Porphyrin-surfactant interactions have been extensively studied for water-soluble porphyrin derivatives, while only a few works describe the aggregation behavior of hydrophobic porphyrin derivatives in ionic surfactant solutions, especially below the cmc. In this report we briefly discuss aggregation behavior of meso-tetra(4-hydroxyphenyl)porphyrin (THPP) in aqueous solutions of anionic surfactant sodium n-dodecylsulfate (SDS) in a wide surfactant concentration range (0.5-50 mM) and focus on the formation of various THPP-SDS supramolecular assemblies.
In submicellar SDS solutions (0.1-1 mM) THPP aggregates of 50-100 nm in diameter are observed, which are stabilized by the adsorbed surfactant molecules. Particle size gradually decreases with the increasing surfactant concentration. In premicellar SDS solutions (1-6 mM) porphyrin-surfactant J-aggregates appear, characterized by the bathochromically shifted absorption band at 490 nm and intense resonance light scattering. Such aggregates are assembled from the diprotonated THPP form, emerging in pH-neutral SDS solutions due to the high local acidity of the porphyrin microenvironment in premicellar THPP-SDS aggregates. Micellar SDS solutions (6-50 mM) provide stabilization of the fluorescent monomolecular THPP form with λ_B = 424 nm and λ_em = 670 nm. Spin probe EPR spectroscopy confirms THPP-SDS aggregate formation in premicellar SDS solutions with the surfactant concentration above 5 mM. In micellar SDS solutions containing THPP the average particle size and degree of polydispersity are higher compared to the pure surfactant solutions, which confirms THPP binding to the micelles. The data obtained from the independent methods suggests on-surface stabilization rather than in-core solubilization of THPP in SDS micelles.

Mikhail Motyakin

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