Lyophobic colloids steric stabilization

There are two general theories of the stability of lyophobic colloids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these colloids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of colloids. 

Fig. 3 Steric stabilization of lyophobic colloidal particles. The particles repel one another because of volume restriction and osmotic pressure effects.

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. 

Electrically stabihzed colloidal dispersions are very sensitive to the addition of electrolytes. If the concentration of ions in the solution increases, decreases as a result of both entropic and electrical screening effects, leading to a reduction in the repulsive potential. On the other hand, colloid particles dispersed in organic media (low dielectric constant) cannot be effectively stabilized by charges because is extremely short. In these cases, steric stabilization is recommended. Steric stabihzation is imparted by nonionic amphiphilic molecules (usually polymeric molecules). The lyophobic moiety will adsorb onto the surface of the colloidal particles, while its lyophilic moiety will be extended in the continuous phase. When two sterically stabihzed particles approach each other, the concentration of the lyophilic segments in the portion of the continuous phase between the particles is increased. This higher local concentration results in an osmotic pressure that... 

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... 

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