Dispersing solid in liquid

Colloid mills are used to grind and disperse solids in liquids and to prepare emulsions. Adjustable clearances are between 0.001 and 0.050in., and peripheral speeds to 10,000ft/min. They are used, for example, to make lubricating greases by dispersion of calcium stearate in hydrocarbon oils. 

Surfactants may be used not only to disperse solids in liquid media, but also to coagulate or flocculate solids already dispersed in liquid media. This may be accomplished with surface-active agents by a number of different mechanisms. 

In terms of the two-phase system which comprises dispersions of solids in liquids, the minimum energy requirement is met if the total interfacial energy of the system has been minimized. If this requirement has been met, chemically, the fine state of subdivision is the most stable state, and the dispersion will thus avoid changing physically with time, except for the tendency to settle manifest by all dispersions whose phases have different densities. A suspension can be stable and yet undergo sedimentation, if a true equilibrium exists at the solid-liquid interface. If sedimentation were to be cited as evidence of instability, no dispersion would fit the requirements except by accident—e.g., if densities of the phases were identical, or if the dispersed particles were sufficiently small to be buoyed up by Brownian movement. 

Dirt is attracted to and held on the skin and clothing mostly by fats and oils that are insoluble in water. Soap, an artificial, human-made (synthetic) substance, cleanses by making fats and oils dispersible in water soap acts as an emulsifier, a substance that disperses solids into liquids in which they are usually immiscible. When used as a body cleanser, for example, soap combines with oils and fats together with dirt particles on the skin, emulsifying and dispersing them in water in such a way that they can be disposed of with the water. 

The solubility of solids in liquids is an important process for the analyst, who frequently uses dissolution as a primary step in an analysis or uses precipitation as a separation procedure. The dissolution of a solid in a liquid is favoured by the entropy change as explained by the principle of maximum disorder discussed earlier. However it is necessary to supply energy in order to break up the lattice and for ionic solids this may be several hundred kilojoules per mole. Even so many of these compounds are soluble in water. After break up of the lattice the solute species are dispersed within the solvent, requiring further energy and producing some weakening of the solvent-solvent interactions. 

COLLOID SYSTEMS. Colloids are usually defined as disperse systems with at least one characteristic dimension in the range 10 7 lo ll> centimeter. Examples include sals (dispersions or solid in liquid) emulsions (dispersion of liquids in liquids) aerosols (dispersions of liquids or solids in gases) /inum (dispersion of gases in liquids or solids) and gels (system, such as common jelly, in which one component provides a sufficient structural framework for rigidity and other components fill the space between the structural units or spaces). All forms of colloid systems are encountered in nature. Products of a colloidal nature arc commonly found in industry and are notably extensive in the food field. Foams, widely used in industrial products, but also the causes of processing problems are described in entries on Foam and Foamed Plastics. 

Pharmaceutical products can be classified as liquid solutions, disperse systems (e.g., emulsions, suspensions), semisolids (e.g., ointments), and solid dosage forms. Liquid solutions are homogeneous mixtures of one or more substances in pharmaceutical liquids. The understanding of the physicochemical properties of liquid solutions and processes to prepare the liquid solutions is an important step in preparing final liquid solution dosage forms. In this chapter, the solutions of gases in liquids, liquids in liquids, and solids in liquids, as well as colligative properties of solutions and their application to pharmacy, are discussed. Disperse systems will be discussed in Chapter 4. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

Control of Precipitate Formation. The problem with the character of the precipitated HML proved to be easily resolved with the particular lignin under study. When the addition of the glacial acetic acid was carried out at temperatures only as high as 50 °C, the reaction product was granular and did not retain large quantities of water in a semisolid mass. However, this condition required a redispersion of the solid after the oxalic acid catalyst had been added. This could be accomplished with mixers designed to homogenize and disperse materials in liquids. 

Unlike a solid-in-liquid suspension, the viscosity of an emulsion may depend upon the viscosity of the dispersed phase. This dependence is especially true when internal circulation occurs within the dispersed droplets. The presence of internal circulation reduces the distortion of the flow field around the droplets (26), and consequently the overall viscosity of an emulsion is lower than that of a suspension at the same volume fraction. With the... 

Bulk Phase Usually refers to a dispersion as a whole. For example, in an emulsion the term bulk phase viscosity refers to the emulsion viscosity, as opposed to the continuous-phase viscosity. Thus the bulk phase is not a separate, single phase at all and may contain dispersed solid and liquid phases. 

Like solutions, dispersed systems do not have to be solids in liquids. Table 4.6.2 lists some different types of colloids that we commonly encounter, probably without knowing that they are colloids. Blood is a colloidal dispersion ... 

Settling and coalescence are common when the dispersed and continuous phases are of different density, and when agitation provides only minimal circulation throughout the vessel. It is, therefore, important to determine the minimum impeller speed, V in to completely incorporate the dispersed phase, as droplets, into the continuous phase (i.e., to remove the initial stratification of the immiscible liquids in a vessel). Most reported work is semiempirical and follows the approach of the just suspended state of solids in liquids described in an earlier entry of this book, as well as by Atiemo-Obeng, Penney, and Armenante. ... 

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