They suggested that altering θ ow, and hence the extent to which the particles are immersed in each liquid, will affect the interaction forces between particles at the interface. How the wettability of micrometre-sized polystyrene latex particles at a planar decane-water interface is altered by adding SDS to the water was investigated by Reynaert et al. They also argued that at high surfactant concentrations, the surfactant adsorbed onto the particle surfaces and made them hydrophilic. observed that isolated bubbles (∼100 μm in diameter) stabilised by micrometre-sized latex particles became unstable to disproportionation and particle detachment after exposure to SDS or Triton X-100 (non-ionic surfactant). As the foam collapsed, the particles drained from the foam into the lower aqueous phase. They argued that adsorption of SDS onto the particle surfaces made them hydrophilic and caused them to detach from the air–water interface. caused particles to detach from foams stabilised by hydrophobic polymer microrods by gently adding a few drops of a concentrated anionic surfactant (sodium dodecyl sulfate, SDS) solution. Then we discuss the materials being fabricated by harnessing destabilisation processes in Pickering emulsions.Īttached particles can be displaced by using surfactants that adsorb to the particle surfaces and modify their wettability in situ ( Figure 1a). We describe how emulsions are broken by particle detachment, mass transfer and drop coalescence. Here we focus on the structural changes that occur in Pickering emulsions as they age. This approach was recently reviewed comprehensively. The particles are used to form emulsions that can be destabilised on demand. One approach to controlling Pickering emulsion stability is to synthesise particles that respond to an external stimulus by detaching from the drop surface. The defects cause the thin films separating drops to rupture and the drops coalesce. first proposed that Pickering emulsions destabilise if there are defects, like fractures or vacancies, in the particle layer coating the drops. The focus of this review is on developments in our understanding of how particle-stabilised emulsions break down. The voids in the solids formed typically lack the desired polyhedral geometry. Although Pickering emulsions are templates for assembling particles into porous solids, the particle networks tend to collapse during drying. Using Pickering emulsions as precursors for assembling films of particles on surfaces, for example, relies on the particle-coated drops or bubbles coalescing with a flat oil-water or air-water interface. Įmulsions and foams are destabilised to make coatings and adhesives by evaporating the volatile components to leave a film of active ingredients on a solid surface. Particle separations using biphasic extractions are also hindered by particles becoming trapped at the liquid interface. They hinder separation of the products from the reaction mixture and recycling of the catalyst. Pickering emulsion formation during biphasic reactions catalysed by nanoparticles increases the reaction yield, but reduces its efficiency. They reduce the volume of oil recovered and generate waste (the unwanted emulsion). Particle-stabilised emulsions that form during the extraction of bitumen from oil sands, for example, are difficult to break. The remarkable stability of Pickering emulsions is a problem for applications that require controlled destabilisation of emulsions. Recent progress has improved our understanding of the mechanisms by which solid particles slow Ostwald ripening and coalescence. Ramsden (and later Pickering ) first described the presence of a membrane of solid particles (proteins or other precipitated colloids) enhancing the lifetime of oil droplets and air bubbles in water. The topic of this review is emulsion stability in the presence of particles. They can be surfactant molecules, polymers, proteins, or particles. Making an emulsion that will not age irreversibly while it is being handled requires addition of stabilising components. They change over time due to Ostwald ripening, flocculation and coalescence of the drops. These properties depend on the volume fraction and size of the droplets in the emulsion. Products like moisturizer creams take advantage of how emulsions yield and flow, their texture and visual appearance. Emulsions are used in cosmetic products, detergents and foods, as well as for liquid extractions and oil recovery. Controlling emulsion stability during their storage and use is a major challenge.
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