A simple method to produce stable aqueous dispersions of spherical anionic or cationic lignin nanoparticles from softwood kraft lignin was developed. A lack of renewable resources and their inefficient use is a major challenge facing the society. Lignin is a natural biopolymer obtained mainly as a by-product from the pulp- and paper-making industries, and is primarily burned to produce energy. However, interest for using lignin in more advanced applications has increased rapidly. In particular, lignin based nanoparticles could find potential use in functional surface coatings, nanoglue, drug delivery, and microfluidic devices. In this work, a straightforward method to produce lignin nanoparticles from waste lignin obtained from kraft pulping is introduced. Spherical lignin nanoparticles were obtained by dissolving softwood kraft lignin in tetrahydrofuran (THF) and subsequently introducing water into the system through dialysis. No chemical modification of lignin was needed. Water acts as a non-solvent reducing lignin's degrees of freedom causing the segregation of hydrophobic regions to compartments within the forming nanoparticles. The final size of the nanoparticles depended on the pre-dialysis concentration of dissolved lignin. The stability of the nanoparticle dispersion as a function of time, salt concentration and pH was studied. In pure water and at room temperature the lignin nanoparticle dispersion was stable for over two months, but a very low pH or high salt concentration induced aggregation. It was further demonstrated that the surface charge of the particles could be reversed and stable cationic lignin nanoparticles were produced by adsorption of poly(diallyldimethylammonium chloride) (PDADMAC).
