Nanoparticles of varying hydrophobicity at the emulsion droplet-water interface:

2019-08-14 08:38:24

contact form nanoparticles droplets coalescence

责任者: Simovic, Spomenka;Prestidge, Clive A. 单位: Ian Work Research Institute, ARC Spec. Res. Ctr. Particle/Mat. I., University of South Australia, Mawson Lakes, SA 5095, Australia 来源出处: Langmuir,2004,20(19):8357-8365 摘要: The coalescence stability of poly(dimethylsiloxane) emulsion droplets in the presence of silica nanoparticles ( [similar to] 50 nm) of varying contact angles has been investigated. Nanoparticle adsorption isotherms were determined by depletion from solution. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Fully hydrated silica nanoparticles adsorb with low affinity, reaching a maximum surface coverage that corresponds to a close packed monolayer, based on the effective particle radius and controlled by the salt concentration. Adsorbed layers of hydrophilic nanoparticles introduce a barrier to coalescence of [similar to] 1 kT, only slightly reduce the coalescence kinetics, and form kinetically unstable networks at high salt concentrations. Chemically hydrophobized silica nanoparticles, over a wide range of contact angles (25 to >90°), adsorb at the droplet interface with high affinity and to coverages equivalent to close-packed multilayers. Adsorption isotherms are independent of the contact angle, suggesting that hydrophobic attraction overcomes electrostatic repulsion in all cases. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics: at or above monolayer surface coverage, stable flocculated networks of droplets form and, regardless of their wettability, particles are not detached from the interface during coalescence. At sub-monolayer nanoparticle coverages, limited coalescence is observed and interfacial saturation restricts the droplet size increase. When the nanoparticle interfacial coverage is >0. 7 and less than or equal 1.0, mesophase-like microstructures have been noted, the physical form and stability of which depends on the contact angle. Adsorbed nanoparticle layers at monolayer coverage and composed of a mixture of nanoparticles with different hydrophobisation levels form stable networks of droplets, whereas mixtures of hydrophobized and hydrophilic nanoparticles do not effectively stabilize emulsion droplets. 关键词: Emulsions;Phase interfaces;Nanostructured materials;Hydrophobicity;Water;Drop formation;Adsorption;Coalescence;Reaction kinetics;Mixtures;Micelles;Proteins;Coalescence stability;Emulsion droplets;Hydrophilic nanoparticles;Droplet interface