Flocculation of Sterically Stabilised Dispersions

3) The stabilising chain A should be highly soluble in the medium and strongly solvated by its molecules. Examples of A chains in aqueous media are poly(ethylene oxide) and poly (vinyl alcohol). 

4) 8 should be sufficiently large ( 5 run) to prevent weak flocculation. 

This occurs when the thickness of the adsorbed layer is small (usually 5 nm), and particularly when the particle radius and Hamaker constant are large. The minimum depth required to cause weak flocculation depends on the volume fraction of the suspension the higher the volume fraction the lower the minimum depth required for weak flocculation. This can be understood by considering the free energy of flocculation that consists of two terms (i) an energy term determined by the depth of the minimum (G ) and (ii) an entropy term that is determined by reduction in configurational entropy on aggregation of particles. 

With dilute suspensions the entropy loss on flocculation is larger than with concentrated suspensions. Hence, for the flocculation of a dilute suspension a higher energy minimum will be required when compared to the case with 

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. 

When X 0.5, Gmix becomes negative (attractive), which, when combined with the van der Waals attraction at this separation distance, gives a deep rninimum, causing flocculation. In most cases, there is a correlation between the critical floe- 

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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.)...

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

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

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. 

Rheological studies of sterically stabilised concentrated polystyrene latex dispersions under conditions of incipient flocculation... 

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... 

As mentioned above, the contribution from the van der Waals attraction at separations corresponding to twice the adsorbed layer thickness is insignificant and, therefore, Es Gs- Thus, r jcan be theoretically calculated from a knowledge of G5 and application of equation (6). Indeed, Neville and Hunter [15] attempted to interpret their versus temperature plots for sterically stabilised dispersions of PMMA particles in the presence of MgS04, using the above simple theory. However, in order to calculate Gs under flocculating conditions, it is necessary to know... 

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... 

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. 

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