Journal of Electroanalytical Chemistry 500 (2001) 356 – 364 www.elsevier.nl/locate/jelechem Kinetics of condensation of adenine at the mercury  electrolyte interface C. Prado a , I. Navarro a , M. Rueda a , H. Franc ¸ois b , C. Buess-Herman b, * a Department of Physical Chemistry, Uniersity of Seille, c / Prof. Garcı ´a Gonza ´lez s /n, Seille 41012, Spain b Uniersite ´ Libre de Bruxelles, Faculte ´ des Sciences, Serice de Chimie Analytique, C.P. 255, Bld. du Triomphe, B-1050 Brussels, Belgium Received 12 June 2000; received in revised form 12 October 2000; accepted 12 October 2000 Dedicated to Professor R. Parsons on the occasion of his retirement from the position of the Editor in Chief of the Journal of Electroanalytical Chemistry and in recognition of his contribution to electrochemistry Abstract The kinetics of phase transitions of adenine adsorbed on mercury are studied by chronocoulometry and chronoamperometry from aqueous 0.1 M KClO 4 and 0.5 M NaF solutions. Experimental conditions have been selected to minimise, in the overall kinetic response, the contribution of the initial current decay, due to double-layer charging and the adsorption step. The transients corrected for these fast initial contributions present a peaked shape and can be described by the classical theory for nucleation and growth. The potential dependences of the rate constants of nucleation and growth have been obtained from double potential step experiments. The analysis of the condensation kinetics according to the classical nucleation theory leads to the evaluation of the line tension, and the Gibbs energy of formation of a critical cluster and its size. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Adenine; Adsorption; Condensation kinetics; Critical nuclei; Nucleation and growth 1. Introduction Two-dimensional (2D) phase transitions on electrode surfaces have been investigated extensively in recent years. Among them, formation of condensed monolay- ers of neutral organic molecules is one of the most representative examples, see Refs. [1 – 5] and references cited therein. These compact layers are interesting as corrosion inhibitors, electroplating brighteners, biosen- sors, etc. Several authors [6 – 10] concluded that adenine exhibits two types of adsorption on mercury. At low concentration, the purine base is adsorbed with its ring system parallel to the electrode, i.e. in a flat orientation. This region is referred to as the dilute region. At higher concentrations, very sharply defined pits are observed in the C – E curves. Brabec et al. [8] ascribed the forma- tion of a capacitance pit of adenine to a reorientation of the molecule from a ‘flat’ to a ‘perpendicular’ orien- tation. However, the structural interpretations based exclusively on electrochemical data were questioned later [11]. In recent years, the combination of electro- chemical techniques with structure-sensitive imaging methods such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) provides informa- tion at atomic-level resolution about organic films in the electrochemical environment. In situ STM and AFM studies of condensed adenine films on solid elec- trodes [12–15] have shown the existence of ordered structures, either stacked polymeric chains on Au (111) or a hydrogen-bonded planar network on graphite elec- trodes. These kinds of study have triggered the recon- sideration of the electrochemical experiments at a more quantitative level with special emphasis on the kinetics of phase transitions. Surprisingly, in spite of the early interest in adenine adsorption because of its importance as a constituent of nucleic acid, the kinetics of its condensation have received very little attention. Only scattered data have been presented where the interven- tion of a nucleation and growth mechanism has been * Corresponding author. Fax: +32-2-6502934. E-mail address: cbuess@ulb.ac.be (C. Buess-Herman). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(00)00462-9