Thermodynamics of Sorption and Tendencies in Retention of Haloadamantanes on the Cyclodextrin-Containing CYCLOBOND Sorbent under HPLC Conditions

The thermodynamic characteristics of sorption (retention, heat, and entropy factors) of various chloro-, bromo- and iodoadamantanes from the water–methanol eluent on the octyl silica gel (SiO₂-C8) and β-CD-octyl silica gel (CYCLOBOND) sorbents were determined experimentally for the first time under HPLC conditions. The sorption of haloadamantanes on the indicated sorbents occurs by different mechanisms: distribution on SiO₂-C8 and macrocyclic mechanism on CYCLOBOND. The isomers of haloadamantanes on the studied sorbents differ in the order of retention under the same elution conditions. The CYCLOBOND sorbent showed high structural selectivity in the separation of positional isomers and stereoisomers of haloadamantanes. The separation factors of positional isomers increase with the van der Waals radii of halogen atoms. A hypothesis is formulated that haloadamantanes that can hardly form inner-spheric complexes with the β-CD fragments can form outer-spheric complexes with the exocyclic OH groups of β‑CD by the sorption mechanism, as in hydrophilic chromatography.