Emulsions and Interfacial Properties

Emulsions are liquid-in-liquid dispersions of sub-micro/micro-sized droplets widespread in nature, technologies, and products.1 They are of great interest in many applied fields2, 3, 4, 5, 6 spanning from pharmaceuticals, cosmetics, and foods to other practical applications like oil recovery or development of new nanostructured soft materials. Ordinary emulsions are thermodynamically unstable, and the constituting liquids tend to separate under the effect of external forces or perturbations. The characteristic time of this process may differ by several orders of magnitude, depending on the chemical composition of the system and the environmental conditions. For this reason, the prediction of the lifetime of emulsions and their microstructure is essential for their utilization in diverse applications. The control of emulsification and emulsion stability is obtained by employing appropriate surface-active components such as low-weight surfactants, biomolecules, polymers, proteins, solid nanoparticles, or more complex associations of surfactants and nanoparticles. These amphiphilic components, or emulsifiers, tend to accumulate at the interfaces between the two liquid phases (typically aqueous and oily phases), modifying their physicochemical and mechanical properties. In this way, they favor emulsification lowering interfacial tension and, at once, contrast the mechanisms at the bases of the emulsion destabilization. As explained in more detail in the following sections, besides gravity-driven creaming/sedimentation, the main mechanisms governing the emulsion evolution, related to droplet interactions, are flocculation, coalescence, and Ostwald ripening (OR).7,8 The mechanisms involved in emulsification and emulsion destabilization have been widely investigated for many years and the basic concepts are well consolidated. Moreover, several studies have been recently carried out dealing with the utilization of classical and emerging emulsifiers in various application fields,8, 9, 10, 11, 12, 13, 14 some of these focusing on how the emulsion controlling mechanisms are influenced by the adsorption properties of the emulsifiers. For example, many studies7, 11, 12, 14 have underlined the importance of dynamic and structural features of the interfacial layers on stabilization against coalescence and Ostwald ripening. Despite the huge number of studies dedicated to emulsions, several aspects are still worthy of investigation because of their relevance to today’s emulsion-based technologies and products. One of the challenging topics in this field concerns, for example, the development of efficient formulations of emulsions with a low impact on the environment and health. In fact, the mainly used emulsifiers present potential negative effects on health and, at once, contribute in a large extent to environmental pollution when spread as domestic and industrial waste, especially for rivers and sea waters. Moreover, despite the advances in the direction of a better understanding of emulsions behavior, most of the formulations adopted by the industry still rely on semi-empirical approaches using an overabundant amount of surface-active components. In this contest, many studies on emulsions nowadays aim to promote a more efficient and safe use of emulsifiers in emulsion formulations via a deeper understanding of their role in emulsification and stabilization. This should contribute to improving the biocompatibility and biodegradability of products, using molecules with lower toxic/pollutant effects and/or significantly lower concentrations. The use of natural surfactants, derived from plants or vegetable wastes, also goes in the direction of more sustainable solutions, even if these compounds are typically complex (macro)molecules with variable and often ill-defined structures, requiring a rational utilization of the development of new more suitable interpretative models. On the basis of these considerations, it is clear that the way to develop a new formulation minimizing the impact on health and the environment is necessary through a deeper understanding of the relation between emulsion behavior and interfacial properties, mainly governed by the equilibrium and dynamic adsorption of emulsifiers. In the following, the basic well-consolidated concepts on emulsions are recalled together with an overview of the principal results obtained during the last years, focusing on the role of the interfacial properties in emulsification and emulsion evolution, the specificity of some emulsifiers, and the classical and emerging experimental methods recently adopted.