Citric Acid as Green Modifier for Tuned Hydrophilicity of Surface Modified Cellulose and Lignin Nanoparticles

This paper proposes one straightforward route for citric acid modification of two different lignocellulosic products, cellulose nanocrystals (CNC) and lignin nanoparticles (LNP). Modified cellulose nanocrystals (MCNC) and lignin nanoparticles (MLNP) were characterized by means of Fourier Transform Infrared Spectroscopy (FT-IR), 13C and 1H nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible/near-infrared (UV–vis/NIR) spectrometry and thermogravimetric analysis (TGA). The reaction mechanism between citric acid and both CNC and LNP was discussed. The resultant MCNC exhibited improved dispersion in polar solvents, better thermal stability as compared with CNC while, in the case of LNP, a slight increase in thermal stability and alteration of MLNP dispersibility in polar solvents were proved. These results confirmed how esterified (MCNC) and etherified (MLNP) biobased nanoparticles with tuned hydrophilicity, obtained by a treatment with a low cost, sustainable and easily soluble cross-linker, have potential for widespread applicability in the field of polymeric based nanocomposites having different polarity.