Enhanced steric stabilisation

In contrast, if >0.5 (i.e., the chains are in a poor solvent condition) then G j will be negative and the mixing interaction will become attractive. G j is always positive, and hence in some cases stable dispersions can be produced in a relatively poor solvent (enhanced steric stabilisation). 

When >0.5, becomes negative (attractive) this, combined with the van der Waals attraction at this separation distance, produces a deep minimum causing flocculation. In most cases, there is a correlation between the critical flocculation point and the 0-condition of the medium. A good correlation is found in many cases between the critical flocculation temperature (CFT) and the 0-temperature of the polymer in solution (with both block and graft copolymers the 0-temperature of the stabilising chains A should be considered) [2]. A good correlation was also found between the critical volume fraction (CFV) of a nonsolvent for the polymer chains and their 0-point under these conditions. In some cases, however, such correlation may break down, and this is particularly the case for polymers that adsorb by multipoint attachment. This situation has been described by Napper [2], who referred to it as enhanced steric stabilisation. 

Adamina M, et al. Encapsulation into sterically stabilised liposomes enhances the immunogenicity of melanoma-associated Melan-A/MART-1 epitopes. Br J Cancer 2004 90 263. 

Concerning the enhanced steric and electrostatic stabilisation mechanism, for example, the secondary DBS layer has the -SO3- groups oriented towards the dispersion medium forming a local electric field with hydrated NH4+ ions around the particles. Consequently, the degree of stability of water-based magnetic fluids depends on the pH of the medium and the stabilization mechanism outlined earlier is specific only to water as the dispersion medium. 

Sterically stabilised vesicles prepared with the addition of triblock copolymers of the poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO) type, namely Poloxamers or Pluronics (PEO-PPO-PEO), have shown enhanced stabilisation [43]. Steric stabilisation of phospholipid vesicles by the copolymer molecules has been attempted by two different techniques (1) Addition of the block copolymer to preformed vesicles (method A) and (2) addition of the block copolymer to the lipid before formation of the vesicles (method I). In the latter, both the lipid and copolymer participate in the construction of the vesicle. A schematic picture of the resulting vesicle structure for the two methods is given in Figure 13.28. 

Finally, some studies have been performed on the addition of salt to the aqueous phase of oil-in-water HIPEs [109]. For systems stabilised by ionic surfactants, increasing salt concentration reduces the double-layer repulsion between droplets however, stability is more or less maintained, probably due to steric and polarisation repulsions. Above a sufficiently high salt concentration, emulsions become unstable due to salting-out of the surfactant into the oil-phase. For nonionic surfactants, the situation is similar, except that there are no initial double-layer forces. In addition, Babak [115] found that increasing the electrolyte concentration reduced the barrier to coagulation between emulsion droplets, and therefore increased coalescence. Generally, therefore, stability of o/w HIPEs is not enhanced by salt addition. 

Acid-catalysed hydrolysis offers a cheap and obvious method for the cleavage of arylmethylene acetals but oftentimes the conditions required (or pH 1) can be quite harsh, e.g. dilute sulfuric acid at elevated temperature. The lability of arylmethylene acetals can be enhanced both electronically or sterically. For example, p-methoxybenzylidene acetals hydrolyse about 10 times faster than benzylidene acetals79 owing to participation of the p-methoxy substituent in resonance stabilisation of the oxonium intermediate as illustrated by the hydrolysis shown in Scheme 3.48.80 p-Methoxybenzylidene acetals can be hydrolysed in 80% aqueous acetic acid at room temperature. 

Lu et al. have reported that the fluorescence intensities of Eu3+ and Tb3+ are markedly enhanced when they are bound on to a poly(acrylamide-acrylic acid copolymer). Dhake et al. have observed interesting differential effects of pressure on the fluorescence of a polymer and the corresponding monomer. Sterically hindered piperidines have for long been of interest as efficient photostabilisers of polymers. Gugumus has now reported that these stabilisers form charge-transfer complexes with molecular oxygen, thereby preventing the formation of such complexes with the polymer. 

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