**Microencapsulation technologies**

More than 200 microencapsulation methods are described in the scientific literature and patents, and most of them include three necessary steps, namely containment of the central component, formation of microparticles, and hardening of the envelope. These methods are generally divided or classified into three main groups, which are based on the mechanisms governing membrane formation, namely mechanical, chemical, and physicochemical processes. The choice of one method over another is often dictated by the cost of treatment, the use or not of organic solvents, and the consideration of health and environmental aspects. The interactions between polymers and solvents in the microencapsulation process probably have the most critical effect on the morphology and properties of the particles obtained. Thus, each encapsulation step is affected by the solvency of the oil phase, and therefore to form a separate membrane or shell, the solvent used must promote the precipitation of the polymer in the early reaction stage, and also allow the continuous diffusion of monomers through the existing membrane to allow its growth.

The most popular methods include interfacial, in-situ, and suspension polymerization methods for chemical processes, simple or complex phase coacervation for physicochemical processes, and spray drying for mechanical processes. Whatever the process selected, it includes two main steps, e.g., the emulsification step, which determines the size and size distribution of the microcapsules, and the formation of the capsules. The first step is affected immediately by physical parameters such as apparatus configuration, stirring rate, and volume ratio of the two phases, and by physicochemical properties such as interfacial tension, viscosities, densities, and chemical compositions of the two phases used. The formation of microcapsules is also related to the use of surfactants, which influences not only the mean diameter but also the stability of the dispersion. The surfactants or the colloid have two leading roles, i.e., to reduce the interfacial tension between oil and aqueous phases to allow the formation of smaller microcapsules, and to limit or prevent coalescence by its adsorption on the oil/water interface by forming a layer around the dispersed droplets. Shell formation is mainly governed by kinetic factors, i.e., the ability of the monomers, pre-polymer, or polymer to react or to cross-link, and thermodynamic factors, i.e., the minimum total free energy exchange in the system. Furthermore, the choice of the polymer system for shell synthesis needs to be considered regarding the application and availability of the material.
