Ion-selective charge separation at the interfaces between surface-active crown ether-incorporated liquid membranes and aqueous sample solutions as studied by resonant optical second-harmonic generation

Resonant optical second-harmonic generation (resonant optical SHG) at the i nterfaces between aqueous sample solutions and liquid membranes, based on i onophores with different surface activities was measured in order to invest igate the influence of the surface activity of ionophores on ion-selective charge separation. As ionophores with different surface activities, but bea ring the same crown ether moiety as a binding site for the Na+ ion, 4'-(p-o ctadecylbenzoateazo)phenoxymethyl-15-crown-5 (1) and 4'-(p-methylbenzoateaz o) phenoxymethyl-15-crown-5 (2) were synthesized. The dependence of the SHG intensities on the polarization angle of the incident laser light showed t hat the average molecular orientations of ionophores 1 and 2 at the liquid/ liquid interface were nearly equal to each other. When the concentration of Na+ ion in the aqueous sample solutions increased, increases in the SHG in tensities were observed for both membranes based on ionophores 1 and 2. How ever, the SHG intensities based on ionophore 1 were stronger, suggesting th at the surface activity of ionophore I is higher than that of ionophore 2. The change in the phase boundary potentials of the ionophore I-based membra ne in contact with the aqueous Naf ion solutions was found to be larger tha n that of ionophore 2. An improvement in the potentiometric detection limit for Na+ ion was also shown for membranes based on surface-active ionophore 1. These results demonstrate that the extent of charge separation at the l iquid/liquid interface, as observed by resonant SHG from primary ion comple xes, is increased by using surface-active ionophores.