Kulsing Chadin, Nolvachai Yada, Matyska Maria T, Pesek Joseph J, Topete Joshua, Boysen Reinhard I, Hearn Milton T W
Australian Centre for Research on Separation Science (ACROSS), School of Chemistry, Monash University, Melbourne, Victoria, 3800, Australia.
Department of Chemistry, San Jose State University, San Jose, CA, 95192, USA.
Anal Chim Acta X. 2018 Dec 28;1:100003. doi: 10.1016/j.acax.2018.100003. eCollection 2019 Mar.
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 -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 ( 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 -alkyl chain lengths of the different analytes. These observations could not be explained in terms of p , log , 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, hydrogen bonding and electrostatic effects.
全氟C8 -（全氟C8）和双齿锚定C8 -（BDC8）修饰的氢化硅胶固定相已用于等度分离不同烷基链长度的同源苯烷醇和苯烷基胺，使用乙腈（ACN）含量从10%到90%（v/v）、以10%增量变化的水 - 乙腈（ACN）流动相。在ACN含量<40%（v/v）的流动相条件下，这两种固定相对这些分析物均表现出反相（RP）保留行为，但BDC8固定相提供更长的保留时间。在典型的水相正相（ANP）模式条件下（ACN含量>80%（v/v）的流动相），全氟C8固定相（而非BDC8固定相）也对这些分析物表现出显著保留，分析物对流动相ACN含量呈现总体U形保留依赖性。此外，这些固定相在不同分析物的烷基链长度方面表现出选择性行为差异。这些观察结果无法用pKa、logP、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一种可能的解释，即这两种固定相的固定化配体在不同溶剂化条件下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模拟表明相邻BDC8配体之间发生相互作用，导致固定相表面更疏水。此外，在有利于分析物在ANP模式下保留的流动相条件下，双齿锚定C8配体的这些相互作用导致分析物难以接近BDC8固定相表面潜在的ANP结合位点。对于全氟C8固定相，DFT模拟显示单个全氟C8配体链之间存在强烈排斥，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（且表面疏水性较低）。这些DFT模拟结果预测了色谱结果，即苯烷醇和苯烷基胺在全氟C8固定相的RP模式下保留时间缩短。此外，全氟C8固定相更开放的配体结构使得在有利于ANP保留模式的流动相条件下，分析物能够更容易地接近固定相表面的水合结合位点，导致分析物保留，特别是较小的苯烷基胺，这是由于氢键和静电作用。
