Polymeric surfactants steric stabilization, particle -adsorbed layer

In addition to ionic surfactants, nonionic surfactant molecules can also adsorb onto the particle surfaces to impart satisfactory stabihty to colloidal dispersions [20, 21]. Some very old examples include India ink and carbon black particles dispersed in the continuous aqueous phase containing a natural gum. This kind of colloidal stabilization mechanism (termed steric stabilization) was first illustrated experimentally by M. Faraday [31, 32]. Some representative polymeric materials (protective colloids) that are effective in preparing steri-cally stabilized aqueous colloidal dispersions are summarized in Table 2.7 [21]. A portion of an effective protective colloid must be hydrophobic enough to show a strong tendency to adsorb onto the hydrophobic particle surface. Furthermore, the adsorbed macromolecules must form a relatively thick hydrophilic layer surrounding the particle, which serves as a steric barrier to prevent the colloidal particles from flocculation. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

As an indirect method toward processability, conducting colloids of polypyrrole and polyaniline have been prepared via a dispersion polymerization route. In this approach, the conducting polymer is che-mically synthesized in the presence of a suitable polymeric surfactant. The surfactant adsorbs or is chemically grafted onto the conducting polymer particle or chain and prevents macroscopic precipitation by a steric stabilization mechanism. The result is a stable dispersion of submicronic conducting polymer particles which consist of a conducting core and a thin outer layer of the non conducting surfactant as shown schematically in Fig. 2. 

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