įigure 1: Optical microscopy images of O/W emulsions prepared at pH 4.7 stabilised by WPM particles of different concentration in water.īeside their use in industry as special emulsions, PEs have other potential applications. If the preparing conditions, such as wettability, charge, concentration, shape and size, of nanoparticles, as well as, the pH and the salt-concentration in aqueous phase, are right, the clothed droplets can be regarded as charged soft-colloids. By a mechanical mixing of the emulsion, each droplet becomes surrounded by discrete nanoparticles arranged on its surface. The two liquids are not chemically and physically compatible. A typical example is schematized in figure 1. PEs are heterogenous liquids which present as a dispersion of droplets of some liquid (dispersed phase) in another one (continuous phase). In addition, they can serve as templates for the advanced materials, as Janus colloids, composite particles, and colloidosomes. The anchoring of the solid particles emanates from a partial wetting of the surface of the charged solid particles by water and oil.ĭue to their remarkable properties, such as high stability with respect to coalescence and their recent use in nanotechnology leading to the creation and the characterization of the nano-scale structures in new ways, PEs have been the subject of much studies, both from experimental and theoretical point of views. Of course, such an energy mainly depends on the value of the wetting (or contact) angle and the droplet-radius. Here, kB is the Boltzmann’s constant and T is the bath temperature. In contrary to surfactants, where the adsorption is rather dynamic (reversible), that of the charged solid particles is irreversible and sufficiently strong, with a very high adsorption energy, between 106kBT and 108kBT, where kBT is the thermal energy. The stabilization of the dispersed droplets within PEs is ensured by a strong adsorption of the solid particles at their surfaces. The stabilization of larger droplets (few millimeters diameter) is possible as well, using micron-sized solid particles. The solid particles that act as emulsifiers are of nanometric size, while the stabilized droplets are as small as few micrometers diameter. Both droplets and continuous phase contain different molecules (chemically incompatible macromolecules, for instance), which are entirely water-soluble. Also, PEs may be water-in-water (W/W) emulsions, which are dispersions formed by droplets of water-solvated molecules moving in another continuous aqueous solution. These particles may be organic or inorganic, according to the nature of their desired use. These dispersions are stabilized by an addition of small solid particles that act as emulsifiers, instead of the surfactant molecules. Pickering emulsions (PEs), are dispersions presenting, very often, as oil-in-water (O/W), water-in-oil (W/O) or double emulsion water-oil-water (W/O/W). Finally, we precise the major role played by grafted polymers onto the spherical oil/water interface. The question (3) is concerned with an exact study of the spherical diffusion of anchored nanoparticles on the surface of the dispersed oil-droplets. The question (2) deals with a quantitative investigation of the clothed oil-droplets dynamics (cage effect and subdiffusion), using a Generalized Langevin Equation, which is successfully tested by Molecular Dynamic Simulations. To this end, the adopted pair-potential is that of Sogami-Ise combining repulsive and attractive forces, and use is made of the so-called Integral Equation Method. For question (1), we recall the essential steps allowing the determination of the structure-factor and the spatial-correlation function, and the thermodynamic properties, as pressure, internal energy, and thermal compressibility of these emulsions. For the study, the emulsions are regarded as colloidal solutions, where the clothed oil-droplets play the role of charged soft-colloids, and in addition, the adsorbed nanoparticles are assumed to be point-like. Here, we are concerned with three important questions: (1) Structure and thermodynamics of these emulsions, (2) cage effect and subdiffusion phenomenon within them, and (3) spherical diffusion of anchored nanoparticles on the curved oil/water interface. In this review paper, we report on some very recent findings dealt with the oil-in-water Pickering emulsions, stabilized by a strong adsorption of charged solid nanoparticles on the surface of the oil-droplets.
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