Adsorption and Retention of Haloadamantanes on the Hypercarb Graphite-Like Adsorbent under HPLC Conditions

Abstract

The thermodynamic characteristics of sorption (retention, heats, and entropy) of various chloro-, bromo-, and iodoadamantanes from a water–methanol eluent on a graphite-like material Hypercarb and octyl silica gel (SiO₂–C8) were determined experimentally under HPLC conditions. Hypercarb is shown to have the highest structural selectivity in HPLC with respect to haloadamantane stereoisomers. It was found that the order of elution of isomeric chloro-, bromo-, and iodoadamantanes from a polar eluent onto Hypercarb in HPLC completely coincides with the tendencies in the retention of these compounds on columns with graphitized thermal carbon black in gas chromatography. The retention of haloadamantanes and haloarenes on Hypercarb is described by the adsorption mechanism of sorption, the main contribution to retention being from the dispersion interactions of sorbate molecules with the adsorbent surface. The dependence of the heats and entropy of sorption on the composition and structure of haloadamantanes is described by molecular polarizability and molecular areas of sorbates in the case of Hypercarb and by the lipophilicity of compounds in the case of sorption on SiO₂–C8. It is concluded that the tendencies found in the sorption of haloadamantanes under HPLC conditions allow us to classify Hypercarb and SiO₂–C8 as sorbents with 2D and 3D types of structural selectivity, respectively.