Total Internal Reflection Resonance Raman Microspectroscopy for the Liquid/Liquid Interface. Ion-Association Adsorption of Cationic Mn(III) Porphine

Abstract

The interfacial adsorption of meso-tetrakis(N-methylpyridyl)porphyrinatomanganese(III) (Mn(TMPyP)5+) with dihexadecyl phosphate (DHP) at the toluene/water interface was studied by means of a total internal reflection resonance Raman microspectroscopy. The intensity of interfacial Raman spectra of Mn(TMPyP)5+ was significantly enhanced by the addition of DHP. Since the Raman spectra were not shifted by the presence of DHP, the enhancement was ascribed to the electrostatic Stern layer adsorption of Mn(TMPyP)5+ to the negatively charged interface by the adsorption of DHP. The interfacial adsorption of Mn(TMPyP)5+ was confirmed by the measurement of interfacial tension lowering under the absence and the presence of DHP. Polarized Raman spectra suggested that the average tilt angle of the porphyrin plane of the Mn(TMPyP)5+ molecule to the interface normal was 65° at the interface adsorbed by DHP. The interfacial ion association adsorption of Mn(TMPyP)5+ was analyzed by the Langmuir isotherm, and the adsorption parameters were obtained.