ION INTERACTIONS AND ELECTROSTATIC EFFECTS ON TMPYP DMPA MONOLAYERS/

In this work, the influence of the different ionic strengths in the aq ueous subphase on the monomerdimer equilibrium of a water-soluble tetr acationic porphyrin, TMPyP, in a complex monolayer containing an insol uble lipid with negatively charged group, DMPA, has been investigated. LiCl, KClO4, and tetraethylammonium perchlorate, TEAP, have been used . Surface pressure-area, pi-A, and surface potential-area, Delta V-A, isotherms have been measured in the presence of salts in the subphase. As reference, the monolayer of DMPA in absence of TMPyP was studied b y pi-A and Delta V-A isotherms on those subphases. Reflection spectros copy, Delta R, was also used to infer the arrangement of the porphyrin in the cospread monolayer of TMPyP/DMPA, molar ratio 1:4, on differen t aqueous salt solutions. A decrease of Delta R with increasing salt c oncentration independent of the type of salt is obtained indicating a loss of porphyrin molecules to the subphase, which magnitude is depend ent on the electrolyte: Further, a slight red shift of lambda(max) wit h respect to that at 420 nm obtained in absence of salt is observed. T he analysis of reflection spectra at the air-water interface in the pr esence of Li+ and K+ ions in the subphase proposes a new semidimer pha se for the porphyrin molecules in the mixed monolayer with a dimer fra ction of alpha(D) approximate to 0.5. The model cannot be applied to t he effect of TEAP molecules on the cospread monolayer due to the big s ize of TEA(+) ion.