ADSORPTION OF 1,10-PHENANTHROLINE IN HCLO4 SOLUTIONS ON COLUMNAR-STRUCTURED GOLD ELECTRODES - THE EXCLUDED-VOLUME EFFECT

The electroadsorption of 1,10-phenanthroline on columnar Au electrodes from x M 1,10-phenanthroline + 0.1 M HClO4 aqueous solutions (5 x 10( -4) less than or equal to x less than or equal to 5 x 10(-3)), at cons tant potential and 298 K, was studied using voltammetry, chronocoulome try, and ac impedance combined with scanning tunneling microscopy to c haracterize the substrate topography. The roughness factor (R) was def ined as the ratio of the O-adatom monolayer electrodesorption charge a t the columnar Au electrode to that at the initial smooth Au electrode . When both R and the molar 1,10-phenanthroline concentration (c) were kept constant, the degree of surface coverage by 1,10-phenanthroline (theta) diminished as R was increased. The theta vs c plots, irrespect ive of R, were interpreted by a Frumkin adsorption isotherm including a constant standard adsorption free energy. The analysis of parameters derived from adsorption isotherms reflects the existence of an exclud ed volume effect, as should be expected from the 1,10-phenanthroline m olecule adsorption cross section, and the size of the smallest voids a nd crevices at the substrate determined by scanning tunneling microsco py.