Exothermodynamic Relationships in Liquid Chromatography

Abstract

Exothermodynamic relationships in reversed-phase and hydrophobic interaction chromatography, with the temperature, organic modifier or salt concentration, and carbon number of the eluite and of the stationary phase ligate as the operating variables, are classified and the links between various linear free energy relationships established. Starting from Martin's relationship based on group additivity, we arrive at two linear free energy relationships: one between the logarithmic retention factors, κ, obtained on two different columns with eluites of close structural similarity and the other between κ and the logarithmic octanol−water partition coefficient. Molecular interpretation of classical enthalpy−entropy compensation is offered by the combination of van't Hoff's relationship with linear exothermodynamic relationships between thermodynamic quantities on one hand and properties of the eluite, the eluent, or the stationary phase on the other. Thus, the compensation temperature is expressed by the enthalpy and entropy changes per unit of the above properties. Furthermore, the criterion for the invariance of the compensation temperature is set within the framework of dual compensation. A generalized compensation model is developed to extend the concept of enthalpy−entropy compensation to phenomena involving compensation by any two chromatographic variables by drawing analogies between the dependence of κ on the reciprocal temperature and on other operating variables, such as the organic modifier concentration in the eluent. The existence of 12 different compensation parameters is revealed, each marking the common intersection point of linear plots of free energy versus variables of the retention process. The compensation model leads to 12 three-parameter equations, each describing the retention behavior in reversed-phase chromatography as a function of two chromatographic variables. The family of exothermodynamic relationships encompasses most characteristic features of chromatographic retention and is expected to facilitate the organization, interpretation, and prediction of retention data.

Thermodynamic and Extrathermodynamic Studies of Enantioseparation of Imidazolinone Herbicides on Chiralcel OJ Column

“…Reversely, an obvious different EEC can certify different retention or enantioseparation mechanisms [23]. Applying Martin's additive-freeenergy relationship [13], a functional group contribution to thermodynamic quantities can be written as…”

“…Many extrathermodynamic relationships have been developed to facilitate the understanding of chromatographic retention and separation mechanism [13]. Typically, it has been demonstrated that the extrathermodynamic relationship derived from homologous series of analytes with additive group such as methylene or phenyl is able to provide useful information on chromatographic behavior [14,15].…”

Conformational temperature dependence of a poly(ethylene‐co‐acrylic acid) stationary phase investigated by nuclear magnetic resonance spectroscopy and liquid chromatography

“…Thermodynamic effects in RPLC have been the subject of previous research [38][39][40][41]. Significant changes in shape selectivity are observed with changes in temperature (Certificate of Analysis, https://srmors.nist.gov/view_de-tail.cfm?srm=869A), and an insight into the temperature dependence of alkyl chain conformations has been provided by Fourier transform infrared spectroscopy studies of chromatographic sorbents [42].…”

Quantitative structure-retention relationships of antimicrobial hydrazides evaluated by reverse-phase liquid chromatography

“…Regularising the structures via MO-PAC version 6, as implemented in SYBYL, using AM1, gave rise to the same CoMFA results. MR.lff 1 compounds, though congeneric depicted (some similar moieties present in molecules) as to the need of QSAR and QSRR studies [19,20]. On the other hand, QSRR studies demonstrate that a single physicochemical descriptor is not easily found for such complex retention mechanisms, mainly when they involve chiral recognition problems.…”