# 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  
All the prepared samples were oil-in-water emulsions, containing mostly an aqueous phase (67.3-69.3% by weight). The maintenance of the proper pH of the skin is crucial. The surface of a healthy stratum corneum is acidic, in the range of pH 4.5 to 6.0 [37]. However, both endogenous (skin moisture, sweat, sebum, age, and genetic predispositions) and exogenous (detergents, cosmetic products, and topical antibiotics) factors may affect the pH value of the skin surface [38]. Topically applied cosmetics may contribute to preserving appropriate skin conditions. Therefore, the selection of skin care products should be considered carefully. As the sunscreen formulations are intended to be applied topically on the skin surface, the pH values of each emulsion were analyzed. The results are gathered in Table 3. The pH values of the formulations range from 6.02 to 6.11, which is acceptable for cosmetic products. The addition of both UV filters only slightly increased the pH value of the emulsions. It should be stressed that the most problematic, for technologists, are sunscreen formulations containing zinc oxide because the pH can be shifted toward 7.5, which reduces the number of preservatives possible to be applied to the product [39]. In the case of the emulsions obtained in this study, this issue was not detected. The stability of the prepared formulations was assessed using a centrifugation test, a multiple light scattering method, and by the determination of the zeta potential value. No phase separation was observed in any of the samples tested after centrifugation. The zeta potential values of all five formulations exceeded \(\pm\)30 mV, which proved their high degree of stability. High negative or positive electrokinetic values give information that the particles in the suspensions tend to repel and thus the flocculation process is prevented [40]. It was found that the addition of UV filters to emulsions decreased the zeta potential value compared to the emulsion without the sunscreen agent; however, it remained at the proper high level. The stability of the formulations was also assessed using multiple light scattering. This method allows to characterize the samples without dilution [41]. Moreover, it is possible to detect any changes occurring in the sample much faster than by the naked eye. Therefore, the aging tests can be shortened significantly [42]. \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **pH** & **Zeta Potential (mV)** & **Refractive Index** \\ \hline E & 6.02 \(\pm\) 0.04 & \(-\)79.0 \(\pm\) 1.3 & 1.3590 \\ \hline E\({}_{0.5\%\text{Avo}}\) & 6.04 \(\pm\) 0.06 & \(-\)75.1 \(\pm\) 1.9 & 1.3543 \\ \hline E\({}_{2\%\text{Avo}}\) & 6.06 \(\pm\) 0.08 & \(-\)73.0 \(\pm\) 1.5 & 1.3555 \\ \hline E\({}_{0.5\%\text{OMC}}\) & 6.09 \(\pm\) 0.07 & \(-\)57.1 \(\pm\) 2.0 & 1.3580 \\ \hline E\({}_{2\%\text{OMC}}\) & 6.11 \(\pm\) 0.10 & \(-\)54.2 \(\pm\) 1.8 & 1.3567 \\ \hline \end{tabular}
\end{table} 
Table 3: Physicochemical characterization of emulsions. As the results of the measurements, the transmission and backscattering (BS) profiles versus the height of the sample were registered and the exemplary graphs are presented in Figure 1. As the emulsions were opaque systems the variation in backscattering was only considered. It should be highlighted that for stable products all backscattering profiles will overlap over time. In turn, for unstable samples, the variations in backscattering profiles will be visible. Furthermore, no changes in particle size occurs if the BS profiles do not exceed 2% [43]. When changes are higher than 10%, the formulation is treated as unstable [44]. It can be observed in Figure 1A,B that the BS profiles of samples E and E0.5%Avo remained almost unchanged during the 35 days so it confirms that the 0.5% of avobenzene did not affect the stability of the emulsion. Whereas for E2%Avo a slight variation in the delta BS (2.8%) was detected.