## 3 Results and Discussion
### Stability of TiO\({}_{2}\) Dispersion upon Adding Surfactants
The qualitative results of the dispersion stability based on photographic images are summarized in Figure 2d. The quantitative values of the degree of dispersion stability are presented in Figure S4. Anionic SDS produced a TiO\({}_{2}\) dispersion with a degree of dispersion stability of \(\sim\)100% at a range of treated concentrations (0.1-2 wt%). In the case of CTAB, TiO\({}_{2}\) was dispersed with a stability of 84-100% at treated surfactant concentrations (0.01-2 wt%), although DDPAS enabled TiO\({}_{2}\) to disperse from 85-95% at relatively low concentrations (0.001-0.05 wt%). In the case of Tween 20, it was not dispersed at low concentrations of 0.001 and 0.005 wt% but then dispersed with a degree of 100% in treated surfactant concentration (0.01-2 wt%). After one month of measurement, TiO\({}_{2}\) dispersion by adding SDS maintained stability  
Figure 2: Photograph of TiO\({}_{2}\) dispersions using different surfactants after 24 h storage. (**a**) 0.01 wt% TiO\({}_{2}\) was added to non-polar oil (DC345): water (8:2) with surfactant. (**b**) 0.01 wt% TiO\({}_{2}\) was added to polar oil (CEH):water (8:2) with surfactant. Table of the degree of TiO\({}_{2}\) dispersion (O: \(>\)80% dispersed, X: \(<\)80% dispersed) in a mixture of (**c**) DC345 and water and (**d**) CEH and water. at concentrations of 0.01-2 wt%, while the case of adding Tween 20 remained stable at concentrations of 0.05-2 wt%. In contrast, most of the CTAB- and DDAPS-added samples showed significant precipitation (Figure S5). Using FT-IR spectroscopy (Nicolet iS20 FTIR Spectrometer, Thermo Fisher Scientific, Waltham, MA, USA), we further characterized the chemical compounds in the well-dispersed TiO\({}_{2}\) dispersions (in the case of adding 1 wt% of the four different surfactants). The band at 600-700 cm\({}^{-1}\) associated with TiO\({}_{2}\) and the C-H bands of the surfactants at 2916 and 2848 cm\({}^{-1}\) appeared, indicating that the surfactants were adsorbed onto the TiO\({}_{2}\) surface, forming TiO\({}_{2}\) dispersions (Figure S6). To examine the surface-charged state of the particle, the size and zeta potential of TiO\({}_{2}\) dispersions were analyzed. Light dynamic scattering (DLS) measurements indicated that the addition of surfactants successfully reduced particle size compared to samples without surfactants. Without surfactants, particles tended to aggregate, increasing in size, whereas surfactant addition prevented this aggregation, resulting in smaller particle sizes. SDS, in particular, formed the smallest and most stable particles. Furthermore, size measurements at varying SDS concentrations showed that particle size increased with higher concentrations, indicating a thicker adsorption layer. This increase in the adsorption layer creates steric hindrance between particles, leading to even greater stability (Figure S7). The zeta potential increased with concentration in the case of SDS and CTAB addition, showing more negative and positive values, respectively, while DDAPS and Tween 20 showed no change in zeta potential. This increase in zeta potential can enhance dispersion stability through ionic repulsion. Previous studies have shown that as the concentration of surfactants increases, the absolute value of zeta potential (negative or positive) also increases [35; 36]. Similarly, in this study, we observed that higher concentrations resulted in greater dispersion stability (Figure S8). To correlate the degree of dispersion stability and the viscosity of the TiO\({}_{2}\)-based W/O formulation, the viscosity of the TiO\({}_{2}\) dispersion was measured based on the concentration of the treated surfactants. For non-polar oil (DC345), ionic surfactants (anionic SDS, cationic CTAB, and zwitterionic DDAPS) showed a decreasing trend at low concentrations (0-1 wt%) but increased at higher concentrations (2-5 wt%) of treated surfactants (Figure 3a).