Interaction of porphyrins with cetyltrimethylammonium bromide micelles in methylene chloride

Abstract

The formation of micelles of cetyltrimethylammonium bromide (CTAB) in CH₂Cl₂ solution has been reported for the first time. The critical micelle concentrations (cmc) of CTAB in CH₂Cl₂ solution were determined using conductivity and fluorescence techniques in both the absence and presence of 1,3,5,7-tetramethyl-2,4,6,8-tetra(2,4,6-triisopropylphenyl)porphyrin (TMTIP). The cmc values obtained by conductivity and fluorescence methods were found to be 75 and 78 mM, respectively. The aggregation numbers (N) of CTAB and porphyrin-CTAB in CH₂Cl₂ solution were determined using pyrene and N-cetylpyridinium chloride (CPC) as the fluoroprobe and quencher, respectively. The N of CTAB in CH₂Cl₂ solution was also determined using the porphyrin (in lieu of pyrene) as the fluoroprobe and CPC as the quencher. The N of CTAB obtained by these probes was found to be 134 ± 3 and 140 ± 5, respectively. This suggests that the porphyrin could be used as an excellent fluoroprobe for determination of the aggregation number of CTAB micelles. The results of both cmc and aggregation number suggest that the micelle formation of CTAB in CH₂Cl₂ solution is favored in the presence of the porphyrin. The standard free energy change for micelle formation of CTAB in CH₂Cl₂ solution was calculated using a biphasic model. The standard free energy transfer (ΔG⁰_m)_tr of CTAB micelles from CH₂Cl₂ to porphyrin environment has been found to be −1.0 kJ mol^-1 at 25°C, suggesting that the above transfer is difficult. However, the reverse situation, i.e. binding of the porphyrin to CTAB micelles, is more probable. The Langmuir-Blodgett (LB) film formation of the porphyrin and metalloporphyrins (Fe, Co, Cu, Zn) has also been studied and the UV-vis spectra of these films have been analyzed.