Binding of a non-ionic azo dye by sodium salt of .BETA.-naphthalenesulfonic acid-formaldehyde condensates - Effect of the degree of the condensation on the binding equilibrium.

Abstract

The binding of a model azo disperse dye by β-naphthalenesulfonic acid and formaldehyde condensates (βNSF) in water was discussed. The binding constants, K, for the bidding by βNSF with different degrees of condensation, N=2, 3, 4, 5, 7.2, 12 and 40 were determined spectrophothometrically. The thermodynamic parameters, the enthalpy change, ΔH° and the entropy change, ΔS° calculated from the values of K in the temperature range 15-40°C revealed following facts.The value of K increases with N up to N=ca. 10 and then level off gradually. This is ascribed to an increase in ΔS° with N; the value is negative for βNSF with N=2, 3 and 4 but it becomes positive for βNSF with N=12, 40 under the low concentration of added salts. The value of ΔH° is negative irrespective of N but its absolute value decreases with the increase in N.The increase in the concentration of added salts gives no appreciable change in the thermodynamic parameters for βNSF with N=3 but for βNSF with N=12 and 40, the increase gives significant changes in the parameters; both values of ΔH° and ΔS° decrease significantly. These results may be explained in terms of the conformational change of polyelectrolytes. βNSF with N=12 and 40 behave as a typical polyelectrolyte; their conformation varies from an expanded form to a compact one on addition of electrolytes. The extended conformation is favorable for the hydrophobic interaction between the linear dye and successive naphthalene nuclei of βNSF with a high degree of the condensation. The increase in the value of ΔS° with N may be explained in terms of the contribution of the hydrophobic bonding to the binding.