Structural and thermodynamic aspects of phase transitions in uracil adlayers. A chronocoulometric study

The phase formation of six alkyl derivatives of 2,4-dihydroxypyrimidine on a Au(111) single crystal electrode from aqueous 0.05 M KClO4 has been investigated using chronocoulometry, cyclic voltammetry and phase-sensitive ac voltammetry. As a function of the applied electrode potential and/or the adsorbate concentration uracil (U), 5-methyluracil (5-MU) and 5,6-dimethyluracil (5,6-DMU) assume four different adsorption regions. These are assigned to a ''dilute'' phase (I), a two-dimensional condensed physisorbed film (II), a partial charge transfer/deprotonation region (III), and a chemisorbed film (IV). Substitution of the N(1)- and/or the N(3)-ring nitrogen with methyl groups prevents the formation of the ordered physisorbed film II. In the case of 1-methyluracil (1-MU) and 3-methyluracil (3-MU) region I is followed immediately by regions III and IV. No indications for the latter two regions were found with 1,3-dimethyluracil, where both ring nitrogen atoms are blocked simultaneously with methyl groups. Quantitative thermodynamic adsorption parameters such as film pressure, Gibbs surface excess, Gibbs energy of adsorption, and electrosorption valency were determined on the basis of chronocoulometric measurements for the ''dilute'' and the two-dimensional condensed physisorbed film. All molecules studied are planar oriented to the electrode surface in both ordered regions. The corresponding values of the Gibbs energies of adsorption Delta G(I)(o) and Delta G(II)(o) of the six uracil derivatives on Au(111) indicate weak chemisorption. On the basis of structural and energetic data the formation of the 2D-condensod physisorbed film of U, 5-MU, and 5,6-DMU is interpreted as a replacement of residual interfacial water molecules accompanied by the creation of a 2D network of planar oriented and, via hydrogen bonds, interconnected molecules. The adsorption and phase formation parameters obtained for the six uracil derivatives on Au(111) in regions I and II will be complementary analyzed with selected results on Ag(111) and mercury electrodes. No quantitative thermodynamic data were extracted in regions III and IV.