Weak flocculation

Kose A and Flachisu S 1976 Ordered structure in weakly flocculated monodisperse latex J. Colloid Interface Sol. 55 487-98... 

The stability of these dispersions has been investigated. A strong dependence of critical flocculation conditions (temperature or volume fraction of added non-solvent) on particle concentration was found. Moreover, there seems to be little or no correlation between the critical flocculation conditions and the corresponding theta-conditions for the stabilising polymer chains, as proposed by Napper. Although a detailed explanation is difficult to give a tentative explanation for this unexpected behaviour is suggested in terms of the weak flocculation theory of Vincent et al. 

One of the characteristics of weak flocculation is that the system is reversible. At low volume fractions the system will form some clusters and some single particles. The clusters can be easily disrupted by gentle shaking. As the concentration is increased the system will reach a percolation threshold . The number of nearest neighbours around any test particle reaches 3 at about (p — 0.25 and the attractive forces between... 

Many different types of interaction can induce reversible phase transitions. For instance, weak flocculation has been observed in emulsions stabilized by nonionic surfactants by increasing the temperature. It is well known that many nonionic surfactants dissolved in water undergo aphase separation above a critical temperature, an initially homogeneous surfactant solution separates into two micellar phases of different composition. This demixtion is generally termed as cloud point transition. Identically, oil droplets covered by the same surfactants molecules become attractive within the same temperature range and undergo a reversible fluid-solid phase separation [9]. 

With nonadsorbing polymer, rheological effects of similar magnitude accompany the phase transitions described earlier (Patel and Russel, 1989a,b). Since macroscopic phase separation takes weeks or months, rheological measurements performed within a few days on samples formulated within the two-phase region, with — Q>miJkT 2 - 20, detect a metastable structure that changes little over time. The systems respond as flocculated dispersions, but the microstructure recovers relatively quickly to a reproducible rest state after shear. Hence these weakly flocculated dispersions are quite tractable materials. 

Weak flocculation occurs above a critical free polymer concentration which decreases with Increase of molecular weight, of the free polymer. The weeik... 

These problems can be avoided in weakly flocculated gels, which attain thermodynamic equilibrium on experimental time scales. Since an understanding of weakly flocculated gels can form the basis for study of strongly flocculated gels, we shall consider the former first. 

Weakly flocculated gels are less strain sensitive than the strongly flocculated gels discussed above. Figure 7-17 shows the elastic modulus G versus strain y for two gels made from weakly flocculated octadecyl-coated silica spheres (Chen and Russel 1991). For the more weakly flocculated of the two, obtained by lowering the temperature to 29 C, nonlinearity occurs above a critical strain yc of around 10%, while for a somewhat more... 

We have seen that the rheological properties of weakly flocculated gels can be predicted at least qualitatively using reasonable particle-particle interaction potentials derived from van der Waals and polymer depletion forces. Can a similar approach succeed in predicting the mechanical properties of strongly flocculated gels ... 

Ogden, A.L. and Lewis, J.A., Effect of nonadsorbed polymer on the stability of weakly flocculated nonaqueous suspensions, Langmuir, 12, 3413, 1996. 

The calculations for hydrophobic silica reveal that with increasing styrene content of the liquid phase the strength of the pair potentials decrease. The magnitudes of the potentials are reasonable and can be assigned to a weakly flocculated gel. 

The adsorbed layer thickness 5 should be sufficiently large (>5 nm) 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. 

Weak flocculation this occurs when the thickness of the adsorbed layer is small (usually <5 nm), particularly when the particle radius and Hamaker constant are large. 

State (i) represents a phenomenon that is referred to as depletion flocculation and is caused by the addition of a free nonadsorbing polymer [35]. In this case, the polymer coils cannot approach the particles to a distance A (this is determined by the radius of gyration of free polymer R f), as the reduction in entropy on close approach of the polymer coils is not compensated by an adsorption energy. Thus, the suspension particles will be surrounded by a depletion zone with thickness A. Above a CFV of the free polymer, the polymer coils are squeezed out from between the particles and the depletion zones begin to interact At this point the interstices between the particles are free from polymer coils, such that an osmotic pressure is exerted outside the particle surface (the osmotic pressure outside is higher than in between the particles), and this results in a weak flocculation [35]. A schematic representation of depletion flocculation is shown in Figure 9.11. 

Depletion flocculation As discussed above, the addition of a free nonadsorbing polymer can produce weak flocculation above a CFV of the free polymer, [Pg.157]

This weak flocculation produces a gel structure that reduces sedimentation. 

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