Surface force lyophobicity

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. 

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-... 

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. 

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... 

Lyophobic (liquid-hating) colloids are those in which the liquid does not show affinity for the particle. The Gibbs free energy increases when the particles are distributed through the liquid so that if attractive forces exist between the particles, there will be a strong tendency for the particles to stick together when they come into contact. This system will be unstable and flocculation will result. A lyophobic colloid can, therefore, only be dispersed if the surface is treated in some way to cause a strong repulsion to exist between the particles. Suspensions of insoluble particles in a liquid (e.g., most ceramic particles dispersed in a liquid) are well-known examples of lyophobic colloids. We therefore need to understand the attractive forces that lead to flocculation and how they can be overcome by repulsive forces to produce colloids with the desired stability. 

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 second strand of recent evidence for slip at the wall is due to Zhu and Granick [7][8]. These authors employed a force balance apparatus to study the hydrodynamic squeeze force between a pair of crossed mica cylinders lubricated by both tetradecane and water. As with the alumina stufaces used by Pit et al. above, the mica surfaces were chemically modified, either by grafting on them a lyophobic monolayer of long chain octadecyl-triethoxysiloxane (OTE) or by adding a surfactant (1-hexadecylamine) to the tetradecane solvent. Contact angle measurements showed that these treatments converted the normally fully-wetted mica surface to one only partially wetted by both test liquids. 

For tetradecane between untreated, wetted mica, the authors found hydrodynamic squeeze forces in accord with conventional theory. However when the mica was rendered lyophobic, the hydrodynamic squeeze force was drastically reduced. This is shown in figure 4, where the ratio of the squeeze force using treated mica surfaces to that for untreated mica, is shown as a function of the minimum film thickness between the cylinders, D [7], It can be seen that when the mica is lyophobic, the squeeze force is reduced by more than an order of magnitude. 

In some sols the particles are stabilized by the preferential attraction of the suspending liquid to the particles surface. These are referred to as lyophillic sols. In other sols the thermod5mamically stable condition is a flocculated or a gelled state, but the particles are held apart by electrostatic forces produced by ions which collect on and near the surface of the particles. Sols of elements, oxides, and salts (including the oxides of uranium and thorium) are generally of the latter type and are referred to as lyophobic sols. 

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