Organization of meso-tetra(4-N-stearylpyridyl)porphine in pure and mixed monolayers at the air/water interface and in Langmuir-Blodgett films

The molecular packing of meso-tetra( 4-N-stearylpyridyl)porphine tetra-p-to luene-toluenesulfonate (PO) was investigated at the air/water interface and in Langmuir-Blodgett (LB) films in progressively more complex systems, nam ely, the pure component PO, PO mixed with sodium hexadecyl sulfate (SHS), a nd PO mixed with SHS and stearic acid (SA). The LB films formed at selected surface pressures have been analyzed by UV-visible absorption spectroscopy . The long transition in the pure PO monolayer was ascribed to changes in t he orientation of PO bases, from parallel to oblique orientation, toward th e water surface. At the interface, the PO/4SHS mixture exhibits two transit ions: a low surface pressure transition, attributed to a closer arrangement favoring electrostatic inter actions; and a high surface pressure transiti on, ascribed to the formation of an interdigitated double layer. The PO/4SH S/4SA system exhibits: the high surface pressure transition in a short rang e of areas. This behavior was ascribed to the coexistence of two mechanisms of long chain dense packing: the filling of PO/4SHS bases by SA molecules and the double layer interdigitation of PO/4SHS. On increasing the SA conte nt in the ternary mixture, the transition disappears and a solid condensed monolayer, induced by the long chain packing, begins at low surface pressur es.