Lyophobic colloids preparation

Lyophobic colloids (sols) may be prepared by grinding crystalline materials or running an electric arc between metallic electrodes, both in the dispersing medium. More commonly, they are prepared by precipitating the solid from a strongly supersaturated solution, which produces a large number of precipitation nuclei. Because there is little attractive interaction between the particles and the medium, attractive forces between the particles would soon lead to their aggregation flocculation). This tendency, however, is counterbalanced by repulsive electrical forces between the particles. 

As a direct consequence of the thermodynamics of lyophobic colloidal systems, their preparation requires... 

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. 

Introduction to colloid science Characterization, size distributions Preparations lyophobic colloids, suspensions... 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

Colloidal systems can be divided into lyophilic and lyophobic systems. Lyophilic colloids have a strong affinity with the dispersion medium by which a solvation shell around the particle is formed. This process is called solvation and if the dispersion medium is water it is called hydration. A polysaccharide dissolved in water is an example of a lyophilic colloidal system. The solvation shell is formed by hydrogen bonds between the hydroxyl groups of the polymer molecules and the water molecules. Pharmaceutical examples are solutions of dextran, used as plasma expanders. Micelles are also lyophilic colloids. Example of such a system is the aqueous cholecalciferol oral mixture (Table 18.15). In these preparations, a lipophilic fluid is dissolved in an aqueous medium by incorporating it in micelles. Because this type of colloids falls apart on dilution to concentrations below the CMC, they are also known as association colloids. Lyophobic colloids have no affinity with the dispersion medium. Thus, in this type of colloids no solvation shell is formed around the particles. An example of lyophobic particles are colloidal gold particles (with a diameter of 1 nm - 1 pm) dispersed in water. There are no... 

Lyophobic colloids are inherentiy unstabie with respect to aggregation of the particles and separation Into macroscopic phases, it, therefore, comes as a surprise that a goid sol prepared by Michael Faraday as far back as 1857 still retains its stability (it is kept in the museum of the Royal Society). Subsequent investigations have demonstrated that Faraday s gold sol owes its astonishing longevity to a protective colloid which Faraday must have added. 

Hydrophobic sols are, like all lyophobic colloids, heterogeneous. The dispersed phase and dispersion medium represent two different phases separated by the phase interface. Hydrophobic sols arise spontaneously, but are unstable. They are difficult to prepare (e.g. by mechanical agitation of solid particles, stirring or condensation of small particles to micelles of colloidal dimensions) and require the presence of surfactants. They are also more stable in the presence of hydrophilic sols. 

In this paragraph the discussion of the preparation of colloidal dispersions will be limited to those in which a liquid is the dispersion medium and a solid is dispersed, consequently to sols. Sols can be divided into two groups lyophobic (Greek for liquid hating) and lyophilic... 

It was observed that lyophobic sols are stable due to their electric charge. As most of the lyophobic sols are prepared by preferential adsorption of ions, the mutual repulsion of particles is responsible for their stability. The sols become unstable, as soon as they are robbed off their charge. However, in case of lyophilic sols, the stability is due to electric charges as well as solvation—a phenomenon in which the colloid particle is surrounded by a thin film of the solvent. The layer of the solvent forms an envelope around each colloidal particle and thus pre-forms an envelope around each colloidal particle and thus prevents the particles from coming together. Groups like... 

The condensation method makes it possible to obtain more highly dispersed systems than the dispersion method, and true lyophobic sols are always prepared by this method. Colloidal solutions are obtained by the condensation method as a result of chemical reactions of nearly all known types. But it should be noted that sols are by no means always formed, but only in the case of certain concentrations of the original substances, order of their mixing, temperatures of interaction, and a combination of several other conditions. The main method of preparing sols of heavy hydroxides is hydrolysis of solutions of salts, which takes place more completely and more rapidly at high temperatures and in dilute solutions. 

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. 

By lyophobic sols are meant colloidal dispersions of insoluble substances in a liquid medium, usually an aqueous solution. They can be prepared in several different ways. It is very typical for these systems that they can never be prepared, by simple contact or slight shaking of macrocrystals of the insoluble substance and the solvent. The roundabout ways by which lyophobic sols can be arrived at may be distinguished according to SvEDBERG into condensation methods and dispersion methods. 

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