Display omitted •Novel comparative exploration on LCNF production from willow bark and wood.•LCNF made of willow bark using p-toluenesulfonic acid were very hydrophobic.•The origin of the unique hydrophobicity of willow bark LCNF was revealed.•Aromatic extractives and hydroxymethylfurfural condensed with lignin in the acid.•The potential of willow bark LCNF in making hydrophobic films was shown. We conducted the first comparative exploration on lignin containing cellulose nanofibril films prepared alternatively from willow bark and wood using a highly recyclable acid hydrotrope, aqueous p-toluenesulfonic acid, as a sustainable means for isolating the nanofibrils. The nanofibrils of willow bark were hydrophobic and produced dense films of high strength under hot pressing. The hydrophobicity resulted from condensation of the residual lignin with low molecular weight aromatic substances of the bark and hydroxymethylfurfural formed in situ from fructose that is also present in the bark.