Lyophobic dispersions

The colloidal structures described above are dictated by thermodynamics, and the resulting structures are thermodynamically stable. Similar thermodynamically stable structures can develop even in a copolymer melt (i.e., there is no other polymer or solvent). Such colloidal systems differ from kinetically stable lyophobic dispersions of the type discussed in Vignettes 1.4 and 1.5. 

Lyophobic dispersions are never stable in the thermodynamic sense, but exhibit some degree of instability. From a practical point of view, the word stable is often loosely used to describe a dispersion in which the coagulation rate is slow in relation to its required shelf life . 

Lyophobic dispersions are not thermodynamically stable. Rather they possess some degree of kinetic stability, and one must consider such things as what degree of change and over what time scale. The dispersed species can come together in very different ways. 

From a practical point of view, in addition to understanding the direction in which reactions will proceed, it is just as important to know the rates at which such reactions will proceed. It is the same in colloid science and its applications. For the many lyophobic dispersions that are not thermodynamically stable, the degree of kinetic stability is very important. Although all of the typical rate processes are important, sedimentation (creaming), aggregation, and coalescence, this section will discuss the rate of aggregation. 

From the various equations given, under the appropriate conditions of Kfl, h, etc. the total potential energy of interaction between the particles can be calculated from the basic DLVO assumption that for lyophobic dispersions. 

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. 

Microemulsions. 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 to 10" mN/m low interfacial tension allows spontaneous or nearly spontaneous emulsification to very small droplet sizes, ca. 10 nm or smaller. The droplets can be so small that they scatter little light the emulsions appear to be transparent and do not break on standing or centrifuging. Unlike coarse emulsions, these microemulsions are usually thought to be thermodynamically stable. The thermodynamic stability is frequently attributed to transient negative interfacial tensions, but this hypothesis and the question of whether microemulsions are really lyophilic or lyophobic dispersions are areas of some discussion in the literature (J 7). As a practical matter, microemulsions can be formed, have some special qualities, and can have important applications. 

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

The soft (electrostatic) and van der Waals interparticle forces are described in the well-established theory of the stability of lyophobic dispersions (colloidal... 

From the preceding discussion of the DLVO theory and equations 9.1-9.5 and 9.8, it is apparent that the stability of a lyophobic dispersion is a function of the particle radius and surface potential, the ionic strength and dielectric constant of the dispersing medium, the value of the Hamaker constant, and the temperature. Stability is increased by increase in the particle radius or surface potential or in the dielectric constant of the medium and by decrease in the effective Hamaker constant, the ionic strength of the dispersing liquid, or the temperature. 

Rebinder (48) investigated the effect of the surface (adsorption) layers on the properties of colloidal systems. When the lyophobic dispersion systems are stable, the structural-mechanical stabilization occurs where the protecting layers of the micelle-forming surface-active agents or high-molecular compounds are formed at the interface boundary. 

This chapter is primarily devoted to the formation of lyophobic disperse systems. It is normally assumed that these systems have been stabilized in some way. Throughout this chapter, along with the discussion of... 

It has been repeatedly emphasized that lyophobic disperse systems are thermodynamically unstable as compared to macroheterogeneous systems. The cause for this instability is a high excess of surface free energy at the interfaces. At the same time, many lyophobic colloids are stable towards aggregation and may maintain such stability for infinite periods of time. Let us now discuss the basic thermodynamic and kinetic factors that favor stabilization in disperse systems. In this sections we will restrict ourselves to just naming some of these factors, and will return to their detailed discussion later on. 

VIII. STRUCTURE, STABILITY AND DEGRADATION OF VARIOUS LYOPHOBIC DISPERSE SYSTEMS... 

These forces originate from entirely different sources and therefore may be evaluated separately. The interplay of (i) and (ii) forms the basis of the classical theory of flocculation of lyophobic dispersions, flrst proposed by Derjaguin and Landau in Russia and independently by Verwey and Overbeek in the Netherlands and hence now known as the DLVO theory. The interplay of (i) and (iii) is commonly termed steric stabilization , and much has been written on this protective mechanism, although a workable understanding has developed only during the last two decades. 

A two-layer bath, with a horizontal rotating cathode. This cathode moves through, and is passivated by, an organic solvent containing a surfactant it then travels into the aqueous electrolyte, A fine powder is deposited at unpassivated sites, and the particles, being lyophobic. disperse in the organic solvent. 

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