pickering emulsion stabilized by nanocomplex

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.

Pickering emulsion stabilized by nanocomplex