Pickering Emulsions: A Novel Tool for Cosmetic Formulators

Emulsions are colloidal dispersions formed by two immiscible liquids, where one of them is dispersed as droplets (dispersed phase) into the second one (continuous phase). However, the high surface energy of the interface between the two immiscible liquids results in thermodynamic instability. This leads to emulsion destabilization through different processes, e.g., flocculation, creaming, coagulation, coalescence, phase inversion, and Ostwald ripening, although they can be kinetically arrested for long periods of time [1]. The most common approach exploited for stabilizing emulsions is the use of surfactants or amphiphilic polymers, which contribute to the reduction in the interfacial tension between the two fluids by forming a molecular layer around the liquid droplets. This enhances the kinetic stability of emulsions, minimizing the destabilization events. However, it must be considered that the preparation of kinetically stable emulsions is a very difficult task due to the complexity of their interfacial and rheological behavior [2,3]. Despite the multiple difficulties associated with emulsion preparation, these systems are versatile tools for the cosmetic industry, finding extensive exploitation in the production of cosmetics for different purposes, e.g., sebum control, encapsulation, color cosmetics, skin whitening or UV protection [4,5,6].

Cosmetic emulsions are commonly very complex polydisperse multicomponent mixtures, consisting on the combination of the two liquids (water/hydrophilic base liquid and oil/hydrophobic base one), more than one active surface molecule (stabilizers), and several additives. The latter can contribute to improving the functionality or the sensorial feeling associated with the use of the formulation, provide fragrance, or simply enhance the quality of the final product (e.g., increase their stability or modify their viscosity and texture) [6,7]. The most common stabilizers of cosmetic emulsions are conventional surfactants, e.g., tween 80, span 80, sodium laureth sulfate (SLES), sodium lauryl sulfate (SLS), sodium dodecyl sulfate (SDS), cocamidopropyl betaine (CAPB) or polyethyleneglycol ethers [8,9,10]. However, in most cases, molecular and polymeric surfactants are not enough to guarantee the long-term stability of cosmetic emulsions and prevent a change in their properties over time [4]. Moreover, the use of synthetic surfactants in cosmetic products may cause adverse effects, e.g., allergies, hemolysis or cytotoxicity, to the final consumer and limit the eco-sustainability profile of cosmetics. The most common emulsifiers used in the cosmetic industry may induce different adverse effects depending on their chemical nature. For instance, cationic emulsifiers are more toxic than anionic ones, and the latter are more toxic than the non-ionic ones.

Despite this toxicity, emulsifiers are widely used in cosmetic products, with anionic surfactants having a prominent role as emulsifying agents. In fact, the low cost and stability of anionic surfactants have expanded their use in a broad range of cosmetic products; unfortunately, they may cause skin irritation due to their ability to modify the multilamellar structure lipid structure of the stratum corneum [11,12]. On the other side, the recovery of surfactant residues is not always easy from a practical point of view [13]. Therefore, it is urgent to seek alternative species, allowing the stabilization of emulsions to manufacture safer and more eco-sustainable cosmetic emulsions with long-term stability and satisfactory consumer perception during their application. A very promising approach for improving the quality of cosmetic emulsions is the replacement of conventional surfactant with colloidal particles to obtain particle-stabilized emulsions, the so-called Pickering emulsions [14]. This type of emulsion has become a very interesting alternative for the cosmetic industry. However, to date, there are many difficulties in manufacturing stable emulsions that provide a suitable sensorial perception upon application. Figure 1 schematizes the main features of emulsions stabilized by surfactants and particles.

Pickering emulsions were first described by Ramsdem and Pickering more than one century ago [15,16] and have undergone a very important development in the last two decades. Nowadays, it is possible to fabricate Pickering emulsions with properties selected at will, which opens up many opportunities for the design of new products in different industries, including the cosmetics industry [7,17]. In fact, the use of particles allows controlling the nature of the emulsions, the organization of the droplets and viscosity, modifying the sensory feeling upon application, product appearance and texture [18]. This makes possible the fabrication of innovative cosmetic formulations and multifunctional delivery systems. Moreover, Pickering emulsions offer many interesting properties compared to conventional ones, including superior stability against coalescence, minimization of the Ostwald ripening phenomena, higher biocompatibility and low cytotoxity [19].

This review tries to present a general overview of the potential interest of Pickering emulsions in cosmetics, highlighting some of the areas where they can contribute most to the current aims of the cosmetics industry. It is true that to date, the number of studies dealing with the use of particles for the stabilization of cosmetic emulsions remains scarce. However, Pickering emulsions can help to open up new avenues for the optimization of new eco-sustainable cosmetic products with enhanced sensorial perception.