Potential of lignin multifunctionality for a sustainable skincare: Impact of emulsification process parameters and oil-phase on the characteristics of O/W Pickering emulsions

Colloidal lignin particles (CLPs) from softwood kraft lignin were evaluated as a multifunctional ingredient to prepare bio-based oil-in-water (O/W) Pickering emulsions. After a preliminary screening, three Pickering emulsions systems were formulated using orange, coconut, and paraffin oils, at varying concentration of CLPs, oil/water ratio, and by applying two-step homogenisation processes (rotor-stator homogenisation followed by ultrasonication). Ultrasonication as a second homogenisation step considerably enhanced the emulsification efficiency, generating emulsions with smaller droplet size and less polydisperse distribution. Furthermore, the effect of ultrasonication on the characteristics of emulsions and the stability of the systems was evaluated over time. The oil content and type, the concentration of CLPs, and the homogenisation methods significantly influenced the characteristics of the emulsions and drop size. Higher concentrations of oil and CLPs favoured the formation of the emulsion and contributed to higher physical stability after 120 days of monitoring. The in vitro Sun Protection Factor (SPF) results demonstrated that the incorporation of natural oils containing phenolic compounds in Pickering formulations improves the SPF value of emulsions, showing an interesting synergic effect between lignin particles and vegetable oils, which was not observed in the case of paraffin oil. Furthermore, the broad-spectrum sun blocker of Pickering emulsions was confirmed by the values of UVA/UVB between 0.74 and 0.90 and Cλ > 380.