14 Various bioderived materials, such as polysaccharides, 15 and protein 13 macromolecules, can be effectively utilised as Pickering particles to stabilise these W/W emulsions.Īlthough numerous reviews have already been published on the role of inorganic-particle-stabilised Pickering emulsions, 16–18 their relatively poor biodegradability and biocompatibility significantly limit their practical applications in many fields. 12,13 However, stabilisation at the W/W interface remains a challenge due to significantly lower interfacial tension. Moreover, in some applications such as biocatalysis, the activity of the biomacromolecules ( e.g., proteins, enzymes) is enhanced in an all-water system because their activity is significantly reduced at the oil–water interface. The W/W emulsions offer an oil-free system suggesting a more sustainable and greener approach than the traditional O/W or W/O emulsions. Water-in-water (W/W) emulsions can also be formed by dispersing one aqueous phase into another aqueous phase, where the aqueous phases contain two types of hydrophilic macromolecules that are immiscible with each other. 4, Copyright 2019, with permission from Elsevier. 2 Schematic representation of O/W and W/O-type Pickering emulsions governed by the wettability (contact angle, θ) of the solid particles. Therefore, there is an option for the researchers to alter the surface properties (for example, wettability) of these solid particles to make them more amphiphilic.įig. However, a more stable Pickering emulsion is formed when the contact angle is close to 90°, where the solid particles balance their dispersion in both phases. 4 A W/O-type emulsion forms if the three-phase contact angle is higher than 90° (usually hydrophobic). More specifically, in an O/W-type emulsion, the contact angle at the three-phase boundary ( i.e., between the disperse phase, solid particles and continuous phase) is less than 90° ( i.e., hydrophilic) (see Fig. Typically, the phase with the better wetting capacity on the solid particles becomes the continuous phase, while the other becomes the dispersed phase. The mechanism of the attachment of the solid particles at the oil–water interface mainly depends on their wettability, which determines the type of emulsion, either water-in-oil (W/O) or oil-in-water (O/W) systems. In Pickering emulsions, the solid particles are irreversibly adsorbed at the oil–water interface to form a steric barrier resulting in a more stable emulsion formulation. It is very important to understand the mechanism of Pickering emulsion formation when selecting a solid emulsifier. 2–4,9–11 The key advantages of using solid particles as emulsifiers are associated with their tunable properties (such as wettability, surface charge, porosity, responsiveness etc.), and the option to select the particles from sustainable sources with biodegradability and low toxicity. 1 Schematic representation of surfactant stabilised emulsion and solid nanoparticle stabilised Pickering emulsions.Ī significant variety of inorganic (such as silica, clay) and organic (polysaccharides and proteins) particles have been effectively utilised as Pickering emulsifiers providing long-term emulsion stability. 8 In a Pickering emulsion, solid particles are allowed to accumulate at the interface between two immiscible phases to reduce the possibility of coalescence by forming a physical barrier (see Fig. Introduction Pickering emulsions are becoming more widely used in many fields, including food, cosmetics, paints, coating, pharmaceutics, and drug delivery, 1–6 although they were first reported a century ago by Ramsden in 1903 (ref. Also, an overview of the various tuneable factors associated with the functionalisation or surface modification of these solid particles, that govern the stability of the Pickering emulsions is provided. Therefore, this review reports recent literature (2018–2021) on the use of comparatively safer biodegradable polysaccharide particles, proteins, lipids and combinations of these species in various Pickering emulsion formulations. However, synthetic surfactants are not always a suitable choice in some applications, especially in pharmaceuticals, food and cosmetics, due to toxicity and lack of compatibility and biodegradability. Usually, emulsions are a dispersion system, where synthetic surfactants or macromolecules stabilise two immiscible phases (typically water and oil phases) to prevent coalescence.

In recent years, the demand for non-surfactant based Pickering emulsions in many industrial applications has grown significantly because of the option to select biodegradable and sustainable materials with low toxicity as emulsion stabilisers.

Pickering emulsion simulation shear