Radiotracer studies on calcium and lithium ion-selective membranes based on poly(vinyl chloride) and poly(acrylate) matrices

Abstract

Radiotracer permeation studies on polymer [mainly poly(vinyl chloride)(PVC)] matrix membranes based on a calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate}(A) liquid ion exchanger and various neutral carrier acyclic ionophores, i.e., ETH-129 (B) and ETH-1001 (C) for calcium, and a crown ether, DB77 (D), and two acyclic diamides [sensor E and ETH-2137 (F)] for lithium are reported. The calcium ion membranes (A, B and C) were studied with ⁴⁵Ca and the lithium sensing membranes with ²²Na tracers. The ion permeability studies were generally based on migration of the tracer from an initially active solution (counts of c′) through the membrane to an initially inactive solution (counts of c″). The resulting change in the count ratio with time [d(c″/c′)dt] showed a clear distinction between the more permeable PVC membranes based on sensor A and the less permeable PVC membranes based on the neutral carriers. The PVC membranes generally yielded better calcium ion permeation through sensor A membranes compared with acrylate polymer matrices except, possibly, for poly(2-methylpropyl methacrylate). Other parameters discussed include membrane diffusion coefficients (D), which relate in the same manner to the d(c″/c′)/dt trends, c″/c′ ratios and counts on membranes at the end of each run. The c″/c′ data for the end of the experiments also distinguish between the organophosphate ion exchanger (sensor A) and the calcium neutral carrier sensors; in addition, together with membrane count data, they show that neutral carrier membranes based on 2-nitrophenyl octyl ether plasticising solvent mediator show enhanced permeability particularly in the presence of potassium tetrakis(4-chlorophenyl)borate. Also, the ²²Na tracer studies for the various lithium membranes gave c″/c′ ratios similar to those observed for the ⁴⁵Ca studies with the calcium neutral carrier membranes.