Li Hongping, Arzhantsev Sergei, Maroncelli Mark
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Phys Chem B. 2007 Mar 29;111(12):3208-21. doi: 10.1021/jp067916y. Epub 2007 Mar 7.
Electronic absorption and emission spectra of 10-bis(phenylethynyl)anthracene (PEA) and coumarin 153 (C153) are measured as functions of composition along the bubble-point curve at 25 degrees C in CO2-expanded cyclohexane (c-C6H12), acetonitrile (CH3CN), and methanol (CH3OH). The nonlinear dependence of the spectral frequencies on composition suggests substantial preferential solvation of both solutes by the liquid components of these mixtures. Estimates of enrichment factors (local mole fraction of a component divided by its bulk value) based on the assumption that spectral shifts are linearly related to local composition are quite large (approximately 10) in the cases of the C153/CH3CN + CO2 and C153/CH3OH + CO2 systems at high xCO2. Computer simulations of anthracene, the chromophore of PEA, and C153 in these three CO2-expanded liquids are used to clarify the relationship between local composition and spectral shift. A semiempirical model consisting of additive electrostatic and dispersive interactions is able to capture the main features observed experimentally in all six solute/solvent combinations. The simulations show that the commonly used assumption of a linear relation between spectral shifts and local compositions grossly exaggerates the extent of preferential solvation in these mixtures. The collective nature of electrostatic solvation and the composition dependence of the solute's coordination number are shown to be responsible for the breakdown of this assumption.
在25摄氏度下，沿着CO₂膨胀的环己烷（c-C₆H₁₂）、乙腈（CH₃CN）和甲醇（CH₃OH）的泡点曲线，测量了10-双（苯乙炔基）蒽（PEA）和香豆素153（C153）的电子吸收和发射光谱随组成的变化。光谱频率对组成的非线性依赖性表明，这两种溶质在这些混合物的液体组分中存在大量优先溶剂化现象。在高xCO₂的情况下，基于光谱位移与局部组成呈线性关系的假设，对富集因子（组分的局部摩尔分数除以其整体值）的估计在C153/CH₃CN + CO₂和C153/CH₃OH + CO₂体系中相当大（约为10）。利用对蒽（PEA的发色团）和C153在这三种CO₂膨胀液体中的计算机模拟，来阐明局部组成与光谱位移之间的关系。一个由加和静电相互作用和色散相互作用组成的半经验模型能够捕捉在所有六种溶质/溶剂组合中实验观察到的主要特征。模拟结果表明，光谱位移与局部组成之间线性关系的常用假设严重夸大了这些混合物中优先溶剂化的程度。静电溶剂化的集体性质和溶质配位数的组成依赖性被证明是导致该假设不成立的原因。
