5-Methyl-2-[3-(4-phenylthiazol-2-yl)triazenyl]benzenesulfonic acid as a chromogenic reagent of N-cetylpyridinium chloride: Synthesis, mechanism and analytical application

Despite that triazene reagents have been widely used for spectrophotometric determinations of cationic surfactants, the mechanism underlying such applications has yet to be studied. We report the synthesis of a new triazene reagent 5-methyl-2-[3-(4-phenylthiazol-2-yl)triazenyl]benzenesulfonic acid (MPTTBSA) and its interaction with N-cetylpyridinium chloride (CPC). The reagent was synthesized by coupling 4-methyl-2-sulfobenzenediazonium salt with 2-amino-4-phenylthiazole. Spectral evidence suggests that the neutral reagent (H2R) exists in a triazenium sulfonate zwitterion form. Two ionizations were detected at weak acidic (H2R/HR−, pKa1 2.71 ± 0.20) and alkaline pH (HR−/R2−, pKa2 ca. 13.5), respectively. In the presence of 3.5 equivalents of CPC, a 2.71 unit decrease in pKa1 and a 3.0 unit decrease in pKa2 were observed. While the optical properties of H2R are essentially unaffected, CPC causes a 53 nm red shift and a 31 nm red shift in maximum absorption wavelengths (λmax), and a 24% increase and a 29% increase in extinction coefficients (ɛ) of HR− and R2−, respectively. These data suggest that CPC forms ion associates with HR− and R2−, but its interaction with H2R is weak. Associations of CPC with HR− and R2− both follow a 3:1 stoichiometry, and the apparent stability constants of the two associates were estimated as 6.02 × 1018 and 2.42 × 1022 M−3, respectively. Consistent with their high stability constants, the two ion associates did not show any changes in optical properties under submicellar and micellar conditions. The strict 3:1 association stoichiometry was interpreted in terms of electrostatic-induced, topology-defined π-stacking and hydrophobic interactions, which not only change the optical properties of the reagent anions, but also provide the driving force to shift the two ionization equilibria to the right and cause the decreases in pKa. Compared to the first ionization, perturbation of the second ionization by CPC provides a color reaction that is more sensitive and has a better color contrast, which was used to develop a new protocol for spectrophotometric determination of CPC. This is the first mechanistic study on the interaction between a triazene reagent and a cationic surfactant.