GLUTATHIONE-LOADED NANOEMULSION 265 The W/O nanoemulsion produced by high-energy homogenization showed no difference in droplet size over the study period at both 25 and 4°C. The NE-19 demonstrated high physical stability, verifying our results for storage conditions of 4 and 25°C. High-energy homogenization is better at producing stable nanoemulsions than its other preparation methods. When nanoemulsions were prepared using a high-pressure homogenizer, the droplet size was initially around 100 nm however, the particles increased in size after 30 d at 25°C. This phenomenon was attributed to the preparation method. The high-energy emulsifi cation method used in our study showed high stability with respect to the droplet size and polydispersity index (29). Zeta potential is a measurement of charge on the nanodroplets when it is high, nanodroplets repel each other because of similar charges. The zeta potential is very signifi cant in deter- mining the stability of nanoemulsions, all the nanoemulsions which have zeta potential greater than ±30 mV are considered more stable (30). The zeta potential of all NE-19 formulations was found to be negative and more stable as zeta potential is greater than ±30 mV. The zeta potential of fresh NE-19 was -37.1 mV after 30 days, it was -34.2 mV and after 60 days, it was -34.8 mV. The mobility of fresh NE-19 was -2.726 μm cm/Vs after 30 days, it was -2.911 and after 60 days, it was -2.59. When the zeta potential is high ( -30 mV), the nanodroplets repel each other because of similar charges and there is no coalescence and aggregation. The charge on the droplet surface causes the transpor- tation of drug molecules through various physiological barriers (31). Rheological properties characterize the spreadability features of the nanoemulsion (32). There is a decrease in viscosity as the shear rate and shear stress increases. NE-19 and B-19 have shown non-Newtonian fl ow and shear-thinning pseudo-plastic behavior on varying shear rates, which occurs in cosmetics (33). The decrease in viscosity may be due to surfactants (span 80 and tween 80), which play an important role in nanoemulsion stability and prevent coalescence and aggregation (34). CONCLUSION In this study, a thermodynamically stable nanoemulsion was successfully prepared. The droplet sizes were less than 200 nm with good rheology and stability of GSH. It main- tained normal skin pH values throughout the 90-d testing period. The zeta potential of -34 mV, which could impede the coalescence and deposit of the oil nanodroplets, generated the stable nanoemulsion formulation. The nanoemulsion was most stable at room tem- perature compared with the higher temperature (40°C/75 RH) and could be evaluated for treatment of oxidative skin related diseases. ACKNOWLEDGMENTS The authors would like to thank Prof. Dr. Nisar Ur Rehman, Chairman, Department of Pharmacy, COMSATS University Islamabad, Abbottabad Campus, for providing cosmetic laboratory services. REFERENCES (1) M. N. Yu kuyama, D. D. Ghisleni, T. J. Pinto, and N. A. Bou-Chacra, Nanoemulsion: process selection and application in cosmetics—a review, Int. J. Cosmet. Sci., 38, 13–24 (2016).

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