Steric stabilizer systems

Prestidge and Tadros (96), Liang et al. (95), and Tadros et al. (209) studied the viscoelastic responses of both electrostatically and sterically stabilized systems. 

Note that in calculating A for sterically stabilized systems, allowance must be made not only for the intervening dispersion medium between the particles but also for the presence of any adsorbed layers (Vincent, 1973b). A is typically of order kT for latex particles, although it can be substantially larger for dispersions of metals or for inorganic sols (Gregory, 1970 Visser, 1972). 

Note that the spontaneous redispersion of dried particles is a characteristic feature of most sterically stabilized systems, in marked contrast to the behaviour of many (though certainly not all) electrostatically stabilized systems. Note, too, that fresco paints were also produced by dispersing suitable pigments in water. This technology presumably had as one of its antecedents the preparation by Palaeolithic man of cmde paints for cave paintings, an activity that chronologically preceded the preparation of ink in Egypt and China by many millennia. These paints", which were often based on iron oxide, were, however, commonly prepared without a binder. 

A general rule governing the thermodynamic limit to stability of sterically stabilized systems... 

The unimportance of van der Waals forces Electrostatically stabilized dispersions are readily coagulated by the addition of electrolyte. This reduces the spatial extension of the double layers to the point where instability is induced as a consequence of the London attraction between the colloidal particles. The question then arises as to whether the van der Waals forces are also responsible for the flocculation that is observed in sterically stabilized systems. A crude calculation of the magnitude of the attraction between the core particles in typical cases casts considerable light upon this question. 

The flocculation discussed above could conceivably arise front a dramatic dimensional collapse of the stabilizing moieties (the so-called coil-globule transition). This would be analogous to the coagulation of electrostatically stabilized dispersions which arises from the dimensional collapse of the double layers on adding electrolyte. It is therefore necessary to review the evidence, both theoretical and experimental, supporting the existence of the coil-globule transition to assess the likelihood of its occurrence in sterically stabilized systems. 

The data presented in Tables 6.1 and 6.2 show unequivocally that neither of the predictions of the Cairns and Neustadter hypothesis are realized for most sterically stabilized systems (see, however. Chapter 9). The results of Dawkins and Taylor (1980) (highlighted in Table 6.8) are especially illuminating in this context. These authors stabilized poly(methyl methacrylate) particles with monodisperse poly(dimethylsiloxane) chains in a n-heptane/effianol (51/49, v/v) mixture. The molecular weight of the stabilizing moieties was varied from... 

In summary, for nonaqueous dispersions, the combinatorial free energy of interpenetration favours stabilization. Both of the corresponding free energies associated with contact dissimilarity and free volume dissimilarity favour flocculation. These conclusions are represented schematically in Fig. 7.2. Since the combinatorial free energy is purely entropic in origin, it is scarcely surprising that nonaqueous sterically stabilized systems are usually found to be entropically stabilized at room temperature and pressure for it is this term that imparts stability. Anticipating the results of the next section, we stress that this does not necessarily imply that all nonaqueous dispersions are entropically stabilized at room temperature. 

The foregoing experimental studies present a pattern of agreement that leaves little doubt that the overall shapes of the potential energy curves for sterically stabilized systems are in qualitative accord with the theoretical predictions. The general shape is determined inter alia by the quality of the solvency of the dispersion medium for the stabilizing moieties, the molecular weight of the stabilizing moieties and the particle size. 

In conclusion it may be stated that there is now abundant experimental evidence to justify the assertion that the potential energy diagram for sterically stabilized systems is qualitatively different from that for electrostatically stabilized systems. The primary maximum that is so important in determining whether or not stability is observed in electrostatically stabilized systems appears to be absent for many sterically stabilized dispersions. 

A characteristic feature of sterically stabilized systems is their different responses to temperature change. Those in (i) flocculate on cooling, while for (ii) flocculation occurs on heating, and for (iii) there is no accessible (critical) temperature for flocculation. Entropic stabilization seems to be more common in non-aqueous media, whereas enthalpic stabilization is more frequently encountered in aqueous media. Examples of the three types of stabilization were given by Napper. ... 

FIGURE 10.12. In a sterically stabilized system cxjntaming low-molecular-weight or weakly adsorbed polymer (a), as two particles approach, the loosely bound polymer may desorb, leaving bare spots on the approaching surfaces, leading to an enhanced flocculation tendency (b). That process is referred to as depletion flocculation. ... 

For sterically stabilized systems the total interaction energy now becomes... 

Figure 4 The total potential energy curve for a sterically stabilized system (sohd hne) comprising a steric interaction (dotted line) and a van der Waals/electrostatic term (dashed line).

The combination of van der Waals attraction with steric repulsion (combination of mixing and elastic interaction) forms the basis of the theory of steric stabilization [20]. Figure 1.3 (b) gives a schematic representation of the force-distance curve of sterically stabilized systems. This force-distance curve shows a shallow minimum at separation distance h comparable to twice the adsorbed layer thickness (28) and when h < 28, very strong repulsion occurs. Unlike the V-h curve predicted by the DLVO theory (which shows two minima), the V-h curve of sterically stabilized systems shows only one minimum whose depth depends on the particle or droplet radius R, the Hamaker constant A and the adsorbed layer thickness 8. At a given R and A, the depth of the minimum decreases with increasing the adsorbed layer thickness 8. When the latter exceeds a certain value (particularly with small particles or droplets) the minimum depth can become < kT and the dispersion approaches thermodynamic stability. This forms the basis of stability of ntmodispersions. 

To keep the particles well dispersed (as single particles) high steric repulsion is required to overcome strong van der Waals attraction. A schematic representation of the layers required for stabilization and the resulting energy-distance curve for such sterically stabilized system is given in Fig. 1.53. 

Fig. 2.30 Energy-distance curves for sterically stabilized systems.

Latex dispersions have attracted a great deal of interest as model colloid systems in addition to their industrial relevance in paints and adhesives. A latex dispersion is a colloidal sol formed by polymeric particles. They are easy to prepare by emulsion polymerization, and the result is a nearly monodisperse suspension of colloidal spheres. These particles usually comprise poly(methyl methacrylate) or poly(styrene) (Table 2.1). They can be modified in a controlled manner to produce charge-stabilized colloids or by grafting polymer chains on to the particles to create a sterically stabilized dispersion. Charge-stabiHzed latex particles obviously interact through Coulombic forces. However, sterically stabilized systems can effectively behave as hard spheres (Section 1.2). Despite its simpHcity, the hard sphere model is found to work surprisingly well for sterically stabilized latexes. 

Figure 5.8 Schematic representation of three alternative effects of the adsorption of stiff hydrocolloid polymers on the surface of spherical emulsion droplets, depending on the hydrocolloid concentration and the nature of the hydrocolloid-protein interaction (a) A sterically stabilized system, b) an emulsion gel, and (c) a system flocculated by macromolecular bridging (Dickinson, 2003).

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