Gritti Fabrice, Guiochon Georges
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, USA.
Anal Chem. 2006 Jul 1;78(13):4642-53. doi: 10.1021/ac0602017.
The effect of temperature on the adsorption and retention behaviors of a low molecular weight compound (phenol) on a C18-bonded silica column (C18-Sunfire, Waters) from aqueous solutions of methanol (20%) or acetonitrile (15%) was investigated. The results of the measurements were interpreted successively on the basis of the linear (i.e., overall retention factors) and the nonlinear (i.e., adsorption isotherms, surface heterogeneity, saturation capacities, and equilibrium constants) chromatographic methods. The confrontation of these two approaches confirmed the impossibility of a sound physical interpretation of the conventional Van't Hoff plot. The classical linear chromatography theory assumes that retention is determined by the equilibrium thermodynamics of analytes between a homogeneous stationary phase and a homogeneous mobile phase (although there may be two or several types of interactions). From values of the experimental retention factors in a temperature interval and estimates of the activity coefficients at infinite dilution in the same temperature interval provided by the UNIFAC group contribution method, evidence is provided that such a retention model cannot hold. The classical Van't Hoff plot appears meaningless and its linear behavior a mere accident. Results from nonlinear chromatography confirm these conclusions and provide explanations. The retention factors seem to fulfill the Van't Hoff equation, not the Henry constants corresponding to the different types of adsorption sites. The saturation capacities and the adsorption energies are clearly temperature dependent. The temperature dependence of these characteristics of the different assorption sites are different in aqueous methanol and acetonitrile solutions.
研究了温度对低分子量化合物（苯酚）在甲醇（20%）或乙腈（15%）水溶液中于C18键合硅胶柱（C18 - Sunfire，沃特世公司）上的吸附和保留行为的影响。基于线性（即总保留因子）和非线性（即吸附等温线、表面非均质性、饱和容量和平衡常数）色谱方法，相继对测量结果进行了解释。这两种方法的对比证实了对传统范特霍夫图进行合理物理解释的不可能性。经典线性色谱理论假定保留是由分析物在均匀固定相和均匀流动相之间的平衡热力学决定的（尽管可能存在两种或几种相互作用类型）。根据温度区间内的实验保留因子值以及由UNIFAC基团贡献法提供的相同温度区间内无限稀释时的活度系数估计值，有证据表明这样的保留模型不成立。经典的范特霍夫图似乎毫无意义，其线性行为只是偶然现象。非线性色谱的结果证实了这些结论并给出了解释。保留因子似乎符合范特霍夫方程，而不是对应于不同类型吸附位点的亨利常数。饱和容量和吸附能明显依赖于温度。在甲醇水溶液和乙腈溶液中，不同吸附位点的这些特性对温度的依赖性是不同的。
