Steric stabilisation of suspensions

The use of natural and synthetic polymers (referred to as polymeric surfactants) to stabilise solid/liquid dispersions plays an important role in industrial applications [27] such as in paints, cosmetics, agrochemicals and ceramics. 

Polymers are particularly important for the preparation of concentrated dispersions, i.e. at high volume fraction of the disperse phase j = (volume of all particles)/(total volume of dispersion). 

Polymers are also essential for the stabilisation of non-aqueous dispersions, since in this case electrostatic stabihsation is not possible (due to the low dielectric constant of the medium). 

To understand the role of polymers in dispersion stability, it is essential to consider the adsorption and conformation of the macromolecule at the solid/liquid interface [1]. 

Interaction Between Particles Containing Adsorbed Polymer Layers 

As this subject was covered in detail in Chapter 8, only a brief summary wiU be given at this point When two particles each with a radius R and containing an 

1) An increase in the osmotic pressure in the overlap region as a result of the unfavourable mixing of the polymer chains, when these are in good solvent conditions [28, 29]. This is referred to as osmotic repulsion or mixing interaction, and is described by a free energy of interaction, 

A combination of G j and G j is usually referred to as the steiic interaction free energy, Gg, that is 

The sign of G j depends on the solvency of the medium for the chains. If in a good solvent-that is, the Flory-Huggins interaction parameter is less than 

---

Firstly, the presence of a solid snrface will restrict the conformation of polymer molecules in its vicinity. This type of effect is widely recognised in colloid science where, as already mentioned, it has been extensively stndied with regard to steric stabilisation of suspensions. Detailed treatment can be found in works such as those of Napper [74].The basic findings are that, in the absence of mnltipoint attachment, chain molecules respond to this restriction by expanding in a direction parallel to the surface. This expansion can double the effective molecular size in that direction. 

These authors studied the steric stabilisation of aqueous BaTiOs suspensions with block copolymers, which fulfil both the stabilising and binder function. Some block copolymers with PVA and polyacrylic acid blocks were found to be very suitable for this purpose. They found further that depletion flocculation occurs with random copolymers. In that case the homogeneity of dried layers prepared was lower. 

Fig. 3.13. Flocculation and stabilisation of suspensions by neutral polymers, (a) Lone particles. The range of electrostatic repulsion has been represented dotted line), (b) When a small quantity of polymer is added, the adsorbed layer remains thin since the chain conformation is highly flattened, (c) When the quantity of polymer is such that the size of loops becomes greater than twice the range of electrostatic repulsion, links can be established between particles. Aggregates form in the solution and precipitate out. (d) The particles are saturated with polymer and repel each other via steric effects, (e) The saturation of particle surfaces by polymers with low molecular weight can favour stability, without increasing the risk of flocculation...

Fig. 2.3 Stabilisation of suspensions of solid particles in water creation of a double laya- by sorption of hydrated anti-ions (a) creation of steric hindrances by sorption of organic chain molecules with hydrophobic and hydrophilic functional groups (b)...

Controlled flocculation is a widely used technique for stabilizing suspended systems. The aim is to alter particle surface charge or to achieve particle separation via steric hindrance with the help of appropriate stabilizing excipients. However this is particularly difficult to achieve in non-polar systems such as suspensions in CFC (or HFA) propellants. Controlled flocculation to optimise the stabilisation of MDIs has been recommended by Ranucci et al. but disputed by Hickey, Dalby, and Byron. ... 

Many of the principles here are the same as for normal oral aqueous suspensions, except that the liquid is essentially non-polar, hence steric stabilisation is thought to be more important than by electrostatic means (DLVO theory). A stabilising agent is normally added to give an adequate suspension. The agent may also act as a valve lubricant, which may be necessary for... 

