Mechanism and Kinetics of Copper(II) Transport through Diaza-crown Ether−Fatty Acid-Supported Liquid Membrane

Abstract

The mechanism of macrocycle-mediated Cu(II) transport through a supported liquid membrane containing 1,10-didecyl-1,10-diaza-18-crown-6 ether−fatty acid in toluene/phenylhexane solvent mixture has been investigated. The mechanism was elucidated by studying the effects of parameters such as stirring speed, membrane thickness, and temperature on Cu(II) flux. The results showed that a countercation transport is controlled by a sodium gradient. The rate-limiting step of Cu(II) transport in the system has been found to be diffusion of metal−carrier complex in the membrane. But, depending on the hydrodynamic conditions and cell geometry, diffusion of metal in the stagnant aqueous Nernst layer is found to be rate limiting. In addition, a linear Cu(II) gradient in the membrane was found by performing Cu(II) transport experiments in a stack of eight membranes and analyzing the copper(II) concentration in each of the membranes separately, supporting diffusion-limited steady-state transport. Comparative studies at optimum operational conditions have been done with two types of supports, Celgard 2500 and Accurel, differing in their pore size and porosity. From lag time measurements, the effective diffusion coefficient, D̄m, of the metal−carrier complex in the membrane was evaluated and the values were found to be 5.2 × 10-8 and 2.55 × 10-7 cm2 s-1 for Celgard and Accurel membranes, respectively. These differences in D̄m between the two membranes has been interpreted in terms of molecular diffusion of the complex in the solvent, Dm, and of the equilibrium constant for its adsorption on the pore wall of the membrane.