Towards understanding size and stability of lignin-based core-shell structures

Micro- and nanostructures of natural polyphenols have received steadily growing interest as a result of their peculiar properties and applications superior to their bulk counterparts. Such structures can be generated in aqueous medium, avoiding thus the use of eventually environmentally non-benign organic solvents. Among the non-fibrous, but particulate systems, micro- and nanocapsules made from polyphenols seem particularly interesting due to the physico-chemical properties of polyphenols. Lignin-based micro- and nanoscaled core-shell structures were realised that allow for the encapsulation of active small molecules of various sizes and lipo- or hydrophobicities. We used a previously introduced ultrasound-based for producing nano- and microscaled capsules, since ultrasonication offers simple ways to tune size-determining conditions. Starting from oil in water emulsions that feature aqueous solutions of various technical lignins and an oil phase that hosts actives to be incorporated, capsules were formed in various sizes and with specific surface characteristics relying solely on self-assembly tendencies apparently exerted by the polyphenols. Screening various lignins, lignin fractions and modified lignins, expectable differences are observed in terms of yields, size and stabilities. More interestingly, however, not only the type of polyphenol, but also certain additives in the oil phase impact surface characteristics of realised capsules and thus the stability of their emulsions. A systematic screening revealed mechanistic insights regarding the molecular built of the shell that can eventually be exploited in the design of second-generation polyphenol capsules.