Perfluorinated C8-(PerfluoroC8) and bidentate anchored C8-(BDC8)-modified silica hydride stationary phases have been employed for the isocratic separation of homologous phenylalkanols and phenylalkylamines differing in their n-alkyl chain length, using aqueous-acetonitrile (ACN) mobile phases of different ACN contents from 10 to 90% (v/v) in 10% increments. These analytes showed reversed-phase (RP) retention behaviour with mobile phases of <40% (v/v) ACN content with both stationary phases but with the BDC8 stationary phase providing longer retention. The PerfluoroC8, but not the BDC8, stationary phase also exhibited significant retention of these analytes under conditions typical of an aqueous normal phase (ANP) mode (i.e. with mobile phases of >80% (v/v) ACN content), with the analytes exhibiting overall U-shape retention dependencies on the ACN content of the mobile phase. Further, these stationary phases showed differences in their selectivity behaviour with regard to the n-alkyl chain lengths of the different analytes. These observations could not be explained in terms of pKa, log P, molecular mass or linear solvation energy concepts. However, density functional theory (DFT) simulations provided a possible explanation for the observed selectivity trends, namely differences in the molecular geometries and structural organisation of the immobilised ligands of these two stationary phases under different solvational conditions. For mobile phase conditions favouring the RP mode, these DFT simulations revealed that interactions between adjacent BDC8 ligands occur, leading to a stationary phase with a more hydrophobic surface. Moreover, under mobile phase conditions favouring retention of the analytes in an ANP mode, these interactions of the bidentate-anchored C8 ligands resulted in hindered analyte access to potential ANP binding sites on the BDC8 stationary phase surface. With the PerfluoroC8 stationary phase, the DFT simulations revealed strong repulsion of individual perfluoroC8 ligand chains, with the perfluoroC8 ligands of this stationary phase existing in a more open brush-like state (and with a less hydrophobic surface) compared to the BDC8 ligands. These DFT simulation results anticipated the chromatographic findings that the phenylalkanols and phenylalkylamines had reduced retention in the RP mode with the PerfluoroC8 stationary phase. Moreover, the more open ligand structure of the PerfluoroC8 stationary phase enabled greater accessibility of the analytes to water solvated binding sites on the stationary phase surface under mobile phase conditions favouring an ANP retention mode, leading to retention of the analytes, particularly the smaller phenylalkylamines, via hydrogen bonding and electrostatic effects.

中文翻译：

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用全氟化 C8 和双齿 C8 改性氢化硅固定相分离的不同正烷基链长的芳香醇和胺的选择性差异的起源

全氟 C8-(PerfluoroC8) 和双齿锚定 C8-(BDC8) 改性氢化硅固定相已用于使用乙腈水溶液 (ACN) 流动相等度分离同系苯基链烷醇和烷基链长度不同的苯基烷基胺10% 到 90% (v/v) 的不同 ACN 含量，增量为 10%。这些分析物显示出反相 (RP) 保留行为，流动相的 ACN 含量小于 40% (v/v)，使用两种固定相，但 BDC8 固定相提供更长的保留时间。在水正相 (ANP) 模式的典型条件下（即流动相的 ACN 含量 >80% (v/v)），全氟 C8 固定相（而非 BDC8）也表现出对这些分析物的显着保留，分析物表现出整体 U 形保留依赖于流动相的 ACN 含量。此外，这些固定相在不同分析物的正烷基链长度方面表现出选择性行为的差异。这些观察结果无法用 pKa、log P、分子量或线性溶剂化能概念来解释。然而，密度泛函理论 (DFT) 模拟为观察到的选择性趋势提供了可能的解释，即在不同溶剂化条件下这两种固定相的固定配体的分子几何形状和结构组织的差异。对于有利于 RP 模式的流动相条件，这些 DFT 模拟显示相邻 BDC8 配体之间发生相互作用，导致固定相具有更疏水的表面。此外，在有利于以 ANP 模式保留分析物的流动相条件下，双齿锚定 C8 配体的这些相互作用导致分析物无法进入 BDC8 固定相表面上的潜在 ANP 结合位点。对于全氟 C8 固定相，DFT 模拟显示单个全氟 C8 配体链的强烈排斥，与 BDC8 配体相比，该固定相的全氟 C8 配体以更开放的刷状状态存在（并且具有更少的疏水表面）。这些 DFT 模拟结果预测了色谱结果，即苯基链烷醇和苯基烷基胺在使用全氟 C8 固定相的 RP 模式下降低了保留。而且，