Formulation and Physical Evaluation of 96% Ethanol Extract Microemulsion Gel from White Watermelon Rind (Citrullus lanatus) as an Antioxidant

Abstract

Oxidative stress induced by free radicals, particularly from ultraviolet radiation, is a primary cause of skin damage, accelerating aging and increasing disease risk. Natural antioxidants offer a safer alternative to synthetic compounds with potential carcinogenic effects. Watermelon (Citrullus lanatus) rind, an agricultural waste product, is rich in phenolic compounds, flavonoids, lycopene, and citrulline, suggesting potent antioxidant capabilities. However, its topical application requires an effective delivery system. This research aims to formulate a stable microemulsion gel containing 96% ethanol extract of white watermelon rind and evaluate its physical characteristics and antioxidant activity. White watermelon rind extract was incorporated into microemulsion gel formulations at three different concentrations: FI (1.5 g), FII (2 g), and FIII (3 g). All formulations were subjected to physical evaluation tests including organoleptic observation, homogeneity, pH, viscosity, spreadability, particle size analysis, and microemulsion type determination over 28 days. Antioxidant activity was assessed using the DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging method with UV-Vis spectrophotometry, calculating the IC50 value. All three formulations (FI, FII, and FIII) demonstrated good physical stability, producing homogeneous, semi-solid preparations with a characteristic odor and pH values (4-5) suitable for skin application (4.0-8.0). Spreadability (5.4-6.8 cm) and viscosity met the required standards for topical gels. The microemulsion gel type was confirmed as oil-in-water (O/W) for all formulations. The particle size analysis showed mean diameters ranging from 41.2 nm (FII) to 740.1 nm (FI). The antioxidant activity assay revealed IC50 values of 67.43 ppm for FI, 64.06 ppm for FII, and 53.85 ppm for FIII, all classified as strong antioxidants. Notably, FIII with 3% extract exhibited the most potent antioxidant activity. The 96% ethanol extract of white watermelon rind (Citrullus lanatus) can be successfully formulated into a physically stable microemulsion gel. The formulation containing 3% extract (FIII) demonstrated the best physical properties and the strongest antioxidant activity (IC50 = 53.85 ppm), indicating its potential as a natural, value-added topical product for skin protection.