High internal phase emulsions and their applications in functional material preparation

Abstract

High internal phase emulsions (HIPEs) are usually defined as a kind of superconcentrated emulsions, in which the minimum internal phase volume fraction reaches to 0.74. Comparing with traditional emulsions, high internal phase emulsions (HIPEs) tend to be highly viscous, solid-like emulsions because of their higher internal phase volume fraction. Thus, focusing on their microstructure, the shape of the oil droplets turned from spheres to irregular polyhedrons separated by a continuous phase film, leading to the deformation of the inner phase droplets. Obviously, the selection of suitable and effective stabilizers based on different stabilization mechanisms is vital for the emulsification process. Up to now, mainly three types of stabilizers based on dissimilar stabilization mechanisms have been successfully developed, including traditional surfactants, solid particles, and low molecular weight or macromolecular gelators. Given that the unique structural features, the wide applicability of HIPEs in pharmaceutical formulations, controlled-release deliverers, cosmetics industries, cold chain logistics and fatty food products has received considerable attentions. Moreover, owing to the characteristics of high viscosity and large interface film area, it is feasible to use high internal phase emulsion as a template to prepare porous functional materials, and the polyhedral structure of the internal droplets endows the final obtained porous material with higher specific surface area, higher porosity and lighter weight. Comparing with the other traditional methods, such as microemulsion method, chemical corrosion method, phase separation method, and so on, the parameters of porous material, including pore distribution, pore capacity and pore density can be easily and precisely controlled by altering the internal and external phase volume fraction, the type of stabilizer and other factors. Overall, the porous materials based on HIPEs template possess many advantages, including low cost, high capacity and excellent reversibility showing huge potential in gas adsorption, separation and enrichment, drug delivery and other practical fields. The present review provides the advanced progresses of preparation of HIPEs and applications in functional materials, with sophisticated porous structures and desirable environmental adaptabilities. Initially, we clearly summarize the synthesizing strategies in regard to both corresponding mechanisms and typical examples developed in recent years, including oil-in-water (O/W) HIPEs, water-in-oil (W/O) HIPEs, oil-in-oil (O/O) HIPEs, high internal phase supercritical fluid emulsions, high internal phase ionic liquid emulsion and high internal phase deep eutectic solvent emulsion. Subsequently, we also clearly discuss in detail the selection of stabilizers including traditional surfactants, solid particles, and low molecular weight or macromolecular gelators, and the influence of various external environmental factors on the stability of high internal phase emulsions (the features of phase, temperature and electrolyte). Meanwhile, we further look forward to their potential applications in functional material preparation, including the functional porous materials, flexible wearable devices and stimulus responsive material. Moreover, the limitations of the existing porous functional materials based on high internal phase emulsion are discussed, including inadequately mechanical performance and unsatisfactory biocompatibility, which will provide significant guidance for synthesizing strategies and practical application in the future. In the end, the current challenges and perspectives on the future development of functional HIPEs materials are also discussed.