Hydrophobic deep eutectic solvent as a multifunctional co-emulsifier and stabilizer in W/O emulsions: interfacial properties, colloidal stability, and enhancement of functional properties

In this study, the hydrophobic deep eutectic solvent (HDES) menthol - levulinic acid (1:1) was assessed for its role as a co-emulsifier, stabilizer, antibacterial and moisturizing agent in a water-in-oil (W/O) emulsions for cosmetic purposes. Surface properties of the HDES, including interfacial tension, contact angle, and surface energy components, were systematically evaluated, while its polarity was determined using a solvatochromic probe, collectively indicating that the HDES possesses physicochemical characteristics favorable for interfacial activity and emulsion stabilization. The resulting emulsion exhibited a zeta potential of –30 to – 40 mV, pH ≈ 4.5, density of 914 – 920 kg m -3 , lamellar morphology observed by SEM, and remained stable without phase separation or precipitation during storage. FTIR spectroscopy indicated the presence of hydrogen-bonding interactions between the HDES components and the emulsion constituents, while SEM confirmed the formation of a lamellar microstructure. These findings suggest that the HDES may contribute to emulsion stabilization through adsorption at the oil–water interface, reducing interfacial tension and forming a steric barrier against droplet coalescence. The presence of HDES in the emulsion contributed to enhanced antibacterial activity against Escherichia coli, Bacillus subtilis , and Candida spp . Using the cyclic voltammetry method, the increase in antioxidant capacity of emulsions with НDES was confirmed. The functional properties of the cosmetic emulsions, including moisture, pigment, sebum and elasticity parameters, were evaluated using a skin analyser. The Menthol–Levulinic acid НDES demonstrated a high DES-GREEN sustainability score of 77/100, confirming its green and environmentally favorable profile and supporting its suitability for incorporation into cosmetic cream formulations.