Chromoionophore-Mediated Imaging of Water Transport in Ion-Selective Membranes

The K+ complex of 2-(4-dipropylaminophenylazo)benzoic acid octadecyl ester (ETH 2418), a new lipophilic chromoionophore, is used to monitor the water uptake of dry solvent polymeric membranes based on 2-nitrophenyl octyl ether/poly(vinyl chloride) (2:1). Upon contact with an alkaline solution, its λmax changes from 535 to 435 nm. This process can be reversed by drying the membranes. From the time-dependent recordings of absorbance changes, an apparent water diffusion coefficient, DH2O*, of 2×10-8 cm2 s-1 is determined. In contrast to results obtained with hydrophilic water indicators, no light scattering due to water droplets is observed in the bulk of the membrane.