Thermodynamics of the Adsorption of Isomeric Dipyridyls and Their Derivatives from Water–Organic Solutions on HYPERCARB™ Porous Graphitic Carbon

Abstract

High-performance liquid chromatography is used under near-equilibrium conditions to study the adsorption of isomeric dipyridyls and their derivatives from water–acetonitrile, water–methanol, and water–isopropanol solutions onto Hypercarb™ graphite-like carbon material in the region of Henry’s law. Hypercarb™ graphite-like carbon material exhibits high adsorption selectivity in separating the investigated isomeric dipyridyl and its derivatives. It is shown that the possibility of forming strong intramolecular C–H–N'-hydrogen bonds in a molecule of 2,2'-dipyridyl or its derivatives strengthens the adsorption bonding of adsorbate molecules and the surface of the graphite-like material due to stabilization of their planar conformation. Destabilizing this intramolecular hydrogen bond by adding substituents in different positions of the pyridine rings enhances the specific intermolecular interaction between adsorbate molecules and the solvent’s components and distorts the planar conformation of dipyridyls, weakening their retention on the Hypercarb™ material. Positive adsorption from the water–organic medium on the carbon adsorbent is observed for all of the investigated dipyridyls, with the exception of 2,2'-dipyridyl-N,N '-dioxide, which is adsorbed weaker than the solvent components. Anomalous medium–property dependences are found for the thermodynamic characteristics of the adsorption of dipyridyls on porous graphitic carbon, and are attributed to the predominance of adsorbate–adsorbent π–π interactions over hydrophobic ones and the resolvation of adsorbate molecules with acetonitrile in proportion to lowering the content of water in the bulk solution.