Particles , polymeric surfactant adsorption

The addition of electrolytes in the continuous phase this has the effect of enhancing the polymeric surfactant adsorption and thus preventing particle entry into the oil droplets. 

The surfactant is an important component of this process and acts to stabilize the growing polymeric particles by surface adsorption. Phase separation and the formation of solid particles occur before or after termination of the polymerization process [42]. Polymerization can occur in some systems without the presence of surfactants [40]. Various particulate systems have been prepared by this method, including poly(styrene) [43], poly(vinylpyridine) [44, 45], poly(acrolein) [46, 47], and poly(glutaraldehyde) [48-50],... 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

When present at high concentration, polymeric surfactant, due to its high adsorption, may form a dense lyophilizing adsorption layer at the particle surface. Under these conditions the same polymer acts as a stabilizer of colloid dispersion, stabilizing the latter by means of structural-mechanical barrier (Chapter VII). 

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). 

Since hydrophobic interactions appear to play a significant role in determining surfactant adsorption it is possible that these interactions may also alter the manner in which a mixture of surface-active materials might adsorb. In a very early investigation, Orr and Breitman [39] found that when mixtures of anionic and nonionic surfactants were adsorbed onto a polymeric latex particle there was a decrease in the adsorption area per molecule for the anionic surfactants. Hay et al. [40], however, observed a decease in total adsorption for sodium dodecyl sulfate and 0-n-octyltetraethylene glycol onto graphitized carbon. 

In Chapter 3, the solution and surface properties of a relatively new class of material, namely, polymeric surfactants, are illustrated in some detail using Flory-Huggins theory and current polymer-adsorption theory. This is followed by a discussion of the phenomenon of steric stabilization of suspended particles and how it is affected by the detailed structure of the stabilizing polymeric species. It concludes with a discussion of the stabilization of emulsions by interfacial and bulk theological effects, and presents closing comments on multiple emulsions. 

This chapter described the basis principles involved in stabilization of dispersions by polymeric surfactants. The first part described polymeric surfactants and their solution properties. The second part described the adsorption of polymeric surfactants and their conformation at the interface. The methods that can be applied to determine the adsorption and conformation of polymeric surfactants were briefly described. The third part dealt with the stabilization mechanism produced using polymeric surfactants. Two main repulsive forces were considered. The first arises from the unfavorable mixing of the chains on close approach of the particles or droplets, when these chains are in good solvent conditions. This is referred to as mixing or osmotic repulsion. The second force of repulsions... 

Okubo (16) used a similar approach to follow the growth of seeded emulsion polymerizations. When polystyrene was used as the seed particles, followed by the addition of methyl methacrylate, overcoating of the seed particles with a layer of poly(methyl methacrylati was observed. In the reverse situation, however, a distinctly different morphology was produced as indicated by the surfactant adsorption variations with monomer conversion. Instead of overcoating the PMMA seed particles with a uniform layer, the PSTY tended to separate into spherical domains eventually producing a... 

The high affinity isotherm obtained with poiymeric surfactants implies that the first added moiecuies are virtuaiiy compieteiy adsorbed and such a process is irreversible. The irreversibility of adsorption is checked by carrying out a desorption experiment. The suspension at the plateau value is centrifuged and the supernatant liquid is replaced by pure carrier medium. After redispersion, the suspension is centrifuged again and the concentration of the polymeric surfactant in the supernatant liquid is analytically determined. For lack of desorption, this concentration will be very small indicating that the polymer remains on the particle surface. 

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