Binding of 4-aminoazobenzene derivatives by sodium salt of .BETA.-naphthalenesulfonic acid-formaldehyde condensates - An interpretation of the binding equilibria by an ion-exchange mechanism.

Effects of the concentrations of sodium and hydrogen ions on the binding equilibria of 4-aminoazobenzene derivatives (model azo disperse dyes) by sodium salts of β-naphthalenesulfonic acid-formaldehyde condensates (βNSF) in water were determined spectrophotometrically; a βNSF with degree of condensation, N=40, was used. The detailed analysis of the results indicates that the binding of dye by the βNSF of a compact conformation is successfully described by an ion-exchange mechanism. At first, a trace amount of hydrogen ion in the solution is attached to the β-azo nitrogen of the dyes, then the protonated dye (DH) competes with sodium (Na) and hydrogen (H) ions for the dyeing sites in the βNSF, -SO3- The values of ion-exchange constants determined in the pH ranges of 4.126-4.9505 and 5.010-5.508 at 25°C were KHDH=0.14-0.60 and KNaDH-3.8-9.2×10-4, respectively. The values of KHDH, obtained indicate the fact that the sulfonic acid group in the βNSF of a compact conformation behaves as a weak acid.