Solid emulsifiers used for in Pickering emulsions can be categorized into three main groups when it comes to particle geometry: spheres or patchy particles featuring spherical lobes ( Fig. Particle geometry The rapid developments in the synthesis of advanced materials have led to a wealth of available uniform (an)isotropic particles that can be studied as model emulsifiers to investigate the interplay between liquid–liquid confinement and particle geometry. 38,39 This holds the advantage of being able to choose, for example, the continuous phase in advanced flow applications (see below). Increased surface roughness of the particle not only dramatically improves PE stability, but also offers the opportunity to produce stable o/w and w/o emulsions by dispersing the same type of particle initially into either the aqueous or oil phase. 38 illustrated this by preparing silica microparticles with tunable surface roughness and demonstrated that surface roughness caused pinning of the contact line around the adsorbed particles, consequently trapping the particle via strong adsorption and thus dramatically promoting emulsion stability. 37 Increasing the roughness of the surface of the solid particles results in very strong adsorption at the interface ( Fig. The strength of adsorption of the particle at the liquid–liquid interface is governed by both the interfacial tensions and the surface morphology of the particles. For example, hydrophilic zeolites are unable to stabilize o/w PEs, while zeolites hydrophobized by silylation proved very efficient. 29,35 Hydrophobization of silica is often used to allow PE catalysis and the same concept has been utilized to tune the contact angle of other types of materials, including catalytically active ones for PIC. 34 Notably, while numerous examples report the use of hydrophobic silica for the generation of highly stable PEs, hydrophilic silica spheres have not been found capable of stabilizing PEs without additional surfactants. For example, stable oil-in-water (o/w) droplets can be easily prepared using hydrophilic goethite particles, while (w/o) droplets are obtained with hydrophobic polystyrene particles. 1, mid top), leading to water-in-oil (w/o) type PEs, while hydrophilic particles lead to inverse o/w emulsions, i.e. Bottom left: Oil-in-water (o/w) PE, stabilized by hydrophilic particles with contact angle 90°, ( Fig. Top left: Water-in-oil (w/o) PE, stabilized by hydrophobic particles with contact angle >90°. 1 Schematic representation of o/w and w/o emulsions with corresponding contact angles measured through the aqueous phase, yellow: oil, blue: water. Other types of emulsions, such as water-in-water (w/w), 4 gas-in-liquid (g/w or g/o) 5 or multiple emulsions, water-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o), 6 have been studied as well.įig. Hydrophilic particles, such as bare silica and metal oxides, are more easily wetted by the aqueous phase, resulting in contact angles <90° and the formation of o/w PEs. Hydrophobic particles, such as carbon nanotubes or silylated silica, are more easily wetted by the oil phase than by the aqueous phase, showing contact angles of >90°, resulting in the formation of w/o PEs. by the contact angle the particles exhibit with, e.g., the water–oil interface. 3 Which type of PE is formed is determined by the type of particles used, i.e. 2 Generally, two types of emulsions can be readily distinguished, water-in-oil (w/o) or oil-in-water (o/w) emulsions stabilized by hydrophilic and hydrophobic particles, respectively ( Fig. Pickering who was second to describe them, 1 a few years after Ramsden in 1903. The latter type of emulsion is a so-called Pickering emulsion (PE), named after S.U. These biphasic systems can be stabilized by surfactants, as is the case in classical emulsions, or by solid micro- or nanosized particles. Introduction Emulsions are dispersions of two immiscible liquids ( e.g. Here, we provide a review of the recent advances in the field of PE catalysis, emphasizing the developments in the design of (functional) solid stabilizing particles, the range of accessible catalytic reactions and reaction conditions, as well as advances in reactor engineering, such as the application of PE catalysis in continuous flow systems. These efforts aim to combine the green credentials of biphasic catalysis with the intrinsic advantages offered by PEs, which include increased stability and interfacial area. More recently, PEs are also being increasingly used as advanced catalytic systems for green chemical transformations. Pickering emulsions (PEs), emulsions stabilized by solid emulsifiers, are already of great importance for the food, pharmaceutical and biomedical industry.
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