Interfacial solubilization of (5,10,15,20-tetraphenylporphyrinato)zinc(II) in cetyltrimethylammonium chloride reversed micellar solutions

The interaction between the zinc(II) complex of 5,10,15,20-tetraphenylporphyrin (Zn(tpp)) and cetyltrimethylammonium chloride (CTAC) at the interface of a CTAC reverse micelle in chloroform and 6:5 (v/v) chloroform-cyclohexane has been investigated spectrophotometric ally. The spectra of Zn(tpp) in the CTAC reversed micellar solutions provide evidence of formation of the associated species Zn(tpp)CTAC; its Soret band is not influenced by solvent polarity. At a fixed molar ratio of water to surfactant (R-w = [H2O]/[CTAC]), the absorbance at the maximum of the Zn(tpp)CTAC band increases with an increase in the CTAC concentration. In the slope of the plot a discontinuity was found, due to formation of reverse micelles at this CTAC concentration, taken as the critical micelle concentration (cmc). The variations of the spectral data were interpreted quantitatively by applying the pseudophase model. Increasing R-w causes a decrease in the Zn(tpp)-solubilizing ability of reverse micelles, implying that the nucleophilicity of Cl- ion to the central zinc of the complex is dropped by the Cl- hydration at the interface. Furthermore, the standard free energy of micellization, calculated from the cmc data, indicates that the micellization is promoted by the Cl- hydration and by decreasing the chloroform fraction in the bulk organic phase.