## 2 Results and Discussion
### Control of the Stability of the TiO\({}_{2}\) NC/Monomer Emulsion Droplets
Considering that most of the polymeric nanoparticles produced industrially from emulsion polymerization are negatively charged, it would be difficult to homogeneously disperse our hybrid nanospheres with those polymeric nanoparticles due to electrostatic attraction, leading to destabilization. For example, such a homogeneous mixing of two different particle dispersions at the nanoscale is crucial for making nanocomposites through shear-driven hetero-aggregation [48]. Thus, we kept using KPS as the initiator but designed a second step to warrant the stabilization of the mini-emulsion droplets. In the second step, the anionic surfactant, SDS, was introduced just after the Tween 20-only stabilized emulsion was formed, so as to have a synergistic stabilization played by both Tween 20 and SDS. In this case, it is evident that the added SDS also neutralizes the positive surface charges of TiO\({}_{2}\). Then, in order to be sure that the added amount of SDS not only neutralizes the positive charges, leading to the charge inversion, but is also sufficient to stabilize the emulsion, we have monitored the hydrodynamic diameter and zeta-potential of emulsion droplets as a function of the added amount of SDS. The typical results are shown in Figure 4 at the concentration of Tween 20 added in the first step, \(C_{\mathrm{T20}}=1.0\) wt\(\%\). It should be mentioned that during the experiments, when a given amount of SDS was added, the emulsion was treated by the probe sonicator at a power of 40 W for 10 min, and then the droplet diameter and \(\zeta\)-potential were measured. We can see that there exist two stages:  
1. Initially, the TiO\({}_{2}\) NCs/monomer droplets are positively charged, and with continuous addition of SDS, \(C_{\mathrm{SDS}}\), the diameter of the droplets, \(D_{p}\), increases while the \(\zeta\) potential decreases;
2. When \(D_{p}\) achieves a local maximum, which is around the location where the \(\zeta\) potential approaches zero, it starts to decrease until reaching a plateau, and the corresponding \(\zeta\) potential becomes negative and its absolute value increases with \(C_{\mathrm{SDS}}\). In Stage 1, as expected, the added SDS molecules affected the positive charges on the TiO\({}_{2}\) surface, leading to charge neutralization, and the droplets are partially destabilized. It follows that the droplet diameter, \(D_{p}\), increases with \(C_{\rm SDS}\), due to aggregation, while the \(\zeta\) potential decreases. When the added SDS molecules fully neutralizes the positive charges, the \(\zeta\) potential approaches zero, which leads to the lowest stability of the droplets, and the aggregation rate reaches the maximum, thus resulting in the largest droplet size. In Stage 2, further increase in the SDS concentration not only neutralizes the positive charges but also contributes to the stability of the droplets. Thus, the droplet diameter decreases with \(C_{\rm SDS}\), and the \(\zeta\) potential becomes negative and its absolute value increases. In Figure 4, when \(C_{\rm SDS}>0.3\) wt%, the \(D_{p}\) value decreases to reach a plateau. This may be related to the synergistic stabilization of Tween 20 and SDS, which involves complex competitive adsorption between Tween 20 and SDS on the surface. The plateau droplet diameter reached at the end is 170 nm, which is smaller than that (240 nm) before adding SDS, implying a lower density of Tween 20 on the surface required to stabilize the droplets due to the contribution of the adsorbed SDS. It should be pointed out that the sequence of the above two steps is crucial for the successful stabilization of the emulsion system, _i.e._, the droplets should be first stabilized by Tween 20 before the anionic SDS is introduced. Otherwise if SDS is used first, the TiO\({}_{2}\) NCs/monomer droplets would immediately aggregate, leading to phase separation, and eventually their re-dispersion would become extremely difficult.