Compliance of retention data on inorganic oxide adsorbents with the solvation parameter model

The solvation parameter model has been used to characterize the retention p roperties of a varied group of solutes in silica gel thin-layer chromatogra phy (TLC) and in silica gel and alumina column liquid chromatography. The m odel was unable to describe retention on silica get TLC plates with the R-M value as the dependent variable for five single-solvent mobile phases. The results were improved by fitting the retention data to the simple competit ion model and separating the solute and solvent contributions (denoted S an d e(o)A(s), respectively) to the free energy of adsorption on the inorganic oxide. Separate models were then constructed to enable estimation of value s of S and A(s) from structure. These models were successful in describing retention in column liquid chromatography on silica gel with mixtures of me thyl t-butyl ether and hexane as a mobile phase. This approach proved less reliable for calculating retention in TLC, probably because of non-equilibr ium in the separation system. Evidence is presented that neither the solute adsorption parameter (S) nor the solute cross-section (A(s)) as used in th ese studies is unambiguously defined. Further refinements aimed at establis hing clearly defined solute and solvent adsorption terms, and possibly incl uding selective solute-solvent interactions in the mobile phase as a second ary contribution to retention, could result in improved model performance. The approach described here should be considered preliminary and thought of as a stepping stone in the direction towards a comprehensive model for str ucture-driven method development in normal-phase separations, which are cur rently less developed than models available for reversed-phase separations.