Lyophobic colloids electrostatic stabilization

There are two methods of stabilization of lyophobic colloids electrostatic and polymeric. Electrostatic stabilization results from charge-charge repulsion, as discussed previously. Polymeric stabilization is achieved by the adsorption of macromolecules (lyophilic colloids) at the surface of a lyophobic colloid. Macromolecules of at least a few thousand molecular weight are required, as they must extend in space over... 

A considerable advance was made in the 1940 s when in the theories of Derjaguin and Landau (11) and Verwey and Overbeek (12) a theory of the stability of lyophobic colloids was obtained by assuming the pairwise additivity of the potential energy of electrostatic repulsion, VR, and the van der Waals attraction,... 

The stability of latexes during and after polymerization may be assessed at least qualitatively by the theoretical relationships describing the stability of lyophobic colloids. The Verwey-Overbeek theory (2) combines the electrostatic forces of repulsion between colloidal particles with the London-van der Waals forces of attraction. The electrostatic forces of repulsion arise from the surface charge, e.g., from adsorbed emulsifier ions, surface sulfate endgroups introduced by persulfate initiator, or ionic groups introduced by using functional monomers. These electro-... 

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. 

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. 

The theory of the kinetic stability of lyophobic colloids is based on the projjerties of the pair potential between charged colloidal particles, which for spherical particles is characterized by an attractive potential due to disjjersion forces of the form of Eq. (6.19), to be discussed in the next section, and a long-range repulsive screened electrostatic potential of the form... 

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. 

The basic idea of the DLVO theory is that the stability of lyophobic colloids in aqueous systems is determined by the combination of van der Waals attraction and electrostatic repulsion and that the two are exactly additive. In other words, the total interaction free energy VT would at any value of h... 

Up to this point we have considered distributed dilute dispersions of colloidal size particles and macromolecules in continuous liquid media. Where the particles are uncharged and of finite size, they are always separated by a fluid layer irrespective of the nature of the hydrodynamic interactions that take place. In the absence of external body forces such as gravity or a centrifugal field or some type of pressure filtration process, the uncharged particles therefore remain essentially uniformly distributed throughout the solution sample. We have also considered the repulsive electrostatic forces that act between the dispersed particles in those instances where the particles are charged. These repulsive forces will tend to maintain the particles in a uniform distribution. The extent to which a dispersion remains uniformly distributed in the absence of applied external forces, such as those noted above, is described in colloid science by the term stability, whereas colloidal systems in which the dispersed material is virtually insoluble in the solvent are termed lyophobic colloids. 

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. 

On the basis of work done in the years just before World War II, Deijaguin and Landau [26] were able to explain in 1941 many of the complex phenomena involved in aggregative stability on the basis of forces of interaction between colloidal particles, namely the van der Waals-London forces of attraction and the electrostatic forces of repulsion. In the meantime, as a result of theoretical investigations and calculations performed in the years 1940-1944 and without the benefit of much of the literature that appeared during the war years, Verwey and Overbeek [7] formulated a theory of stability of lyophobic colloids and published it as a book in 1948. Because their... 

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... 

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. 

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

---