Birch Outer Bark-derived Betulin Hydrogels as Sustainable Feedstock for Pickering Emulsions

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In this study, particles derived from birch outer bark extractives were used to formulate Pickering emulsions as potential ecological cosmetic cream base. Two types of extractive particles were evaluated – purified (99.4 wt% triterpenes) and unpurified (72 wt% triterpenes, 1.4 wt% polyphenols). Using a hydrogel-based-size-reduction method, the average particle size decreased from 1373 to 205 nm for unpurified and from 1080 to 230 nm for purified particles. Both particle types were polydisperse (polydispersity index was 0.20-0.25) and negatively charged, indicating their suitability for stabilizing emulsion. Emulsion performance was assessed through storage modulus (G’) in the linear viscoelastic region and thermal stability after 3 days at 50 °C. The highest G’ values were obtained for emulsions containing 60% water and 2.2 wt% unpurified (G’ = 11.5 kPa) and purified (G’ = 23.1 kPa) particles. Optical microscopy results indicated that slow coalescence does occur in all systems. However, emulsions stabilized with unpurified particles exhibited a threefold increase in oil-phase oxidative stability due to the presence of polyphenols. Overall, results demonstrate that stable Pickering emulsions can be produced from both purified and unpurified betulin hydrogels.