Colloidal dispersions of solids

Would you walk up to a soda fountain and order a triple, chocolate-flavored colloidal dispersion No Yet that s what you do when you ask for a chocolate sundae. Ice cream is a colloidal dispersion of solids in a liquid so is chocolate syrup. Whipped cream is a colloidal dispersion of air in a liquid. 

SN1 and reactions Two varieties of nucleophilic substitution, with unimolecular and bimolecular ratedetermining stages, respectively, sol A colloidal dispersion of solid particles in a liquid, solid A rigid form of matter that maintains the same shape whatever the shape of its container, solid emulsion A colloidal dispersion of a liquid in a solid. Example butter, an emulsion of water in butterfat. solid solution A solid homogeneous mixture of two or more substances. 

An important technique is that in which it is the precursor of the final colloidal particle that is reduced to a colloidal size. Thus a liquid reactant may be emulsified and then caused to react to form a colloidal dispersion of solid particles whose particle size distribution is related to that of the emulsion precursor. The commonest application of this method is in suspension polymerisation, in which an emulsion of monomer droplets, stabilised by a surfactant, is polymerised by adding an initiator which is soluble in the monomer. Polymerisation occurs within the monomer droplet, leading to the formation of a polymer latex. 

Colloidal systems exist in both nature and industry and can consist of either solids or liquids dispersed in either fluids or gases. Blood is a dispersion of the red blood cells (which are similar to self-assembling colloids) in serum and emulsions or microemulsions (see Chapter 8) are dispersions of oil in water or water in oil. Fog, mist, and smoke are dispersions of small particles in gases, while pollution control deals with dispersions of solid particles in air. Foams (dispersion of liquid in a gas at relatively high volume fractions of liquid) are familiar from toothpastes to beer. Many industrial processes make use of colloidal dispersions of solid particles in fluids to tailor the hydrodynamic properties of the fluid or sometimes to produce a system with large amount of internal surface area for catalytic applications. 

In the chapter on Pigmentation (Chapter 8), under the heading Dispersion, we considered how to make stable colloidal dispersions of solid and found that, for stability, it was necessary to keep the particles apart. This could be done by using polymer molecules, anchored strongly to the particle, but also extending out into the solvent, in which they were soluble. These polymer molecules provide a steric barrier around the particle and this method of stabilization is called steric stabilization. We also learnt that aqueous pigment dispersions could be stabilized by adsorbed surfactant molecules, which ionized in the water to produce an electrical charge barrier around the particle ionic stabilization). Exactly the same techniques are used to stabilize emulsions. 

A particular focus of this chapter is colloidal dispersions of solid particles in a liquid. These are both industrially important but also scientifically interesting since model systems can be prepared with which we can probe the intermolecular interactions responsible for colloidal aggregation. As indicated in Table 3.1, such systems are termed sols. Sometimes they are also known as lyophobic solids. This reflects a now-outmoded classification of colloids into those that are solvent hating (lyophobic) and those that are solvent loving (lyophilic). Some examples of sols are described in Section 3.9, whilst the aggregation of model sols is discussed in Section 3.15. Other examples of commonly encountered colloids are described in Sections 3.10 to 3.14. 

As already described, a magnetic fluid is a stable colloidal dispersion of solid ferromagnetic particles with subdomain sizes in a dispersant. The concentration of particles in a dispersant can be as high as 10 /m. Historically, materials similar to magnetic fluids were prepared by dispersing a ferromagnetic powder, such as iron carbonyl, with diameters (d) of... 

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. 

Another family of polyols is the filled polyols.llb There are several types, but die polymer polyols are die most common. These are standard polyether polyols in which have been polymerized styrene, acrylonitrile, or a copolymer thereof. The resultant colloidal dispersions of micrometer-size particles are phase stable and usually contain 20-50% solids by weight. The primary application for these polyols is in dexible foams where the polymer filler serves to increase foam hardness and load-bearing capacity. Other filled polyol types diat have been developed and used commercially (mainly to compete with die preeminent polymer polyols) include the polyurea-based PEID (polyhamstoff dispersion) polyols and the urethane-based PIPA (poly isocyanate polyaddition) polyols. 

