Ester-Containing Imidazolium-Type Ionic Liquid Crystals Derived from Bio-based Fatty Alcohols

The
need to take into account the life cycle of ionic liquids (ILs),
from the sourcing of the raw materials involved in their synthesis
to their disposal and degradation, has become paramount in the design
of new IL-type molecular structures. In the case of 1-alkyl-3-methylimidazolium
salts, one of the prominent IL families, there is an increasing demand
for synthetic methods involving (i) substitution of the petro-based
alkyl derivatives by readily available bio-sourced surrogates and
(ii) functionalization of the alkyl tail with heterofunctional groups
enabling the (bio)­degradation of ILs after use. Herein, a straightforward
and industrially viable synthesis of lipidic imidazolium salts is
reported, starting from different bio-sourced fatty alcohols, including
oleic, stearyl, and lauryl alcohols. This procedure is based on the
acrylation of fatty alcohols, followed by the aza-Michael addition
of the imidazole group onto the acrylate moiety. Subsequent quaternization,
using either methyl iodide or methyl tosylate, provides a library
of 1-alkylpropionate-3-methylimidazolium salts with various alkyl
chain lengths (C18, C12, and C11) and incorporating different types
of counteranions (iodide, tosylate, and tetrafluoroborate). These
ester-containing analogues of classical 1-alkyl-3-methylimidazolium
salts are all ILs, that is, with a melting point below 100 °C.
In addition, most of them exhibit a liquid-crystal behavior and can
be referred to as IL crystals (ILCs). The thermal stability, as well
as the phase transitions of these ILs, has been investigated by thermogravimetric
analysis, as well as differential scanning calorimetry, respectively,
while the molecular structure into the crystalline phase and the mesophase
is studied by X-ray scattering. Interestingly, ILCs featuring unsaturated
alkyl tails exhibit a low melting point, close to room temperature,
and the presence of the ester function is shown to provide an enhanced
stabilization of the mesophase.