Lyophobic colloids emulsions

The process in which small amounts of added hydrophilic colloidal material make a hydrophobic colloid more sensitive to coagulation by electrolyte. Example the addition of polyelectrolyte to an oil-in-water emulsion to promote demulsification by salting out. Higher additions of the same material usually make the emulsion less sensitive to coagulation, and this is termed protective action or protection . The protected, colloidally stable dispersions that result in the latter case are termed protected lyophobic colloids . 

Emulsion systems can be considered a subcategory of lyophobic colloids. Like solid-liquid dispersions, their preparation requires an energy input, such as ultrasonication, homogenization, or high-speed stirring. The droplets formed are spherical, provided that the interfacial tension is positive and sufficiently large. Spontaneous emulsification may occur if a surfactant or surfactant system is present at a sufficient concentration to lower the interfacial tension almost to zero. 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

Lyophobic Colloid An older term used to refer to two-phase colloidal dispersions. Examples suspensions, foams, and emulsions. 

The stability of an emulsion, once formed, towards electrolytes added to the system shows close resemblance to that of sols or suspensions of solid particles. In this respect emulsions may be compared with normal lyophobic colloids. Accordingly... 

Lyophobic colloids, which include all emulsions other than the microemulsions, are not formed spontaneously on contact of the phases, because they are thermodynamically unstable when compared with the separated states. These dispersions can be formed by other means, however. Most emulsions that will be encountered in practice contain oil, water, and an emulsifying agent. The emulsifier may comprise one or more of the following simple inorganic electrolytes, surfactants. 

The previous chapters have introduced several classes of colloids and some of the important surface aspects of their formation, stabilization, and destruction. Emulsions, foams, and dispersions are the most commonly treated and intensely studied examples of colloidal systems. They constitute the majority of practical and ideal systems one encounters. There exists one other class of true, lyophobic colloids—the aerosols—which, although seemingly less important in a theoretical or applied sense, are of great practical importance. 

Emulsions and foams belong to the lyophobic colloids and are therefore thermodynamically unstable. Colloidal stability is achieved by adding one or more compounds that adsorb or otherwise accumulate at the interface between the dispersed and the continuous phase, the so-called emulsifiers, foaming agents, or, more generally, stabilizers. 

The stabihty of emulsions, as well as of other lyophobic colloids in the presence of emulsifiers, is limited. Therefore emulsion stabihsers are often added. These can be low molecular weight hydrophihc substances (such as glycerol) or hydrophihc coUoids (proteins or polysaccharides, for example, gelatine, pectin, vegetable gums or modified celluloses, respectively), which either increase the viscosity of the emulsions or interact with particles of hydrophobic coUoids and thus allow their association with water. 

Most kinds of emulsions that will be encountered in practice are lyophobic, metastable emillsions. However, there remain some grey areas in which the distinction between lyophilic and lyophobic dispersions is not completely clear. A special class of aggregated surfactant molecules termed micelles and the microemulsions of extremely small droplet size are usually but not always considered to be lyophilic, stable, colloidal dispersions and will be discussed separately. 

In systems with liquid dispersion medium, i.e. in foams, emulsions, sols and suspensions, there is a broad variety of means to control colloid stability. In these systems the nature of colloid stability depends to a great extent on the aggregate state of dispersed phase. Similar to aerosols, foams are lyophobic, but in contrast to them can be effectively stabilized by surfactants. Properties of emulsions, and, to some extent, those of sols may be quite close to the properties of thermodynamically stable lyophilic colloidal systems. In such systems a high degree of stability may be achieved with the help of surfactants. 

A. A. Noyes, unstable colloids and stable colloids by V. Henri, hydrophobic and hydrophilic colloids by Perrin, and lyophobic and lyophilic colloids by Freundlich and W. Neumann. The names suspensoids and emulsoids were proposed by P. P. von Weimarn, on the basis of the names suspension and emulsion for dispersions of larger solid and liquid particles, but an emulsion of very fine oil drops behaves towards electrolytes like a suspensoid. 

In this paper we review principles relevant to colloids in supercritical fluids colloids in liquids are discussed elsewhere [24]. Thermodynamically unstable emulsions and latexes in CO2 require some form of stabilization to maintain particle dispersion and prevent flocculation. Flocculation may be caused by interparticle van der Waals dispersion forces (Hamaker forces). In many of the applications mentioned above, flocculation of the dispersed phase is prevented via steric stabilization with surfactants, in many cases polymeric surfactants. When stabilized particles collide, polymers attached to the surface impart a repulsive force, due to the entropy lost when the polymer tails overlap. The solvent in the interface between the particles also affects the sign and range of the interaction force, and the effect of solvent is particularly important for highly compressible supercritical solvents. Since the dielectric constant of supercritical CO2 and alkanes is low, electrostatic stabilization is not feasible [24] and is not discussed here. For lyophobic emulsion and latex particles (-1 xm), the repulsive... 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems, the addition of a fourth component, a cosurfactant, to an oil-water-surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10 - 10 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, 10 nm or smaller. The droplets can be so small that they scatter little light, and the emulsions appear to be transparent and do not break on standing or centrifuging. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable. The thermodynamic stability is frequently attributed to transient negative interfacial tensions, but this, and the question of whether microemulsions are really lyophilic or lyophobic dispersions are areas of some discussion in the literature. As a practical matter, microemulsions can be formed, have some special qualities, and can have important applications in areas such as enhanced oil recovery, soil and aquifer remediation, foods, pharmaceuticals, cosmetics, herbicides, and pesticides (13,16,45,59-61). 

Emulsions are heterogeneous dispersed systems (hydrophobic colloids) similar to lyophobic sols. They are the result of dispersions of immiscible liquids. One of the liquids forms a dispersed phase, which is dispersed in the liquid dispersion medium of the second liquid into small particles (drops). Depending on the size of dispersed particles and the associated stability, emulsions can be distinguished as ... 

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