Impedance spectroscopic study on ionic transport in a pH sensitive membrane

Abstract

The characteristics of ionic transport under the influence of an AC electric field in a pH sensitive membrane made of poly(acrylamidocaproic acid) were studied using impedance spectroscopy. The dielectric spectra were obtained at various temperatures and for a range of pH values of HCl aqueous solutions in which the membrane was equilibrated. The contribution from the ionic transport which can be identified by a power dependency on frequency (i.e. omega(-n); where 0 < n < 1) in the dielectric loss spectra, provided a means of determining the tortuosity of the long range ionic transport pathways. Values for n were in the range of 0.97-1.00 for high pH values and temperatures above 20 degrees C. Lower values of n (approximately 0.88-9.94) were observed at very low pH values (1-2) and temperatures (20-50 degrees C). These observations indicate that hydrophobic aggregation effectively hinders the transport of the ions and results in a more tortuous pathway through the membrane at low pH and low temperatures. A relaxation peak was observed at about 10(7) Hz and was attributed to the relaxation of water absorbed in the membrane.