Transparent lignin nanoparticles for superhydrophilic antifogging coatings and photonic films

Lignin nanoparticles are useful in multiple applications, but their opaqueness remains an obstacle in optical applications. In this study, we present a method to prepare optically clear lignin nanoparticle dispersions from acetylated lignin. Thin lignin nanoparticle films remained transparent when deposited on glass and other smooth surfaces, and monolayered particle films provided effective antifogging properties. The particles could also be used to prepare multilayered films with bright structural colors that could be controlled via the film-thickness and were retained in dry conditions. We also developed an improved energy- and cost-efficient esterification method for controlled and quick lignin acetylation. The reaction could be selectively controlled to acetylate only aliphatic groups or to also include phenolic groups. We show here that stable nanoparticle dispersions can be produced from acetylated lignin at very high initial lignin concentrations without the formation of aggregates. Both the very short reaction time and high possible lignin concentration make the process industrially feasible as shown with a preliminary techno-economic assessment.