# Incorporation of UV Filters into Oil-in-Water Emulsions--Release and Permeability Characteristics
## 3 Results and Discussion
### Physicochemical Characterization of Oil-in-Water Emulsions with and without UV Filters
Much greater changes in the backscattering profiles were noticed when octyl methoxycinnamate was added to the formulation. These results are in accordance with the zeta potential measurements (Table 3). To better visualize the changes in the samples, the stability index was calculated. It takes into consideration all the destabilization processes occurring in the tested product [45] and enabled to compare the stability of formulations. In Figure 2 changes in the stability index over time are depicted. The variations in the samples decreased in the following order E2%Avo > E0.5%Avo >E2%Avo >E0.5%Avo >E. In the next step, the refractive indices (R\({}_{\ell}\)) of the formulations were measured. Based on the results presented in Table 3, the values of R\({}_{\ell}\) ranged from 1.3543 to 1.3590, which is typical for oil-in-water emulsions [46]. These data were then applied to determine the particle size distribution of the topical formulations. The results are depicted in Figure 3  
Figure 1: Transmission and backscattering profiles of emulsions without UV filter (**A**) and with avobenzene E0.5%Avo (**B**), E2%Avo (**C**), and octyl methoxycinnamate E2%AMC (**D**). and Table 4. It was proved that the addition of UV filters did not significantly change the particle size distribution of the samples compared to the emulsion without sunscreen agents. The particle sizes ranged from 0.3 to 100 \(\upmu\)m. Bimodal particle size distribution with one predominant peak (average size around 1 \(\upmu\)m) was detected for E, E\({}_{0.5\%\text{Avo}}\), E\({}_{2\%\text{Avo}}\), and E\({}_{2\%\text{OMC}}\). Whereas for E\({}_{0.5\%\text{OMC}}\) the predominant peak was shifted towards slightly larger particle sizes. The slight differences in particle size distribution between the formulations with avobenzene and without this UV filter may be related to the time of the homogenization process. The emulsion containing 2% of avobenzene was homogenized much longer compared to E\({}_{0.5\%\text{Avo}}\) and E, to disperse completely the UV filter. Therefore, the particle size of E\({}_{2\%\text{Avo}}\) was smaller than two other formulations. It is generally known that the increase in the duration of homogenization can lead to the reduction in particle size [47]. In turn, when a smaller amount of UV filter (E\({}_{0.5\%\text{Avo}}\)) was used, the action time of the dispersion tool was the same as for the formulation without Avo. Therefore, the particle sizes of these two emulsions were similar. Additionally, the analyses were performed with and without ultrasound treatment to observe if potential aggregates that occurred in the emulsions can be broken down. All the formulations had bimodal particle size distribution regardless of the condition of the measurement. Ultrasound treatment reduced only slightly the size of the aggregates of the oil-in-water emulsions. The effect of ultrasound treatment on the particle size distribution of each sample was presented in Supporting Information (Figure S1). The biggest changes in the average particle size of samples D [3,2] were noted for E\({}_{2\%\text{OMC}}\). These results are in line with the data obtained by the zeta potential measurements and multiple light scattering, where variation in the stability index for E\({}_{2\%\text{OMC}}\) was the most noticeable. Figure 2: Changes in the stability index of formulations obtained. The effect of sunscreen agent addition to the oil-in-water emulsions was also studied by an optical microscope. It is a tool that enables the observation of any first signs of instabilities in the formulations [48]. In Figure 4, the images of all the oil-in-water emulsions prepared are displayed. No significant differences in droplet sizes between all the samples containing Avo and OMC were detected.