# Gemini Surfactant as a Template Agent for the Synthesis of More Eco-Friendly Silica Nanocapsules
## 3 Results and Discussion
### Characterization of Silica Nanocapsules  
SEM and STEM pictures of synthesised silica nanocapsules are presented in Figure 3. Obtained nanocontainers have a spherical shape with a diameter ranging between 100 nm and 200 nm. In STEM images (Figure 3c,d), a distinction (red arrow) between the wall and the core of the nanocapsules can be noticed, which confirms that obtained nanomaterials are capsules. In the FTIR spectra, presented in Figure 4a-c, some characteristic bands for silica capsules can be observed, such as Si-O-Si stretching at 1049 cm\({}^{-1}\), Si-OH stretching at 935 cm\({}^{-1}\) and Si-O-Si bending at 800 cm\({}^{-1}\). By overlapping the spectra of SiNC_CTAB and SiNC_QSB2-12 (Figure 4c) it is possible to observe their similarity. FTIR spectra for CTAB and QSB2-12 show characteristic wavenumbers values of the bonds: C-H stretching absorption vibrations of long alkyl chains between 2840 cm\({}^{-1}\) to 3000 cm\({}^{-1}\), C-H bending vibration of the (CH\({}_{3}\))\({}_{4}\)N\({}^{+}\) cation at around 1490 cm\({}^{-1}\) and rocking vibrations of (CH\({}_{2}\))\({}_{n}\), when n \(\geq\) 4, at around 720 cm\({}^{-1}\) (present only in case of long alkyl chains) [25]. In the silica capsules' spectra, lower intensity of signals associated with CTAB and QSB2-12 are observed  
Figure 3: SEM (**a,b**) and scanning transmission electron microscopy (STEM) (**c,d**) images of silica nanocapsules based on _N_-hexadecyl-_N_,_N_-trimethylammonium bromide (CTAB) (**a,c**) and QSB2-12 (**c,d**). suggesting that traces of cationic surfactants remained on the capsules' surface, as can be seen in the overlapped spectra of silica nanocapsules and surfactants (Figure 4a,b). Washing silica nanocapsules with an ethanolic solution of hydrochloric acid (1.5 mL of HCl in 150 mL of ethanol) at 60 \({}^{\circ}\)C allows the removal of cationic surfactants completely [16]. This route can be used for empty nanomaterials but applying it to nanocapsules loaded with active molecules may cause the unwanted release of the active molecules during washing steps. Another possibility to remove traces of cationic surfactants is calcination. However, this process removes all the organic compounds and cannot be applied to nanomaterials loaded with organic molecules [9; 11]. Figure 4: Overlapped Fourier-transformed infrared spectroscopy (FTIR) spectra of (**a**) SiNC_CTAB and CTAB, (**b**) SiNC_QSB2-12 and QSB2-12, and (**c**) SiNC_CTAB nd SiNC_QSB2-12. Regarding the size distribution of the synthesised capsules in distilled water (Table 1), determined by DLS, peaks centred at 157.1 nm and 191.6 nm were observed for SiNC_CTAB and SiNC_QSB2-12, respectively (Figure 5). These data agree with the size determined by SEM, although polydispersion index (PdI) shows values higher than 0.5, indicating heterogeneity of the samples and the presence of larger aggregates. For both SiNC_CTAB and SiNC_QSB2-12 larger particles are observed probably due to the presence of polymer residues (TEOS polymerization) [14]. DLS data also confirm this assumption by the presence of secondary peaks, indicating the agglomeration of the nanocapsules and the polymer residues into larger particles [14]. The zeta potential associated with SiO\({}_{2}\) is typically \(-28\) mV [31], so the recorded positive values of Z-potential for the synthesised silica nanocapsules in this work (cf. Table 1) are associated with the traces of cationic surfactants remaining in the capsules, which is consistent with FTIR spectra. Data for SiNC_CTAB are also in agreement with the literature where the hydrodynamic size for that conventional nanomaterial range from 80 nm to 180 nm [10; 11; 14]. Thermogravimetric (TG) experiments were performed in order to verify and compare the thermal stability of synthesized nanocapsules. Tests with calcined samples (after thermal treatment) were carried out as a reference.