Polyimides are most conveniently prepared by condensation of a bis-aromatic anhydride with a 6w-aromatic amine (Figure 6.13). The first step yields a polyamic acid which can be conveniently dehydrated and cyclised to a polyimide by treatment with acetic anhydride and pyridine under rather mild conditions. This can now be achieved in an organic suspension to yield spherical particulate polyimides [67]. In a typical procedure a polyamic acid precursor is formed in solution in dimethylacetamide (DMAc 10 wt%) by reaction of pyromellitic dianhydride with /7-phenyle-nediamine at room temperature for 2 h. This pre-polymer solution is then suspended in paraffin oil containing a dissolved polymer chosen to function as a steric stabiliser for the DMAc solution. A copolymer of maleic anhydride and octadec-l-ene is very effective. The suspended droplets are kept dispersed by overhead stirring while acetic anhydride and pyridine are added dropwise to induce imidisation, typically reaction proceeds for... 

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

In practice, therefore, the objective is to achieve an intermediate form by the addition of a controlled amount of electrolyte or surfactant. When the particles strongly repel each other, an electrolyte can be added. By decreasing the zeta-potential, the repulsive forces will decrease. When the particles attract each other too strongly a surfactant can be added. As the lyophobic part of the surfactant molecule adsorbs onto the surface of lyophobic colloids its lyophilic part will be oriented into the dispersion medium. By steric stabilisation, the attraction forces are decreased. The properties of flocculated and deflocculated suspensions are summarised in Table 18.18. 

All the same, steric interaction is related to the presence of at least one additional component (adding to continuous and dispersed phase). In that regard, it depends much more on the specific material properties than on the double layer interaction. Steric interaction is relevant for the stabilisation of colloidal suspensions with high electrolyte background or with organic solvents. A good introduction is, e.g., given by Napper (1983). 

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

As discussed before, addition of free non-adsorbing polymer can produce weak flocculation above a critical volume fraction of the free polymer, This weak flocculation produces a gel structure that reduces sedimentation. As an illustration, results were obtained for a sterically stabilised suspension [using a graft copolymer of poly(methyl methacrylate) with poly(ethylene oxide) side chains] to which hydroxyethyl cellulose with various molecular weights was added to the suspension. The weak flocculation was studied using oscillatory measurements. Figure 7.52 shows the variation of the complex modulus G with p. 

A-B, A-B-A block and BAn graft type polymeric surfactants are used to stabilise emulsions and suspensions [18]. B is the anchor chain that adsorbs very strongly at the 0/W or S/L interface, whereas the A chains are the stabilising chains that provide steric stabilisation. These polymeric surfactants exhibit surface activity at the 0/W or S/L interface. The adsorption and conformation of these polymeric surfactant at the interface has been described in detail in reference 18. 

Of particular interest in sterically stabilised dispersions is the study of flow behaviour of such suspensions under the condition of incipient flocculation. This is obtained by reducing the solvency of the dispersion medium through the addition of sufficient electrolyte or by raising the temperature of the suspension, at constant electrolyte concentration [18]. The simplest case to consider is where the adsorbed polymer forms a thick layer (i.e. high molar mass) so that one can neglect any contribution from the van... 

This potential force occurs in microstructured fluids like microemulsions, in cubic phases, in vesicle suspensions and in lamellar phases, anywhere where an elastic or fluid boundary exists. Real spontaneous fluctuations in curvature exist, and in liposomes they can be visualised in video-enhtuiced microscopy [59]. Such membrane fluctuations have been invoked as a mechanism to account for the existence of oil- or water-swollen lamellar phases. Depending on the natural mean curvature of the monolayers boimding an oil region - set by a mixture of surfactant and alcohol at zero -these swollen periodic phases can have oil regions up to 5000A thick With large fluctuations the monolayers are supposed to be stabilised by steric hindrance. Such fluctuations and consequent steric hindrance play some role in these systems and in a complete theory of microemulsion formation. 

In w/o emulsions the hydrocarbon chains of the adsorbed molecules protrude into the oily continuous phase. Stabilisation arises from steric repulsive forces as described in section 7.2.2. Emulsions are more complex than suspensions, because of the possibility (a) of movement of the surfactant into either the continuous or disperse phase, (b) micelle formation in both phases, and (c) the formation under suitable conditions of liquid crystalline phases between the disperse droplets. 

---