## 4 Microencapsulation Techniques
### Physicochemical Process
#### 4.3.2 Sol-Gel  
Sol-gel is the abbreviation for \"solution-gelling\". It is a process by which essentially inorganic materials are synthesized. Sol (solution) is a stable dispersion of polymers or colloidal particles with diameters of (1-100 nm) in a solvent prepared at low pH. This solution is prepared through polycondensation reactions of monomers (or precursors) already hydrolyzed to polymerize and form sol nanoparticles. To maintain the resulting particles suspended, they should have van der Waals-type interactions that require sufficiently small particles. The formed amorphous \"gel\" is characterized by a large number of pores (<1 um) containing liquids (solvents and non-reacting monomers) and smaller polymer chains [152]. As the reaction progresses, the smaller polymer chains are consumed, and solvents and monomers diffuse less in the pores. The material densifies and stabilizes when some parts depolymerize and polymerize again, resulting in well-compact gel and releasing some solvent that needs to be eliminated by drying. The formation of long macromolecular chains can induce their aggregation. As a result, the viscosity increases, and the solution undergoes a transition to a gel characterized by an infinite three-dimensional network structure (Figure 11). Various physicochemical parameters such as the phases (solvents, precursors), temperature, pH, or drying, influence the sol-gel transition process (gelling). These parameters determine the reaction mechanisms and kinetics and thus the final properties of the material. Parameters must be studied and adapted to specific applications. Different studies used the sol-gel method to encapsulate PCM materials by inorganic shells. Therefore, inorganic materials have attracted significant attention thanks to their excellent properties, such as high thermal conductivity, non-flammability, and durability. * Silica shell  
Many studies have reported synthesizing encapsulated PCMs with an inorganic shell (SiO\({}_{2}\)) using the sol-gel process. Most of the resulting capsules with SiO\({}_{2}\) shells showed a well-defined core/shell structure with excellent shape stability. It was indicated that PCM/SiO\({}_{2}\) microcapsules revealed a high thermal energy storage capacity and an improved thermal conductivity. [PERSON] et al. used silica as a shell material to encapsulate \(n\)-octadecane [153]. They indicated that the microcapsules exhibited a spherical shape of 7 to 16 \(\upmu\)m in diameter, with a smooth and compact shell structure. An enhancement of \(n\)-octadecane thermal conductivity was observed after encapsulation with the silica shell. Moreover, it was also mentioned that some low-cost silicon sources such as sodium silicate and oil shale ash could be used to synthesize encapsulated PCMs, making them more cost-effective [154,155]. [PERSON] et al. developed \(n\)-alkanes microcapsules using sodium silicate as a silica source [156]. They investigated the influence of pH values, and they obtained microcapsules having a regular spherical shape with a mean diameter of 8 \(\upmu\)m by adjusting pH values between 2.95 and 3.05 (Figure 12). The synthesis of paraffin/SiO\({}_{2}\) was also carried out in acidic conditions by [PERSON] et al. [157]. The pH adjustment in the 2-3 pH range led to the formation of the particles with a PCM content of 87.5%. Figure 11: The condensation of precursor and gelation step [37]. The weight ratio precursor to PCM in the formulation influences the morphology of the microcapsules and the landing capacity and, therefore, heat storage properties [158]. An increase in silica precursors, such as methyltriethoxysilane (MTES), led to increased surface particle roughness due to the silica aggregates deposition onto it. Thus, the core to shell ratio decreased, decreasing the phase change enthalpy of the mPCMs. On the other hand, [PERSON] et al. have shown the increase in emulsifier and dispersed to continue phases volume ratio allowed to enhance the encapsulation efficiency [159].