Solubilization of carboxyl derivatives of porphyrin molecules by micelles of cationic surfactants: Solubilization capacity and localization sites

Abstract

The article is dedicated to the processes of solubilization of virtually insoluble carboxy-substituted tetraphenylporphyrin macrocycles by micelles of cationic surfactants (CTAB, CPC, and MTAB) to enhance their solubility in water. The influence of various structural parameters on the efficiency of the solubilization process is discussed, specifically the nature of the surfactant’s polar group (trimethylammonium group, ⁺N(CH₃)₃, and pyridinium group, ⁺NC₅H₅), the length of the surfactant’s hydrocarbon tail (−C₁₆H₃₁ and −C₁₄H₂₉), and the position of the functional group (–COOH) in the phenyl rings of 5,10,15,20-tetraphenylporphyrin. The ability of two carboxy-substituted porphyrin isomers to localize within the micelles of CTAB, CPC, and MTAB was determined using UV–visible, fluorescence, and ¹H NMR spectroscopy, as well as dynamic light scattering. The data indicate that the hydrodynamic radii of empty surfactant micelles increase in the order: MTAB < CPC < CTAB, with the size of MTAB being comparable to the dimensions of the porphyrin macrocycles. The solubilization of carboxy-substituted tetraphenylporphyrins by the studied surfactants in all cases leads to a decrease in the critical micelle concentration (CMC) and an increase in the hydrodynamic radius of the micelles by 12–30 %. The maximum increase is observed for MTAB micelles. The calculated solubilization capacity of CTAB, CPC, and MTAB micelles for these porphyrin molecules ranges from 1 to 18 porphyrin macrocycles per 10 micelles, depending on the surfactant and porphyrin. Using NMR spectroscopy, the most likely localization sites of porphyrin isomers within the micelles were analyzed. In terms of porphyrin positioning, the data suggest that porphyrins with –COOH groups in para-positions are localized in the hydrophobic core of all three ionic micelles, while the –COOH groups themselves occupy more polar domains of the cationic micelles—specifically, the Stern layer. This arrangement is energetically favorable, as the macrocycle is retained within the micelle through a combination of electrostatic and hydrophobic interactions. In contrast, porphyrins with –COOH groups in the ortho-positions localize either predominantly or exclusively in the hydrophobic core or in the polar region and are retained within the micelles solely by one type of interaction. The results can be used to develop therapeutic drugs with increased solubility and bioavailability (for example, photosensitizers for PDT or antimicrobial photodynamic therapy of localized infections).