Complexation kinetics of 5-alkyloxymethyl-8-quinolinols at liquid-liquid interfaces as studied by dynamic interfacial tensiometry

The effect of alkyl chain length of chelating reagents on the complexation kinetics of 8-quinolinol (HQ) and 5-alkyloxymethyl-8-quinolinol (HC_nQ; n = 1, 2, 3, 5, 8) with transition metal ions (M²⁺: Ni²⁺, Zn²⁺) at liquid-liquid interfaces is studied by dynamic interfacial tensiometry. A decrease in the dynamic interfacial tension is caused by the interfacial adsorption of a 1:1 complex (M(C_nQ)⁺). The interfacial complexation kinetics for Zn²⁺ is controlled by the diffusion of ligands from the bulk organic phase to the : interface. For Ni²⁺, the kinetics is also diffusion-controlled for the ligands with short alkyl chains; however, it is controlled by the complexation reaction at the interface for those having longer alkyl chains. The interfacial complexation rate is found to be faster for Zn²⁺ than for Ni²⁺, and it correlates closely with the partition coefficient of the ligands. For the reaction-controlled complexation kinetics, a new procedure for calculating the interfacial complexation rate constant (k₁) is proposed, and k₁ values for the reaction with Ni²⁺ at the heptane-water interface are calculated as 1.7 × 10^-5 m s^-1 M^-1 (1 M = 1 mol dm^-3) for HC₅Q and 2.2 × 10^-5 m S^-1 M^-1 for HC₈Q.