Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 454

(a) (b) TEOS

surfactant
cyclohexane
solution

acidic
NPG
solution

W/O emulsion

(c)

(d)

NPG
with SiO2 shell

Fig. 4 Process of synthesizing NPG microcapsules. a mix water into cyclohexane with
surfactant; b add TEOS; c hydrolysis of TEOS to form silica shell; and d collection of
microcapsules [ 32 ]

phase transition. One interesting feature of these polyalcohol PCMs is that their
phase transition temperature can be continuously tuned by mixing two or more
polyalcohols. This flexibility is important in order to address many applications
that have different operation temperatures.
In some applications, NPG microcapsules are needed to protect the NPG core
from the reactive base fluids. The NPG microcapsules can be synthesized by the
method of interfacial hydrolysis of TEOS in W/O emulsions [ 32 ]. The synthesis
process is illustrated in Fig. 4 . Highly concentrated NPG aqueous solution with
HCl was used as the water phase of the W/O emulsion. The silicon oxide shell is
formed by the hydrolysis of TEOS, as given in the equation below:

ð C 2 H 5 O Þ 4 Si þ H 2 O ! C 2 H 5 OH þ SiO 2 xH 2 O ð 3 Þ

As the precursor TEOS is hydrophobic and the product silica is hydrophilic, the
hydrolysis reaction of TEOS occurs at the interface between aqueous droplets and
the bulk cyclohexane. During this reaction, water in the aqueous solution is