Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608 Taiwan.
J Chromatogr A. 2024 Nov 8;1736:465383. doi: 10.1016/j.chroma.2024.465383. Epub 2024 Sep 17.
This study investigated the adsorption mechanisms in a normal-phase system using a cyano-based stationary phase as the sorbent. The minor disturbance method was used to measure the adsorption isotherms of acetone and alcohols with various structures. Excluding data in pure n-hexane revealed that the adsorption behaviors on cyano sites were well described by the Langmuir model. The adsorption equilibrium constants, ranging from 8.86 to 11.15 at 25 °C, showed no significant differences across alcohol structures and decreased with increasing temperature. The saturation adsorption concentration decreased with increasing alcohol molecule size, with branched-chain alcohols showing a lower saturation adsorption amount compared to straight-chain alcohols. The standard state adsorption enthalpies and entropies calculated from the equilibrium constants for various alcohols ranged from -29 to -22 kJ/mol and -78 to -55 J/K·mol, respectively, showing enthalpy-entropy compensation. A discrepancy was observed between these adsorption enthalpies and those obtained from the retention factors of alcohols using pure n-hexane as the mobile phase. This discrepancy may result from the affinity energy distribution of the adsorbent. In pure n-hexane, the adsorption behaviors of adsorbates were considerably affected by high-affinity sites. Moreover, acetone and these alcohol molecules were used as solvent modifiers to investigate the relationship between the retention factor, modifier concentration, and temperature for various solutes with distinct functional groups. The retention curves were converted to enthalpic curves using the van't Hoff equation. A theoretical model was proposed to describe the relationship between the van't Hoff enthalpy change and mobile phase composition. The proposed model effectively described the enthalpic curves, indicating that the enthalpy change follows a saturation curve with increasing modifier concentration. This trend is primarily due to competitive adsorption and complexation behaviors between the solute and modifier molecules.
本研究采用氰基键合固定相作为吸附剂，在正相体系中研究了吸附机理。采用微扰法测定了不同结构的丙酮和醇在正相体系中的吸附等温线。排除纯正己烷中的数据后，表明在氰基位上的吸附行为可用朗缪尔模型很好地描述。在 25℃下，吸附平衡常数在 8.86 到 11.15 之间，其值与醇的结构无明显差异，且随温度升高而降低。随着醇分子尺寸的增大，饱和吸附浓度降低，支链醇的饱和吸附量低于直链醇。从各种醇的平衡常数计算得到的标准态吸附焓和熵分别在-29 到-22 kJ/mol 和-78 到-55 J/K·mol 范围内，表现出焓熵补偿。从用纯正己烷作为流动相测定的醇的保留因子得到的这些吸附焓与用标准状态下吸附平衡常数计算得到的吸附焓之间存在差异。这种差异可能是由于吸附剂的亲和能分布不同。在纯正己烷中，吸附质的吸附行为受到高亲和能位的显著影响。此外，还使用丙酮和这些醇作为溶剂改性剂，研究了具有不同官能团的各种溶质在不同保留因子、改性剂浓度和温度下的关系。通过 van't Hoff 方程将保留曲线转换为焓曲线。提出了一个理论模型来描述 van't Hoff 焓变与流动相组成之间的关系。该模型有效地描述了焓曲线，表明随着改性剂浓度的增加，焓变遵循饱和曲线。这种趋势主要是由于溶质和改性剂分子之间的竞争吸附和络合行为。
