RESUMO
Micelle structural properties were determined for five poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer solutions at 25°C by using light scattering techniques. Size distributions for the Pluronic series P103, P104, P105, F108, and P123 were found from dynamic light scattering. Aggregation numbers from both dynamic light scattering and static light scattering were determined, and the results are interpreted using fundamental hydrodynamic and statistical thermodynamic models. The trends in aggregation behavior are discussed in terms of the surfactant composition. For a fixed poly(propylene oxide) content, surfactants with a higher poly(ethylene oxide) content were found to be less aggregated. For a fixed poly(ethylene oxide)/poly(propylene oxide) ratio, surfactants with a higher molecular weight were more aggregated. These results are shown to be in agreement with previous self-consistent mean-field calculations for the same series of polymers and are compared with "star" and "brush" model predictions for the effect of poly(ethylene oxide) chain length on the thickness of the micelle corona.
RESUMO
Pulsed nuclear magnetic resonance was used to monitor the crystallization of supercooled liquid droplets in 30 wt% n-hexadecane oil-in-water emulsions at 6°C. Crystallization was induced in the liquid droplets when solid droplets of the same material were present. The rate of induced crystallization increased as the concentration of free non-ionic surfactant (polyoxyethylene sorbitan monolaurate) in the aqueous phase increased from 0 to 14 wt%. Differential scanning calorimetry measurements indicated that free surfactant had no effect on crystal nucleation of individual droplets. These results indicate that the surfactant enhances induced crystallization by altering colloidal interactions between droplets. Creaming measurements showed that flocculation was enhanced in emulsions when the free surfactant concentration was increased. We propose that the presence of free surfactant micelles increases the attraction between droplets because of an osmotic effect, and this attraction facilitates the ability of solid crystals from one droplet to induce crystallization in an adjacent liquid droplet.
