Steric suspensions

Figure 16. Steady shear pr variation with Pee and T for a steric suspension of d = 0.8 pm and 0 = 0.282 (99).

The behaviour of tliese systems is similar to tliat of suspensions in which short-range attractions are induced by changing solvent quality for sterically stabilized particles (e.g. [103]). Anotlier case in which narrow attractions arise is tliat of solutions of globular proteins. These crystallize only in a narrow range of concentrations [104]. 

Disc-like particles can also undergo an Onsager transition—here tire particles fonn a discotic nematic, where tire short particle axes tend to be oriented parallel to each other. In practice, clay suspensions tend to display sol-gel transitions, witliout a clear tendency towards nematic ordering (for instance, [22]). Using sterically stabilized platelets, an isotropic-nematic transition could be observed [119]. 

Suppose we have a physical system with small rigid particles immersed in an atomic solvent. We assume that the densities of the solvent and the colloid material are roughly equal. Then the particles will not settle to the bottom of their container due to gravity. As theorists, we have to model the interactions present in the system. The obvious interaction is the excluded-volume effect caused by the finite volume of the particles. Experimental realizations are suspensions of sterically stabilized PMMA particles, (Fig. 4). Formally, the interaction potential can be written as... 

Steric Stabilization. Steric stabilization was a term first introduced by Heller to explain how adsorbed polyethylene oxide polymers increased the salt concentration required for flocculation of negatively charged aqueous suspensions.(6) Heller s systems were stabilized by both mechanisms, as are most commercial dispersions today, aqueous and non-aqueous. Much of the more recent literature on steric stabilizers has been preoccupied with solubility requirements, for the solubility of polymers is a delicate matter and very sensitive to temperature and solvent... 

Viscosities of concentrated suspensions of carbon black in a white mineral oil (Fisher "paraffin" oil of 125/135 Saybolt viscosity) were measured with a Brookfield viscometer as a function of OLOA-1200 content. Figure 13 shows the viscosities of dispersions with 30 w%, 35 w% and 70 w% carbon black. In all cases the viscosity fell rapidly as the 0L0A-1200 content increased from 0 to 1%, then fell more gradually and levelled off as the 0L0A-1200 content approached 2%. In many respects the reduction in viscosity with increasing OLOA-1200 content parallels the conductivity measurements both phenomena are sensing the buildup of the steric barrier, and this steric barrier weakens, softens, and lubricates the interparticle contacts. As evidenced in foregoing sections, the particles are still flocculated but can be easily stirred and separated mechanically. The onset of electrostatic repulsion at OLOA-1200 contents in excess of 2.5% did not affect viscosities. 

Polymer brushes were found to minimize adsorption of proteins by the soft or steric repulsion of the flexible yet immobihzed macromolecules [179], although a generally valid explanation of the protein resistant properties of some hydrophihc brushes is not available. A similar explanation can be formulated for the improvement of the colloidal stability of particle suspensions, when polymer brush-type layers are bound to small particles. This and other intriguing features of polymer brushes prompted a remarkable experimental and theoretical research activity in order to understand and exploit the unique properties of polymer brushes. 

The amount of adsorbed chemical is controlled by both properties of the chemical and of the clay material. The clay saturating cation is a major factor affecting the adsorption of the organophosphorus pesticide. The adsorption isotherm of parathion from an aqueous solution onto montmorillonite saturated with various cations (Fig. 8.32), shows that the sorption sequence (Al > Na > Ca ) is not in agreement with any of the ionic series based on ionic properties. This shows that, in parathion-montmoriUonite interactions in aqueous suspension, such factors as clay dispersion, steric effects, and hydration shells are dominant in the sorption process. In general, organophosphorus adsorption on clays is described by the Freundhch equation, and the values for parathion sorption are 3 for Ca +-kaoUnite, 125 for Ca -montmorillonite, and 145 for Ca -attapulgite. 

The kinetics of stabilization must match the kinetics of aggregation. While ion diffusion and adsorption is fast (electrostatic stabilization), the diffusion and adsorption of polymers (steric stabilization) is slow and may be a limiting factor which can only be overcome by applying high polymer concentrations. The stability of the electrostatic stabilized suspension in moderately fast processes can for example be monitored and controlled in-line by modem electro-acoustic techniques [13]. 

The most important factor influencing the degree of steric stabilization is the thickness of the adsorbed layer in comparison with the size of the particles [292], The term protection has also been used because the steric stabilizing effect can cause significant salt tolerance on the part of a colloidal dispersion. Some suspensions have been prepared, using high concentrations of polyelectrolytes, that are quite stable in concentrated salt solutions [49]. 

The second contribution to the steric interaction arises from the loss of configurational entropy of the chains on significant overlap. This effect is referred to as entropic, volume restriction, or elastic interaction, Gei. The latter increases very sharply with a decrease in h when the latter is less than 8. A schematic representation of the variation of Gmix, Gei, G, and Gj =G X + Gei + Ga) is given in Fig. 10. The total energy-distance curve shows only one minimum, at h 25, the depth of which depends on 5, R, and A. At a given R and A, G decreases with an increase in 5. With small particles and thick adsorbed layers (5 > 5 nm), G, becomes very small (approaches thermodynamic stability. This shows the importance of steric stabilization in controlling the flocculation of emulsions and suspensions. 

Kinetic control was observed when the reduction proceeded in a suspension or when the substrates contained a methyl or phenyl group at C-6 (68TL735, 68ZOR1096 69ZOR1135). In these cases decreased solvation (favoring C-6) or steric hindrance (favoring C-4), respectively, led to a different product ratio. 

Colloidal suspensions are often stabilized by the adsorption of polymers that are expected to exert additional configurational-steric repulsive forces. The additional, potentially significant van der Waals interactions between polymer coatings... 

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