Colloidal dispersions subdivision/dispersion

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

As previously pointed out, this book deals mostly with colloidal silicas, that is, disperse systems in which the disperse phase is silica in the colloidal state of subdivision. The colloidal state of subdivision comprises particles with a size sufficiently small (<1 fim) not to be affected by gravitational forces but sufficiently large (>1 nm) to show marked deviations from the properties of true solutions. In this particle size range, 1 nm (10 A) to 1 /xm (1000 nm), the interactions are dominated by short-range forces, such as van der Waals attraction and surface forces. On this basis the International Union of Pure and Applied Chemistry (IUPAC) suggested that a colloidal dispersion should be defined as a system in which particles of colloidal size (1-1000 nm) of any nature (solid, liquid, or gas) are dispersed in a continuous phase of a different composition or state (6). If the particles are solid they may be crystalline or amorphous. The disperse phase may also be small droplets of liquids, as in the case of emulsions, or gases, as for example in foams. 

Colloidal A state of subdivision in which the particles, droplets, or bubbles, which are dispersed in another phase, have at least one dimension between about 1 and 1000 nm. A colloidal dispersion is a system in which colloidal species are dispersed in a continuous phase of different composition or state. 

The microparticles that make up the coating can be of any desired substance composition wise which can be reduced to a colloidal state of subdivision however, they must be dispersible in a medium as a colloidal dispersion. Water is the best medium for dispersions of particles of varying ionic charges. Examples of suitable aqueous sols are amorphous silica, iron oxide, alumina, thoria, titania, zirconia, zircon, and alumina sihcates, including colloidal clays such as montmorillonite, colloidal kaolin, attapul-gite, and hectorite. Silica is preferred material because of its low order of chemical activity, its ready dispersibility, and the easy availabihty of aqueous sols of various concentrations. 

Further subdivision of the colloids concerned many researchers. For example, studies of coagulation processes lead Miiller to connect suspensions with physical disintegration processes and large molecules with chemical precipitation methods. He designated as high molecular such substances as albumin and colloidal silica. The later classification by Staudinger into colloidal dispersions, micellar colloids (assocation colloids), and colloidal molecules (macromolecules) proved to be very suitable and forms the foundation of modern textbooks on colloid science. ... 

For convenience, 1 list the topics of colloid and interface science under two main headings disperse systems and interfacial phenomena. This subdivision does not imply any separation for the following reasons. All disperse systems involve an interface. Many interfacial phenomena are precursors for the formation of disperse systems, e.g. nucleation and growth, emulsiflcation, etc. The main objective of Vol. 1 is to cover the following topics the basic principles involved in interfacial phenomena as well as the formation of colloidal dispersions and their stabilization surfactants and polymer adsorption at various interfaces and interfacial phenomena in wetting, spreading euid adhesion the subject of particle deposition and adhesion is also discussed in detail in Vol. 1. 

Nomenclature. Colloidal systems necessarily consist of at least two phases, the coUoid and the continuous medium or environment in which it resides, and their properties gready depend on the composition and stmcture of each phase. Therefore, it is useful to classify coUoids according to their states of subdivision and agglomeration, and with respect to the dispersing medium. The possible classifications of colloidal systems are given in Table 2. The variety of systems represented in this table underscores the idea that the problems associated with coUoids are usuaUy interdisciplinary in nature and that a broad scientific base is required to understand them completely. 

In industrial and laboratory settings the subdivision process more commonly involves the comminution of large particles or aggregates into smaller sizes, either dry with subsequent dispersion (size reduction to the order of a few pm) or directly in a slurry (size reduction to as small as a few tenths of pm). Examples of comminution machines include agitator ball mills, colloid mills, cutting mills, disk mills, homogenizers, jet mills, mechanical impact mills, ring-roller mills, and roll crushers. 

It is because of the subdivision of matter in colloidal systems that they have special properties. The large surface-to-volume ratio of the particles dispersed in a liquid medium results in a tendency for particles to associate to reduce their surface area, so reducing their contact with the medium. Emulsions and aerosols are thermodynamically unstable two-phase systems which only reach equilibrium when the globules have coalesced to form one macro-phase, for which the surface area is at a minimum. Many pharmaceutical problems revolve around the stabilisation of colloidal systems. 

Figure 1.1 Schematic representation of the subdivision of a cube to give colloidal systems of different kinds (a) slicing of a cube leads to a laminated disperse system with one dimension in the colloid range, (b) cutting a sheet into narrow strips leads to fibrillar disperse systems with two dimensions in the colloid range, (c) cutting of rods or fibrils into particles leads to corpuscular disperse systems with all three dimensions in the colloid range.

An alternative subdivision of colloids which has been widely used in the past is into lyophobic (or hydrophobic, if the dispersion medium is water) and ly op hi lie (hydrophilic, in water) colloids, depending on whether the particles can be described in the former ease as solvent hating or in the latter case as solvent loving . These characteristics are deduced from the conditions required to produce these colloids and from the means available for their redispersion after flocculation or coagulation. It will become apparent later that, while this subdivision has many useful aspects, it is neither entirely logical nor sufficiently all-embracing, and we shall make only limited use of it. 

Dispersions of metallic nanoparticles can be obtained by two main methods (i) mechanic subdivision of metallic aggregates (physical method) or (ii) nucleation and growth of metallic atoms (chemical method). The physical method yields dispersions where the particle size distribution is very broad. Traditional colloids are typically larger (>10nm) and not reproducibly prepared, giving irreproducible catalytic activity. Chemical methods such as the reduction of metal salts is the most convenient way to control the size of the particles. Today, the key goal in the metal colloid area is the development of reproducible nanoparticle (or modem nanocluster) syntheses in opposition to traditional colloids. As previously reported, nanoclusters should be or have at least (i) specific size (1-10 nm), (ii) well-defined surface composition, (iii) reproducible synthesis and properties, and (iv) be isolable and redissolvable ( bottleable )- ... 

As stated earlier, colloids represent a state of subdivision of matter. The matter, finety divided, is uniformly distributed in a continuous medium. However, the dispersed particles are neither so large that they separate on standing, nor so small that they can be said to be in solution. This means that the colloidal state is an intermediate state between a suspension and a true solution. 

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