A REVISED STUDY ON FORMATION AT AIR-WATER-INTERFACE OF METALLOTETRAPHENYLPORPHYRIN MONOLAYERS

In this paper, an experimental review of the isotherms of the metallotetraphenylporphyrin (M-TPP) molecules has been carried out in the air-water interface. The isotherms are a function of the M-TPP concentration in the organic solvent used (chloroform) before spreading on the aqueous subphase. For very low M-TPP concentrations (10(-5) M) in this solvent, isotherms whose extrapolated areas to pi = 0 mN/m are on the order of 100-110 Angstrom(2) per molecule are obtained. This value is intermediate between those expected for a flat orientation of the molecules (160 Angstrom(2)) and a perpendicular one (70 Angstrom(2)). When a constant pressure is applied, such monolayers are initially not stable. However, after waiting sufficient time, one can observe a sudden condensation of this monolayer (decrease of the area at a constant pressure) which leads to the formation of a stable monolayer, with an area per molecule on the order of 67 Angstrom(2) for H-2-TPP, 69 Angstrom(2) for Mg-TPP, and 86 Angstrom(2) for Zn-TPP (pi = 7 mN/m). Monolayers were characterized by visible spectroscopy following transfer to glass slides. In this paper, a possible explanation for this phenomenon in relation to the molecular structure of the M-TPP is reached. (C) 1995 Academic Press, Inc.