Thermodynamic aspects in the adsorption of polynuclear aromatic hydrocarbons on chrysotile and silica – possible relation to synergistic effects in lung toxicity

This study is to compare the adsorption of phenanthrene — a polynuclear aromatic hydrocarbon — (apolar molecule) and water (polar molecule) on chrysotile, a mainly ionic solid and on one silica (Aerosil), a covalent solid. The adsorption of water shows that the adsorption capacity and the interaction energies are much higher for chrysotile than for silica. With phenanthrene, the adsorption mechanism on these two materials is very different: on chrysotile — a solid with a surface having a basic character — there is a multilayer adsorption, with lateral interactions between PAH molecules vertically oriented with regard to solid surface, whereas on silica (acidic solid), the PAH molecules are adsorbed flat and form only one layer. Chrysotile turns out to be a much more powerful adsorbent than silica towards both polar molecules and apolar molecules. The calorimetric data concerning the adsorption of phenanthrene on chrysotile in xylene reveal three thermal stages, showing the influence of preadsorbed water and solvent and permit to propose a model of adsorption mechanism. The formation of multilayers of phenanthrene on chrysotile converts its marked hydrophilic surface into a hydrophobic one which may act as a carrier of organic molecules and slowly release them into the biological medium. These phenomena may provide a possible interpretation for some synergistic effects concerning lung cancers observed after asbestos and cigarette-smoke exposure. Keywords: adsorption, phenanthrene, chrysotile, silica, water.