Effect of Trace Ions on Self-assembly in Ionic Liquids

In our earlier work with cationic surfactants (dodecyltrimethylammonium, DTA+) in ethylammonium nitrate (EAN, an ionic liquid), we found that the critical micelle concentration (CMC) and micelle morphology is sensitive to the choice of surfactant counterion. This is surprising as the nitrate concentration in EAN is ~100 x higher than counterions of added surfactant. Based on stoichiometry, ~99 % of DTA+ surfactant in solution should predominately interact with nitrate regardless of their original counterion. This observed dependency on surfactant counterion highlights that ion specific effects are present, and can be used as an effective tool to regulate self-assembly processes in high ionic strength solutions. The origin of specific ion effect in high ionic strength solutions is not well understood and potentially relates to the anomalous long screening length observed in these solutions. This experiment will examine dodecyltrimethylammonium bromide and nitrate self-assembly in EAN, ethylammonium thiocyanate (EASCN), and ethanolammonium nitrate (EtAN) to reveal the origin of ion specific effects in ILs, as well as their dependency on IL composition and structures.

我们此前针对硝酸乙酯胺（ethylammonium nitrate, EAN，一种离子液体）中阳离子表面活性剂十二烷基三甲基铵（dodecyltrimethylammonium, DTA+）开展的研究发现，临界胶束浓度（critical micelle concentration, CMC）与胶束形态对表面活性剂反离子的选择具有显著敏感性。这一现象颇为出人意料：尽管EAN体系内硝酸根浓度较外加表面活性剂的反离子高出约100倍，按照化学计量比计算，溶液中约99%的DTA+表面活性剂应优先与硝酸根结合，而非其原始反离子。上述对表面活性剂反离子的依赖性表明，离子特异性效应（specific ion effect）确实存在，且可作为调控高离子强度溶液中自组装过程的有效手段。目前学界对高离子强度溶液中离子特异性效应的起源尚未形成清晰认知，其可能与这类体系中观测到的反常长屏蔽长度相关。本实验将探究溴化十二烷基三甲基铵与硝酸十二烷基三甲基铵在EAN、硫氰酸乙酯胺（ethylammonium thiocyanate, EASCN）以及硝酸乙醇胺（ethanolammonium nitrate, EtAN）中的自组装行为，以揭示离子液体（ionic liquids, ILs）中离子特异性效应的起源，及其与离子液体组成和结构的依赖关系。