Electrolytes colloid stability

Figure 6.9 Influence of electrolyte concentration on colloid stability ( denotes change from a stable to an aggregated state)...

Among the various branches in colloid and interface science, polymer adsorption and its effect on the colloid stability is one of the most crucial problems. Polymer molecules are increasingly used as stabilizers in many industrial preparations, where stability is needed at a high dispersed phase volume fraction, at a high electrolyte concentration, as well as under extreme temperature and flow velocity conditions. 

As we shall see colloid stability can be affected by electrolytes and by adsorbates that affect the surface charge of the colloids and by polymers that can affect particle interaction by forming bridges between them, or by sterically stabilizing them. 

Physical model for colloid stability. Net energy of interaction for spheres of constant potential surface for various ionic strengths (1 1 electrolyte) (cf. Verwey and Overbeck). 

Some of the pertinent interactions that affect colloid stability are readily apparent from Figs. 7.4 and 7.12. The main effect of electrolytes is a more rapid decay of the repulsion energy with distance and to compact the double layer (reducing k 1). Experimentally it is known that the charge of the counterion plays an important role. The critical electrolyte concentration required just to agglomerate the colloids is proportional to z 6 Aj for high surface potential, and to z 2 A, 2, at low potentials [(4) and (5) in Table 7.3]. This is the theoretical basis for the qualitative valency rule of Schulze and Hardy. 

The use of surface charge to provide colloid stability to particles dispersed in dilute electrolytes in aqueous solution, or even in media of intermediate polarity, is an effective means of stabilising particles against van der Waals forces of attraction. Figure 3.16 shows typical potential... 

Schulze and Hardy (1882-1900) studied the effects of electrolytes on colloid stability. 

Throughout most of this chapter the emphasis has been on the evaluation of zeta potentials from electrokinetic measurements. This emphasis is entirely fitting in view of the important role played by the potential in the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. From a theoretical point of view, a fairly complete picture of the stability of dilute dispersions can be built up from a knowledge of potential, electrolyte content, Hamaker constants, and particle geometry, as we discuss in Chapter 13. From this perspective the fundamental importance of the f potential is evident. Below we present a brief list of some of the applications of electrokinetic measurements. 

As we noted above, the evaluation of W for given values of dispersion properties such as surface potential, Hamaker constant, pH, electrolyte concentration, and so on, forms the goal of classical colloid stability analysis. Because of the complicated form of the expressions for electrostatic and van der Waals (and other relevant) energies of interactions, the above task is not a simple one and requires numerical evaluations of Equation (49). Under certain conditions, however, one can obtain a somewhat easier to use expression for W. This expression can be used to understand the qualitative (and, to some extent, quantitative) behavior of W with respect to the barrier against coagulation and the properties of the dispersion. We examine this in some detail below. 

The role of polymers on colloid stability is considerably more complicated than electrostatic stability due to low molecular weight electrolytes considered in Chapter 11. First, if the added polymer moieties are polyelectrolytes, then we clearly have a combination of electrostatic effects as well as effects that arise solely from the polymeric nature of the additive this combined effect is referred to as electrosteric stabilization. Even in the case of nonionic... 

The question to be discussed is whether saturation of the electric field (asserted by Proposition 2.1) implies saturation of the interparticle force of interaction. Consider for definiteness repulsion between two symmetrically charged particles in a symmetric electrolyte solution. In the onedimensional case (for parallel plates) the answer is known—the force of repulsion per unit area of the plates saturates. (This follows from a direct integration of the Poisson-Boltzmann equation carried out in numerous works, primarily in the colloid stability context, e.g., [9]. Recall that again in vacuum, dielectrics, or an ionic system with a linear screening, the appropriate force grows without bound with the charging of the particles.)... 

Polyacrylic acid stabilised latices have been prepared by aqueous dispersion polymerisation. The method used is analogous to the non-aqueous dispersion (NAD) polymerisation methods originally used to prepare polymethyl methacrylate particles in aliphatic hydrocarbons (1. In effect the components of a NAD polymerisation have been replaced as follows aliphatic hydrocarbon by aqueous alcohol, and degraded rubber, the stabiliser, by polyacrylic acid (PAA). The effect of various parameters on the particle size and surface charge density of the latices is described together with details of their colloidal stability in the presence of added electrolyte. 

Figure 5.9 Illustration of the effect of electrolyte on colloid stability. The photomicrographs A through D show how 1.1 tm size silica particles are progressively coagulated by increasing additions of alum (0, 10, 30, 40 ppm, respectively). The corresponding zeta potentials are -30 mV (A), -14 mV (B), -6 mV (C), and -0 mV (D). From Zeta-Meter [544], Courtesy L.A. Ravina, Zeta-Meter, Inc., Staunton, Va.

In the framework of the traditional double layer theory, when there is a strong adsorption of a multivalent cation of the electrolyte, the stability ratio of a colloidal dispersion first decreases, passes through a minimum, followed by a maximum, after which it decreases as the concentration of the (multivalent) electrolyte increases. When the adsorption is weak, the stability ratio decreases mono-tonically with increasing electrolyte concentration. For a monovalent electrolyte, the stability ratio calculated in the framework of the classical double layer theory decreases monotonically with increasing electrolyte concentration. Consequently, in the traditional theory, the ion screening becomes dominant at high ionic strength, whereas experiment appears to show that the colloidal dispersion is restabilized. 

A quantitative treatment of the effects of electrolytes on colloid stability has been independently developed by Deryagen and Landau and by Verwey and Over-beek (DLVO), who considered the additive of the interaction forces, mainly electrostatic repulsive and van der Waals attractive forces as the particles approach each other. Repulsive forces between particles arise from the overlapping of the diffuse layer in the electrical double layer of two approaching particles. No simple analytical expression can be given for these repulsive interaction forces. Under certain assumptions, the surface potential is small and remains constant the thickness of the double layer is large and the overlap of the electrical double layer is small. The repulsive energy (VR) between two spherical particles of equal size can be calculated by ... 

FIGURE 10.27 (a) Zeta potential as a function of pH for A1203 in an indifferent 1 1 electrolyte solution (i.e., l Os). (b) Colloid stability ratio for the same AI2O3 sol as a function of pH. The minimum values correspond to the isoelectric point at pH == 9. Data from Wiese and Healy [65]. 

Mixtures of anionic and nonionic surfactants are usually employed. The anionic emulsifiers are the less water soluble and control the number and size of the particles. The nonionic surfactants are often ethylene oxide condensates of alkyl phenols their water solubility is proportional to ihedcgree of polymerization of the poly(ethylene oxide) component. Their function is primarily to provide colloidal stability against electrolytes, mechanical shearing, and freezing. 

The most striking difference between the group of silicas and most other oxides Is that over several pH units above pH° the oxide Is reluctant to dissociate protons, but beyond that charging becomes very easy. This observation does not stand on its own for a number of silicas the colloid stability Is inversely related to 0° In that uncharged sols are very resistant against coagulation by Indifferent electrolytes whereas they become less stable with increasing pH 2-3 4). This... 

But often more important than the effects of the electrolytes that influence the thickness of the electric double layer are many solutes that, upon adsorption onto the colloid surface, reduce or modify the surface charge. The specific adsorption of H, OH, metal ions, and ligands (as well as the attachment of polymers) to the colloid surface affects the surface charge and the surface potential and, in turn, the colloid stability. 

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