Adsorbed steric stabilisation

Dispersions of fine mineral particles can be stabilised by direct electrical charging of the particles or by steric/electrosteric protection from adsorbed polymers. Stabilisation by direct charging is well described by the classical DLVO theory. ... 

Already the ancient Egyptians knew that one can keep soot particles dispersed in water when they were incubated with gum arabicum, an exudate from the stems of acaia trees, or egg white. In this way ink was made. The reason for the stabilizing effect is the steric repulsive force cause by adsorbed polymers. In the first case these are a mixture of polysaccharide and plycoprotein, in the second case it is mainly the protein albumin. Steric stabilisation of dispersions is very important in many industrial applications. Direct quantitative measurements were... 

The stability of many protected colloidal dispersions cannot be explained solely on the basis of electric double layer repulsion and van der Waals attraction other stabilising mechanisms must be investigated. Steric stabilisation is a name which is used (somewhat loosely) to describe several different possible stabilising mechanisms involving adsorbed macromolecules. These include the following ... 

Steric stabilisers are usually block copolymer molecules (e.g. poly (ethylene oxide) surfactants), with a lyophobic part (the anchor group) which attaches strongly to the particle surface, and a lyophilic chain which trails freely in the dispersion medium. The conditions for stabilisation are similar to those for polymer solubility outlined in the previous section. If the dispersion medium is a good solvent for the lyophilic moieties of the adsorbed polymer, interpenetration is not favoured and interparticle repulsion results but if, on the other hand, the dispersion medium is a poor solvent, interpenetration of the polymer chains is favoured and attraction results. In the latter case, the polymer chains will interpenetrate to the point where further interpenetration is prevented by elastic repulsion. 

Surfactants or polymers adsorbed on the particle surface are able to keep particles that far ap-part that the Van der Waals attraction cannot become effective. This phenomenon is called steric stabilisation. 

Steric stabilisation. Particles with large molecules adsorbed on the surface are repelled by each other because the freedom for chain movement decreases if particles approach (this... 

The sterically stabilised dispersions produced can be weeikly flocculated by the addition of free (non-adsorbing) polymer such as poly(ethylene oxide). 

The distribution of segments in loops and tails, p(z), which extend in several layers from the surface. p(z) is usually diflBcult to obtain experimentally, although recently the application of small-angle neutron scattering has been used to obtain such information. An alternative and useful parameter for assessing steric stabilisation is the hydrodynamic thickness, Sf, (the thickness of the adsorbed or grafted polymer layer plus any contribution from the hydration layer). Several methods can be applied to measure 5, as will be discussed below. 

Stability of Disperse Systems Containing Adsorbed Nonionic Surfactants or Polymers Steric Stabilisation... 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

Interaction between Particles Containing Adsorbed Nonionic and Polymeric Sur ctant Layers (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. 

The efficiency of steric stabilisation depends on both architecture and the physical properties of the stabilising molecule. Steric stabilisers should have an adsorbing anchor with a high affinity for the particles and/or be insoluble in the medium. The stabiliser should be soluble in the medium and highly solvated by its molecules. For aqueous or highly polar oil systems, the stabiliser block can be ionic or hydrophilic... 

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. 

Interaction between Oil or Water Droplets Containing an Adsorbed Polymeric Surfactant Steric Stabilisation... 

Figure 12.9), which shows a shallow minimum, G (weak attraction) at h 28 that is, at h 20 nm for the present W/O emulsion based on PHS-PEO-PHS block copolymer. When h < 25, Gj is increased very rapidly with further decreases in h. The depth of the minimum, G , will depend on the adsorbed layer thickness. In the present W/O emulsion, based on a PHS layer thickness of about 10 nm, G j is very small (fraction of kT). This shows that, with the present sterically stabilised W/O emulsion, there is only a very weak attraction at a relatively long distance of... 

Steric Stabilisation and the Role of the Adsorbed Layer Thickness 281... 

As most nanoemulsions are prepared using nonionic and/or polymeric surfactants, it is necessary to consider the interaction forces between droplets containing adsorbed layers (steric stabilisation). As this was described in detail in Chapter 10, only a summary will be given here [15, 16]. 

With a sterically stabilised dispersion, weak flocculation can also occur when the thickness of the adsorbed layer decreases. Again, the value of E can be used as a measure of the flocculation the higher the value of E, the stronger the flocculation. 

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