## 4 Discussion
Both stearic fit and the solubility of the drug may affect the particle size of NLC formulations. The smallest particle size for the SBO-NLCs was 105.27 nm, which was predicted and experimentally proved. The results showed that the experimental design could predict the conditions to develop the optimum SBO-NLCs with respect to particle size (Table 2). The PDI values provided information about the homogeneity of prepared NLCs [30]. Vegetable oil-loaded NLCs showed a PDI value of 0.281 to 0.382, and in the study in question it was claimed that the formulation was homogenous [31]. Curcumin-loaded NLC preparations with different oils (coconut, fish, black seed, and linesed oils) showed PDI values of 0.35 to 0.38 [32]. Argan oil-loaded NLCs and sillybin-loaded NLCs showed PDI values of 0.21 \(\pm\) 0.01 and 0.109 \(\pm\) 0.008, respectively [33; 34]. Increased drug concentration increases PDI values; for example, lycopene-loaded NLCs exhibited increases in PDI values corresponding to drug concentrations [35]. In the present study, the actual PDI values were found to be between 0.16 and 0.37, which reflects the relatively homogeneous size of the SBO-NLCs (Table 2). As with PDI, ZP also reflects the homogeneity of NLC formulations. The ZP of lycopene-loaded NLCs was found to be between \(-\)73.8 and \(-\)74.6 mV, depending on the concentration of the drug [35]. The ZP of curcumin-loaded NLCs (\(-\)26.5 \(\pm\) 1.8 to \(-\)32.0 \(\pm\) 1.9 mV), organ oil-loaded NLCs (53.37 \(\pm\) 0.92), and sillybin-loaded NLCs (\(-\)19.5 \(\pm\) 1.1 mV) were documented previously [32; 34]. In the present study, actual ZPs between \(-\)15.63 (the optimum ZP) and \(-\)36.87 mV were observed for the SBO-NLCs (Table 2). We may conclude that the prepared SBO-NLC was a homogenized optimal NLC, according to particle size, PDI, and ZP values. In the development of NLCs, EE percentage is an important factor to be considered. Compared to other drug delivery systems, NLCs have high EE percentages [29]. The drug solubility in the surfactant and lipid matrix used affect the EE. The protective surfactant layer of the NLC prevents drug leakage from the particles, thus helping to increase the EE percentage. The type of solid lipid used in NLCs greatly affects EE through the formation of different morphologies of NLCs and drug solubility in the lipid matrix. Approximately 99.98% of EE was documented with Precirol(r) ATO 5 as a solid lipid in the development of paclitaxel-loaded NLCs [36], while EE was observed to be between 82 and 98% when usingCOMPRITOL(r) 888 ATO as lipid [29]. The EE percentage using glyceryl monostearate was between 48 and 87% [37; 38]. Interestingly, in the present study, a 90.88% actual EE was observed using glyceryl monostearate as solid lipid (Table 2), which may be due to the combination of lipids, surfactant, and co-surfactant used in this study. Thermal characterization of developed SBO-NLCs was performed using DSC. The method used for the characterization of biomolecules includes nucleic acid, carbohydrate protein, lipid, and thermal characterizations of NLCs [39]. The melting point of the SBO-NLCs was higher than 65 \({}^{\circ}\)C (Figure 5), which is one of the desirable properties of topical products [40]. The result showed a reduction in the melting peak of the SBO-NLCs (Table 5). This could be associated with the smaller size of the particles [34]. The results indicate a lower possibility of drug repulsion from the core matrix during storage due to the crystallinity deduction of the lipid matrix [41]. DSC suggested that the NLCs had less crystallinity, thereby reducing the amount of drug expulsion from the nanoparticle and resulting in a high EE percentage. Increased trans-epidermal water is associated with disturbance to the stratum corneum and intercellular lipid loss, and skin dehydration causes a variety of skin problems. Thus, protecting the skin from dehydration is necessary and is one of the key characteristics of skin moisturizing agents.