Stability of colloidal suspensions

The stability of iron oxide suspensions is relevant to fields as varied as the paint industry, extraction of iron from its ores, the structure of soils, hydrometallurgy and waste water treatment. The ease of homogensisation of paint, for example, is controlled by proper adjustment of the stability of the pigment suspensions. In ground waters, the settling behaviour of small iron oxide particles influences transportation of trace elements and radio-nuclides. The stability of a dispersion of magnetic particles can determine the quality of ferrofluids and magnetic tapes. 

The most widely used theory of suspension stability, the DLVO theory, was developed in the 1940s by Derjaguin and landau (1941) in Russia and by Verwey and Overbeek (1948) in Holland. According to this theory, the stability of a suspension of fine particles depends upon the total energy of interaction, Vt, between the particles. Vf has two components, the repulsive, electrostatic potential energy, Vr, and the attractive force, Va, i. e. 

The repulsive force depends on the double layer potential and thickness, the particle radius and the dielectric constant of the medium, whereas the attractive force arises from retarded London/van der Waals forces. 

1) An alternative theory originally proposed by Langmuir (1938) is discussed in the article by McBride (1997). 

Repulsive forces between Fe oxide particles can be established by adsorption of suitable polymers such as proteins (Johnson and Matijevic, 1992), starches, non-ionic detergents and polyelectrolytes. Adsorption of such polymers stabilizes the particles at electrolyte concentrations otherwise high enough for coagulation to occur. Such stabilization is termed protective action or steric stabilization. It arises when particles approach each other closely enough for repulsive forces to develop. This repulsion has two sources. 1) The volume restriction effect where the ends of the polymer chains interpenetrate as the particles approach and lose some of their available conformations. This leads to a decrease in the free energy of the system which may be sufficient to produce a large repulsive force between particles. 2) The osmotic effect where the polymer chains from two particles overlap and produce a repulsion which prevents closer approach of the particles. 

The zeta potential, then, is the dominant factor in stability, and it is determined almost entirely by the type of exchangeable ions that are present on the clay. Referring to the equation for the zeta potential  

In general, stability means the quality of a substance or a system to remain at the same state. In practice, one needs to specify if state refers to the isotopes of elements, the chemical composition, the state of matter, or something else. When talking about the stability of colloidal suspensions or suspension stability, one usually addresses the size distribution of the particles, the homogeneity of the disperse system, and/or the stmcture of the suspension. Hence, the meaning of suspension stability is somewhat ambiguous. Moreover, a suspension can be 

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The stability of colloid suspensions is an important criteria in the manufacture of a large number of industrial products where these are the basic building blocks (food colloids, pollution control, emulsions, wastewater treatment). 

The DLVO-theory is named after Derjaguin, Landau, Verwey and Overbeek and predicts the stability of colloidal suspensions by calculating the sum of two interparticle forces, namely the Van der Waals force (usually attraction) and the electrostatic force (usually repulsion) [19],... 

TABLE 5 Chemical Factors that Affect the Stability of Colloidal Suspensions... 

While forming methods are the focus of the present section, two factors have a significant effect on the ability to control the geometry and structure of the green body the use of additives that are commonly polymeric and the stability of colloidal suspensions. 

It has been shown that the stability of colloidal suspensions can also be influenced by a pure alcohol-water mixture, without the addition of any surface active agent. In a study of the flocculation of polystyrene emulsions in ethanol-water mixtures (42), the concentration of sodium chloride required to produce rapid flocculation increases with increasing ethanol concentration up to 0.09 molar fraction, beyond this composition, the concentration of sodium chloride required for flocculation decreases rapidly. It will be very informative, therefore, to compare our coagulation rate obtained in the microemulsion media to that in pure IPA + water mixture. The results can be used to further delineate the role of inverted micellar structure on the enhancement of coagulation. 

J. Liu and E. Luijten (2004) Stabilization of colloidal suspensions by means of highly charged nanoparticles. Phys. Rev. Lett. 93, p. 247802 S. Asakura and F. Oosawa (1954) On interaction between two bodies immersed in a solution of macromolecules. J. Ghem. Phys. 22, pp. 1255-1256... 

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. 

Let us tinaily mention that the assumption of the existence of the three contributions to the total potential energy is known as extended DLVO theory of the stability of colloidal suspensions, as opposed to the classical model, originally developed by Derjaguin and Landau, and Verwey and Overbeek. universally called DLVO theory of stability. 

According to the classic DVLO theory (from the names Deriagin, Landau, Ver-wey and Overbeck) the stability of colloidal suspension is determined by the equilibrium between the van der Waals attraction forces (VJ and the electrostatic repulsion, occurring between the electric double layers ( F ). The changes of these forces vs the distance between the colloid particles is shown in Fig. 5.16. The repulsion forces are directly proportional to the product of charges of both particles and decreases with the second power of the distance between them as follows ... 

In many ceramic systems it is not possible to create a stable suspension simply by controlling pH. Large additions of acid or base can result in dissolution of the particles, or provide a too high ionic strength. Hence, addition of suitable polymeric dispersants is commonly used to create colloidally stable suspensions. These polymeric additives can induce an interparticle repulsion that prevents coagulation. Upon the close approach of two particles covered with adsorbed polymer layers, the interpenetration of the polymer layers give rise to a repulsive force, the so-called steric stabilization (10). There are some simple requirements for steric stabilization of colloidal suspensions, as follows ... 

The study of polymer solutions at interfaces has most often been motivated by the effects of polymers on the stability of colloidal suspensions. 

In the case of ionic substances the stabilization of colloidal suspensions in their sol form is based on electrostatic repulsion and the development of electrie double layers. Besides the chemical interaction between the dispersed and continuous phases, gelation can be induced by lowering the temperature of the system and increasing the concentration of the dispersed phase. 

Each of these properties may be beneficial or unwanted. The large size of aggregates, when compared to the primary particles, improves powder handling, but may deteriorate the optical properties (colour/opacity) of the final product or may require intensive dispersion when, e.g., used in polishing slurries. Sometimes just the presence or absence of aggregates is of interest, e.g., when evaluating the stability of colloidal suspensions. 

The kinetic of aggregation determines the morphology of the evolving aggregates and is a decisive factor for the formation and strucmre of particle networks (gelling systems). Last but not least, the aggregation kinetics is used as a measure for the stability of colloidal suspensions (cf. Sect. 5.2.4). 

The stability of colloidal suspensions is important for their processing and the final product quality (shelf life). It is related to size and concentration of the particles and is essentially affected by the particle-particle interactions. 

The stability of colloidal suspensions is frequently examined for a certain variety of suspension properties (e.g. solid content, liquid phase, concentration of ionic, or polymeric additives). That is, for instance, relevant for developing suspension formulas and preparation procedures for particle characterisation or for predicting the particle behaviour in environmental milieus. A typical problem of such parameter studies is that a variation in the concentration of the charge determining additive (e.g. a pH variation as in Fig. 5.10, p. 261) coincides with a significant variation of the total electrolyte content. This problem is most pronounced for dense suspensions (pv 1 vol%) of very small particles (or particles with a high specific surface area). 

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