Pickering emulsion

Abstract: The preparation and properties of emulsions, stabilised solely by the adsorption of solid particles at the oil–water interface, are reviewed especially in the light of our own work with particles of well-controlled surface properties. Where appropriate, comparison is made with the behaviour of surfactant-stabilised emulsions. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions. Many of the properties can be attributed to the very large free energy of adsorption for particles of intermediate wettability (contact angle at the oil–water interface, say, between 50 and 130°). This effectively irreversible adsorption leads to extreme stability for certain emulsions and is in contrast to the behaviour of surfactant molecules which are usually in rapid dynamic equilibrium between the oil–water interface and the bulk phases. There is evidence that, in some systems, weak flocculation of the particles improves the emulsion stability. Phase inversion from w/o to o/w can be brought about by increasing the volume fraction of water. Emulsions close to this inversion point tend to be the most stable, again in contrast to surfactant systems. The volume fraction needed for inversion depends on the particle wettability and the nature of the oil and these effects have been rationalised in terms of surface energy components. Stable multiple emulsions (w/o/w and o/w/o) can be made using two types of particles with slightly different wettability. Similar multiple emulsions prepared with two types of surfactant tend to be much less stable. The possibility of preparing novel solid materials by evaporating solid-stabilised emulsions is also proposed. Finally we report on some extensions to the work of Levine et al. who obtained expressions for the free energy of formation of emulsion drops covered with close-packed monolayers of monodisperse spherical particles. In particular in the light of the observations that nanoparticles can act as excellent emulsion stabilisers, we have considered potential effects on the free energy of emulsion formation of the action of small (physically realistic) positive and negative line tensions in the 3-phase contact lines skirting particles adsorbed at the droplet interfaces. We also explore the possibility that curvature properties of close-packed particle monolayers can affect emulsion properties in much the same way that surfactant monolayer properties influence emulsion type and stability.

Abstract: Pickering emulsions are attractive formulations because they are simple and bear strong similarities with the well-known surfactant-based emulsions Pickering emulsions have been largely ignored since their early disclosure in 1907 and arouse a renewed interest quite recently Since this unintelligible time gap raises suspicion, the first aim of the present review is giving the simple fundamental rules as an introduction for newcomers in the topic The basic physical chemistry of Pickering emulsions is explained and the ways to control the parameters of higher relevance with respect to development of applications are given This first part covers the choice of the solid nanoparticles used as stabilizers and their surface properties, the control of emulsion type, droplet size, and rheology A second part gives examples of some applications in drug delivery and manufacturing of porous nanomaterials as illustrations of the potential of such emulsions

Abstract: The preparation, type, and stability of emulsions of oil and water stabilized solely by spherical, monodisperse polystyrene latex particles of different size is described. Two types of behavior occur depending on whether particles remain intact (in the case of cyclohexane) or dissolve to give free polymer chains (in the case of toluene). Emulsions formed with cyclohexane and either “hydrophilic” aldehyde/sulfate particles or “hydrophobic” sulfate particles are water-in-oil (w/o) over a wide range of salt concentrations and water volume fractions. Average emulsion drop diameters initially increase from 35 to 75 μm with increasing particle diameter and then remain constant. Although such emulsions sediment, there is no sign of coalescence for over 6 months. We show evidence of the transition from nonflocculated to flocculated emulsions upon increasing the water volume fraction, as predicted theoretically for charged drops in oil. By use of toluene and “hydrophilic” particles however, emulsions can be invert...

Abstract: We studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge density, forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diameter stable for several months. We characterized the emulsion and visualized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.