## 3 Results and Discussion
### Qualitative Evaluation of Fragrance  
We confirmed that silica fragrance capsules maintain fragrance for an extended period of time, even in harsh conditions. The silica capsules designed for the fabric softener also had to be evaluated to ensure that they worked as intended. The fragrance evaluation was carried out with 20 panelists to determine the residual fragrance. The ISO 5.12 and 5.3 methods were used to assess the degree of fragrance retention in the silica fragrance capsules at different time durations and temperatures in the cationic polymer solution that was used for the fabric softener [35]. In this evaluation, T1 was not included. The number of samples should be minimized because the olfactory sense, which is the easiest to exhaust among the five senses in humans, causes difficulty in accurately measuring the intensity of fragrance. Although T1 revealed superior fragrance preservation to control the fragrance retention test, T3 and T5 were assumed to be more significant.  
Similar to the fragrance retention test, the fragrance intensity of the T5 microcapsule sample was more than moderate after 60 weeks and the fragrance intensity of the T3 microcapsule sample was weak, while the fragrance intensity of the T0 microcapsule sample was very weak within 8 weeks (Figure 6A). The fragrance of the sample after 8 weeks at 60 \({}^{\circ}\)C was similar to that in the previous in vitro fragrance retention test (Figure 6B). The fragrance retention in T5 was higher than that in T3, and the fragrance in T0 almost vanished after two weeks. The fragrance intensity could not be further evaluated, since DTHM hydrolyzes at 60 \({}^{\circ}\)C after 8 weeks; however, the silica fragrance capsules preserved the fragrance during the period of use as a fabric softener product. Silica fragrance capsules are used to confer fragrance in fabric softeners, and it has been concluded that they can be used for fabric softeners because of their superior long-term fragrance preservation and effective fragrance release based on user demand.  
Silica microcapsules have been synthesized elsewhere [33]. In the inverse phase, it was possible to synthesize microcapsules with dense silica shells composed only of silica, which, in turn, prolonged content retention. Although their research revealed that sustained release might be feasible, most of the compounds present in microcapsules released within 100 h. However, for our purposes, the results were insufficient because we had to keep the ingredient in microcapsules for a long time. In contrast to the hydrophobic fragrance oil used in our study, the encapsulated hydrophilic solute is supposed to release readily in water, which is an external phase. Another distinction is that, as stated in Section 3.1, the formation rate of the shell of silica capsules is slow below pH 4, whereas their reaction conditions were 0.1 M HCl (pH 1). It is difficult to compare them because there are no data  
Figure 5: Schematic representation of the mechanism of silica microcapsule synthesis in the presence of TEOS.  
on their capsule thickness. However, due to differences in the synthesis environment, the thickness of the silica shell is presumably thinner than our result; therefore, the encapsulated compound is likely to be released quickly. Considering that the purpose of this study was to maintain fresh fragrance for extended periods even in harsh environments, microcapsules that achieve this objective were successfully synthesized in this study. Furthermore, the synthesis of these microcapsules is facile, requiring little energy and no pH adjustment, rendering the procedure an efficient and eco-friendly manufacturing approach.