Dispersions sterically stabilised

Key words Rheology, concentrated dispersions, sterically stabilised dispersions, polystyrene latex, incipient flocculation... 

Jansen J W, de Kruif C G and Vrij A 1986 Attractions in sterically stabilised silica dispersions. I. Theory of phase separation J. Colloid Interface Sc/. 114 471-80... 

If the polymer layers increases the stability of the dispersion, it is denoted steric stabilisation. The polymer must fulfil two key criteria (i) the polymer needs to be of sufficient coverage to coat all the particle surfaces with a dense polymer layer, and (ii) the polymer layer is firmly attached to the surface. How this is engineered is beyond the scope of this article, but the consequences of not satisfying these criteria are informative in understanding the effect that polymers have on the overall interparticle interaction. Since complete or incomplete coverage of the particles results in very different properties (i.e stability or instability), this is clearly one way in which minimal change in initial conditions can lead to major differences in product. 

Vincent, B., Edwards, J., Emmett, S. and Jones, A. (1986) Depletion flocculation in dispersions of sterically-stabilised particles (soft spheres). Colloids Surf,... 

In studying the stability of colloidal dispersions it is of considerable advantage if the particles concerned are monodisperse and spherical. For aqueous, charge-stabilised systems polymer latices have proved invaluable in this regard. With non-aqueous systems, steric stabilisation is usually required. In this case it... 

Many dispersions are stabilised by polymers. The underlying interaction is often called the steric force. For the understanding of steric interactions it is necessary to know some fundamentals of polymer physics (a good introduction is the book of Strobel [190]). Here we are mainly concerned about linear polymers because these are commonly used for steric stabilization. Fortunately, in many applications we do not need to consider the detailed molecular chemical nature of the polymer such as effects of bond lengths, bond angles, rotation energy, etc. In many discussions we can use simpler models to describe the polymer. 

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. 

Table 8.4 Classification of sterically stabilised dispersions and comparison of critical flocculation temperatures (c.f.t) with theta-temperatures112 (By courtesy of Academic Press Inc.)...

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

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

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. 

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). 

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. 

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. 

The aromatic nuclei of the bisphenol A segment have a high affinity for the aromatic nuclei of styrene - ACN copolymer styrene units, the polyether chains having a strong interaction with the liquid poly ether medium. As an immediate consequence, the structure 6.15 assures a good steric stabilisation of polymeric dispersions in liquid polyether polyols (see the structure in Figure 6.6). 

Ethoxylated nonionics show the lowest interfacial tension as well as high solubilisation and detergency around their cloud points [36, 54]. Sterically, their polyoxyethylene chains can assume two different conformations in aqueous solution at the low ethoxylation is has a fully extended trans or "zig-zag" form and at higher ethoxylation it has a more compact gauche or "meander" form. In the second case the saturation adsorption is not so high but the steric stabilisation of dispersions may be quite effective [7, 8]. 

The jetness properties of dispersant D are also the best across the three chosen pigments, which indicates excellent compatibility of the steric stabilisation chain with the binder system and that particle stability is maintained in the dry film after removal of water. 

Vincent B, Young CA, Tadtos TF (1978) Equilibrium aspects of heteroflocculation in mixed sterically stabilised dispersions. Faraday Discuss Chem Soc 65 296... 

---