sum of the electrostatic and van der Waals forces. However, this has been extended now to include the steric forces as well. The total interaction energy can be expressed as... 

Many experimental studies have shown DLVO theory is not sufficient to describe particle stability in environmental systems, and additional non-DLVO interactions (hydrophobic/hydrophilic and steric) have been applied to an extended DLVO theory. The unique and size-dependent properties of nanoparticles may require additional modification of DLVO theory to model their interactions in aqueous environments. 

Probably the most direct and, in principle, the most powerful method for determining the distance dependence of steric interactions is the compression of polymer chains attached to macroscopic objects as they undergo close approach. The most successful method to-date has unquestionably been the crossed mica cylinders technique. This was originated by Tabor and coworkers (Tabor and Winterton, 1969 Israelachvili and Tabor, 1972) and improved by Israelachvili and Adams (1976 1978). In the hands of Pashley and Israelachvili (1981), it has been successfully exploited to test the predictions of the Deqaguin-Landau-Verwey-Overbeek (DLVO) theory of electrostatic stabilization. It is also likely in the future to prove to be the most sensitive method for testing the predictions of theories for the distance dependence of steric interactions. 

These forces originate from entirely different sources and therefore may be evaluated separately. The interplay of (i) and (ii) forms the basis of the classical theory of flocculation of lyophobic dispersions, flrst proposed by Derjaguin and Landau in Russia and independently by Verwey and Overbeek in the Netherlands and hence now known as the DLVO theory. The interplay of (i) and (iii) is commonly termed steric stabilization , and much has been written on this protective mechanism, although a workable understanding has developed only during the last two decades. 

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