Colloidal dispersions flocculation

In general, A and B subchains in an AB- or an ABA-type block copolymer have different solubilities or affinities for a solvent or other polymers. Therefore, it is expected that a block copolymer is surface-active when dissolved in a suitable solvent or mixed in polymer melts108. This property of block copolymers is now utilized to stabilize or flocculate colloidal dispersions. Blocks A, which are insoluble in a given solvent, are anchored in an insoluble polymer particle, and blocks B, which are soluble in the solvent, form a surface layer around the particle. 

Russel, W., Distinguishing between dynamic yielding and wall slip in a weakly flocculated colloidal dispersion , Colloid and Surfaces, A Physiochem. and Eng. Asp., Vol. 161, No. 2 (December 1999)... 

Jenkins P and Snowden M 1996 Depletion flocculation in colloidal dispersions Adv. Colloid Interface Sc/. 68 57-96... 

Chen M and Russel W B 1991 Characteristics of flocculated silica dispersions J. Colloid Interface Sol. 141 564-77... 

Problems which arise with certain precipitates include the coagulation or flocculation of a colloidal dispersion of a finely divided solid to permit its filtration and to prevent its re-peptisation upon washing the precipitate. It is therefore desirable to understand the basic principles of the colloid chemistry of precipitates, for which an appropriate textbook should be consulted (see the Bibliography, Section 11.80). However, some aspects of the colloidal state relevant to quantitative analysis are indicated below. 

Polyacrylamides are nonionic polymers, usually with much higher molecular weights (MW from 100,000 up to 12 or 15 M). They often are copolymerized with polyacrylates. Depending on the MW ratios employed, they may act as colloidal dispersants, sludge conditioners, or flocculants. Nonionics such as polyacrylamides (and isobutylenes) are particularly useful at dispersing uncharged particles. 

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

Stability" is a ubiqitous word as applied to colloidal dispersions it may apply to more than one physical process (e.g. flocculation, settling, Ostwald ripening) and may be used in either the kinetic or thermodynamic senses. We restrict discussion here to flocculation and consider first the thermodynamic aspects. 

LA Mer, V. K. Disc. Faraday Soc. 42 (1966) 248. Filtration of colloidal dispersions flocculated by anionic and cationic polyelectrolytes. 

Van der Woude, J.H.A. Rijnbout, J.B. De Bmyn, P.L. (1984 a) Formation of colloidal dispersions from supersaturated iron(III) nitrate solutions. IV. Analysis of slow flocculation of goethite. Colloids Surfaces 11 391-400... 

Porous inkjet papers are in general created from colloidal dispersions. The eventual random packing of the colloid particles in the coated and dried film creates an open porous structure. It is this open structure that gives photographic-quality inkjet paper its apparently dr/ quality as it comes off the printer. Both the pore structure and pore wettability control the liquid invasion of the coated layer and therefore the final destination of dyes. Dispersion and stability of the colloidal system may require dispersant chemistries specific to the particle and solution composition. In many colloidal systems particle-particle interactions lead to flocculation which in turn leads to an increase in viscosity of the system. The viscosity directly influences the coating process, through the inverse relation between viscosity and maximum coating speed. 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

The spontaneous shrinking of a colloidal dispersion due to the release and exudation of some liquid frequently occurs in gels and foams but also occurs in flocculated suspensions. Mechanical syneresis refers to enhancing syneresis by the application of mechanical forces. Micro-syneresis is a special case of syneresis in which the polymer molecules cluster together while retaining some of the original bulk gel structure. This process creates regions of free liquid within the gel network. 

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

The presence of free polymer in sterically stabilized colloidal dispersions is shown to be responsible for the occurrence of phase sepa-ration/flocculation in dilute dispersions. The theoretical calculations show that the limiting polymer concentration for the onset of phase separation decreases with increasing molecular weight of the free polymer and with increasing particle concentration, as observed experimentally. The two limiting cases of no penetration and total penetration of the free polymer into the adsorbed layers represent extreme ends with the real situation being somewhat intermediary between the extreme cases. 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

How do potential determining ions affect colloid flocculation or dispersion ... 

Many processes in soil are controlled by colloid flocculation or dispersion. One such process is hydraulic conductivity. The data in Figure 10.3 show that for a Mg2+-saturated soil containing a solution of 3.16 x 10 2 M MgCl2, its hydraulic conductivity decreased by 35% after 5 hr of leaching with distilled water (Quirk and Schofield, 1955). This demonstrates that as solution ionic strength approaches zero, soil hydraulic conductivity decreases significantly owing to soil dispersion induced by a decompressed electric double layer. 

The asymptotic approach has not yet been extended to interactions between stretched layers in theta and poor solvents, which control the incipient flocculation of polymerically-stabilized colloidal dispersions. Application of the mean-field theory to poor solvents produces an attractive minimum only for —nll2v/w112 > new112/l, when layers begin to interpenetrate rather than... 

The interaction potentials described in previous sections for adsorbing homopolymer and terminally anchored layers in good solvents clearly indicate the ability of polymers to stabilize colloidal dispersions against flocculation due to van der Waals dispersion forces. Indeed, the practice preceeded the analyses by centuries in some cases and decades in others, since the use of adsorbing polymers dates to ancient times, and block copolymer stabilizers emerged from industrial laboratories in the 1960s (Napper, 1983). 

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