Cationic surfactant-selective potentiometric sensors based on neutral ion-pair carrier complexes

Abstract

Poly(vinyl chloride) (PVC) membrane electrodes to determine monomer concentrations of tetradecyltrimethylammonium ion (TTA+) and hexadecylpyridinium ion (HPy+) based on neutral ion-pair carrier complexes of tetradecyltrimethylammonium dodecyl sulfate (TTA +-DS-) and hexadecylpyridinium dodecyl sulfate (HPy +-DS-), respectively, are reported here. The electrodes exhibit a Nernstian slope of 59 mV per decade for TTA+ and a sub-Nernstian slope of 34.5 mV per decade for HPy+ ions. The TTA + ion-selective electrode (ISE) and the HPy+-ISE can determine the monomer units down to concentrations as low as 4.0 × 10 -4 M and 1.66 × 10-5 M, respectively. The effect of various additives, such as the anionic polyelectrolyte sodium dextran sulfate and macrocyclic β-cyclodextrin, on the surface activity exhibited by the cationic surfactants TTA+-DS- and HPy+-DS -, in the presence of background concentrations of NaCl was also examined with surfactant-selective sensors. The Gibbs free energy of micelle formation (ΔGm) of both surfactants in the presence of various additives was calculated and found to be less favorable with respect to an increase in the amount of additives due to less availability of cationic surfactant monomer units. The ion-pair complexes TTA+-DS- and HPy+-DS- were found to behave as selective carrier compounds in PVC membranes in determining the concentration of monomer units of both TTA+ and HPy+, respectively. The proposed sensors worked well at a fairly acidic pH of 1-6.5 with response time of 60 s. The sensors responded well to the surfactant ions even in the presence of additives at lower concentration. The lifetime of the sensors is 3 mon. Copyright © 2004 by AOCS Press.