Note A colloidal suspension is colloidal dispersion of a solid in a liquid. 

Colloid chemistry investigates substance mixtures. These substance mixtures can be heterogenous, such as emulsions (in which tiny droplets of one liquid are dispersed in another), suspensions (consisting of a fine dispersion of solid particles in a liquid volume phase), and aerosols (in which liquid droplets are dispersed in the gas phase). However, there are also homogenous mixtures in which the solute is present in larger, supermolecular aggregates. These homogenous mixtures include micellar solutions and liquid crystalline... 

The general usage of the terms lyophilic and lyophobic in describing colloidal systems is somewhat illogical. Lyophobic traditionally describes liquid dispersions of solid or liquid particles produced by mechanical or chemical action however, in these so-called lyophobic sols (e.g. dispersions of powdered alumina or silica in water) there is often a high affinity between the particles and the dispersion medium - i.e. the particles are really lyophilic. Indeed, if the term lyophobic is taken to imply no affinity between particles and dispersion medium (an unreal situation), then the particles would not be wetted and no dispersion could, in fact, be formed. Lyophilic ... 

A surfactant used in froth flotation to adsorb onto solid particles, make them hydrophobic, and thus facilitate their attachment to gas bubbles. See also Froth Flotation. A high-shear mixing device used to prepare colloidal dispersions of particles or droplets by size reduction (comminution). Also termed dispersion mill . 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

Liquid sol—A colloidal system of solids dispersed in a hquid. 

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). 

TEX-WET colloidal silica is an aqueous colloidal dispersion of silica particles varying in solids content and particle size. 

Pharmaceutical suspensions are dispersions of solid particles in a suspending medium or vehicle (usually aqueous in nature). When the suspended solids are less than 1 pm, the system is referred to as a colloidal suspension. When the particle sizes are greater than about 1 pm, the system is called a coarse suspension. The practical upper limit for particles in a coarse suspension is approximately 50-75 pm. Depending on the affinity or interaction between the dispersed phase and the dispersion medium, a colloidal dispersion can be classified as lyophilic (hydrophilic) or lyophobic (hydrophobic). ... 

A reconstituted colloidal dispersion of latex particles rather than solvent solution coating material of cellulose acetate phthalate is also available. This white, water-insoluble powder is composed of solid or semisolid submicrometer-sized polymer spheres with an average particle size of 0.2 pm. A typical coating system made from this latex powder is a 10-30% solid-content aqueous dispersion with a viscosity in the 50-100 mPa s range. 

The analysis of cow s milk shows it to be a mixture consisting of 78 per cent water and 12 per cent solid substances in solution or colloidal dispersion. Of this 12 per cent casein, the most important, belongs to a complex group of organic compounds called proteins a sugar (lactose), butter-fat, and small amounts of inorganic salts comprise the remainder. By addition of a small amount of acetic acid the dispersed protein is precipitated. By evaporation of the filtrate the crude sugar is obtained. 

Gel A colloidal dispersion of a solid in a liquid a semirigid sol. Hard water Water containing Fe, Ca +, or Mg + ions, which form precipitates with soaps. 

Solid emulsion A colloidal dispersion of a liquid in a solid. 

A suspension is a dispersion of solid particles in a liquid. A colloidal suspension is a sol. Colloidal properties become significant when the size of the parhcles is of the order of a few micrometer or less. In suspensions of large particles, for example, of some 10 pm or higher, hydrod5mamic interactions dominate the suspension flow properties emd particle packing behaviour. In colloidal suspensions interaction forces between the particles as well as hydro-dynamic interactions play a role in determining the flow and particle packing properties. 

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