Structural Properties of Coniferyl Alcohol-Based Low Transition Temperature Mixtures

This study examines the structural organization of low transition temperature mixtures (LTTMs) based on coniferyl alcohol, a primary monolignol, at the macroscopic and microscopic levels. The work presents the first investigation into the use of lignin-derived monomers as constituents of LTTMs. The LTTMs prepared with 2:1 and 3:1 molar ratios of coniferyl alcohol to choline chloride exhibited glass transition temperatures (<i>T</i> _g) of -24.6 and -16.8 °C, respectively, and with no observable melting points (<i>T</i> _m). The respective water contents were 0.4790 and 0.7564 wt %, similar to that reported for other natural deep eutectic solvents. Time-domain nuclear magnetic resonance analysis revealed that these LTTMs are heterogeneous and exhibited two regions between 20 and 90 °C due to differences in molecular mobility. Furthermore, Fourier-transform infrared spectroscopy and proton NMR demonstrated that the region of restricted molecular mobility was stabilized by strong hydrogen bonding, mainly involving the γ-OH group of the coniferyl alcohol. The weak hydrogen bonds were gradually broken with an increase in temperature, leading to structural reorganization. As the temperature increased, the system transitioned to a state in which the hydrogen-bonded clusters and more freely dispersed molecular regions were present in approximately equal proportions.