Ion Interactions and Electrostatic Effects on TMPyP/DMPA Monolayers

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Abstract

In this work, the influence of the different ionic strengths in the aqueous subphase on the monomer−dimer equilibrium of a water-soluble tetracationic porphyrin, TMPyP, in a complex monolayer containing an insoluble lipid with negatively charged group, DMPA, has been investigated. LiCl, KClO4, and tetraethylammonium perchlorate, TEAP, have been used. Surface pressure−area, π−A, and surface potential−area, ΔV−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 by π−A and ΔV−A isotherms on those subphases. Reflection spectroscopy, ΔR, was also used to infer the arrangement of the porphyrin in the cospread monolayer of TMPyP/DMPA, molar ratio 1:4, on different aqueous salt solutions. A decrease of ΔR with increasing salt concentration independent of the type of salt is obtained indicating a loss of porphyrin molecules to the subphase, which magnitude is dependent on the electrolyte. Further, a slight red shift of λmax with respect to that at 420 nm obtained in absence of salt is observed. The analysis of reflection spectra at the air−water interface in the presence of Li+ and K+ ions in the subphase proposes a new semidimer phase for the porphyrin molecules in the mixed monolayer with a dimer fraction of αD ≈ 0.5. The model cannot be applied to the effect of TEAP molecules on the cospread monolayer due to the big size of TEA+ ion.

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