Depolymerization and modification
of lignin have been achieved
simultaneously in a one-pot chemical reaction. Two heteroelement-rich
modifiers, imidazol-1-yl phosphonic dichloride and 1H-1,2,4-triazol-1-yl
phosphonic dichloride, were selected to react with lignin in this
work. The modified lignin (m-lignin) is demonstrated as an effective
lubricating additive for [choline][amino acid] ([CH][AA]) bioionic
liquids. Different characterization techniques have been utilized
to study the lignin depolymerization, reaction between lignin and
modifiers and m-lignin/[CH][AA] interaction. The effect of the molecular
structure of the modifiers on the rheological and tribological properties
of m-lignin/[CH][AA] lubricants was systematically investigated. Density
function theory is used to calculate the electronic structure of lignin,
m-lignin, and [CH][AA]. The atomic natural charge analysis revealed
the most negative charge on nitrogen bonded to a phosphorus atom and
the strongest capability of forming hydrogen bonding with [CH][AA].
The introduced nitrogen and phosphorus elements not only increase
the hydrogen bonding density in m-lignin/[CH][AA] but also enhance
the polarity of the m-lignin, both of which facilitate a strong adhesion
of lubricant on a metal surface and thus promote lubrication. A larger
fraction of heteroatom groups in m-lignin contributes to a better
lubrication property of these lubricants.