The Mechanism of Steric Stabilization

FIGURE 10.8. As two surfaces having adsorbed polymer approach, two phenomena occur that produce a net repulsive force between the surfaces, (a) At relatively close approach, but before actual interpenetration of the layers, the local concentration of polymer chains (between particles) increases above the normal equilibrium value giving rise to an osmotic pressure effect solvent molecules move into the area between the surfaces and pushes them apart, (b) At distances where layer interpenetration occurs, the polymer chains begin to lose degrees of freedom (an entropy decrease) and thermodynamic factors introduce a second repulsive term. 

FIGURE 10.9. A polymer may be adsorbed at one end of the chain to produce tails  

On the other hand, for loops, once interpenetration begins, there will be twice as many units affected by the volume restriction effect, leading to a stronger entropic effect. One cannot say, therefore, that one configuration is better than another. In most practical systems, both configurations will be involved. 

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Flocculation studies (6) indicated that the mechanism of steric stabilization operates for the PMMA dispersions. The stability of PMMA dispersions was examined further by redispersion of the particles in cyclohexane at 333 K. Above 307 K, cyclohexane is a good solvent for PS and PDMS, and if the PS-PDMS block copolymer was not firmly anchored, desorption of stabilizer by dissolution should occur at 333 K followed by flocculation of the PMMA dispersion. However, little change in dispersion stability was observed over a period of 60 h. Consequently, we may conclude that the PS blocks are firmly anchored within the hard PMMA matrix. However, the indication from neutron scattering of aggregates of PS(D) blocks in PMMA particles may be explained by the observation that two different polymers are often not very compatible on mixing (10) so that the PS(D) blocks are tending to... 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

Immobilization has also been shown to stabilize against solvent dena-turation of enzymes. However, here we presented suggestive data on the mechanisms of this stabilization. Only the CPO immobilized in 200-A sol-gel showed any solvent or temperature stabilization. CPO bound to matrices with pores smaller than the protein showed little or no stabilization effect owing to surface immobilization alone. This supports the concept that steric hindrance to protein unfolding within a pore is part of the stabilization mechanism. An unresolved question for the future applications of this research is to increase the overall enzyme activity or loading. 

The term steric stabilization has been used by colloid scientists to describe how a lyophilic substance, located on the surface of a lyo-phobic colloid, can prevent aggregation of the dispersion. The phenomenology of steric stabilization has been recognized and put to use over many millenia one notable example is the use, by the ancient Egyptians, of casein as a steric stabilizer of carbon (lamp black) in the production of inks for writing on papyrus. Only in the last 50 years or so has a scientific understanding of steric stabilization mechanisms emerged. 

An example of the commonly used polymer in pharmaceutical systems is the A-B-A block copolymer of polyethylene oxide-polypropylene oxide-polyethylene oxide, PEO-PPO-PEO, commercially available as Poloxamers, Pluronics (BASF). On hydrophobic drug particles or oil droplets, the polymer adsorbs with the B hydrophobic chain (PPO) close to the surface, leaving the two hydrophilic A chains dangling in solution. These nonionic polymers provide stabilization against flocculation and/or coalescence by a mechanism usually referred to as steric stabilization [9]. In order to understand the principles of steric stabilization, one must first consider the adsorption and conformation of the polymer at the solid/liquid or liquid/liquid interface. The PPO chain adsorbs on the surface with many attachment points forming small loops , whereas the A chains (sometimes referred to as tails ) extend to some distance (few nm) from the surface [10]. 

The mechanism of action of the hydrophilic PEG chains can be explained in terms of steric interaction that is well known in the theory of steric stabilization. Before considering the steric interaction one must know the polymer configuration at the par-ticle/solution interface. The hydrophilic PEG chains can adopt a random coil (mushroom) or an extended (brush) configuration. This depends on the graft density of the PEG chains as will be discussed below. The conformation of the PEG chains on the nanoparticle surface determines the magnitude of steric interaction. This configuration determines the interaction of the plasma proteins with the nanoparticles. 

The reason that solubility of the tail is important lies in the mechanism for steric stabilization whereby the tail acts as a molecular spring. Long-chain tails adopt a random coil configuration, where the size of that coil (radius of gyration) increases depending upon how well the chain and the surrounding polymer interact. Although the shape and size of the polymer coil are perturbed by the proximity... 

In our view, Rehbinder s doctrine on the structure-mechanical barrier had been proposed much earher than the idea of steric stabilization. The latter, related (at least, initially) to the conformational statistics of hydrophilic tails and loops, presents only the entropic part of elastidty and is not responsible for the mechanical strength. 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

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