# Timed-Release Silica Microcapsules for Consistent Fragrance Release in Topical Formulations
## 3 Results and Discussion
### Mesoporous-Controlled Silica Microcapsules for Commercial Products  
For effective timed release, the microcapsule walls must have an appropriate thickness to achieve the desired fragrance-release profile. It is also important that the fragrance is retained within the microcapsules in the formulation. In our previous study [10], additional silica coatings were applied to the surfaces of pre-synthesized microcapsules using post-TEOS concentrations of 1% (T1), 2% (T2), and 4% (T4) to prevent or delay fragrance leakage. As described elsewhere [10], silica microcapsules exhibit type II adsorption isotherms (Figure 3A), indicating the presence of mesopores (2-50 nm) and macropores (>50 nm), with a type H3 hysteresis loop, which is characteristic of slit-like pore structures [10; 44]. As the TEOS concentration increased up to 4%, the additional silica coatings likely filled the surface pores of the microcapsules, improving fragrance retention. As shown in Table 2, the porosity of the microcapsules was assessed using BET analysis. The T2 microcapsules demonstrated a 76.54% reduction in total surface area (BET) and 84.89% reduction in total pore volume compared to the control (0 wt.% TEOS; T0). The T4 microcapsules exhibited a similar surface area to the T2 microcapsules, suggesting saturation of the mesopores within the silica microcapsules. The average pore size versus differential pore area and volume analysis also revealed a reduction in both pore area and volume for both macropores and mesopores (Figure 3B,C). The pores were distinguishable by their distinct maxima centered at 3.83, 4.90, and 8.13 nm. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Additional TEOS**} & **Total Surface Area** & **Total Pore Volume** & **Average Pore** \\  & **(BET) (m\({}^{2}\)/g)** & **(m\({}^{3}\)/g)** & **Diameter (nm)** \\ \hline
0 wt.\% (T0) & 155.365 & 0.4870 & 9.978 \\ \hline
1 wt.\% (T1) & 63.061 & 0.1521 & 9.672 \\ \hline
2 wt.\% (T2) & 36.449 & 0.007361 & 8.079 \\ \hline
4 wt.\% (T4) & 37.495 & 0.007953 & 8.484 \\ \hline \end{tabular}
\end{table} 
Table 2: Surface areas and porosities of fragrance-containing silica microcapsules synthesized following the addition of various amounts of TEOS. Figure 3: (**A**) Nitrogen adsorption/desorption isotherms of silica microcapsules synthesized following the addition of various concentrations of TEOS. (**B**) Pore areas and (**C**) pore volumes as functions of average pore size. The storage stability of the microcapsules after surface modification using TEOS was tested by measuring the fragrance retention. Our previous study [10] revealed that the fragrance retention time for silica microcapsules was approximately 120 d at room temperature and 80 d at 60 \({}^{\circ}\)C. To accelerate the stability evaluation, we conducted the experiment under harsh conditions (2% SDS solution at 60 \({}^{\circ}\)C), which reduced the test time by approximately five times compared to the test at room temperature. After 14 d, more than 50% of the fragrance compound remained within the T4 microcapsules, whereas 30% remained within the T2 microcapsules, and only 15% remained within the T1 microcapsules (Figure 4A). The fragrance encapsulated within the T0 microcapsules was completely released after 10 d. Silica microcapsules with pore-controlled shells (T4) exhibited more effective fragrance retention. To confirm the timed-release performance of the T4 microcapsules, the fragrance capsules were incorporated into a cream formulation and evaluated by a panel of participants. As described in Section 2.6, Formulation B contained 2 wt.% microcapsules with 4 wt.% TEOS (T4), resulting in a formulation with 0.2 wt.% fragrance concentration. Formulation A (control) contained pure (unencapsulated) fragrance at a 0.2 wt.% concentration.