Taguchi optimization of multi-plant saponin-based natural surfactants from Northeast India for reduced contact angle, critical micelle concentration and enhanced wettability

Surfactants play a critical role in cosmetic formulations, particularly in shampoos, yet conventional options such as sodium lauryl sulfate (SLS) raise environmental and dermatological concerns. This study explores the development and optimization of natural surfactants derived from saponin-rich plants indigenous to Northeast India, a biodiversity hotspot. Five ethnobotanically significant species, Albizia lucidior , Albizia chinensis , Acacia auriculiformis , Acacia pennata , and Gymnocladus assamicus were selected for hydrothermal extraction of saponins, followed by spectroscopic and physicochemical characterization. The extracts were evaluated for surfactant properties, including foaming capacity, critical micelle concentration (CMC), and wettability. Foam stability tests revealed superior performance from Acacia auriculiformis pod and Albizia lucidior leaf extracts. In addition, UV-Visible spectroscopic and Fourier Transform Infrared (FTIR) analyses confirmed the presence and structural integrity of saponins. Notably, bark extracts exhibited the lowest CMC values, indicating potent surface activity. Wettability was assessed via contact angle (CA) measurements, and an L8 Taguchi design optimized extract combinations to minimize CA values that are key to enhancing spreading behaviour on surfaces. The optimal blend with Albizia chinensis bark, Acacia auriculiformis pod and leaf, and Acacia pennata bark achieved a CA of ∼74°, closely aligning with predicted values. The optimized formulation also demonstrated excellent emulsification (E₂₄ = 56.8%), superior wetting on both virgin and bleached hair. The study underscores the potential of region-specific botanical resources as sustainable alternatives to synthetic surfactants. The Taguchi-based formulation strategy offers an efficient pathway to optimize multi-plant systems for cosmetic applications, contributing to greener and safer personal care products.