Study of the reorientation of quinolinium ions in the mercury electrode | acidic aqueous solution interphase using Raman intensity/distance profiling microprobe spectroscopy

In a previous study, the adsorption of quinoline from a neutral and an alkaline solution onto a mercury electrode surface was investigated by Raman microprobe spectroscopy. Potential-dependent orientations of quinoline adsorbed upon the mercury surface and film formation were observed, depending upon the applied potential. In this work, where specific adsorption was not observed, the Raman intensity/distance profile of the quinolinium ion in a 40 mM quinoline+0.1 M HClO4 solution was studied. The dependence of the relative peak intensity R of the quinolinium ion (760 cm−1) to that of the perchlorate ion (923 cm−1) was measured. The Raman peak positions did not shift, but the relative peak intensity changed with the applied potential. The data are interpreted in terms of a change of orientation of the quinolinium ion on approaching the electrode from perpendicular (NH+ site pointed toward the aquated mercury surface) at E<−400 mV, to the opposite perpendicular orientation (the NH+ site pointed toward the bulk solution) at E>+200 mV, through random orientations for−300<E<+100 mV.