| This dissertation describes measurements of Ostwald ripening in concentrated oil-in-water emulsions. Ostwald ripening is a mechanism that drives oil from smaller droplets into larger ones, because of the difference in the droplets' chemical potentials, and thus causes coarsening of the emulsion. The effect of oil solubility, surfactant type, surfactant concentration, dispersed phase volume fraction, and initial drop size distribution on Ostwald ripening in emulsions was studied. The Ostwald ripening rate was calculated by the change in the drop size distribution, which was measured by nuclear magnetic resonance (NMR) and light scattering methods. This dissertation adds to the rich body of work already available of experimental Ostwald ripening measurements of emulsion systems.;It was found that the Ostwald ripening rate increased as the solubility, volume fraction, interfacial tension, or the surfactant concentration increased. All of these variables affect the ripening rate in a similar fashion for highly concentrated or dilute emulsions. It was found that the qualitative effect of solubility and interfacial tension predicted by the classic model by Lifshitz, Slyozov, and Wagner (LSW)1, 2 agreed well with our experimental results in highly concentrated emulsions, despite the fact that the assumptions in the model are quite restrictive, and rarely match real emulsified systems. The emulsions studied here were far from the dilute condition assumed in LSW theory, and it was found that the experimentally measured ripening rate for these highly concentrated emulsions was higher than the LSW prediction. Further, the LSW solution applies in the limit of asymptotically long times, yet the emulsions studied in this work did not reach this steady state asymptotic limit, even after 14 days of observation. Since the emulsions systems did not reach the asymptotic limit, the initial distribution did affect the ripening rate. Emulsions with a larger polydispersity had a larger ripening rate, as the chemical potential driving force is larger.;When surfactant micelles were added to the system, the ripening rate was enhanced in similar fashion for all the surfactants studied in this work (SDS, Tween 20, C12E8, and Brij 35). Furthermore, there was a surprising similarity in the enhancement factor between systems of different oils and volume fraction. It was concluded that the mechanism for micelle mediated ripening was similar for most emulsions systems.;Overall, NMR measurement techniques compared well to light scattering methods for measuring the drop size distributions and Ostwald ripening rates. However, care must be taken in order to ensure accurate weighting of the drop size distribution in order to correctly calculate the experimental ripening rates.;1. Lifshitz, I. M.; Slyozov, V. V., Journal of Physics and Chemistry of Solids 1961, 19 (1-2), 35-50. 2. Wagner, C., Zeitschrift Fur Elektrochemie 1961, 65 (7-8), 581-591. |
