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.

在设计新型离子液体（ionic liquids, ILs）类分子结构时，考量其全生命周期——从合成所用原材料的获取，直至最终处置与降解——已成为至关重要的环节。作为一类主流离子液体家族，1-烷基-3-甲基咪唑鎓盐的合成方法正面临日益迫切的改进需求：一是采用易得的生物基替代物取代石油基烷基衍生物，二是在烷基尾链上引入杂官能团，以实现离子液体在使用后的（生物）降解。本文报道了一种简便易行且具备工业可行性的脂基咪唑鎓盐合成方法，以油醇、硬脂醇、月桂醇等多种生物基脂肪醇为起始原料。该工艺先对脂肪醇进行丙烯酸酯化，随后将咪唑基团通过氮杂迈克尔加成反应连接至丙烯酸酯位点。后续再以碘甲烷或对甲苯磺酸甲酯进行季铵化反应，即可得到一系列1-烷基丙酸酯-3-甲基咪唑鎓盐库，其烷基链长度涵盖C18、C12与C11，并包含不同种类的抗衡阴离子（碘离子、对甲苯磺酸根与四氟硼酸根）。这类含有酯基的经典1-烷基-3-甲基咪唑鎓盐类似物均属于离子液体，即熔点均低于100 ℃。此外，其中多数样品展现出液晶行为，可归类为离子晶体（IL crystals, ILCs）。本研究分别通过热重分析与差示扫描量热法，对这些离子液体的热稳定性及相变行为进行了考察；同时借助X射线散射，解析了其晶相与介观相的分子结构。值得注意的是，带有不饱和烷基尾链的离子晶体熔点较低，接近室温；而酯基官能团的存在则被证实可有效增强介观相的稳定性。