If the particles are too hydrophobic, they will prefer to remain in the oil phase. If the particles are too hydrophilic, they will remain in the aqueous phase instead of migrating to the interface. The wettability of the particles determines how strongly the particles are adsorbed at the oil/water-interface. What effect does the wettability of particles have on the stability of a Pickering emulsion? Suitable hydrocolloids such as Dehydroxanthan gum (Amaze XT, Nouryon) have to fulfil important protective functionalities to prevent the emulsion droplets from creaming or coalescening. The bigger the emulsion droplet, the higher the risk of emulsion instability. O/w-Pickering emulsions based on coated titanium dioxides, starches, silicas or even mica usually have bigger emulsion droplet sizes, up to 30 µm diameter. Particles that are not wettable with water stabilise emulsions of the w/o-type. O/w-emulsion-stabilising particles are immersed in water extremely quickly, or at least with a short time delay. Simple wetting tests can be used to measure the time required for the particles to completely penetrate the water surface. One challenge is the identification of potentially suitable particles for the production of stable Pickering emulsions. What are the major challenges involved when formulating Pickering emulsions? Pickering emulsions can be produced in a cold/cold manufacturing process, but this applies generally to the production of o/w-emulsion gels. In addition, a higher viscosity of the outer phase compared to the inner oil phase is essential to improve emulsification efficiency.Īre there any advantages in the production or is it more complicated? During emulsification, the particles migrate from the oil phase to the interface to be adsorbed there. The inverse production method, in which the particles are pre-dispersed into the oil phase, has proved to be successful. Therefore, the production process must be adapted in such a way that the smallest possible emulsion droplets are formed during the emulsification process. This makes it difficult to obtain smaller emulsion droplets. The particles are not able to develop significant spreading pressures at oil/water interfaces. In principle, o/w-Pickering emulsions can be produced by cold/cold manufacturing. In what way is the production of a Pickering emulsion different? This makes them particularly suitable for sun protection products. Because there are less emulsifiers in Pickering emulsions, they also have a higher water resistance. Furthermore, particle-stabilised interfaces are extremely stable against coalescence. This type of emulsion is named after Percival Spencer Umfreville Pickering, who published his work in 1907.Īdvantages of o/w-Pickering emulsions are their light skin feeling and fluid texture which are particularly preferred for men’s cosmetic products. They are not an invention of modern times Walter Ramsden, British physiologist and biochemist, described the phenomenon of an emulsion stabilised by boundary layer adsorbed solid particles as early as 1903. For the first time, we link the pigment surface energy to the propensity of emulsions to phase invert transitionally or catastrophically.COSSMA: What is a Pickering emulsion and what are its advantages in comparison to other emulsions?ĭr Holger Seidel: Pickering emulsions are particle-stabilised emulsions. For pigments of intermediate hydrophobicity however (indigo and violet), catastrophic phase inversion becomes possible with emulsions inverting from w/o to o/w upon increasing ϕw. For the most hydrophilic pigment orange, emulsions are o/w at all ϕw, whereas they are w/o for the most hydrophobic pigments (red, yellow, green and blue). At constant particle concentration, the influence of the volume fraction of water (ϕw) on emulsions was also studied. In a few cases, close-packed primary particles are visible around emulsion droplets. Emulsions are shown to undergo limited coalescence from which the coverage of drop interfaces by particles has been determined. Their stability to coalescence increases with particle concentration. The emulsion type is in line with calculated contact angles of the particles at the oil–water interface being either side of 90°. At equal volumes of oil and water, preferred emulsions were water-in-oil (w/o) for six pigment types and oil-in-water (o/w) for pigment orange. As expected, most of the pigments are relatively hydrophobic but pigment orange is quite hydrophilic. Their solubility in water or heptane was determined using a spectrophotometric method and their surface energies were derived from the contact angles of probe liquids on compressed disks of the particles. Seven pigment types each possessing a primary colour of the rainbow were selected. The possibility of stabilizing emulsions of water and non-polar alkane with pure, coloured organic pigment particles is explored.
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