Thermodynamic study of the interaction between 5,10,15,20-tetrakis-(N-methyl-4-pyridyl)porphyrin tetraiodine and sodium dodecyl sulfate

• Dodecylsulfate interacts with cationic porphyrin in a binding ratio equal to 3.1.
• Dodecylsulfate-cationic porphyrin aggregates are formed by electrostatic interactions mainly.
• By increasing the porphyrin concentration the SDS micellization becomes enthalpy-driven.
• In the pre-micelle region the structure of porphyrin ranges from H-aggregates to micellized monomers.

The interaction between 5,10,15,20-tetrakis-(N-methyl-4-pyridyl)porphyrin tetraiodine (TMePyPI4) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in aqueous solutions using electrical conductivity, UV–vis and fluorescence spectrophotometries. The electrical conductivity data suggests that the addition of SDS to TMePyPI4 solutions is accompanied by strong interactions (mainly electrostatic) with the formation of aggregates. These aggregates have a stoichiometry somewhat lower than the simple charge neutralization, possible due to the formation of supramolecular structures. At SDS/TMePyPI4 ratios higher than the stoichiometry ratio, those SDS-TMePyPI4 redissolve and at higher SDS concentrations formation of micelles is observed. The effect of both porphyrin surfactant and temperature allows a detailed thermodynamic analysis of the SDS micellization process in this multicomponent system.

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