Solid-liquid separation colloidal forces

Removal of cationic impurities from water. Careful analysis of water purified by various methods (see Table 7.10) indicates that the water that is obtained by passing ordinary distilled water through a small monobed deionizer (contained in polyethylene) and a submicrometer filter is equal or superior (with respect to cations) to water obtained by distillation in conventional quartz stills, and is distinctly superior to the product from systems constructed of metal.70 From the data available in the literature, simple distillation clearly does not produce high-purity water. In practice, two effects cause contamination of the distillate. Entrainment is the major factor that prevents the perfect separation of a volatile substance from nonvolatile solids during distillation. Rising bubbles of vapor break through the surface of the liquid with considerable force and throw a fog of droplets (of colloidal dimensions) into the vapor space... 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

ULTRAFILTRATION. Ultrafiltration is a pressure-driven filtration separation occurring on a molecular scale. See also Dialysis Filtration Hollow-Fiber Membranes Membrane Separations Technology and Reverse Osmosis. Typically, a liquid including small dissolved molecules is forced through a porous membrane. Large dissolved molecules, colloids, and suspended solids that cannot pass through the pores are retained,... 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

Closely related to this is the general problem in the whole chemical industry of the separation of solids from liquids. The processes of thickening, flocculation, dewatering, and filtration are all intimately controlled by the forces and structures arising in colloidal systems. The improvements brought about by the use of electroseparation processes depend on the exploitation of the electrokinetic effects discussed in Chapter 6. 

FIGURE 4.30 The forces acting on a large (convex) particle in the course of its separation from the substrate with a constant velocity of0.005 nnits the external force acted from the side of the surface (solid line) and was equal to the external force in a vacuum (1) the force acting from the side of a liquid on the lower surface of the particle (2). The resulting force acting on the particle from the side of a liquid (i.e., the disjoining pressure) can be seen from a comparison of these two cnrves. (Redrawn from Yushchenko, V.S. et al.. Colloids Surf., no, 63, 1996.)... 

The measurement of forces between surfaces at small separation is of great importance in gaining a fundamental understanding of the complex behaviour of colloidal systems. Interactions of solid surfaces (coated or uncoated) across a fluid medium have been made through development of both the surface forces apparatus (SFA) and the atomic force microscope. Interactions between liquid surfaces are generally discussed in terms of the variation of the disjoining pressure with surface separation, and the majority of studies relate to foam (i.e. vapour-liquid-vapour) Investigations of various aspects of emulsion (i.e. liquid-liquid-... 

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