Chester Thomas L, Coym Jason W
The Procter and Gamble Company, Miami Valley Laboratories, P.O. Box 538707, Cincinnati, OH 45253-8707, USA.
J Chromatogr A. 2003 Jun 27;1003(1-2):101-11. doi: 10.1016/s0021-9673(03)00846-x.
In analysis of the thermodynamics of the transfer of a solute from the mobile phase to the stationary phase in reversed-phase liquid chromatography, it is nearly always assumed that the phase ratio is constant. This type of analysis is typically performed by applying a form of the van't Hoff equation, which relates the retention factor to temperature via the enthalpy and entropy of transfer. When non-linear van't Hoff plots are observed, it is often assumed that the enthalpy and entropy of transfer change with temperature. However, when the possibility of a change in the phase ratio is considered, it becomes apparent that non-linear van't Hoff behavior may or may not be due to changes in enthalpy or entropy. In this work, we present mathematical evidence that phase ratio changes, if they occur, can cause deviations from linearity in a van't Hoff plot. We also show that the phase ratio influence can be eliminated by considering the molecular difference between two solutes instead of the solutes themselves. The resulting selectivity van't Hoff plots may be linear, even when the van't Hoff plots of the two solutes are non-linear. In such cases, temperature-dependent phase ratio changes, and not necessarily changes in the transfer enthalpy, may be responsible for the curved van't Hoff plots of the individual solutes. In addition, we present chromatographic evidence that different solutes may "see" different thermodynamic phase ratios. It is clear that the concept of a phase ratio in reversed-phase chromatography is not nearly as well defined as a phase ratio in a bulk system like a liquid-liquid extraction.
在反相液相色谱中分析溶质从流动相转移至固定相的热力学过程时，几乎总是假定相比是恒定的。这类分析通常通过应用范特霍夫方程的一种形式来进行，该方程通过转移焓和转移熵将保留因子与温度联系起来。当观察到非线性的范特霍夫图时，通常假定转移焓和转移熵随温度变化。然而，当考虑相比变化的可能性时，很明显非线性的范特霍夫行为可能是也可能不是由焓或熵的变化引起的。在这项工作中，我们给出数学证据表明，如果发生相比变化，可能会导致范特霍夫图出现偏离线性的情况。我们还表明，通过考虑两种溶质之间的分子差异而非溶质本身，可以消除相比的影响。由此得到的选择性范特霍夫图可能是线性的，即使两种溶质的范特霍夫图是非线性的。在这种情况下，单个溶质的弯曲范特霍夫图可能是由温度依赖性的相比变化而非转移焓的变化导致的。此外，我们给出色谱证据表明不同溶质可能“看到”不同的热力学相比。显然，反相色谱中相比的概念远不如液 - 液萃取等本体系统中的相比定义明确。
