Stabilisation energy-distance curves

A schematic representation of the variation of G j, G p Ga, and Gj with surface-surface separation distance h is shown in Figure 8.4. G j increases very sharply with a decrease in h, when h<28 likewise, G i increases very sharply with a decrease in h, when h<8 and Gj versus h shows a minimum, G j , at separation distances comparable to 28. When h < 28, Gj shows a rapid increase with decrease in h. The depth of the minimum depends on the Hamaker constant A, the particle radius R, and the adsorbed layer thickness 8. G p increases with increases of A and R and, at a given A and R, also increases with a decrease in 8 (i.e., with decrease in the molecular weight, M, of the stabiliser). This is illustrated in Figure 8.5, which shows the energy-distance curves as a function of S/R. The larger the value of 5/R, the smaller the value of G j in this case, the system may approach thermodynamic stability, as occurs with nanodispersions. 

Figure 8.4 Energy-distance curves for sterically stabilised systems.

Figure 11.3 Energy-distance curves for three stabilisation mechanisms (a) electrostatic (b) steric and (c) electrosteric.

For sterically stabilised dispersions, the resulting energy-distance curve often shows a shallow minimum at particle-particle separation distance h comparable to twice the adsorbed layer thickness 5. For a given material, the depth of this minimum depends upon the particle size R, and adsorbed layer thickness S consequently, decreases with increase in S/R, as illustrated in Figure 11.4. 

A and R. At a given A and R, Gmm increases tvith decrease in S (i.e. with decrease of the molecular weight, Mw, of the stabiliser). Figure 7.31 illustrates this with the energy-distance curves for poly(vinyl alcohol) with various molecular weights (1). Mw varies with 3 as tabulated below. 

Fig. 7.32. Influence of reduction in solvency on the energy-distance curves for sterically stabilised dispersions.

Such a repulsive energy can be produced by charge separation and the creation of electrical double layers, as discussed in detail in Chapters 6 and 7. Combination of the van der Waals attraction and double layer repulsion at various separation distances between the particles produce an energy-distance curve, which will have an energy barrier at intermediate separations [10] and this is the origin of electrostatic stabilisation. The energy-distance curve is controlled by the following parameters. 

For systems stabilised by nonionic surfactants or macromolecules, the energy-distance curve also shows a minimum whose depth depends on particle size, the Hamaker constant and the thickness of the adsorbed layer [94, 95]. This is illustrated in Figure 14.15, which shows the energy-distance curves for polystyrene latex particles containing poly(vinyl alcohol) (PVA) layers of various molecular... 

As discussed above, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at a relatively long distance of separation between the droplets. However, by adding small amounts of electrolyte, such minima can be made sufficiently deep for weak flocculation to occur. The same applies to stericaUy stabihsed emulsions, which show only one minimum, but whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or the addition of a nonsolvent for the chains (e.g., an electrolyte). 

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