Abstract
Abstract Total internal reflection (TIR) spectra were measured in the visible region at an interface between glass and an aqueous solution of 1.00 × 10−6 mol dm−3 5,10,15,20-tetraphenylporphyrintetrasulfonic acid (TPPS) in the presence of admicelles of cetyltrimethylammonium bromide (CTAB). Comparison of the TIR spectra with transmission spectra of TPPS solutions with different CTAB concentrations revealed that the deprotonated ions, TPPS4−, are simultaneously adsorbed at the interface as two complexes with TPPS4-/CTA+ ratios of 1 : 4 and 1 : 7. Interfacial concentrations of the 1 : 7 complex were estimated by using a linear relation between the absorbance and the interfacial concentration derived from a theory of TIR spectroscopy. The concentration data indicated that the adsorption of TPPS hardly occurs below pH 3, is saturated at pHs between 4 and 6, and continuously decreases above pH 6. Assuming an aggregative structure of the admicelle, the pH dependence of the adsorption was mathematically treated with a model including the dissociation, complex formation, and adsorption equilibria.
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