Two-dimensional phase transitions of chemisorbed uracil on Ag(111): modeling of short-and long-time behavior

The short-time behavior of uracil two-dimensional phase transition on Ag(111) from aqueous 0.05 M KClO4 at 25°C was investigated at uracil concentrations ranging from 1 × 10-4 to 1 × 10-3 M by measuring the time dependence of the charge following potential steps from -1.5 V/SCE, where there is indirect evidence that uracil is completely desorbed, to potentials positive to -0.5 V/SCE, where uracil is chemisorbed, over a period of 0.6 s. The corresponding long-time behavior was followed on the basis of differential capacity vs time curves over a period of a few minutes. The short-time behavior is interpreted by a model that accounts for diffusion-controlled random adsorption of uracil molecules according to a Frumkin isotherm, followed by their progressive nucleation and by growth of the resulting clusters; the effect of nucleation with exponential decay of the nucleation rate and ingestion effects are also considered. The long-time behavior is explained by a model that considers the formation of a new two-dimensional phase and the nucleation and growth of water holes within this new phase, up to the attainment of a steady state.