It was clarified by previous experiments that Hap nanoparticles could help form O/W Pickering emulsions when the oil contained an ester group or the oil phase contained other polymers with ester groups, whereas Hap nanoparticles alone could not work as an emulsifier for Pickering emulsions ( Fujii et al., 2007, 2009). Besides, Hap can be simply synthesized through different approaches, such as wet chemical process, sol-gel process, emulsion process, and so on ( Fujii et al., 2007). Due to the excellent adsorbability with many compounds, Hap nanoparticles have already been extensively used in the formation of Pickering emulsions, which can be applied to a variety of applications such as biomaterials, adsorbents, and catalysts. Hydroxyapatite is an important component in human bodies, especially in bones and teeth, as the main mineral. In this review, various kinds of commonly used solid particles, including Hap, silica, clay, magnetic nanoparticles, chitosan (CS), cyclodextrin (CD), nanotube, and some food-grade stabilizers types of materials fabricated from Pickering emulsions, including microsphere (MS), microcapsule (MC), and Janus colloidal particles (JCPs) morphology of solid particles being used and materials being fabricated, as well as applications of Pickering emulsions in delivery vehicles, porous scaffolds, stimuli-responsive materials, catalysts, and so on will be discussed in detail, with some recent researches. In this respect, many researches have been done to modify these particles with different molecules or to different degrees, in order to make them more amphiphilic. Since particles tend to remain dispersed in either phase if they are too hydrophilic (low θ) or too hydrophobic (high θ) ( Binks and Lumsdon, 2000b). However, only when θ is relatively close to 90° can the particle effectively act as a Pickering stabilizer. O/W emulsions will come into being if the three-phase contact angle θ (angle at the three-phase boundary of solid particles, continuous phase and dispersed phase) is less than 90° (e.g., silica, clay), and W/O emulsions should form if θ > 90° (e.g., carbon black). Whether oil-in-water (O/W) Pickering emulsion or water-in-oil (W/O) Pickering emulsion can be formed is determined by the wettability of solid particles at the oil–water interface: if one of the liquids wets solid particles more than the other one, the better wetting liquid becomes the continuous phase and the other becomes the dispersed phase. This mechanism was supported by many scientists through theoretical approaches and some thermodynamic calculations ( Menon and Wasan, 1988 Binks and Clint, 2002 Aveyard et al., 2003). That is, particles are able to irreversibly attach to the oil–water interface, leading to a more efficient stabilization than surfactant adsorption. Theories that demonstrate the mechanism of stabilization in Pickering emulsions have been proposed, and the commonly accepted one is based on the formation of a steric barrier by solid particles adsorbing at the oil–water interface ( Monegier du Sorbier et al., 2015). It has been demonstrated by many researches that numerous types of inorganic particles including silica, clay, and hydroxyapatite (Hap), as well as some organic particles, can effectively serve as Pickering emulsifiers. It was discovered a century ago, but has recently drawn significant research interests as templates in many fields due to the following advantages: (i) solid particles reduce the possibility of coalescence, bringing about higher stability to emulsions (ii) many solid particles can endow as-prepared materials useful characteristics such as conductivity, responsiveness, porosity, and so on (iii) some food-grade solid particles have lower toxicity, thus leading to higher safety for in vivo usage. Pickering emulsion ( Pickering, 1907) utilizes solid particles alone as stabilizers, which accumulate at the interface between two immiscible liquids (typically denoted as oil and water phase) and stabilize droplets against coalescence. It is commonly known that emulsions can be stabilized by small molecular emulsifiers and some macromolecules, yet some of them may cause allergy-like reactions and carcinogenicity. Emulsions are widely used in many different fields including pharmaceutics, drug delivery, cosmetics, food industry, and so on, especially after the advancement of methods for preparing various kinds of emulsions.
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