Hydrophobized lignin nanoparticle-stabilized Pickering foams : building blocks for sustainable lightweight porous materials

Pickering particles play an essential role in stabilizing Pickering foams that can be utilized as templates for making lightweight porous materials for thermal insulation purposes. With the shift from petroleum to renewable-source-derived materials, particles synthesized from biomass are emerging but are typically too hydrophilic to function as Pickering particles in foams. Here, we report the hydrophobization of lignin nanoparticles (LNPs) by adsorption of an oppositely charged surfactant for air-in-water Pickering foam stabilization. The surface tension and complex viscoelasticity of the aqueous dispersions were tunable by varying the concentration of LNPs and the adsorption ratio of hexadecyltrimethylammonium bromide (CTAB) onto LNPs, which were systematically studied with the pendant drop technique (DPT). Under the optimum conditions, the achieved air-in-water Pickering foams were remarkably stable against coalescence and coarsening, i.e., the bubble size distribution remained unchanged over 30 days. We further utilized the Pickering foams as templates for making dry lightweight composite foams with the introduction of cellulose nanofibrils (CNFs). The closed-cell composite foams, with lignin as the major component, exhibited good thermal insulation properties and mechanical properties that were comparable to commercial rigid polyurethane (PU) foams. We envision that the renewable Pickering particles could find applications in many other areas beyond the templates for porous materials such as enhanced oil recovery.