Flocculation, polymeric surfactant stabilization

From the above discussion, one can summarize the most important criteria for effective steric stabilization when using polymeric surfactants. First, there should be enough polymer to ensure complete coverage of the particle surface by the chains. This will prevent any attraction between the bare patches of the particles, while it also prevents any bridging flocculation (simultaneous adsorption of the chain on more than one particle). 

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... 

As will be discussed later, one of the main features of effective steric stabilization is strong adsorption (anchoring) of the chains to the interface. This anchoring is produced by the use of polymeric surfactants, the main topic of this chapter. These materials have attracted considerable attention in recent years for stabilization of many o/w and w/o emulsions. Apart from their effectiveness in prevention of flocculation and coalescence of the droplets, they are also expected to cause no skin irritation. The high molecular weight of the surfactants prevents their penetration through the skin and hence they do not cause any disruption of the stratum corneum. 

The last part of the chapter dealt with the preparation of stable water-in-oil-in-water (w/o/w) multiple emulsions that are suitable for application in cosmetics. The main criterion for producing stable w/o/w systems is to use two polymeric surfactants one with a low HLB number for preparation of the primary w/o emulsion and one with a high HLB number for preparation of the final w/o/w multiple emulsion. The primary emulsifier should produce a viscoelastic film fhat prevents leakage from the internal water droplets to the outside continuous phase. It will also ensure high stability (minimum coalescence) of the internal water droplets. The secondary emulsifier should also provide an effective barrier to prevent flocculation and coalescence of the multiple emulsion droplets on storage. It is also essential to balance the osmotic pressure of the internal aqueous droplets and the outside continuous medium. 

Various types of amphiphilic water-soluble polymers have been a focus of considerable interest over the past decade from both scientific and practical perspectives. The scientific interests are derived from their molecular selforganization phenomena, which are relevant to biological polymer systems and to their nanoscopic molecular frameworks as a basis of materials science. Practical interests stem from their usefulness in a variety of applications, such as polymeric surfactants, emulsifiers, solubilizers, associative thickeners, rheology modifiers, flocculants, and colloid stabilizers. These applications are particularly important in such industries as paint, coating, printing, paper, ceramic, drug, and cosmetic or personal care goods [1-3]. In addition, these water-based polymers have become more important than ever as environmentally friendly materials. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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