Adsorptivities of azaarenes on Florisil

Abstract

Adsorptivities of azaarenes on Florisil were investigated; the study included the use of Florisil: (1) to analyze azaarenes in ambient air; (2) to evaluate substituent effects of various azaarenes on retention factors; and (3) to characterize localized adsorbate-adsorbent interactions. For determining polynuclear azaarenes in ambient air, a high-volume sampling procedure was developed to distinguish particulate and vapor-phase azaarenes. Glass fiber filters were used to collect the former, and Florisil the latter. The analytical procedure provided simple and reproducible determinations of trace concentrations of two- and three-ring heterocyclic bases. For the first time, vapor-phase concentrations of polynuclear azaarenes were reported, and they were 1-2 orders of magnitude higher than concentrations in particulate matter in earlier reports. Earlier reports excluded measurements of the more abundant vapor-phase azaarenes. Retention of azaarenes in liquid chromatography on Florisil was mainly controlled by effects of substituents on the basicity of the ring nitrogen atom. Thus, with pyridines and some quinolines, linear free energy relationships were observed between log k' (retention factors) and aqueous free energies of protonation. The results suggest that the mechanism of adsorption involves Bronsted acid sites. These data show that Florisil can be effectively used for liquid chromatographic separations within some classes of basic nitrogen heterocycles. Localized adsorbate-adsorbent interactions were investigated by studying equilibrium adsorption of pyridine and n-butylamine on Florisil. The discontinuity of the Langmuir isotherms and heat of adsorption of pyridine were analyzed by the "patch" theory of adsorption on heterogeneous surfaces. Thus, at low surface coverages, the solutes appeared to be strongly localized, possibly chemisorbed, on specific active sites. Final surface coverge appeared to result in a surface monolayer of adsorbed molecules. The results suggested that Florisil might be a model adsorbent for studying localized adsorbate-adsorbent interactions in liquid chromatography.