Selectivity of Pb(II) transport across polymer inclusion membranes doped with imidazole azothiacrown ethers

Cellulose triacetate membranes doped with imidazole azothiacrown ethers as fixed carriers have been prepared and applied for examination of facilitated transport of Zn(II), Cd(II), and Pb(II) ions from aqueous nitrate source phase (cMe = 0.001 M, pH 5.5). For the polymer inclusion membranes (PIMs) doped with the 18-membered azothiacrown ethers (1 and 3) the following patterns of transport selectivity were found: Pb(II) > Zn(II) > Cd(II), whereas for the 21-membered azothiacrown ether (2) selectivity was Pb(II) > Cd(II) > Zn(II). The highest initial flux of Pb(II) ions (1.265 μmol/m2 s) was found for PIM doped with phenylimidazole azothiacrown (3). For this compound also the best Pb(II)/Cd(II) and Pb(II)/Zn(II) selectivity coefficients equal to 105.4 and 19.5, respectively, were found. The influence of admixed Cu(II), Co(II), and Ni(II) ions on the initial fluxes of Pb(II) ions across PIMs was also studied. The results are discussed on the basis of the hydrophile–lipophile balance (HLB) theory. The initial fluxes of Pb(II) and Zn(II) ions decrease with the increase of hydrophile–lipophile balance values for imidazole azothiacrown ethers in the order 3 > 1 > 2. The cellulose triacetate-o-NPOE-azothiacrown ether membranes were characterized by non-contact atomic force microscopy. Analyses have shown homogeneous texture and uniform distribution of pores in the polymer inclusion membranes. The roughness of PIMs doped with azothiacrown carriers 1–3 decreases accordingly to the increase of the molar intrinsic volumes of the azothiacrown ethers with the following sequence 1 > 2 > 3.