Stability of dispersions

P. C. Hiemen2, Principles of Colloid and Suf ace Chemisty, 2nd ed., Marcel Dekker, Inc., New York, 1986 R. D. Void and M. J. Void, Colloid and Inteface Chemisty, Addison-Wesley, Reading, Mass., 1983 H. Sonntag and K. Strenge, Coagulation and Stability of Disperse Systems, Halsted, New York, 1972 D. J. Shaw, Introduction to Colloid and Suf ace Chemisty, 3rd ed., Butterworth, London, 1980. 

Dispersion Stability of Disperse Dyes at High Temperature. A disperse dye dyebath is treated under the desired test conditions at 130°C in a special apparatus (Gaston County Lab Dye and Chemical Tester) and filtered through cotton and polyester filters. The filter with the heaviest residue is then compared with a series of standard photographs of standard performance and rated equal to the one it most resembles (1 poor, 5 excellent). 

The steric stabilization of dispersed particles by both grafted chains and by physically adsorbed polymers has been much studied in recent years. In the present paper we extend earlier work on... 

Stumm, W., C. P. Huang, and S. R. Jenkins (1970), "Specific Chemical Interaction Affecting the Stability of Dispersed Systems", Croat. Chem. Acta 42, 223-245. 

Lee et al. s study also investigates the hydrophilicity of the heterocatalyst. They mention that the highly acidic surface of the material is more hydrophobic than the pure titanium oxide surface. They theorize that this is because the acidic surface results in fewer adsorbed OH ions and thus a weaker interaction with water. As expected, this increased hydrophobicity leads to an increase in the stability of dispersions of nanoscale powders of this material. Saltiel et al. showed that WOs-coated titanium oxide powders were much more stable than their uncoated counterparts. Even after agglomeration, the agglomerates of the coated powders were more porous than those of pure titanium oxide (the coated powders had a fractal dimension of 1.55 while the pure titanium oxide powders had a fractal dimension of 1.60). 

Specific chemical interactions affecting the stability of dispersed systems. Croatica Chem. Acta 42 223-245 Su, C. Puls, R.W (2001) Arsenate and arsenite removal by zerovalent iron Kinetics, Redox transformation, and implications for in situ groundwater remediation. Environ. Sd. 

G.J. Fleer, J.M.H.M. Scheutjens and B. Vincent, The Stability of Dispersions of Hard Spherical Particles in the Presence of Nonadsorbing Polymer in Polymer Adsorption and Dispersion Stability, E.D. Goddard and B. Vincent (eds.), American Chemical Society, Washington DC, 1984, ACS Symposium Series 240, Chapter 16, pp. 245-263. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

Izmailova, V.N., Yampolskaya, G.P., Tulovskaya, Z.D. (1999). Development of Reh-binder s concept on structure-mechanical barrier in stability of dispersions stabilized with proteins. Colloids and Surfaces A Physicochemical and Engineering Aspects, 160, 89-106. 

Sonntag, H., and Strenge, K., Coagulation and Stability of Disperse Systems, Halsted, New York, 1964. 

Our primary focus in this chapter is on kinetic stability of dispersions arising due to either electrostatic forces or polymer-mediated forces. 

Clearly, W is a function of any property of the dispersion that affects the strength of the interparticle forces and the energy barrier that slows down (or prevents) coagulation. A classical goal of colloid science has been to develop the equations necessary to predict the extent of stability of dispersions so that the results could be used in combination with the theories of interaction forces developed in previous chapters to promote or prevent the stability of dispersions. 

FIG. 13.11 A simplified representation of the effects of polymer additives on the stability of dispersions. See the text for explanation. (Redrawn with permission from Hunter 1987.)... 

One of the first theoretical attempts to understand steric stabilization of dispersions was based on an entropic mechanism that resembles the elastic contribution to AGR. We consider this mechanism in Example 13.3. 

Discuss the differences between the thermodynamic and kinetic factors that determine the structure and stability of dispersions. Give examples of dispersions with stability that are... 

Comminution also may be used to examine the stability of dispersed phases such as oil droplets. Depending on the viscosity of the product one simply mixes it or breaks it up in a solvent (usually water but, for example, use fresh soybean oil for chocolate), a buffer or the appropriate dyes (below). For instance, we mix easily dispersible foods (cream cheese, ice cream mix or tablespreads) with dyes on slides in a ratio of about 1 1 before examination. Where the dye is a diachrome (that is, highly colored) or is fluorescent in the absence of the substrate (for example, Acridine Orange) some attempt must be made to remove excess, uncomplexed dye molecules which might confound the interpretation. This can be done by reduction of the dye concentration or by making the preparation thinner. The advantage of these simple techniques is that a battery of microchemical tests to identify protein, lipid and carbohydrate can be completed on multiple samples in a very short time period. 

The work was planned on the basis of a model of a dispersed solid particle onto which one type of sequences of a BG copolymer is adsorbed selectively while the other type sequence is dissolved in the dispersion medium. A sketch of this model is shown in Figure 1. The model is the result of applying the same arguments which had been advanced (12) in discussing the mechanism of stabilization of polymeric oil-in-oil emulsions by BG copolymers to the problem of stabilization of dispersions of solid particles in organic media. Previously, essentially the same arguments had led to the demonstration of micelle formation of styrene-butadiene block copolymers in organic media under certain conditions (15). 

Most reports over the past 4 years have dealt with the manipulation of display-related parameters such as electro-optic response and alignment, but increasingly also with thermal effects, pattern formation, nanoparticle-liquid crystal compatibility (i.e., enhancing the stability of dispersions), and to some degree with nanoparticle organization. 

A special, but nevertheless important, case is the interaction between two identical spheres. It is important to understand the stability of dispersions. For the case of two spheres of equal radius R, the parameters x and r are related by (Fig. 6.5)... 

When two such surfaces approach each other, layer after layer is squeezed out of the closing gap (Fig. 6.12). Density fluctuations and the specific interactions then cause an exponentially decaying periodic force the periodic length corresponds to the thickness of each layer. Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [168], Periodic solvation forces across confined liquids were first predicted by computer simulations and theory [168-171], In this case, however, the experimental proof came only few years afterwards using the surface forces apparatus [172,173]. Solvation forces are not only an important factor in the stability of dispersions. They are also important for analyzing the structure of confined liquids. 

The stability of dispersions in aqueous media can often be described by the DLVO theory, which contains the double-layer repulsion and the van der Waals attraction. In some applications other effects are important, which are not considered in DLVO theory. At short range and for hydrophilic particles the hydration repulsion prevents aggregation. Hydrophobic particles, in contrast, tend to aggregate due to the hydrophobic force. 

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