Ostwald ripening polymeric surfactants

Wang and Schork [73] used PS, PMMA and PVAc as the costabilizers in miniemulsion polymerizations of VAc with PVOH as the surfactant. They found that, while PMMA and PS were effective kinetic costabihzers (at 2-4%wt on total monomer) for this system, PVAc was not. While the polymeric costabilizers did not give true miniemulsions, Ostwald ripening was retarded long enough for predominant droplet nucleation to take place. 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

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. ... 

A lack of knowledge regarding the ingredients that can be incorporated to overcome Ostwald ripening. For example, the addition of a second oil phase with very low solubiHty and/or the incorporation of polymeric surfactants that strongly adsorb at the O/W interface (which are also insoluble in the aqueous medium). 

As discussed above, the incorporation of an oil-soluble polymeric surfactant that adsorbs strongly at the O/W interface would be expected to cause a reduction in the Ostwald ripening rate. To test this hypothesis, an A-B-A block copolymer of poly(hydroxystearic acid) (PHS, the A chains) and PEO (the B chain) PHS-PEO-PHS (Arlacel P135) was incorporated in the oil phase at low concentrations (the ratio of surfactant to Arlacel was varied between 99 1 and 92 8). For the hexadecane system, the Ostwald ripening rate showed a decrease with the addition of Arlacel PI 35 surfactant at ratios lower than 94 6. Although similar results were... 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

As mentioned above, the best procedure to follow Ostwald ripening is to plot versus time, following Eq. (6.68). This gives a straight line, from which the rate of Ostwald ripening can be calculated. In this way one can assess the effect of the various additives that may reduce Ostwald ripening, e.g. addition of highly insoluble oil and/or an oil-soluble polymeric surfactant. 

As discussed above, on incorporating an oil-soluble polymeric surfactant that adsorbs strongly at the O/W interface, one would expect the Ostwald ripening rate to... 

---