Sarkar Abhra, Ali Maroof, Baker Gary A, Tetin Sergey Y, Ruan Qiaoqiao, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi - 110016, India.
J Phys Chem B. 2009 Mar 12;113(10):3088-98. doi: 10.1021/jp8098297.
In this work, an array of molecular-level solvent features--including solute--solvent/solvent--solvent interactions, dipolarity, heterogeneity, dynamics, probe accessibility, and diffusion--were investigated across the entire composition of ambient mixtures containing the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4], and pH 7.0 phosphate buffer, based on results assembled for nine different molecular probes utilized in a range of spectroscopic modes. These studies uncovered interesting and unusual solvatochromic probe behavior within this benchmark mixture. Solvatochromic absorbance probes--a water-soluble betaine dye (betaine dye 33), N,N-diethyl-4-nitroaniline, and 4-nitroaniline--were employed to determine ET (a blend of dipolarity/polarizability and hydrogen bond donor contributions) and the Kamlet-Taft indices pi* (dipolarity/polarizability), alpha (hydrogen bond donor acidity), and beta (hydrogen bond acceptor basicity) characterizing the [bmim][BF4] + phosphate buffer system. These parameters each showed a marked deviation from ideality, suggesting selective solvation of the individual probe solutes by [bmim][BF4]. Similar conclusions were derived from the responses of the fluorescent polarity-sensitive probes pyrene and pyrene-1-carboxaldehyde. Importantly, the fluorescent microfluidity probe 1,3-bis(1-pyrenyl)propane senses a microviscosity within the mixture that significantly exceeds expectations derived from simple interpolation of the behavior in the neat solvents. On the basis of results from this probe, a correlation between microviscosity and bulk viscosity was established; pronounced solvent-solvent hydrogen-bonding interactions were implicit in this behavior. The greatest deviation from ideal additive behavior for the probes studied herein was consistently observed to occur in the buffer-rich regime. Nitromethane-based fluorescence quenching of pyrene within the [bmim][BF4] + phosphate buffer system showed unusual compliance with a "sphere-of-action" quenching model, a further manifestation of the microheterogeneity of the system. Fluorescence correlation spectroscopic results for both small (BODIPY FL) and macromolecular (Texas Red-10 kDa dextran conjugate) diffusional probes provide additional evidence in support of microphase segregation inherent to aqueous [bmim] [BF4].
在这项工作中，基于在一系列光谱模式下使用的九种不同分子探针收集的结果，研究了一系列分子水平的溶剂特征，包括溶质 - 溶剂/溶剂 - 溶剂相互作用、偶极矩、非均质性、动力学、探针可及性和扩散，这些特征涵盖了含有离子液体1 - 丁基 - 3 - 甲基咪唑四氟硼酸盐（[bmim][BF4]）和pH 7.0磷酸盐缓冲液的环境混合物的整个组成范围。这些研究揭示了该基准混合物中有趣且不寻常的溶剂化显色探针行为。使用溶剂化显色吸收探针——一种水溶性甜菜碱染料（甜菜碱染料33）、N,N - 二乙基 - 4 - 硝基苯胺和4 - 硝基苯胺——来确定表征[bmim][BF4] + 磷酸盐缓冲液体系的ET（偶极矩/极化率和氢键供体贡献的混合）以及Kamlet - Taft指数π*（偶极矩/极化率）、α（氢键供体酸度）和β（氢键受体碱度）。这些参数均显示出明显偏离理想状态，表明[bmim][BF4]对各个探针溶质存在选择性溶剂化作用。荧光极性敏感探针芘和芘 - 1 - 甲醛的响应也得出了类似结论。重要的是，荧光微流动性探针1,3 - 双（1 - 芘基）丙烷检测到混合物中的微粘度显著超过了根据纯溶剂中行为的简单插值所预期的值。基于该探针的结果，建立了微粘度与本体粘度之间的相关性；这种行为暗示了明显的溶剂 - 溶剂氢键相互作用。本文研究的探针与理想加和行为的最大偏差始终出现在富含缓冲液的区域。[bmim][BF4] + 磷酸盐缓冲液体系中芘基于硝基甲烷的荧光猝灭表现出对“作用球”猝灭模型的异常符合，这是该体系微非均质性的进一步体现。小分子（BODIPY FL）和大分子（Texas Red - 10 kDa葡聚糖共轭物）扩散探针的荧光相关光谱结果为水性[bmim][BF4]固有的微相分离提供了额外证据。
