O/W nanoemulsions underwent organoleptic inspections to evaluate their external appearance. The size, polydispersity index (PDI), and zeta potential were determined using a Zeta sizer instrument (Malvern Instruments Ltd., Malvern, UK). Results have been reported as the mean and standard deviation (S.D.) of at least ten measurements of each sample. In addition, their pH was measured using a pH meter. The viscosity of nanoemulsions was determined using a Brookfield rheometer equipped with a bob and cup (R/S Rheometer, Brookfield Viscometer Ltd., Middleboro, MA, USA) at 25 °C. The nanoemulsion morphology was analyzed using transmission electron microscopy (TEM), following the procedure outlined by Anantaworasakul et al. (2020) [11 (link)]. A single drop of nanoemulsion was placed onto a copper grid and stained with 1% phosphotungstic acid. TEM imaging was conducted at 100 kV to examine their nanostructure. The stability of each nanoemulsions was assessed following 6 cycles of heating–cooling. During each cycle, the nanoemulsions were subjected to storage at 45 °C for 24 h, followed by storage at 4 °C for 24 h. In addition, a long-term stability test was also performed by storing the formulation at room temperature for 3 months. Subsequently, the physical appearance, particle size, PDI, and zeta potential were evaluated.
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