Formation and interfacial adsorption of the μ-oxo dimer of (5,10,15,20-tetraphenylporphyrinato)iron(III) in dodecane/aqueous acid systems

The specific adsorption behavior of iron(III) complexes of 5,10,15,20-tetraphenylporphyrin (H2TPP) at the dodecane/water interface has been determined by means of the high-speed stirring method in systems including hydrochloric acid, perchloric acid, nitric acid and sodium thiocyanate as the aqueous phases. In the systems of perchloric acid and nitric acid, Fe(TPP)Cl, which was initially dissolved in the dodecane phase, was transformed to the mu-oxo dimer, [Fe(TPP)](2)O, after stirring. The adsorptivity of [Fe(TPP)](2)O is more pronounced at pH 4. It was postulated that the interfacial species, [{Fe(TPP)}(2)(OH)(H2O)](+), was reversibly formed by the reaction between [Fe(TPP)](2)O and H3O+ at lower pH. The interfacial absorptivity of the iron(III) porphyrin complexes decreased in the order [{Fe(TPP)}(2)(OH)(H2O)](+) much greater than Fe(TPP)Cl > [Fe(TPP)](2)O > Fe(TPP)(SCN). The adsorption constants of these species and the formation constants of [{Fe(TPP)}(2)(OH)(H2O)](+) were determined from the observed adsorption isotherms and the pH dependences.