STUDY OF ADSORPTION OF WATER-SOLUBLE PORPHYRIN AT GLASS-SOLUTION INTERFACE IN THE PRESENCE OF CATIONIC SURFACTANT ADMICELLES BY MEANS OF TOTAL INTERNAL-REFLECTION SPECTROSCOPY

Total internal reflection (TIR) spectra were measured in the visible r egion at an interface between glass and an aqueous solution of 1.00x10 (-6) mol dm(-3) 5,10,15,20-tetraphenylporphyrintetrasulfonic acid (TPP S) in the presence of admicelles of cetyltrimethylammonium bromide (CT AB). Comparison of the TIR spectra with transmission spectra of TPPS s olutions with different CTAB concentrations revealed that the deproton ated ions, TPPS4-, are simultaneously adsorbed at the interface as two complexes with TPPS4-/CTA(+) ratios of 1:4 and 1:7. Interfacial conce ntrations of the 1 :7 complex were estimated by using a linear relatio n between the absorbance and the interfacial concentration derived fro m a theory of TIR spectroscopy. The concentration data indicated that the adsorption of TPPS hardly occurs below pH 3, is saturated at pHs b etween 4 and 6, and continuously decreases above pH 6. Assuming an agg regative structure of the admicelle, the pH dependence of the adsorpti on was mathematically treated with a model including the dissociation, complex formation, and adsorption equilibria.