Sarkar Abhra, Trivedi Shruti, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2009 May 28;113(21):7606-14. doi: 10.1021/jp901338x.
Poly(ethylene glycols) (PEGs) and room-temperature ionic liquids (ILs) are both projected as possible alternatives to volatile organic compounds (VOCs). Their potential usage in chemical applications, however, is often hampered by their limited and, in some cases, undesired individual physicochemical properties. Properties of mixtures of PEG with a common IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) are assessed via responses of three fluorescence probes: pyrene (Py) and pyrene-1-carboxaldehyde (PyCHO) are the dipolarity sensing probes and 1,3-bis-(1-pyrenyl) propane (BPP) is the probe for microfluidity. All three probes demonstrate anomalous fluorescence behavior within the mixture of [bmim][PF6] with four different PEGs of average molecular weight (MW) 200, 400, 600, and 1500 g.mol(-1), respectively, across complete composition range. Cybotactic region dipolarity of the probe Py within the mixtures is observed to be higher than that expected from ideal additive behavior. PyCHO lowest energy fluorescence maxima implying the static dielectric constant around the cybotactic region shows values within the mixtures to be even higher than that in neat PEG, the component having higher static dielectric constant of the two, clearly indicating the milieu to have anomalously high dipolarity. "Hyperpolarity" inherent to the PEG+[bmim][PF6] mixture is confirmed. Intramolecular excimer-to-monomer fluorescence intensity ratio of BPP indicates the microfluidity within the mixture to be even lower than that within neat [bmim][PF6], the component with lowest microfluidity. Presence of strong solvent-solvent interactions within the mixture is proposed to be the major reason for the anomalous fluorescence probe responses. Specifically, extensive hydrogen-bonded network involving termini hydroxyls of PEGs and PF6- as well as ethoxy/hydroxyl oxygens of PEGs and the C2-H of bmim+ is proposed to be responsible for the unusual outcomes. Fluorescence probe responses are shown to be adequately predicted by a four-parameter simplified combined nearly ideal binary solvent/Redlich-Kister (CNIBS/R-K) model. Unusually altered physicochemical properties are demonstrated to be the key feature of the "hybrid green" PEG+IL systems.
聚乙二醇（PEGs）和室温离子液体（ILs）都被视为挥发性有机化合物（VOCs）的可能替代品。然而，它们在化学应用中的潜在用途常常因其有限的、在某些情况下不理想的个别物理化学性质而受到阻碍。通过三种荧光探针的响应来评估PEG与常见离子液体1-丁基-3-甲基咪唑六氟磷酸盐（[bmim][PF6]）混合物的性质：芘（Py）和芘-1-甲醛（PyCHO）是偶极感应探针，1,3-双（1-芘基）丙烷（BPP）是微流动性探针。在整个组成范围内，所有三种探针在[bmim][PF6]与平均分子量（MW）分别为200、400、600和1500 g·mol⁻¹的四种不同PEG的混合物中均表现出异常的荧光行为。观察到混合物中探针Py的近程有序区域偶极比理想加和行为预期的更高。PyCHO最低能量荧光最大值表明近程有序区域周围的静态介电常数在混合物中的值甚至高于纯PEG中的值，而PEG是两者中具有较高静态介电常数的组分，这清楚地表明该环境具有异常高的偶极性。证实了PEG + [bmim][PF6]混合物中存在“超极性”。BPP的分子内准分子与单体荧光强度比表明混合物中的微流动性甚至低于纯[bmim][PF6]中的微流动性，而[bmim][PF6]是微流动性最低的组分。混合物中存在强烈的溶剂 - 溶剂相互作用被认为是荧光探针异常响应的主要原因。具体而言，提出了涉及PEG末端羟基与PF6⁻以及PEG的乙氧基/羟基氧与bmim⁺的C2 - H的广泛氢键网络是导致异常结果的原因。荧光探针响应通过四参数简化组合近理想二元溶剂/Redlich - Kister（CNIBS/R - K）模型得到了充分预测。异常改变的物理化学性质被证明是“混合绿色”PEG + IL系统的关键特征。
