Effects of Polarization on the Extents and Kinetics of Adsorption of Ionic and Neutral Aromatic Heterocyclics Exhibiting Orientation Transitions at a Porous C Electrode

Abstract

Comparative investigations of the adsorption and electrosorption of the heterocyclic compounds, pyridine, 1,4-pyrazine and 1-quinoline, and their orientation behaviours in an high specific-area C-cloth/H₂O interphase, directly related to the replacement of preadsorbed and field-oriented solvent (H₂O) molecules by oriented adsorbates themselves, have been carried out by means of in situ UV spectrophotometry conducted under positive or negative electric polarization at the C-cloth as an electrode. Surface charge, dipole moment of the adsorbate, polarization mode, acidity of the solution (leading to cation formation) and hydrophilic/hydrophobic properties of the adsorbed molecules in relation to the solvent (H₂O vs. non-aqueous) are found to have important influences on these processes and hence govern the experimentally measurable extents and rates of their adsorption. The adsorption and orientation effects were indicated by evaluation of surface-dipole potential changes represented by Esin and Markov (EM)-type effects. Different adsorption and orientation processes amongst the three adsorbates, together with the possible rearrangement of the adsorbed molecules, were revealed by changes of the observed EM effects. The preferred adsorption/electrosorption behaviour arises by either “combined adsorption” (CAD) (i.e. combination of adsorption on open-circuit with electrosorption upon subsequent polarization) or by “alternate polarization electrosorption” (APE) (i.e. electrosorption by periodically reversed direction of polarization, between positive and negative, in one adsorption experiment). Negative-current polarization in a single transient also led to similar results for effective adsorptive removal of 1-quinoline from aqueous solution. Major differences of adsorption arise between neutral pyridine and its cation in acidic solution, including the effect of electrode polarization.