Structure and Stability of Cytidine Adlayers on Au(111), Carbon Electrodes with thin Film of Mercury (MFE) and Hg Electrode
The formation, structure and stability of cytidine adlayers on the mercury electrode, carbon electrodes with mercury thin film (MFE) and on the Au(111) electrode has been studied by capacitance-potential, current-potential and transient (C-t and j-t) measurements.
There have been investigated influence pH, temperature, ionic strength on the adsorption of cytidine. Depending on the potential applied to the electrodes, cytidine forms highly ordered two dimensional adlayers, which were imaged in the broad range pH. These ordered states are displayed like capacitance pits on the C-E curve an sharps peak on the cyclic voltammogram. On the ability form 2D adlayers has majority influence value pH and composition of solution.
In the acid solution has been formed only one 2D condensed adlayer (region II). In the neutral and alkaline solution 0.5 M NaCl + BR buffer cytidine forms two different 2D adlayers. The first adlayer (region II) is more negative. More positive has been observed next 2D adlayer (region III). These state is independence on the temperature. Region II in the alkaline solution has lower value differential capacitance that region II in the acid solution. At the pH 7 has been discovered ? metastabile" state (region IIa and IIb). Capacitance pit has two different level differential capacitance. Negative edge this region is independence on the temperature. More negative part this pit is equivalent situation at pH 8. More positive part the pit is corresponding with situation at pH 5. The temperature dependence a phase transient between states IIb and IIa is very interesting. Region IIb is more stable with increase temperature. At the higher temperature region IIa has been vanished.
The kinetics of structural changes in cytidine adlayers on the mercury electrode were investigated by means of current and capacitance transients using the potential step technique. At the hand of j-t transients we studied initial process at the growth/or dissolution physical condensed and highly ordered adlayers. C-t transients revealed changes within physical condensed cytidine adlayers at the long time. The experimental curves were analysed by comparison with a few model based on the nucleation and growth in combination with a parallel Lanqmuir-type desorption process.
2D condensation of cytidine has been observed on the carbon electrodes with thin mercury film. We used the glassy carbon electrode and pyrolytic carbon electrode. We compared quality and structure thin mercury film on the adsorption of cytidine. The nucleation process was more quickly that on the mercury electrode. The kinetics of the interfacial rearrangements were better observed on the pyrolytic carbon electrode with mercury film compared with glassy carbon electrodes coated mercury film.
Adsorption of cytidine on Au(111) from 0.1 M KClO4 electrolyte solution is characterised by formation of various films delimited by 2D phase transitions. Formation and properties 2D condensed cytidine physical adsorbed adlayers is very dependence on the pH solution. In the acid solution exist 2D condensed cytidine adlayer which behaviours on the temperature and concentration of cytidine are different in comparison with neutral solution. The kinetics of dissolution of the ?chemisorbed" adlayer IV was studied with current vs. time transients employing a potential-step technique. The experimental curves were analysed by comparison with a few model based on the nucleation and growth in combination with a parallel Lanqmuir-type desorption process.
This work was supported by grant A 4004002 a Grant agency AV CR 204/97/K084
Stanislav Hason1 a Vladimir Vetterl1,2
1Department of physical electronic, Masaryk University of Brno, Czech Republic