Hsieh Han-Yu, Wu Shyuan-Guey, Tsui Hung-Wei
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3,Zhongxiao E. Rd., Taipei 10608 Taiwan.
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3,Zhongxiao E. Rd., Taipei 10608 Taiwan.
J Chromatogr A. 2017 Apr 21;1494:55-64. doi: 10.1016/j.chroma.2017.03.011. Epub 2017 Mar 8.
Stoichiometric displacement models have been widely used for understanding the adsorption mechanisms of solutes in chromatography systems. Such models are used for interpreting plots of solute retention factor versus concentrations of polar modifier in an inert solvent. However, these models often assume that dispersion forces are negligible and they are unable to account for solutes with significant aromatic interactions. In this study, a systematic investigation of the relationship between retention behavior and aromatic groups was performed using five simple aromatic molecules-benzene, naphthalene, mesitylene, durene, and toluene-with a commercially available amylose tris(3,5-dimethylphenylcarbamate)-based sorbent. The enthalpy changes of adsorption, determined from van't Hoff plots, were obtained separately in pure n-hexane and in pure isopropanol (IPA). In pure n-hexane, the solute adsorptions were driven by electrostatic interactions, favoring a T-shaped binding configuration (edge-to-face π-π interaction). The order of enthalpy change indicated the amount of effective T-shaped π-interactions. In pure IPA, solute adsorption was dominated by dispersion forces, favoring a sandwich binding configuration (face-to-face π-π interaction). The adsorption isotherms of toluene revealed that in pure IPA and in pure n-hexane, the isotherms were linear. The results suggested that the high solvent strength of IPA weakened the interactions between aromatic molecules. The retention behavior of the benzene, naphthalene, mesitylene, and durene as a function of IPA concentration was investigated. U-shaped retention curves were found for all aromatic solutes. A new retention model for monovalent aromatic solutes was developed for describing the U-shaped curves. Three key dimensionless groups were revealed to control the retention behavior. The models suggested that solvophobic interactions should be accounted for in the retention models used to investigate the retention behaviors of solutes associated with aromatic groups.
化学计量置换模型已被广泛用于理解色谱系统中溶质的吸附机制。此类模型用于解释溶质保留因子与惰性溶剂中极性改性剂浓度的关系图。然而，这些模型通常假定色散力可忽略不计，并且它们无法解释具有显著芳香相互作用的溶质。在本研究中，使用五种简单的芳香分子——苯、萘、均三甲苯、杜烯和甲苯——与一种市售的基于直链淀粉三（3,5-二甲基苯基氨基甲酸酯）的吸附剂，对保留行为与芳香基团之间的关系进行了系统研究。通过范特霍夫图确定的吸附焓变，分别在纯正己烷和纯异丙醇（IPA）中获得。在纯正己烷中，溶质吸附由静电相互作用驱动，有利于T形结合构型（边对面π-π相互作用）。焓变顺序表明有效T形π相互作用的量。在纯IPA中，溶质吸附以色散力为主，有利于夹心结合构型（面对面π-π相互作用）。甲苯的吸附等温线表明，在纯IPA和纯正己烷中，等温线均为线性。结果表明，IPA的高溶剂强度削弱了芳香分子之间的相互作用。研究了苯、萘、均三甲苯和杜烯的保留行为随IPA浓度的变化。发现所有芳香溶质的保留曲线均为U形。开发了一种用于描述单价芳香溶质U形曲线的新保留模型。揭示了三个关键的无量纲基团来控制保留行为。这些模型表明，在用于研究与芳香基团相关的溶质保留行为的保留模型中，应考虑疏溶剂相互作用。
