Effect of the eluent composition in HPLC on the retention of bi- and trifunctional aromatic solutes

Summary Adsorption equilibria of aromatic compounds containing two or three polar groups (NH₂, OH, NO₂, CN, aromatic nitrogen) are investigated by high-performance liquid chromatography in chromatographic systems consisting of silica and a binary mobile phase consisting of heptane and a polar modifier (isopropanol or 1,4-dioxane). The relationships between the log k values and the concentration of the modifier (expressed as the logarithm of its mole fraction) are interpreted from the basis of the assumed molecular mechanism of adsorption, including solvation effects, i.e., solute-modifier interactions. Correlation of the log k values obtained for the two modifiers indicates that proton donor solutes can be retained by the adsorbed monolayer of dioxane molecules due to an interaction with the exposed ether oxygen atoms.