Structural effects on metal ion migration across polymer inclusion membranes: Dependence of transport profiles on nature of active plasticizer

Structural modifications promoted by the nature of the plasticizer that affect metal ion migration in polymer inclusion membranes (PIMs) were evaluated using transport data, transmission infrared mapping microspectroscopy (TIMM) and electrochemical impedance spectroscopy (EIS). An analysis of the effects of different plasticizers on indium(III) transport across cellulose triacetate membranes with bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272) as carrier revealed differences in transport profiles that can be explained on the basis of the nature of plasticizer used. While a transport profile of the type carrier-diffusion was observed for tris(2-ethylhexyl)phosphate (TEHP), a transport profile of the type chained-carrier with reduced mobility was suggested by the presence of a percolation threshold for PIMs with tris(2-butoxyethyl)phosphate (TBEP), 2-nitrophenyloctylether (NPOE) and without plasticizer under the experimental conditions used in this work. Accordingly, diffusional equations and percolation theory were used to model permeation and to gain insight into the transport processes occurring in these systems. A correlation between the structural conformation of the PIMs and the transport profiles was successfully achieved using the aforementioned characterization techniques and theoretical frames. Values of the percolation parameters were rationalized considering the distribution of the membrane components observed by TIMM and PIM resistances evaluated by EIS. Membrane behavior for metal extraction was characterized by the determination of the equilibrium constants via solid–liquid extraction experiments. EIS measurements allowed correlating the equilibrium constants with membrane resistances as well.