STRUCTURAL AND THERMODYNAMIC ASPECTS OF PHASE-TRANSITIONS IN URACIL ADLAYERS - A CHRONOCOULOMETRIC STUDY

The phase formation of six alkyl derivatives of 2,4-dihydroxypyrimidin e 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 pote ntial 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-dimensio nal condensed physisorbed film (II), a partial charge transfer/deproto nation region (III), and a chemisorbed film (IV). Substitution of the N(1)- and/or the N(3)-ring nitrogen with methyl groups prevents the fo rmation of the ordered physisorbed film II. In the case of 1-methylura cil (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 bloc ked simultaneously with methyl groups. Quantitative thermodynamic adso rption parameters such as film pressure, Gibbs surface excess, Gibbs e nergy of adsorption, and electrosorption valency were determined on th e basis of chronocoulometric measurements for the ''dilute'' and the t wo-dimensional condensed physisorbed film. All molecules studied are p lanar oriented to the electrode surface in both ordered regions. The c orresponding 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 w eak chemisorption. On the basis of structural and energetic data the f ormation of the 2D-condensod physisorbed film of U, 5-MU, and 5,6-DMU is interpreted as a replacement of residual interfacial water molecule s 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(11 1) in regions I and II will be complementary analyzed with selected re sults on Ag(111) and mercury electrodes. No quantitative thermodynamic data were extracted in regions III and IV.