RESUMO
The influence of particle adsorption on liquid/liquid interfacial tension is not well understood, and much previous research has suggested conflicting behaviors. In this paper we investigate the surface activity and adsorption kinetics of charge stabilized and pH-responsive polymer stabilized colloids at oil/water interfaces using two tensiometry techniques: (i) pendant drop and (ii) microtensiometer. We found, using both techniques, that charge stabilized particles had little or no influence on the (dynamic) interfacial tension, although dense silica particles affected the "apparent" measured tension in the pendent drop, due to gravity driven elongation of the droplet profile. Nevertheless, this apparent change additionally allowed the study of adsorption kinetics, which was related qualitatively between particle systems by estimated diffusion coefficients. Significant and real interfacial tension responses were measured using â¼53 nm core-shell latex particles with a pH-responsive polymer stabilizer of poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (pMMA-b-pDMAEMA) diblock copolymer. At pH 2, where the polymer is strongly charged, behavior was similar to that of the bare charge-stabilized particles, showing little change in the interfacial tension. At pH 10, where the polymer is discharged and poorly soluble in water, a significant decrease in the measured interfacial tension commensurate with strong adsorption at the oil-water interface was seen, which was similar in magnitude to the surface activity of the free polymer. These results were both confirmed through droplet profile and microtensiometry experiments. Dilational elasticity measurements were also performed by oscillation of the droplet; again, changes in interfacial tension with droplet oscillation were only seen with the responsive particles at pH 10. Frequency sweeps were performed to ascertain the dilational elasticity modulus, with measured values being significantly higher than previously reported for nanoparticle and surfactant systems, and similar in magnitude to protein stabilized droplets.
RESUMO
Coalescence between oil caps with irreversibly adsorbed layers of nonionic surfactant is characterized in deionized water and electrolyte solution. The coalescence is characterized using a modified capillary tensiometer allowing for accurate measurement of the coalescence time. Results suggest two types of coalescence behavior, fast coalescence at low surface coverages that are independent of ionic strength and slow coalescence at high coverage. These slow coalescence events (orders of magnitude slower) are argued to be due to electric double layer forces or more complicated stabilization mechanisms arising from interfacial deformation and surface forces. A simple film drainage model is used in combination with measured values for interfacial properties to quantify the interaction potential between the two interfaces. Since this approach allows the two caps to have the same history, interfacial coverage and curvature, the results offer a tool to better understand a mechanism that is important to emulsion stability.
RESUMO
In the 2010 Deepwater Horizon rig explosion and subsequent oil spill, five million barrels of oil were released into the Gulf over the course of several months. Part of the resulting emergency response was the unprecedented use of nearly two million gallons of surfactant dispersant at both the sea surface and well head, giving rise to previously untested conditions of high temperature gradients, high pressures, and flow conditions. To better understand the complex interfacial transport mechanisms that this dispersant poses, we develop a model surfactant-oil-aqueous system of Tween 80 (a primary component in the Corexit dispersant used in the Gulf), squalane, and both simulated seawater as well as deionized water. We measure surfactant adsorption dynamics to the oil-aqueous interface for a range of surfactant concentrations. Using techniques developed in our laboratory, we investigate the impact of convection, step changes in bulk concentration, and interfacial mechanics. We observe dynamic interfacial behavior that is consistent with a reorganization of surfactant at the interface. We demonstrate irreversible adsorption behavior of Tween 80 near a critical interfacial tension value, as well as measure the dilatational elasticity of equilibrium and irreversibly adsorbed layers of surfactant on the oil-aqueous interface. We report high values of the surface dilatational elasticity and surface dilatational viscosity, and discuss these results in terms of their impact regarding oil spill response measures.
