Stabilization electrostatic colloidal

In addition to electrostatic colloid stabilization generated by anionic surfactants, liquid dispersions are also made from nonionic surfactants. Stabilization of the emulsion is achieved by electrosteric stabilization or by pure steric stabilization (2,13). Polyoxyethylene dodecyl ethers, polyoxyethylene nonyl phenyl ethers, and polyoxyethylene nonyl phenol ethers are a few surfactants typically used in emulsion polymerization with nonionic surfactants (14-16). Non-ionic emulsion polymerizations are characterized by lower critical micelle concentration than their ionic counterparts. Thus, the emulsion particle sizes are generally much larger than in the ionic polymerizations. The mechanism of radical entry and exit in polymeric surfactant stabilizer systems are different than in anionic systems. With water-soluble initiators, the kinetics depends on initiator concentration. 

Those species capable of stabilizing a nanosized metal phase can be classified in three classes, according to their mechanisms of action (i) electrostatic, based on the classical theory of electrostatic colloidal stabilization, (ii) steric and (iii) electrosteric, which is a combination of the electrostatic and steric modes (Ott Finke, 2007). 

Based on the application of the established theory of colloid stability of water treatment particles [8,85-88], the colloidal particles in untreated water are attached to one another by van der waals forces and, therefore, always tend to aggregate unless kept apart by electrostatic repulsion forces arising from the presence of electrical charges on the particles. The aggregation process... 

Many investigators of steric stabilization have measured colloidal stability without taking the effort to find out whether the stability actually resulted from electrostatic stabilization. In many published articles it has been concluded that steric stabilization had been attained and further study showed this was not the case. One such example is a recent paper on "steric" stabilization by an additive of the same type used in this work. (12) The published photograph shows the silica particles in oil stabilized at interparticle separations several times the distances provided by the adsorbed films no electrical measurements had been made, but it they had, this particular dispersant would have provided about -200 mV of zeta-potential and given excellent electrostatic repulsion. The reader should be wary of any claims of steric stabilization unless the electrostatic contribution has been measured. 

In such systems the requirement of the electrostatic contribution to colloidal stability is quite different than when no steric barrier is present. In the latter case an energy barrier of about 30 kT is desirable, with a Debye length 1/k of not more than 1000 X. This is attainable in non-aqueous systems (5), but not by most dispersants. However when the steric barrier is present, the only requirement for the electrostatic repulsion is to eliminate the secondary minimum and this is easily achieved with zeta-potentials far below those required to operate entirely by the electrostatic mechanism. 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

Table 7.3 Colloid Stability as Calculated from van der Waals Attraction and Electrostatic Diffuse Double-Layer Repulsion >b)...

Electrostatic vs. Chemical Interactions in Surface Phenomena. There are three phenomena to which these surface equilibrium models are applied regularly (i) adsorption reactions, (ii) electrokinetic phenomena (e.g., colloid stability, electrophoretic mobility), and (iii) chemical reactions at surfaces (precipitation, dissolution, heterogeneous catalysis). 

For colloidal solutions, as a general rule, a barrier of 15-25kT is sufficient to give colloid stability, where the Debye length is also relatively large, say, greater than 20 nm. This electrostatic barrier is sufficient to... 

Once the dirty spot is removed from the substrate being laundered, it is important that it not be redeposited. Solubilization of the detached material in micelles of surfactant has been proposed as one mechanism that contributes to preventing the redeposition of foreign matter. Any process that promotes the stability of the detached dirt particles in the dispersed form will also facilitate this. We see in Chapter 11 how electrostatic effects promote colloidal stability. The adsorption of ions —especially amphipathic surfactant ions —onto the detached matter assists in blocking redeposition by stabilizing the dispersed particles. Materials such as carbox-ymethylcellulose are often added to washing preparations since these molecules also adsorb on the detached dirt particles and interfere with their redeposition. 

In contrast to the situation in the case of van der Waals and electrostatic forces, very little is known about polymer-induced forces. The development of the surface force apparatus and scanning tunneling and atomic force microscopies have begun to shed light on this very difficult topic. In Section 13.6, we take a brief look at some of the polymer-induced forces of interest in colloid stability and structure. 

Despite the fact that there is much that is unknown about colloid stability, the topics covered in the chapter are sufficient to solve many routine problems of industrial interest, particularly in the case of electrostatic stability. More advanced information on polymer-induced forces is available in specialized monographs (Napper 1983 Israelachvili 1991 Sato and Ruch 1980) and in other texts on colloid science (Hunter 1987). 

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... 

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