Response surface optimisation of a strobilanthes crispus-loaded red palm oil nanoemulsion with enhanced physicochemical stability

Plant-derived lipophilic bioactive compounds often show poor dispersion and instability in a water system; this limits their use in topical and pharmaceutical formulations. This study aimed to develop and optimise an oil-in-water nanoemulsion containing Strobilanthes crispus extract dissolved in red palm oil using Response Surface Methodology (RSM). A three-factor Box-Behnken design was applied to evaluate the effects of Tween® 80 concentration, glycerol concentration, and homogenisation pressure on droplet size, polydispersity index (PDI), and creaming index. The results show that the surfactant concentration was the most important factor affecting the droplet size and overall stability. The optimised nanoemulsion formulation comprised 5.06% (w/w) Tween® 80, 10.4% (w/w) glycerol, and a homogenisation pressure of 617.1 bar. Experimental validation produced a nanoemulsion with a droplet size of 151.8 ± 1.3 nm, PDI of 0.127 ± 0.021, and no visible creaming. The regression models showed strong predictive ability for droplet size (R2 = 0.984) and creaming index (R2 = 0.999), but moderate prediction for PDI (R2 = 0.7516). Stability studies under different pH (2-6), temperature (40-60°C), ionic strength (0.5-2.5mM), and centrifugal conditions (1000-5000rpm), as well as four-month storage, showed that the nanoemulsion remained physically stable without phase separation. This stability is mainly attributed to steric stabilisation from the non-ionic surfactant. Overall, the study demonstrates that Response Surface Methodology is an effective tool for optimising nanoemulsion formulations for the delivery of lipophilic plant bioactives.

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