Gritti Fabrice, Guiochon Georges
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, USA.
Anal Chem. 2005 Jul 1;77(13):4257-72. doi: 10.1021/ac0580058.
The adsorption isotherms of phenol and caffeine were acquired by frontal analysis on two different adsorbents, Kromasil-C18 and Discovery-C18, with two different mobile phases, aqueous solutions of methanol (MeOH/H2O = 40/60 and 30/70, v/v) and aqueous solutions of acetonitrile (MeCN/H2O = 30/70 and 20/80, v/v). The adsorption isotherms are always strictly convex upward in methanol/water solutions. The calculations of the adsorption energy distribution confirm that the adsorption data for phenol are best modeled with the bi-Langmuir and the tri-Langmuir isotherm models for Kromasil-C18 and Discovery-C18, respectively. Because its molecule is larger and excluded from the deepest sites buried in the bonded layer, the adsorption data of caffeine follow bi-Langmuir isotherm model behavior on both adsorbents. In contrast, with acetonitrile/water solutions, the adsorption data of both phenol and caffeine deviate far less from linear behavior. They were best modeled by the sum of a Langmuir and a BET isotherm models. The Langmuir term represents the adsorption of the analyte on the high-energy sites located within the C18 layers and the BET term its adsorption on the low-energy sites and its accumulation in an adsorbed multilayer system of acetonitrile on the bonded alkyl chains. The formation of a complex adsorbed phase containing up to four layers of acetonitrile (with a thickness of 3.4 A each) was confirmed by the excess adsorption isotherm data measured for acetonitrile on Discovery-C18. A simple interpretation of this change in the isotherm curvature at high concentrations when methanol is replaced with acetonitrile as the organic modifier is proposed, based on the structure of the interface between the C18 chains and the bulk mobile phase. This new model accounts for all the experimental observations.
通过前沿分析法，在两种不同的吸附剂（Kromasil-C18和Discovery-C18）上，使用两种不同的流动相（甲醇水溶液（MeOH/H₂O = 40/60和30/70，v/v）以及乙腈水溶液（MeCN/H₂O = 30/70和20/80，v/v））获取了苯酚和咖啡因的吸附等温线。在甲醇/水溶液中，吸附等温线始终严格向上凸。吸附能量分布的计算证实，对于Kromasil-C18和Discovery-C18，苯酚的吸附数据分别用双朗缪尔和三朗缪尔等温线模型拟合效果最佳。由于咖啡因分子较大，被排除在键合层中埋藏最深的位点之外，因此在两种吸附剂上，咖啡因的吸附数据均遵循双朗缪尔等温线模型行为。相比之下，在乙腈/水溶液中，苯酚和咖啡因的吸附数据与线性行为的偏差要小得多。它们用朗缪尔等温线模型和BET等温线模型的总和拟合效果最佳。朗缪尔项表示分析物在C18层内高能位点上的吸附，BET项表示其在低能位点上的吸附以及在键合烷基链上乙腈吸附多层体系中的积累。通过在Discovery-C18上测量的乙腈过量吸附等温线数据证实了形成了包含多达四层乙腈（每层厚度为3.4 Å）的复合吸附相。基于C18链与本体流动相之间界面的结构，对用乙腈替代甲醇作为有机改性剂时高浓度下等温线曲率变化提出了一种简单解释。这个新模型解释了所有实验观察结果。
