Lyophobic colloids surfactants

For further information see reviews on colloid stability in non-aqueous media besides the review by Parfitt and Peacock, already mentioned, see J. Lykiema, Principles of the Stability of Lyophobic Colloidal Dispersions in Non-Aqueous Media. Adu. Colloid Interface Set 2 (1968) 65 and P.C. van der Hoeven. J. Lykiema, Electrostatic Stabilization in Non-Aqueous Media. Adv. Colloid Interface Set 42 (1992) 205 A. Kitahara, Non-aqueous Systems in Electrical Phenomena at Interfaces. (Surfactant Series No. 15). A. Kitahara. A. Watanabe, Eds.. Marcel Dekker (1984) 119. 

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. 

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. 

In spite of many advances in the theory of stability of lyophobic colloids, resort has still to be made to an empirical approach to the choice of emulsifier, devised in 1949 by Griffin. In this system we calculate the hydrophile-lipophile balance (HLB) of surfactants, which is a measure of the relative contributions of the hydrophilic and lipophilic regions of the... 

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. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

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. 

In practice, therefore, the objective is to achieve an intermediate form by the addition of a controlled amount of electrolyte or surfactant. When the particles strongly repel each other, an electrolyte can be added. By decreasing the zeta-potential, the repulsive forces will decrease. When the particles attract each other too strongly a surfactant can be added. As the lyophobic part of the surfactant molecule adsorbs onto the surface of lyophobic colloids its lyophilic part will be oriented into the dispersion medium. By steric stabilisation, the attraction forces are decreased. The properties of flocculated and deflocculated suspensions are summarised in Table 18.18. 

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. 

Colloids can be classified according to the phase (gas, liquid, solid) of the dispersed phase and the dispersion medium or according to their stability. Colloidal dispersions are thermodynamically unstable, while association colloids (surfactants) and poly-mer/protein solutions are stable. The former are often called lyophobic (hydrophobic if the dispersion medium is water) and the latter lyophihc (hydrophilic) colloids. These terms can be also used for surfaces. 

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. 

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... 

Surface Activity The chemical species given the general name of surface-active agents or surfactants have a special tendency to adsorb at interfaces, or to form colloidal aggregates in solution at very low molar concentrations. A surface-active material possesses lyophobic part, which has little attraction for the solvent, and lyophilic part, which has a strong attraction for the solvent, in its chemical structure. In water-based systems, the terms hydrophobic and hydrophilic are quite frequently employed in place of lyophobic and lyophilic, respectively. 

The zeta-potential can also be influenced by the absorption of specific ions from the dispersion medium onto the surface of the colloidal particle. For example, if a positively charged surfactant adsorbs onto a positively charged colloidal lyophobic particle, the zeta-potential becomes larger than the Nemst potential. 

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). 

Latexes constitute a subgroup of colloid systems known as lyophobic sol. Sometimes they are called polymer colloids. The stability of these colloids is determined by the balance between attractive and repulsive forces affecting two particles as they approach one another. Stability is conferred on these latexes by electrostatic forces, which arise because of the counterion clouds surrounding the particles. Other forces of an enthalpic or entropic nature arise when the lyophilic molecules on the surfaces of the latexes interact on close approach. These can be overcome by evaporation of the water, heating, freezing, or by chemically modifying the surfactant, such as by acidification. 

Dr. Zelenev s professional interests include industrial applications of colloid and surface science, pulp and paper, oil and gas production, coagulation and flocculation, lyophobic and lyophilic colloidal systems, surfactant phase behavior, interaction of surfactants with surfaces, microencapsulation, particle deposition and aggregation, particle and surfactant transport in porous media, wetting and spreading, development of novel experimental methods for studying colloidal systems, and physical-chemical mechanics. Dr. Zelenev is an inventor on four issued U.S. patents and five pending patent applications, coauthor of 22 scientific publications, and coauthor of the textbook Colloid and Surface Chemistry (Elsevier, 2001). 

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