## 4 Microencapsulation Techniques
### Physicochemical Process
#### 4.3.2 Sol-Gel
Sol-gel processes also allow the designing of PCM submicromic capsules. In this way, the emulsion step and the hydrolysis and condensation reactions conditions played a key role in the particle properties. Thus, the formulation parameters such as the concentration of silica precursor, the effect of ethanol, and the ammonia to TEOS ratio were investigated to determine their impact on the particle size. According to [PERSON] et al., the mean diameter of the submicromic particles increases with the increase in TEOS or ammonia in the system [160]. [PERSON] et al. have realized their emulsion from an ultrasonic dispersion with a high surfactant content to ensure the formation of submicromic droplets in an alcohol medium [161]. The condensation of the silica species was initiated by dripping ammonia. The obtained rough particles with a mean diameter of 200 to 500 nm exhibited a latent heat of about 45.5 J.g\({}^{-1}\), corresponding to a loading content of 31.7%. The pH of the condensation step has a strong influence on the particle size and, therefore, on the heat storage capacity related to the PCM loading content [162]. Improving the structural and thermal performance of encapsulated PCMs is a challenge that researchers are exploring. In this context, many works have been directed toward the synthesis of the titanium oxide (TiO\({}_{2}\))-based membrane to avoid leakage problems and protect the PCM after many heating/cooling cycles. TiO\({}_{2}\) is mainly chosen because of its thermal and mechanical properties and as a chemically inert material. The formation of TiO\({}_{2}\) shell consists of the hydrolysis of titania organic precursors (titanium (IV) isopropoxide, titanium (IV) butoxide (TNBT), titanium (IV) ethoxide, etc.) before the poly-condensation of the TiO\({}_{2}\) sol on the PCM droplets (Figure 13). Particle size is controlled by formulation parameters such as weight ratios of surfactant to PCM, PCM to solvent, and PCM to precursor, and by process parameters such as stirring time and speed. Figure 12: SEM micrographs of _n_-octadecane/sodium silicate mass ratios and pH values: (**a,b**) 50/50 and pH between 2.95 and 3.05; (**c,d**) 50/50 and pH between 7 and 8 [156]. [PERSON] et al. encapsulated SA using TiO\({}_{2}\) as a shell material through the sol-gel method under alkaline pH conditions [163]. Titanium tetraisopropoxide was used as a TiO\({}_{2}\) precursor. They obtained particles with a nano spherical shape with a size distribution between 583 and 946 nm and showed suitable chemical stability and thermal reliability. The influence of the SA/TiO\({}_{2}\) weight ratio and pH values were investigated. They noticed that the encapsulation efficiency of the microcapsules increased by increasing the SA/TiO\({}_{2}\) weight ratio, and the most suitable pH for the formation of nanocapsules was about 10. However, [PERSON] et al. encapsulated stearic acid with amorphous TiO\({}_{2}\) composites, and they indicated that acidic pH was more effective in preparing the composite PCMs than under alkaline pH conditions [164]. [PERSON] et al. prepared LA/TiO\({}_{2}\) microcapsules. They established that encapsulation efficiency depended on the amount of anhydrous ethanol used to prepare the precursors mixture. It influenced the kinetics of the hydrolysis and condensation reactions of tetrabutyl titanate [165]. [PERSON] et al. reported the encapsulation of PA using the same method [166]. Diluting the precursor in ethanol solution decreases the hydrolysis pull kinetics, which promotes the formation of a denser and more even membrane upon condensation of the hydrolyzed precursors. On the other hand, an excess of ethanol also limits polymerization. Thus, control of membrane formation is achieved by adjusting the amount of ethanol. The encapsulation efficiency is controlled by the choice of precursor. Furthermore, the surfaces of the microcapsules become more homogeneous and smoother in decreasing the mass ratio of the TNBT to ethanol [167].