Interactions between cationic surfactants and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin tetrasodium salt as seen by electric conductometry and spectroscopic techniques

Thais Carmona, Marta Pineiro, Carlos J.P. Monteiro, Mariette M. Pereira, Artur J.M. Valente

CQC, Department of Chemistry, University of Coimbra, 3004 – 535 Coimbra, Portugal.

Abstract

The interaction between the anionic porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin tetrasodium salt (Na₄H₂TPPS₄) and alkyltrimethylammonium bromides (C_nTAB), with different alkyl chain lengths (n = 12 and 16), in aqueous solutions have been studied by using electrical conductivity, potentiometry and spectroscopic (UV–vis and fluorescence) techniques. The effect of surfactant concentration in a concentration range involving the pre- and post-micelle region and temperature was evaluated. Interactions between surfactant and porphyrin lead to a three well-defined regions. At the lowest molar ratios, r = [C_nTAB]/[Na₄H₂TPPS₄] occur the disaggregation and deprotonation of porphyrins concomitantly with the formation of surfactant–porphyrin aggregates. These aggregates have a stoichiometry lower than the simple charge neutralization as due to the occurrence of multiple equilibria involving porphyrins. This also suggests that although electrostatic interactions are important, they are not the only driven-force for interaction. When surfactant is in excess (porphyrin saturation concentration < [C_nTAB] < critical micelle concentration), occur the stabilization of porphyrin monomers H₄TPPS^2- and J-aggregates, in agreement with the ability of surfactants for redissolution of surfactant–porphyrin complexes. At surfactant concentrations above the critical micelle concentration (cmc) the non-protonated form of Na₄H₂TPPS₄seems to be included in the surfactant micelle.

The effect of porphyrin concentration and temperature on the micellization properties of surfactants was also analyzed, showing that temperature has no significant effect on the cmc of C₁₆TAB. However, looking to the free energy of micellization the effect of temperature favours the micellization process, being such effect more pronounced for the most hydrophobic surfactant. This suggests that the degree of dissociation of counter ions in the micelle plays an important role on the micellization process in the presence of porphyrins.

Graphical abstract

Keywords: Anionic porphyrin; Alkyltrimethylammonium bromide; Porphyrin–surfactant interaction; Aggregation; Micellization.