Structural Properties of Coniferyl Alcohol-Based Low Transition Temperature Mixtures

TL;DR

This paper explores the structural properties of mixtures made from coniferyl alcohol and choline chloride, revealing how their molecular organization changes with temperature.

Contribution

The study is the first to investigate lignin-derived monomers in low transition temperature mixtures (LTTMs).

Findings

LTTMs with coniferyl alcohol and choline chloride have glass transition temperatures of −24.6 and −16.8 °C with no melting points.

The mixtures are heterogeneous, showing two regions of molecular mobility between 20 and 90 °C.

Strong hydrogen bonding involving the γ-OH group stabilizes the restricted molecular mobility region.

Abstract

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 (T
g) of −24.6 and
−16.8 °C, respectively, and with no observable melting
points (T
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.

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