Bulk phase colloidal dispersion

Although most colloidal systems are metastable or unstable with respect to the separate bulk phases, they can have an appreciable kinetic stability. That is, the state of dispersion can exist for an appreciable length of time. Colloidal species can come together in very different ways therefore, kinetic stability can have different meanings. A colloidal dispersion can be kinetically stable with respect to coalescence but unstable with respect to... 

Colloidal dispersions that required a relatively large amount of eIectrol)rte in order to obtain conventional coagulation in the bulk phase were not susceptible to surface coagulation. 

Polymers in the form of latex (colloidal dispersions) acquire a novel quality not observed in bulk or solution and which is due to the strongly developed interface with the aqueous phase, The properties of this interface va ry specifically with the nature of the polymer, the latter varying in a wide range for different latexes and other polymer dispersions. 

The high pressures, so conveniently applied to surface films, would be much more difficult to attain in bulk systems. A force of 20 dynes cm. acting on a monomolecular film is equivalent to a pressure of 10 dynes cm., about 100 atmospheres. The high concentrations in the surface are obtained in bulk phases only in pure liquids and sohds. The high electrical fields near charged surfaces are probably never found in bulk solutions except for colloidal dispersions. 

Colloidal dispersions and other related systems are present in many applications, e.g., in paints and coatings and detergents. Here, phase equilibrium and surface phenomena are equally important. A unified representation of such phenomena, e.g., of adhesion phenomena and liquid-liquid equilibria with the same model/concepts is of interest. Thermodynamic models can be used to calculate certain surface properties such as surface tension. hi addition, properties such as the solubility parameters can be equally well employed for bulk and surface thermodynamic properties. ... 

The formation of colloidal dispersions by growth from the molecular state is often controlled by a nuclcation step. An embryo has to achieve a certain minimum size before growth can proceed spontaneously (Chapter 4) but is then limited by the availability of material in the bulk phase from which the particle is growing. 

Viscous, sticky, or waxy materials are easier to dispense in the form of emulsions, as are solids in snspended form. Consequently numerous consumer products are greatly influenced by the knowledge of how to make stable colloidal dispersions. Breaking snch dispersions also has many interesting applications. In secondary oil recovery, for instance, petrolenm is flushed from underground oil fields with water. The material that is extracted is frequently in the form of an emulsion oil-in-water or water-in-od, depending on the relative amounts of the two liquids. As refinery feed streams shonld be free of water, it is necessary to know how to break the emulsion into the two bulk phases. 

The bulk lipid mixtures and the corresponding nanoparticles were intensively studied by Schubert et al. using different methods (e.g. DSC, X-ray, NMR, electron microscopy) to get a deeper insight into the structure of this kind of lipid matrices. The lecithin was dissolved in the molten hard fat (Softisan 154) and colloidal dispersions were prepared after solidification of the lipid matrices by high-pressure homogenization. Solutol was added to the water phase for the stabilization of the nanoparticles. 

H. Lowen, E. Allahyarov, J. Dzubiella, C. von Ferber, A. Jusufi, C. N. Likos, and M. Heni, Interactions and phase transitions of colloidal dispersions in bulk and at interfaces, Phil Trans. R. Soc. London A 359, 909-920 (2000). 

The flow properties of a colloidal system are very much dependent on its microstructure, as determined by the molecular arrangement and interaction of its components. ME systems show flow typical of a Newtonian liquids, for which the shear stress is directly proportional to the shear rate. Since viscosity measurements are dynamic experiments, they will give information on dynamic properties of the ME. These will depend on the miCTOStructure, type of aggregates, or interactions within the ME, which in turn are determined by the concentration of the various components and the temperature. The dispersion of one component in another, e.g., water in oil, will generally increase the bulk viscosity in comparison to the individual components (oil and water) [58]. For at true colloidal dispersion, viscosity will increase with increasing volume fraction of dispersed phase according to the formula generated by Einstein ... 

Another unique phenomenon involving colloidal dispersions stabilized by low molecular weight, weakly adsorbed polymer chains is the depletion flocculation mechanism [41], as shown in Figure 2.12. When an isolated pair of the particles approach each other, the weakly adsorbed polymer chains are squeezed out of the overlap volume due to the greatly reduced space available for these polymer chains. This then results in the imbalance of the local osmotic pressure that is, the concentration of the adsorbed polymer is lower than that in the continuous bulk phase. Thus, water molecules are forced to diffuse out of the overlap region to counterbalance the osmotic pressure effect. The net effect is that the particles are pulled together and flocculation takes place. 

An emulsion may be defined as an opaque, heterogeneous system of two immiscible liquid phases ( oil and water ) where one of the phases is dispersed in the other as drops of microscopic or colloidal size (typically around 1 pm). There are two kinds of simple emulsions, oil-in-water (O/W) and water-in-oil (W/O), depending on which phase comprises the drops. Emulsions made by agitation of the pure immiscible liquids are very unstable and break rapidly to the bulk phases. Such emulsions may be stabilised by the addition of surface-active material which protects the newly formed drops from re-coalescence. An emulsifier is a surfactant which facilitates emulsion formation and aids in stabilisation through a combination of stfrface activity and possible structure formation at the interface. 

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