Ostwald ripening emulsion concentrates

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. 

Transfer of water molecules (in evaporation control), transport of solvent across monolayers at oil-water interfaces (in Ostwald-ripening of emulsions) and transfer of ions across such interfaces (as models for ion conduction in bilayers and membranes) can often be treated in terms of surface concentration fluctuations. Their magnitudes can be expressed in standard deviations (cr for the standard deviation in the surface concentration), which are measures of the probability that random holes are formed in the layer, allowing material transport. We have presented the formal treatment in sec. 1.3.7. From this section we can immediately obtain = IcTOr / 0 ), for a Gibbs monolayer, with... 

In practice, the emulsions are formed in the presence of surfactants. At concentrations above the critical micellization concentration (CMC) the swollen micelles can serve as carriers of oil between the emulsion droplets of different size. In other words, surfactant micelles can play the role of mediators of the Ostwald ripening. Micelle-mediated Ostwald ripening has been observed in solutions of nonionic surfactants. In contrast, it was found that the micelles do not mediate the Ostwald ripening in undecane-in-water emulsions at the presence of an ionic surfactant (SDS). It seems that the surface charge due to the adsorption of ionic surfactant (and the resulting double layer repulsion) prevents the contact of micelles with the oil drops, which is a necessary condition for micelle-mediated Ostwald ripening. 

Compound Droplets. If the droplets contain various components that differ significantly in solubility, Ostwald ripening is slowed down. The reason is that a less soluble compound will leave a small droplet at a slower rate than a more soluble one, which implies that the concentration, and hence the chemical potential, of the less soluble compound in the drop increases. This produces a driving force for the more soluble compound to diffuse back to the small droplet. This would play a part in slowing down Ostwald ripening in the essential oil emulsions mentioned. 

Coalescenceis especially typical in concentrated emulsions. In such systems coalescence mainly determines the lifetime of emulsions prior to phase separation. In finely dispersed emulsions, both dilute and concentrated, the average size of drops may noticeably increase due to Ostwald ripening. At the same level of dispersion Ostwald ripening of emulsion droplets is a slower process than mass transfer of bubbles in foams [60]. This is due to a rather low interfacial energy, and consequently, low difference in chemical potentials of substance in droplets of different size, as well as due to a lower mutual solubility of liquids as compared to the solubility of gases in liquids. 

In case of a constant contact of emulsion droplets, which is typical of highly concentrated systems, diffusive transfer (isothermal distillation, recondensation, Ostwald ripening) is of great importance, along with coalescence. This phenomenon has been extensively studied in [46- 7]. 

Emulsion droplets provide the large interfacial area necessary for efficient mass transfer during emulsion polymerisation. Most monomers have slight solubility in water so that they may be transported across the aqueous phase from the monomer droplets to the sites of polymerisation (i.e., the polymer particles). During polymerisation, the monomer concentration gradient will overcome Ostwald ripening forces, and diffusion of monomer from large drt lets to smaller monomer-swollen particles will occur. 

In the case of oil-in-water emulsion, xf can be the concentration of the oil dissolved in the water. In particular. Equation 4.126 predicts that the large emulsion droplets will grow and the small droplets will diminish. This phenomenon is called Ostwald ripening (see Section 4.3.1.4). If the droplets (phase a) contain a component, which is insoluble in phase p, the small droplets will be protected against complete disappearance a counterpart of Equation 4.125 can be derived ... 

All nano-emulsions showed an increase in droplet size with time, as a result of Ostwald ripening. Figure 9.10 shows plots of versus time for all the nanoemulsions studied. The slope of the lines gives the rate of Ostwald ripening w (m s ), which showed an increase from 2 x 10 to 39.7 x 10 m s as the surfactant concentration is increased from 4 to 8 wt%. This increase could be due to several factors (1) A decrease in droplet size increases the Brownian diffusion... 

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