| Miniemulsion technology offers possible applications such as encapsulation of pigments, oils, and polymers, and polymerization of highly water-insoluble monomers not possible via conventional emulsion polymerization. Fundamental understanding of miniemulsions has been hindered by ignorance of their droplet size distribution (DSD). In this work, the droplet size and size distribution of miniemulsions have been characterized using, with adaptation, particle sizing techniques such as capillary hydrodynamic fractionation (CHDF), acoustic attenuation spectroscopy (AAS), surfactant titration, dynamic light scattering (DLS), and microscopy.;AAS has the advantage of being able to characterize the DSD of concentrated dispersions, unusual in typical sizing techniques. This makes it particularly well-suited for sizing monomer droplets in miniemulsions. Another advantage is its wide size measurement range of 10 nm to 100 microns. However, for systems with relatively soft, deformable dispersed particles, or systems with low density contrast between the dispersed and continuous phases, such as monomer miniemulsions and polymer latexes, accuracy is somewhat compromised at the micron scale. A disadvantage is the need to know many thermodynamic properties of both the dispersed and continuous phases of the dispersion being measured. AAS results are reproducible and reliable, and agree fairly well with CHDF and surfactant titration results.;A CHDF instrument was modified to enable detection of styrene miniemulsion droplets, by using styrene-saturated eluent. This proved to be a reliable sizing technique for characterizing miniemulsion DSDs. However, use of eluent that is completely saturated with monomer was found to be critical to this technique. Agreement between CHDF and AAS was generally good, except when the DSDs were very broad and multimodal.;A critical ingredient in miniemulsion recipes is the costabilizer, a water-insoluble compound that helps prevent diffusion of the slightly soluble monomer from the smaller droplets through the continuous phase to the larger droplets via Ostwald ripening. The DSDs of styrene miniemulsions with both hexadecane (HD) and cetyl alcohol (CA) as costabilizer were characterized by surfactant titration, CHDF, and AAS periodically after preparation to observe changes in DSD. When HD was used as the costabilizer, droplets were initially smaller, and their DSDs slowly broadened and shifted to larger sizes, when less costabilizer was present. These changes were slowed when additional surfactant was added after preparation. After several days, the miniemulsions' average droplet size increased to about 150 nm regardless of how much HD or surfactant was present. When CA was used as the costabilizer, the miniemulsions had multimodal DSDs, and average droplet sizes in the micron range. The DSDs shifted to larger sizes within several hours after preparation.;Diffusional degradation by Ostwald ripening in styrene miniemulsions occurred to a significant extent, causing HD costabilizer to concentrate in small droplets while becoming diluted in large droplets. Less composition change occurred in styrene miniemulsions with either higher initial HD content or with additional surfactant added after sonification. Both of these conditions led to a minimization of free energy, increasing stability. It was shown to be feasible that Ostwald ripening occurred within minutes of preparation of the miniemulsion.;Styrene miniemulsions were found to have fairly similar polymer PSDs after polymerization, despite large differences in their initial monomer DSDs. This indicates that, in addition to the droplets, polymerization occurred in the aqueous phase, and transport of monomer from large droplets to polymerizing sites occurred, to varying extents. This is in contrast to miniemulsion polymerization with a highly water-insoluble monomer, such as octadecyl methacrylate, where it was found that the final latex PSD exactly matched the initial monomer DSD, indicating polymerization was solely in the droplets. |
